Coconut and Areca Nut

Abstract

The plantation crops, owing to their perennial nature, live through the cycles of several types of stresses. Abiotic stresses such as droughts, dry spells, high and low temperatures and floods significantly affect the growth, development and yield of the plantations. At seedling stage, the high light intensity stress causes seedling mortality if no protective measurements are taken. Studies indicated that plantation crops such as coconut impart drought tolerance through morphological, anatomical, physiological, biochemical and molecular mechanisms. The tolerant genotypes with better revival capacity are also identified. Though genotypic improvement through breeding is important, it is more important to improve the population by incorporating the progeny of identified in situ drought-tolerant and high-yielding palms. This should hasten the selection process for abiotic stress tolerance as well as for high yield as the in situ drought-tolerant palms have withstood the naturally occurring stresses in their life cycle. Apart from genetic improvement, the agronomic management becomes very important in plantation crops for sustaining the yield. Therefore, the soil moisture conservation measures and drip irrigation become very important. In this chapter, these issues are discussed in detail with research evidences with special reference to coconut.

1 Introduction

Growth and productivity of coconut and areca nut palms are influenced by the external factors such as rainfall, temperature, relative humidity and sunshine duration apart from the soil characteristics and management practices. The optimum weather conditions for good growth and nut yield in coconut are well-distributed annual rainfall between 130 and 230 cm, mean annual temperature of 27 °C and sunlight ranging from 250 to 350 Wm⁻² with at least 120 h per month of sunshine period (Child 1974; Murray 1977). Application of NPK is at 500N:320P₂O₅:1200 K₂O per palm/year and optimal irrigation is at 200 L/palm once in 4 days or at 66 % E as drip irrigation. Similarly, areca nut requires a temperature range of 14–36 °C, though it can grow in temperatures ranging from 5 to 40 °C. Also, it requires a well-distributed annual rainfall ranging above 1,200 mm with relative humidity neither very high nor low. Any deviations from the optimal conditions cause the palms to experience the stress conditions. The wide variation in nut yield implies that there is a wide gap in potential and realized yield showing the poor exploitation of production potential. Coconut plantations are seldom grown under ideal management conditions as well as they are continuously exposed to vagaries of nature as perennial crops. While coconut plantations face abiotic and biotic stresses, areca nut plantations in India are generally well managed but have major issues of biotic stresses.

Amount and distribution of rainfall have significant influence on nut yield in coconut (Lakshmanachar 1963; Abeywardena 1968). The relationship between rainfall and nut yield varied (Mathai and Panicker 1979) due to magnitude of rainfall and other factors (Vijayakumar et al. 1988). Weather data-based descriptive models for yield forecasting are developed for different agroclimatic zones (Naresh Kumar et al. 2008). These models had high R² with 4-year lag, suggesting the possibility of forecasting first year nut yield using weather data of first to fourth year. However, with the development and validation of the process-based InfoCrop coconut simulation model (Naresh Kumar et al. 2008), now it is not only possible for yield forecast but also the model is applied for various studies such as climate change impacts, adaptation gains (Naresh Kumar and Aggarwal 2013) and also for yield gap analysis (Naresh Kumar et al. 2008) as well as for assessing the carbon sequestration potential with high reliability and also under multifactor influence. In this chapter, major findings of research on abiotic stresses in coconut are summarized. Since the information on areca nut is not as exhaustive and scientifically analysed as in the case of coconut, relevant available information is provided.

2 Coconut

In general, coconut plantations are exposed to abiotic stresses such as drought, temperature (high and low), light intensity (high and low), cyclonic wind and flooding stress. The environmental stresses affect coconut yield in almost all coconut-growing areas (Coomans 1975; Mathes 1988; Bhaskara Rao et al. 1991).

2.1 Drought Stress: Extent of the Problem

Drought is the major constraint for coconut productivity in rainfed plantations in India as well as in other coconut-growing countries. Hence, the research efforts and the depth of availability of information are more. Coconut is mainly grown as a rainfed crop and the productivity is about 50 % less as compared to irrigated gardens. As a rainfed crop, it faces summer dry spells each year apart from the frequent occurrence of drought years. Water deficit at seedling stage could lead to seedling death. Though coconut seedlings are generally irrigated for their establishment, aftercare is not optimal. Tall palms flower in about 6 years after planting while the dwarfs and hybrids flower by about fourth year. The duration from the initiation of inflorescence primordium to nut maturity is about 44 months. Out of this, the prefertilization period is about 32 months and nut development phase (postfertilization) is 12 months. The primordial initiation, ovary development and button size nut stage are very sensitive to water stress. Coincidence of drought with critical stages affects nut yield (Rajagopal et al. 1996, 2000a) not only in the current year but also in the subsequent 3 years to follow, thus making the problem more severe across all agroclimatic regions of India (Naresh Kumar et al. 2007b). Severely affected palms take at least 4 years to recover after going through stress period. Thus the economic loss to the growers is perennial and significant mainly in rainfed zones of Kerala, Karnataka, Orissa, the northeastern region and Lakshadweep Islands. Average productivity of these regions is significantly low (5,000–8,000 nuts/ha/year) compared to the predominantly irrigated regions (13,000–20,000 nuts/ha/year). The rainfed coconut plantations in intermediate and dry zones of Sri Lanka (Peiris and Peries 1993; Peiris et al. 1995; Peiris and Thattil 1988), Zanzibar (Juma and Fordham 1998) and Indonesia (Bonneau and Subagio 1999) are affected by droughts. Thus, drought is the major constraint for productivity across coconut-growing countries.

In order to sustain the productivity, planting drought-tolerant cultivars with faster recovery potential and agronomical management becomes utmost important. Hence, the research efforts were focused on development of drought-tolerant cultivars as well as agronomic management strategies. In nature, the physiology of a plant’s response to drought stress is rather complex. Because of the dynamics of soil water depletion and atmospheric water demand, the length and severity of stress have spatio-temporal variations. Moreover, the response of a plant to abiotic stress is a multidimensional one with responses starting at the cellular and intercellular level to the organ and phenological stage level varying with space and time. Over generations, plants developed adaptive mechanisms, i.e. molecular, biochemical, physiological, anatomical, morphological and phenological, to overcome drought stress.

The length and intensity of dry spell and influence of rainfall and dry spell on the nut yield in major coconut-growing areas in different agroclimatic zones, viz., in the western coastal area, hot, subhumid and per-humid (Kasaragod, Kerala; Ratnagiri, Maharashtra); in Western Ghats, hot, subhumid and per-humid (Kidu, Karnataka) and hot and semiarid (Arsikere, Karnataka); and in eastern coastal plains, hot and subhumid (Veppankulam, Tamil Nadu; Ambajipeta, Andhra Pradesh), in India has indicated that the dry spell was longer in Ratnagiri (216 days) and Arsikere (202 days) and shorter at Kidu (146 days), which differentially affected the nut yield (Naresh Kumar et al. 2008). The annual nut yield under rainfed conditions varied at different zones. Fluctuations in coconut yield during different years could be explained on the basis of rainfall distribution. However, the length and number of dry spells are more important than the total rainfall per se which influences the nut yield (Rajagopal et al. 2000b; Naresh Kumar et al. 2008). In view of the long duration (44 months) between the inflorescence initiation to nut maturation, the occurrence of dry spell in any year would affect the yield for the subsequent 3–4 years, depending on the critical stages. Consecutive droughts in Coimbatore District (Tamil Nadu) during 1998–2002 reduced the coconut production by about 3 lakh nuts/year for 4 years. The productivity loss was to the tune of about 3500 nuts/ha/year (Naresh Kumar et al. 2008) (Fig. 15.1) (Photo 15.1).

Fig. 15.1

Trends in coconut production in two districts of Tamil Nadu

Photo 15.1

Drought affected coconut garden

2.2 Nut Yield in Relation to Coincidence, Intensity and Length of Drought Stress

The coconut palm is influenced considerably by the environmental variables in its productive features especially under rainfed condition. Of all the climatic factors, rainfall has the maximum influence on the seasonal fluctuation in yield (Abeywardena 1968). In coconut palm, there is a long duration (~44 months) between inflorescence primordial initiations to nut maturity with the prefertilization period (~32 months) being longer than the postfertilization (12 months) period. Hence, the impact of drought occurring during any of the critical stages of the development of inflorescence affects nut yield (Rajagopal et al. 1996). The impact of drought on the ontogeny of coconut inflorescence integrating the overall occurrence of dry spell and growth stages of the developing nut has been delineated (Rajagopal et al. 1996). The intricate relationship between dry spell and stages of nut development right from inflorescence initiation to the nut maturity as well as annual nut yield in different agroclimatic zones has been well described (Rajagopal et al. 1996, 2000a; Naresh Kumar et al. 2008). Physiological traits responsible for drought tolerance correlated with yield performance under stress conditions, and some of the cultivars identified as drought tolerant also proved to be good yielders (Bhaskara Rao et al. 1991; Rajagopal et al. 1992). There were genotypic variations for the drought index in coconut (Pomier and de Taffin 1982). By providing life-saving irrigation during summer months, the adverse effects of dry spells especially on the development of the inflorescence primordium can be reduced (Naresh Kumar et al. 2003).

The influence of drought on nut yield becomes manifest in the subsequent year (Bhaskara Rao et al. 1991). Seven lag periods were worked out to relate 11 weather variables with coconut yield (Vijayakumar et al. 1988). Among the variables tested, rainfall had positive influence on five of the seven lag periods while temperature and relative humidity on two or three lag periods. The weather-based descriptive models for different agroclimatic zones were developed (Naresh Kumar et al. 2009). The prediction models with 3- and 4-year lag had high R² values. The models differed for usage of parameters in different agroclimatic zones, indicating the relative importance of these parameters in respective conditions for realizing the nut yield in coconut. Interestingly, the parameters used in models for the western coastal area (hot, subhumid and per-humid) are temperature and relative humidity, as indicated even in the classification of these areas. Models were verified for 2 years and prediction of yield during 1998–1999 and 1999–2000 within 10 % confidence level validated these models. The study indicates that the relative humidity and temperature play an important role during the ontogeny of inflorescence and nut development. The descriptive models, developed based on weather data, can be used for the prediction of coconut yield 2–4 years in advance within acceptable range of accuracy. The yields to be realized can be bracketed within the predicted range obtained from models using 1-, 2-, 3- and 4-year lags (Naresh Kumar et al. 2009; Fig. 15.2).

Fig. 15.2

Annual nut yield and variation in annual yield as influenced by annual rainfall and dry spell over a period of time at Kasaragod

Among the hybrid combinations in India, two of them performed well under drought conditions in which yield stability was found maintained through lower reduction in bunch production, pistillate flower production, nut setting and yield (Bhaskara Rao et al. 1991; Table 15.1).

Table 15.1 Performance of hybrids during good and drought-influenced years

Among the three hybrid combinations, viz., Chandrasankara (COD × WCT), Chandralaksha (LCT × COD) and Lakshaganga (LCT × GBGD), Chandrasankara outyielded both but Lakshaganga and Chandralaksha performed well under drought situations than Chandrasankara (Table 15.1). Physiological traits responsible for drought tolerance correlated with yield performance under stress conditions and some of the cultivars identified as drought tolerant also proved to be good yielders (Rajagopal et al. 1992). However, stability analysis needs to be done assessing the suitability of cultivars across the agroclimatic zones (Siju Thomas et al. 2005).

2.3 Button Shedding

Flowering in coconut is a continuous process and the development and growth of inflorescence is intimately associated with the development and growth of the leaf (Patel 1938). Once the inflorescence (spadix) opens, it takes about 12 months for full maturity of the buttons and harvest of nuts. Being a tree crop of indeterminate growth habit, the partitioning of metabolites towards vegetative and reproductive growth is a continuous process and interlinked during the entire span of its life. Shedding of buttons is one of the major constraints in coconut production. Under normal conditions, within 3 months of the opening of the inflorescence, about 80 % of the pistillate flowers are shed. Extensive reports are available on the impact of various factors on the shedding of buttons (Sudhakara 1990; Ramadasan et al. 1991; Kasturi Bai et al. 2003).

During summer months under unirrigated condition, soil water deficit is the major cause for the shedding of buttons, which gets aggravated with the changes in the micrometeorological variables. Although high rainfall is not harmful to the growth and productivity of palms, shedding can be observed due to the impairment of pollination and fertilization (Menon and Pandalai 1958). Significant differences have been observed in pistillate flower production, shedding of buttons and nut production between the cultivars (Kasturi Bai et al. 2003). High correlation has been observed between the pistillate flower production and shedding of buttons. This implies that the increase or decrease in the button shedding depends on the number of pistillate flowers in the spadix which show variation between cultivars.

There are also seasonal variations in pistillate flower production and button shedding under irrigated and unirrigated conditions (Table 15.2). In WCT and its two hybrids, WCT × COD and COD × WCT, pistillate flower production and percentage button shedding did not differ significantly under irrigated and rainfed conditions. However, increasing trend in pistillate flower production was observed under irrigated conditions. In general, in all the varieties, lower pistillate flower production is observed in the post-monsoon season as compared to summer (February to May) and monsoon (June to September) months.

Table 15.2 Seasonal variation in pistillate flower production (PFP-No.) and button shedding under irrigated (I) and rainfed (RF) conditions (mean values)

Irrespective of irrigation or rainfed condition, button shedding occurred in two peaks in all the varieties, one during summer months and the other during monsoon. During post-monsoon season, the number of pistillate flower production is low; consequently, the drop percentage is also low in all the varieties. Significant differences in pistillate flower production and percentage drop were observed only between different months (Kasturi Bai et al. 2003).

2.4 Response of Palms to Field Drought Stress

Exposure to excess radiation (>265 Wm⁻²), temperature (>33 °C) and vapour pressure deficit (>26 m bar) cause stress in coconut palms (Kasturi Bai et al. 1988) and are further aggravated by water deficit. The duration of dry spell during initiation of inflorescence primordium, ovary development and button size nut stages in that order has greater influence on nut yield than other stages (Rajagopal et al. 1996). Critical levels of water deficit vary with soil type. Soil types and compaction levels influenced the water stress in seedlings (Nainanayake and Bandara 1998). A water deficit of 110 mm is critical in sandy loam soil, at which the stomata close (Rajagopal et al. 1989). The photosynthetic rates and dry matter production and its partitioning are influenced by the soil water status (Kasturi Bai 1993; Naresh Kumar et al. 2002a). Soil water deficit coupled with atmospheric evaporative demand during dry months affected coconut palms more in red sandy loam than laterite soil (Voleti et al. 1993a). Hybrids had higher stomatal resistance resulting in maintenance of high water potential during stress in laterite soil than in sandy loam. However, the hybrid, COD × WCT, was sensitive to water stress in sandy loam soil. Exposure of palms to moisture stress for 3–4 weeks led to reduction in the vegetative dry matter by 15–18 % and reproductive dry matter by 20–22 %, as compared to irrigated palms (Rajagopal et al. 1989). Soil water deficit in unirrigated conditions reduced the leaf water potential and enhanced the activity of stress-sensitive enzymes (Shivashankar et al. 1991). Four general types of leaves were found in coconut canopies, viz., (i) leaves with higher photosynthesis rate (Pn) and water use efficiency (WUE) than mean performance of canopy leaves, (ii) leaves with higher Pn and lower WUE, (iii) leaves with lower Pn and higher WUE and (iv) leaves with lower Pn and lower WUE (lower leaves) (Naresh Kumar and Kasturi Bai 2009a). An oval-shaped canopy is more suitable for higher photosynthesis efficiency, WUE and productivity as compared to X-shaped and semicircle-shaped canopies. These results indicate that the canopy shape plays a role in the overall performance of photosynthesis and water use efficiencies and productivity in coconut. Results also indicate coconut as a source-limited plant (Naresh Kumar and Kasturi Bai 2009a).

2.4.1 Morphological Symptoms

In rainfed conditions, a prolonged dry spell affects the palm. An ‘aridity index’ of 100 % for a prolonged period of 5–10 weeks severely affects the productivity of coconut palms (Table 15.3). When exposed to such severe moisture stress, adverse effects such as bending and breaking of dry leaves, poor spathe development and most bunches with only one or two nuts are seen (Rao 1985).

Table 15.3 Relationship between aridity index and symptoms of drought in coconut

The drought index, calculated based on the morphological symptoms, is related to nut yield (Ramadasan et al. 1991). The drought index was found to be the lowest in the hybrids WCT × PHOT and WCT × GBGD and the highest in WCT × MOD among ten hybrids. The ‘index to drought tolerance’, calculated based on the percentage of dry leaves (n), compared to the number of living ones (N), i.e. (n/N) X 100, was higher for the hybrid PB-121 and least for Rennell Tall × WAT among five hybrids (Pomier and de Taffin 1982).

The characterization of drought, in different coconut-growing areas, in six states of India falling under different agroclimatic zones, revealed that the length and intensity of dry spell adversely affect the source-sink relationship and consequently the nut yield (Naresh Kumar et al. 2003; 2006a). These agroclimatic zones varied for annual rainfall from 718 to 3,338 mm. Similarly dry spell varied from 146 to 216 days and consequently annual nut yield varied from 30 to 68. In view of the long duration (44 months) between the inflorescence initiation to nut maturation, the occurrence of dry spell in any 1 year would affect the yield for the subsequent 3–4 years. The longer dry spell affects the nut yield for next 4 years to follow with stronger impact on fourth year, irrespective of the total rainfall. Apart from this, current year nut yield is also affected (Naresh Kumar et al. 2007).

2.4.2 Leaflet Anatomical Adaptations in Relation to Drought Stress

Anatomical basis of physiological efficiency for drought tolerance in coconut is delineated. The increase in leaflet thickness is mainly due to increase in parenchyma cell size. It is also associated with lowered stomatal frequency, an indication of adaptation to drought. Increased leaf thickness and thick cuticle are some of the xeromorphic characteristics as observed in WCT and FMS cultivars. An increase in thickness of the leaflet causes a decrease in the ratio of the external surface to its volume. Water in leaves is conducted not only by the veins and bundle sheath extensions but also by the mesophyll cells, epidermis and intercellular spaces. Water transport towards the epidermis is much higher through the palisade tissue than the spongy parenchyma. Increased parenchyma cell size, as observed in WCT and FMS, means less intercellular space/unit area. This may help in reducing the water conductance towards epidermis thus reducing the transpiration rates and maintaining high water potentials, a characteristic feature of drought tolerance. The volume of intercellular spaces in xeromorphic leaves is low thus reducing the water transport to epidermal cells. The structure favourable to high photosynthetic rates (large palisade parenchyma tissue surface due to small parenchyma cells, Table 15.4) induces at the same time high transpiration rates because of higher intercellular space (Naresh Kumar et al. 2000). Stomatal frequency (SF) and index (SI) also play a major role in plant water relations. Variations in SF and SI were observed among the talls, dwarfs and hybrids (Rajagopal et al. 1990).

Table 15.4 Correlations between anatomical features and morpho-physiological parameters

The WCT and FMS have thick leaflets, thick cuticle on both sides and larger parenchyma, hypodermal and water cells compared to PHOT, WCT × COD, COD × WCT, GBGD and MYD (Naresh Kumar et al. 2000). This implies adaptability or tolerance of the former cultivars for drought condition than the latter. The water storage tissues supply water to other tissues when water is limited. Cultivars having thick cuticle are able to maintain higher leaf water potentials (Table 15.4). Drought-tolerant types also have more scalariform thickening on xylem tracheids in vascular bundles and large sub-stomatal cavities. These traits are less favourable in PHOT and WCT × COD implying moderate tolerance to drought. The values for these traits are the least in COD × WCT, GBGD and MYD indicating susceptibility to drought. However, the difference between moderately tolerant and susceptible cultivars for these traits is narrow. Large sub-stomatal cavities will help in maintaining enough internal CO₂ concentrations required for sustaining the photosynthetic rates during the stress period when the stomata are partially closed. High internal CO₂ concentrations may help in reducing the water loss through stomata. Epidermal cell size (upper and lower) and guard cell size are related to the drought tolerance characteristic of a cultivar. It is possible that the cumulative effect of all these traits contributes to the adaptation to drought stress (Naresh Kumar et al. 2000).

In coconut, the leaf (epicuticular wax) ECW, which influence the energy balance of leaf, did not differ significantly among cultivars and hybrids during favourable conditions. Thin-layer chromatographic analysis revealed the qualitative difference in the composition of wax during different stages of palm growth. Major components identified were hydrocarbons and esters while the alcohols were identified in the stress period. On the other hand, fatty acids occurred only during post-stress (Voleti and Rajagopal 1991). Almost a three to fourfold increase in ECW was noticed during dry season in some of the coconut hybrids, viz., WCT × GBGD, WCT × COD, LCT × COD and LCT × GBGD (Voleti and Rajagopal 1991; Kurup et al. 1993). The physiological age of palms and leaves influenced the formation of wax on leaf surface. Leaves of coconut seedlings have almost 50 % less ECW than those from adult palms even at same degree of stress.

2.4.3 Physiological Responses to Drought Stress

2.4.3.1 Stomatal Gas Exchange and Water Relations

Coconut responds to water stress by stomatal regulation and deposition of ECW to maintain leaf water potential. Osmotic adjustment is also found to be key in imparting tolerance to drought (Kasturi Bai and Rajagopal 2000). Stomatal regulation has been found to be a key factor in controlling the water balance in coconut (Milburn and Zimmermann 1977; Rajagopal et al. 1986, 1988), and it is used to monitor development of stress in different coconut cultivars (Kasturi Bai et al. 1988; Juma et al. 1997; Voleti et al. 1993a). Rainfed palms had higher leaf-to-air vapour pressure deficit (LAVPD), leaf temperature (T _leaf ) and leaf-to-ambient temperature difference (ΔT), whereas the irrigated palms had higher photosynthetic rate (Pn), Ψ _leaf and transpiration rates (E) (Rajagopal et al. 2000a). A reduction in Fv/Fm (photochemical efficiency) and Fm/Fo (quantitative indicator of the Ψ _leaf ) with decreasing leaf water potential during stress period indicated damage to photosynthetic apparatus (Kasturi Bai et al. 2006b). Based on principal component loadings, it is found that chlorophyll fluorescence transients, viz., F0 and t½ and Ψ_leaf, are important traits which can be used to differentiate and screen the coconut seedlings that can adapt to water stress condition (Kasturi Bai et al. 2008).

The leaf water potential (Ψ _leaf ), an indicator of plant water status, showed a vertical gradation from middle leaf upwards, with magnitude being higher under rainfed condition (Voleti et al. 1993b). The spindle leaf maintains higher ψ _leaf throughout the day irrespective of rainfed or irrigated conditions (Voleti et al. 1993b). Seasonal variations in the ψ _leaf occur depending on the weather, type of soil and soil water availability (Shivashankar et al. 1991; Voleti et al. 1993a). A rapid screening method was developed based on Ψ _leaf in excised leaflets (Rajagopal et al. 1988) for easy handling of a large number of genotypes. The Ψ _leaf declined with time to different degrees of stress among the genotypes, indicating the degree of tolerance/susceptibility to stress. In general, ψ _leaf is lower in palms grown in red sandy loam than those in laterite soil. In irrigated conditions, the ψ _leaf is maintained at relatively high level even during the non-rainy period. A diurnal fluctuation in the ψ _leaf has been observed in coconut (Rajagopal et al. 2000a). The daytime trends in Pn and gs rates show two peaks, one at 9:00 and another at 15:00 hours under rainfed condition. But, under irrigated condition, the peaks were less distinct. The inverse relationship between Ci/Ca and instantaneous WUE (Pn/E) indicates that during rainfed growth non-stomatal limitations reduce the Pn rates compared to irrigated conditions (Rajagopal et al. 2000a). The instantaneous WUE (Pn/E) also had two peaks during the day in rainfed palms. In irrigated palms, instantaneous WUE was significantly higher than rainfed ones. The intrinsic WUE (Pn/gs) also had almost similar trends (Rajagopal et al. 2000a). Stomatal regulation has been found to be the first line of adaptive mechanisms of coconut to withstand the water deficit conditions (Rajagopal et al. 2000a). In dwarfs, low rs resulted in high E in turn lowering ψ _leaf . On the other hand, in talls and hybrids like WCT × WCT, high rs is associated with high ψ _leaf . Among the cultivars and hybrids, ψ _leaf is relatively high (−1.20 MPa) in FMS, Andaman Giant and SSAT among the talls, in COD among the dwarfs and in WCT × GBGD and WCT × WCT among the hybrids. Thus, behaviour of palms in drought conditions depends on several factors, viz., water relation components like E, gs and ψ _leaf and agrometeorological factors like solar radiation, rainfall and humidity, as well as soil factors.

The photosynthetic rates (Pn) reduced due to water stress mainly because of increase in stomatal or mesophyll resistance, with higher reduction noticed in susceptible types than in tolerant cultivars (Kasturi Bai et al. 1998). The Pn rate is independent of ψ _leaf until a threshold ψ _leaf is reached. The Pn/gs increased while Pn/E (instantaneous WUE) decreased during stress period. Drought-tolerant hybrids such as WCT × COD, LCT × GBGD and LCT × COD exhibited higher increase in Pn/gs ratio as well as higher WUE than that of susceptible types during stress period. The relation between Pn/gs and WUE is linear in both tolerant and susceptible types. The relation between Pn/Ci and Pn is significant and linear in both drought-tolerant and susceptible types (Kasturi Bai et al. 1998).

The potential of palms for higher DM production is reflected in WUE. WUE, determined based on dry matter accumulation (g. DM mm^–1 water used) as well as by gas exchange measurements (μmol CO_2. mmol⁻¹ H₂O), ranged between 28.8 and 69.3 gDM mm⁻¹ water (Kasturi Bai et al. 1996a) among the cultivars/hybrids. The three hybrids, viz., WCT × COD, LCT × GBGD and LCT × COD and WCT, had higher WUE than the others. WUE was higher in unirrigated palms than in irrigated ones (Rajagopal et al. 1989), and under mild stress conditions, the WUE is improved in coconut juvenile palms (Rajagopal et al. 2000a). Preliminary results from carbon and oxygen isotope discrimination studies at the CPCRI in India, Coconut Research Institute (CRI) in Sri Lanka and Essex University in the United Kingdom indicated strong negative relation between carbon isotope discrimination and WUE (Anonymous 2008).

When high evaporative demand in the atmosphere prevails, cultivars exhibit differential adaptability through stomatal regulation (Jayasekara et al. 1993), which is high in hybrids followed by talls. In dwarfs, it is almost 50 % less than that in hybrids. This indicates the higher transpiration loss of water in dwarfs than in talls and hybrids. Among the hybrids studied, COD × WCT had significantly low rs. The transpiration rates (E) are significantly lower in WCT × WCT, FMST, JVGT, LCT × COD, LCT × GBGD, WCT × COD and WCT than in other cultivars and hybrids. Based on leaf water status, gas exchange and membrane lipids at the nursery stage, PB-121 was found to be tolerant and WAT moderately tolerant to drought among five coconut varieties (Repellin et al. 1994). Similarly, Kasturi Bai et al. 2001 reported the extent of variation in physiological characters among the parents and hybrids at the nursery stage.

2.4.3.2 Membrane Stability and Osmotic Adjustment

Solute leakage differed among the cultivars and with the maturity of leaf. WCT and hybrid WCT × COD had higher stability of membranes indicating relative tolerance, while the hybrid COD × WCT had higher leakage indicating susceptibility. The membrane stability is also influenced by the potassium (K+) and phenol levels. Less K+ leakage was seen in WCT × COD as compared with the COD × WCT hybrid. Higher phenol content in leachate is seen also due to heat stress which varies among cultivars and leaf positions. Coconut palms accumulate organic solutes such as sugars and amino acid during stress period. However, during moderate stress period, solute accumulation did not differ significantly between the drought tolerant and susceptible. Accumulation of these solutes was more in WCT × WCT than COD × WCT during severe stress condition (Kasturi Bai and Rajagopal 2000).

2.4.3.3 Root-Shoot Signals

Roots in drying soil are known to overproduce abscisic acid (ABA) thus providing signals to shoot for closure of stomata for water regulation in plants (Zhang and Davies 1989). In coconut also, root-shoot relationship was reported to be an effective indicator of soil compaction and water stress for coconut seedlings (Nainanayake et al. 2000). A high ABA/cytokinin ratio in the leaf has positive influence on water use efficiency (WUE), whereas a high ABA/cytokinin ratio in the root has a negative influence on WUE in coconut seedlings (Kasturi Bai et al. unpublished data).

2.4.3.4 Role of K⁺ and Cl⁻ Nutrition in Relation to Drought Tolerance

The K⁺ and Cl⁻ nutrition impart drought tolerance to coconut through stomatal regulation (Braconnier and D’Auzac 1990; Braconnier and Bonneau 1998). Unlike in most of the crops where malate serves as a balancing ion for K⁺, in coconut, the absence of chloroplasts in the guard cells deprives the availability of malate (Braconnier and D ’Auzac 1985). Probably Cl⁻ replaces malate as an osmoticum for maintaining the turgidity and thereby resisting the effects of water stress. Hence it becomes the most essential ion required for coconut growth, particularly under dry conditions. The Cl⁻ ion has been shown to increase water absorption and reduce transpiration by stepping up osmotic pressure within the cells in coconut (Ollagnier et al. 1983). Chlorine is important in coconut nutrition and for resistance to water stress; the critical level of Cl was identified as 0.7 % in 14th leaf (Bonneau et al. 1993, 1997). Potassium nutrition also plays important role in drought tolerance in coconut (Quencez and de Taffin 1981; Rajagopal et al. 2000b; Rajagopal and Naresh Kumar 2001). Application of KCl increased the drought tolerance of palms under dry conditions (Ollagnier et al. 1983; Rajagopal and Naresh Kumar 2001). Palms fertilized with higher levels K₂O under rainfed condition shown higher stomatal regulation (Lubina 1990). An inadequate supply of K⁺ and Cl⁻ results in symptoms like yellowing and drying of leaves caused by an imbalance in the water relations of palms.

2.4.4 Biochemical Responses to Drought Stress

The biochemical response of coconut palm to drought stress includes up-regulation or synthesis of scavenging enzymes to maintain cell membrane integrity thus enabling cells to tolerate stress. Concentrations of leaf epicuticular wax, proline, reducing sugars and amino acids increased during summer and tall cultivars, viz., WCT and LCT, exhibited more accumulation of these compounds. This resulted in wider adaptability of tall cultivars to different environments (Siju Thomas et al. 2006a, b).

2.4.4.1 Activities of Enzymes

Decrease in leaf water status during drought stress or during prolonged dry spell caused an increase in the activities of the stress-sensitive enzymes, viz., peroxidase (PO), polyphenol oxidase (PPO), superoxide dismutase (SOD), acid phosphatase (APh) and L-aspartate: 2-oxoglutarate amino transferase (AAT), in WCT palms, while activities of malic dehydrogenase (MDH) and nitrate reductase (NR) were decreased (Shivashankar et al. 1991; Kasturi Bai et al. 1996b, 2003). Among five hybrids, WCT × SSGT was more tolerant to drought in terms of water relation components and activities of enzymes such as malic dehydrogenase, peroxidase, SOD and catalase (Kasturi Bai et al. 2005a). Thus, regulation of enzyme activities imparted tolerance to drought stress. Similar responses were observed with PEG-induced osmotic stress, except that the intensity of change was much higher (Shivashankar 1988) due to the rapid induction of stress. Increase of APh and AAT activities in coconut leaves subjected to osmotic stress is correlated with decline in ψ _leaf (Rajagopal et al. 1988). The threshold values of leaf water potential for sudden increase in activities of APh and AAT are −1.6 M Pa and −1.8 M Pa, respectively, indicating that APh is affected earlier in the sequence. Changes in the activity or appearance of isozymes represent the relative tolerance of coconut cultivars to water stress (Shivashankar 1988). Increased intensity of APh isozyme II shows the susceptibility of the cultivar to water deficits since APh is a hydrolytic enzyme (Shivashankar and Nagaraja 1996). In osmotically stressed coconut leaves, peroxidase isozymes change quantitatively, while PPO (Shivashankar 1988) and APh isozymes change both quantitatively and qualitatively. During stress, one of the isozymes of APh (APh II) undergoes alterations in its kinetic parameters, especially in its Km value towards PNPP and other naturally occurring phosphate compounds like ATP, NADP and glucose-6-phosphate (Shivashankar and Nagaraja 1996). The variability in the isozyme patterns of enzymes like esterase, peroxidase, phosphoglucoisomerase, alcohol dehydrogenase, glutamate oxaloacetate transaminase and acid phosphatase was also reported in coconut germplasm (Fernando and Gajanayake 1997). Soluble protein levels also increased during stress indicating increased solubilization of enzyme protein or additional synthesis in response to stress.

The NR activity decreases even when there is no perceptible change in relative water content (RWC) of leaves and the fall in activity is higher in the drought-susceptible cultivars than in the drought-tolerant cultivars. Thermal stability of NR in vivo in several coconut cultivars was found to be correlated with drought tolerance (Shivashankar 1992). An increase in the intensity of peroxidase bands could be considered as an adaptive mechanism. Two additional fast-moving bands of PPO were located in the drought-susceptible varieties under stress, while the drought-tolerant cultivars showed no change (Shivashankar 1988). It is clear that the drought-tolerant varieties are endowed with a biochemical mechanism to prevent the adverse effects of drought by appropriate regulation of enzyme activities.

2.4.4.2 Membrane Stability in Relation to Drought Stress

At the cellular level, the integrity of membranes is affected, and the extent of solute leakage is regulated by the membrane stability. Normal cell functions are affected due to changes in peroxidation of cell wall lipids (LP) during stress resulting in increased cell permeability and solute leakage. In coconut, lipid peroxidation was high in drought-susceptible cultivars as compared to tolerant ones (Chempakam et al. 1993). An increase in the activities of peroxidase and SOD during stress appears to be essential for maintaining the integrity of the cell, since they along with catalase protect the cell against oxidative damage. Higher levels of PPO in susceptible cultivars are due to a higher rate of cell injury, resulting in the release of PPO from plastids to the cytoplasm under induced stress (Chempakam et al. 1993; Kasturi Bai et al. 1996b) and also under water stress (Shivashankar 1988). Thus, tissue peroxidation levels are negatively correlated with activities of SOD, catalase and peroxidase, while PPO is positively correlated. In seedlings, a negative relationship between ψ _leaf and peroxidation of cell wall lipids has been found. As ψ _leaf decreased, an increase in lipid peroxidation has been observed during stress period, while relief of stress reversed this indicating recovery (Kasturi Bai et al. 2001).

Drought tolerance is thus characterized by higher activities of the protective enzymes like SOD, catalase and peroxidase and consequently coupled with lower levels of lipid peroxidation and higher membrane integrity (Shivashankar 1988; Chempakam et al. 1993; Kasturi Bai et al. 1996b). Coconut seedlings of the tolerant group maintained lower water loss and lipid peroxidation than the susceptible group and a negative correlation between leaf water potential and lipid peroxidation was observed (Kasturi Bai et al. 2001). Total leaf lipid and chloroplastic major lipid (monogalactosyldiacylglycerol) contents reduced in drought-susceptible cultivars (Repellin et al. 1997).

2.4.5 Molecular Markers for Drought Tolerance Traits

Studies on stress-responsive proteins in coconut indicated expression of low-, medium- and high-molecular-weight stress proteins in seedlings exposed to drought stress (Naresh Kumar et al. 2007a). Studies using RAPD and ISSR analysis indicated correlation between leaf water potentials and molecular markers in coconut cultivars (Manimekalai et al. 2004). Water stress-related MAPK genes were sequenced in coconut (Bobby et al. 2010). However, development of molecular markers for drought tolerance is still in progress. Efforts are on to link the drought tolerance to molecular diversity to find putative molecular markers, which can be useful for marker-assisted selection (MAS). Even though lack of a viable regeneration technique is a bottleneck for genetic engineering of coconut palm, the molecular markers should be identified for use in large-scale rapid screening of germplasm. This will not only increase the efficiency for selection of parental material but also will reduce the gestation period for breeding improved varieties with drought tolerance (Batugal 1999). The RFLP analysis indicated that tall and dwarf ecotypes from Pacific and Far East Asia were different from those from India, Sri Lanka and West Africa (Lebrun et al. 1998, 1999). An in vitro screening technique was developed using NaCl as the osmoticum at different concentrations in coconut embryo culture medium (Karunaratne et al. 1991).

Plants have earliest signalling pathways after perception of stimuli. The evolutionary conserved mitogen-activated protein kinase (MAPK) cascade is a major pathway by which extracellular signals are transduced into internal responses. A cDNA clone of 467 bp encoding an MAPK (designated CnMAPK1) was isolated from leaves of coconut plantlets subjected to water stress using polyethylene glycol (PEG). CnMAPK1 was significantly homologous to other plant MAPKs, viz., MPK7 and MPK5 from Zea mays, SIPK and MAPK6 from Oryza sativa and FLRS from Triticum aestivum (93 % identity). Coconut MAPK belongs to the serine/threonine kinases (STKs), plant TEY MAPK subfamily group A (Bobby et al. 2012) (Fig. 15.3 and Photo 15.2).

Fig. 15.3

(a) RNA isolation. (b) RT-PCR amplification of coconut transcript for MAPK. (c) RT-PCR amplification of coconut transcript for rbcL. M, 100 bp DNA ladder; M1, 1 kb DNA ladder; PEG-tr, 20 % PEG treated; C, control (Bobby et al. 2012)

Photo 15.2

Predicted 3-D structure of coconut CnMAPK1 protein generated using MODELLER program (Bobby et al. 2012)

A total of 129 transcripts are differentially expressed or up-regulated during water stress in coconut seedlings. Out of ten amplified water stress-responsive candidate genes, five, viz., AP2, CBF, MAPK, NAC and 14-3-3, were up-regulated during water stress. Many of these have significant homology with closely related species in Arecaceae. From the overall experimentation, 50 highly significant gene sequences based on the similarity, identity and E value were chosen for designing functional markers, which will be used for future studies to screen coconut germplasm collection for tolerance/susceptibility to water stress. From these, 40 markers were designed from DDRT-PCR analysis and 10 markers were RT-PCR amplified. These functional markers may be used for screening germplasm and in marker-assisted selection for breeding drought-tolerant coconut cultivars (Bobby et al. 2012).

It is possible to link the in vitro and nursery screening techniques to molecular techniques for the development of molecular markers. Once the markers are established, they will be of prime importance to identify the parental material in breeding for drought tolerance. At the same time, it is essential that the stability of drought tolerance through pheno-phases is also to be established. Thus development of molecular markers and application of biotechnological tools for development of drought-tolerant coconut varieties needs more emphasis and concerted efforts. The future challenge is in overcoming the bottlenecks in implementation of genetic engineering for development of drought-tolerant coconut varieties.

2.5 Drought Tolerance Mechanism in Coconut

Based on all the above, the mechanism of drought tolerance and stability in yield of coconut under water stress conditions has been deciphered (Schematic Diagram 15.1). Drought tolerance in coconut is the cumulative effect of several inductive morphological, anatomical, physiological and biochemical mechanism (Rajagopal and Kasturi Bai 2002; Naresh Kumar et al. 2000). Genotypes possessing the above traits of drought tolerance can be used in breeding programmes. Further, the genetics of these important traits are being looked into for developing future coconut improvement strategies.

2.6 Coconut and Other Abiotic Stress

Apart from drought, coconut is affected by other stress like flooding, cyclone, etc. Loss due to 1996 cyclone in Konaseema led to a reduction in coconut yields by about 3350 lakh nuts/year for 6 years. The loss in East Godavari district alone was to the tune of about 2200 lakh nuts/year in 6 years. The productivity was reduced by 6200 nuts/ha/year in E. Godavari District and by ~4100 nuts/ha/year in AP. Similarly, a supercyclone in Orissa severely damaged the yield (Naresh Kumar et al. 2008) (Fig. 15.4 and Photos 15.3 and 15.4, Table 15.5).

Table 15.5 Loss in nut production in AP due to 1996 cyclone

Fig. 15.4

Trends in coconut production in three districts of AP

Photo 15.3

Cyclone affected palms

Photo 15.4

Flood affected palms

2.6.1 Flood-Affected Palms

Coconut palms generally are not affected by short-term (10–15 days) flooding but if water stagnation prolonged, palms suffer physiological drought, a condition where palms will not be able to uptake the water or nutrients due to hampered root activity as a consequence of lack of oxygen for root respiration.

2.6.2 Abiotic Stress Responsive Protiens in Coconut

Coconut seedlings of different cultivars and cross combinations subjected to stresses like water, high temperature and flooding stresses independently showed an increase in the concentration of heat-stable protein fraction (HSPF) in leaf tissue even when the total protein concentration reduced due to stress (Naresh Kumar et al. 2007). The percentage of HSPF in leaf tissue increased with the decrease in leaf water potentials (Table 15.6). Quantitative changes in proteins also were observed in leaf and root tissue due to temperature and flooding. In HSPF, proteins of ~66 KDa are found in root and leaf tissue. Leaflet tissue have specific proteins in the range of ~10 and ~14 KDa which are not present in the root tissue. Two extra proteins of 66 KDa and ~76 KDa appeared in water-stressed WCT seedlings. They were also present in seedlings exposed to high light intensity (~1,500 mmol/m²/s) apart from an extra LMW HSP of 14.4 KDa. Among the MMW proteins, a protein of 53 KDa is present in all WCT samples. The LMW protein of 20.1 KDa present in non-stress seedlings disappeared during water stress period. In temperature-induced and flooded leaflet tissue, new proteins (LMW range ~17 KDa) were observed. In temperature-induced root tissue, a protein band in the range of 30 KDa was observed but not in flooded root or in control root. Results indicate quantitative and qualitative variations in stress proteins in coconut seedlings subjected to different abiotic stresses (Naresh Kumar et al. 2007).

Table 15.6 Quantitative and qualitative changes of proteins in coconut seedlings subjected to different stresses

2.6.3 Response of Coconut to Elevated CO₂ and Temperature

Elevated CO₂ (550 and 700 umol/mol) and temperature conditons caused an increase in the activities of superoxide dismutase and catalase, whereas they reduced polyphenol oxidase activity. On the other hand, peroxidase (POX) activity decreased in elevated temperature, while it increased under elevated CO₂ conditions. The POX activity and membrane stability index (MSI) were positively correlated. By virtue of greater MSI and lower MDA content, WCT and COD × WCT were rated to be tolerant to oxidative stress among the seedlings of three cultivars, viz., WCT, LCT and COD, and two hybrids, viz., WCT × COD and COD × WCT, under elevated CO₂ and temperature conditions. These cultivars may adapt better to changing climates (Naresh Kumar et al. 2008; Sunoj et al. 2014). After exposure to elevated CO₂ and temperature treatments for about 2 years, the elevated CO₂ increased the accumulation of proline in leaf tissue in spite of maintenance of high leaf water potentials (Fig. 15.5). On the other hand, elevated temperature caused only a slight increase in proline concentration, despite the reduced leaf water potentials (Fig. 15.6). These results suggest a greater role for proline in coconut adaptation to high CO₂ concentrations (Sunoj et al. 2009a; 2013). Elevated CO₂ and temperature reduced total phenolic concentration in majority of cultivars during both the seasons (Fig. 15.7). However, in WCT, a popular cultivar extensively grown in Kerala, India, the concentration of total phenolic compounds increased in elevated CO₂ and T conditions. On the other hand, reduction in concentration of phenolic compounds is more in COD, a dwarf cultivar. The effect of elevated temperature in reducing the concentration of phenolic compounds is found to be more during the post-monsoon period than in the pre-monsoon period. Overall results indicate that in climate change scenarios, WCT is likely to remain relatively more tolerant while COD, LCT and COD × WCT may become more predisposed to pests and diseases (Naresh Kumar et al. 2008; Sunoj et al. 2013). Rhizosphere enzymatic activity also varied in elevated CO₂ and temperature conditions (Sunoj et al. 2015).

Fig. 15.5

Proline concentration (mg/g DW leaf tissue) in coconut seedlings grown at three atmospheric CO₂ concentrations. The leaf water potential of seedlings was similar at ~ −7.2 bars

Fig. 15.6

Proline concentration (mg/g DW leaf tissue) and leaf water potential of coconut seedlings grown in different conditions (shade net (SN), control open top chamber (CC) and elevated temperature (+2 °C over ambient temperatures) at atmospheric CO₂ concentrations (380 ppm))

Fig. 15.7

Variation in total phenol concentration (mg phenol g^–1 dry plant tissue) in coconut seedlings under elevated (CO₂) and (T) during pre- and post-monsoon seasons

Even though anatomical changes provide plants with long-duration or lifetime adaptation thereby bringing about stability to adaptation of plants, these parameters are less studied. Studies indicated that (1) elevated CO₂ and temperature reduced stomatal aperture area, stomatal complex area and guard cell area, (2) elevated T significantly reduced stomatal index and leaflet thickness, (3) elevated CO₂ and T increased the thickness of upper cuticle and diameter of meta xylem and (4) elevated CO₂ increased thickness of leaflets and of the upper epidermal layer (Table 15.7 and Photo 15.5). Some of the parameters such as stomatal index was not significantly influenced by elevated (CO₂), while thickness of lower cuticle and upper epidermal layer was not significantly influenced by elevated (T). However, there exists a variable response of anatomical features of cultivars and hybrids to these external factors. These anatomical modifications in coconut seedlings are likely to make them more adapted to climate change situations. Results indicate that coconut seedlings have the capacity to adapt to future climate by modifying their leaf anatomy (Naresh Kumar et al. 2008; Murali Krishna et al. 2009, 2013).

Table 15.7 Influence of elevated temperature and CO₂ on leaflet anatomical parameters of coconut seedlings. Mean values of five cultivars. Each datum is a mean of 100 observations

Photo 15.5

Epidermal impression showing the stomatal response of coconut leaf to elevated (CO₂) 550 and 700 and temperature (+2 °C over control OTC) conditions (Murali Krishna et al. 2013)

2.6.4 Photo-oxidative Stress in Coconut Seedlings

The main cause for damage due to high light intensity is photo-oxidative stress, and studies on chlorophyll fluorescence indicated a clear case of excess light energy under high light conditions causing stress to coconut seedlings (Naresh Kumar and Kasturi Bai 2009b). Quantum yield of photochemistry of leaflets exposed to high light was significantly less than those under shade. Seedlings exposed to high light and then shifted to shade have shown significant improvement in quantum yield. The Fv/Fm ratios indicated stress in seedlings under high light intensity. Excess light energy harvested by chlorophyll antenna caused high non-photochemical quenching resulting in production of biologically toxic super oxide, hydrogen peroxide and hydroxyl radicals, which damaged the cell membrane integrity as indicated by increased lipid peroxidation (Fig. 15.8) and chlorophyll bleaching. It is apparent that photoinhibition of photosynthesis takes place at (1) PSII down-regulation and (2) damage to PS II system in initial stages of light exposure, and under prolonged exposures, inhibition is caused due to (3) chlorophyll bleaching and (4) damage to chloroplast and cell membrane integrity, followed by reduction in photosynthetically active leaf area due to scorching thus reducing canopy photosynthesis (Fig. 15.9, Tables 15.8 and 15.9) (Naresh Kumar and Kasturi Bai 2009b). Protein concentration in leaf tissue was higher in seedlings under exposed conditions. Three distinct LMW proteins were found in seedlings exposed to high light intensities. These three LMW proteins have pI of 4.9, 8.4 and 10.15 with MW less than 20 kDa. Results clearly demonstrate the events that take place at early stage to subsequent cascading effects leading to the scorching and death of leaf under severe conditions (Naresh Kumar and Kasturi Bai 2009b). Coconut seedlings are sensitive to moisture stress and to high light intensity stresses. The seedlings when exposed to high light intensities experience photo-oxidative stress. Release of oxygen and superoxide radicals causes membrane damage thus causing the leaf scorching and, in severe cases, seedling death (Naresh Kumar et al. 2007). Studies indicated that the seedlings need to be supplied with adequate water and maintained under shade. Even field-planted seedlings need to be protected from high sunlight intensities, which may be by tying the leaves together or by providing a shield of dried coconut leaves around the coconut seedling canopy, which is tied. Further, field-planted seedlings need to be irrigated not only for better establishment but also to protect them from moisture stress. Moisture stress in combination with high light intensity causes the seedling death.

Table 15.8 Physiological and biochemical characteristics of leaflet under different light conditions

Table 15.9 Chlorophyll fluorescence, gas exchange and microclimate parameters of leaflets of the same frond under different light conditions

Fig. 15.8

Lipid peroxidation of leaflet under different light conditions

Fig. 15.9

Mechanism of photo-oxidative stress in coconut seedlings: early events to seedling death (Source: Naresh Kumar and Kasturi Bai 2009b)

2.7 Approaches for Enhancing Drought Tolerance in Coconut

Drought is a major constraint for coconut productivity. Thus, screening of germplasm for drought tolerance attracted the research efforts and those led to generation of a lot of scientific knowledge. However, the breeding for drought tolerance in coconut started only recently. Recent encouraging results on regeneration techniques further the genetic engineering approach to impart drought tolerance in high-yielding cultivars.

Classical approach to improve crop performance in water-limited environments is to select the genotypes that survived and have an improved yield in these environments. Another approach is identification and selection of traits that contribute to drought tolerance and high water use efficiency. Identification of in situ tolerant palms and using their selected progeny for population improvement in plantations are possibly the most promising and stable method.

2.7.1 Screening for Drought Tolerance in Coconut

Drought-tolerant coconut palms can be selected at seedling stage in a nursery and at adult palm stage. Apart from these, one can use an in vitro screening technique as well. The screening of coconut germplasm can be done using morphological, anatomical, physiological and biochemical traits. Further, molecular marker-assisted selection criterion is one, which is to be developed. It is essential to note that one has to develop the threshold levels for development of stress in given climatic and soil conditions. The functional relationship in mechansim of drought tolerance should form the basic criteria for selecting screening method in coconut breeding strategy (Schematic Diagram 15.2).

2.7.2 Traits for Drought Tolerance

Cell size and number, sub-stomatal cavity size, stomatal frequency, epicuticular wax content and thickness, leaf thickness, stomatal resistance, water potential components, cell membrane stability, water use efficiency and activity levels of scavenging enzymes are the essential anatomical, physiological and biochemical traits for assessing moisture stress tolerance in plants (Rajagopal et al. 1991; Kasturi Bai 1993; Champakam et al. 1993; Shivashankar et al. 1991; Naresh Kumar et al. 2000; Siju Thomas et al. 2006, 2008). Based on these, coconut germplasm collections comprising talls, dwarfs and hybrids were screened under field conditions for drought tolerance (Rajagopal et al. 1990).

2.7.3 Ranking of Cultivars for Drought Stress Tolerance

The ranking for drought tolerance was done based on all stress-sensitive parameters, viz., stomatal regulation, leaf water potential and ECW content (Rajagopal et al. 1990), and on biochemical traits, viz., lipid peroxidation, polyphenol oxidase, superoxide dismutase, catalase and peroxidase (Chempakam et al. 1993), as well as the anatomical characteristics (Naresh Kumar et al. 2000). All dwarf (s) performed badly, whereas all hybrids except COD × WCT and all tall(s) except the SSAT, ADOT and LMT were highly ranked. Based on anatomical features such as thicker leaflets, thick cuticle on sides, larger palisade and spongy parenchyma cells, larger hypodermal cells, water cells and sub-stomatal cavity, genotypes like WCT, FMS and PHOT and WCT × COD hybrid were identified as relatively tolerant to drought stress (Naresh Kumar et al. 2000). Two cultivars, viz., San Ramon and Ambakelle Special, were identified as drought tolerant in Sri Lanka (Wikremaratne 1987). In Ivory Coast, PB-121 was identified as tolerant while WAT was classified as moderately tolerant and Rennell Tall × WAT as the most sensitive to drought based on the drought tolerance index and effect of edaphic drought on the leaf water status, gas exchange and membrane lipids (Pomier and de Taffin 1982; de Nuce de Lamothe and Benard 1985; Repellin et al. 1994). Thus, coconut cultivars with different levels of drought tolerance could be identified based on the desirable traits, which reflect on the overall water relations of palms. Presence or absence of desirable traits imparts higher degree of drought tolerance (e.g. WCT × WCT; FMST; LCT; WCT × COD, LCT × GBGD and LCT × COD) or drought susceptibility (e.g. MYD) (Rajagopal et al. 2000a). Although the expression of physiological traits are influenced by weather variables (Kasturi Bai et al. 1988; Rajagopal et al. 2000a; Gomes et al. 2002), diversity analysis in coconut varieties for drought-responsive physiological traits indicated that both rs and ECW are important variables, which contribute to diversity. This implies that in coconut, grouping could also be formed based on their tolerance to stress when appropriate traits are specifically used in diversity analysis Kasturi Bai et al. (2006a).

2.7.4 Identification and Characterization of In Situ Tolerant Palms

The plants that can withstand the natural occurrence of drought and other stresses and still produce good yield are of premium value, as they may possess the desirable genes. Surveys in hotspot areas were conducted to identify the palms yielding very high compared to others in their vicinity. Palms at different agroclimatic regions were identified in farmers’ plots with desirable canopy shape and leaf number with better yield. The physiological water use efficiency of these palms was also found to be high (Photo 15.6, Table 15.10) (Naresh Kumar et al. 2002b). This type of in situ tolerant plants with desirable traits should be used in breeding programmes, which will help in reducing the time gap in breeding for drought-tolerant cultivars in coconut (Naresh Kumar et al. 2002b).

Photo 15.6

In situ tolerant palm

Table 15.10. Physiological, biochemical and morphological characters of in situ drought-tolerant palms (mean of 2 years data collected on palms identified in farmers’ plots under different agroclimatic zones – adopted from Naresh Kumar et al. 2002b)

2.8 Genetics of Drought Tolerance-Related Physiological and Biochemical Traits

Coconut cultivars with desirable characters were selected and crossed in a 2 × 4 line x tester mating design to study the combining ability and gene action with respect to drought-responsive physiological traits (Rajagopal et al. 2007). Seedling transpiration rate and leaf water potential showed higher specific combining ability (sca) effects than general combining ability (gca) effects due to predominance of nonadditive gene action indicating heterosis for this character. The Pn under stress was additive with good combining ability, while the Pn during non-stress and recovery were governed by nonadditive gene action that could be exploited for heterosis. In case of lipid peroxidation, gene action was unpredictable in non-stress with additive gene action being nil with low dominance. These indicate that the nature of gene action governing drought-sensitive traits can be exploited by selecting proper breeding strategies for drought tolerance.

In changing climates, efficient management of available water is very important for sustainable crop production. Growing cultivars with high water use efficiency is even more important in the case of coconut, a perennial plantation crop mainly grown under rainfed and marginal conditions which faces annual summer stress. Drought tolerance research at the Central Plantation Crops Research Institute (CPCRI) in Kasaragod, India, indicated variability for WUE, dry matter production and yield in coconut cultivars (Rajagopal et al. 1989; Kasturi Bai et al. 1996a; Naresh Kuamr et al. 2000). Seedling stage screening of 75 cross combinations, reciprocal crosses, and their parents for drought tolerance and revival capacity at CPCRI indicated that in general, tall(s) and hybrids with tall(s) as mother palms had higher drought tolerance compared to dwarf(s) and dwarf as a mother palm. Heterosis was observed for some of the desirable characters for drought tolerance. Earlier studies (Rajagopal et al. 2000a) indicated the possibility of exploitation of heterosis of some of the drought-tolerant traits in evolving the drought-tolerant hybrids.

2.9 Drought Management

In the rainfed areas, drought is the major constraint for the crop productivity, more so in coconut since they are widely grown in different soil types such as sandy, sandy loam, laterite and forest soils in the states of Kerala, Karnataka, Tamil Nadu, Andhra Pradesh, West Bengal, Orissa, Maharashtra, northeastern states, Andaman and Nicobar Islands and Lakshadweep Islands of India. As this is mainly grown under rainfed condition, productivity is affected due to the dry summer months starting from December/January to April/May. During the period, the soil water deficit coupled with increase in atmospheric water demand aggravates the situation leading to soil as well as atmospheric drought. Ideally, this is the time when they should receive adequate water supply in order to get better yields. Being perennial in nature, the water requirement is also fairly high and the approach then has to be to use the available water source with high productivity efficiency. Thus it is important not only to identify the varieties, which can withstand moisture stress conditions in the field, but also to evolve management strategies for conserving available water sources in order to mitigate adverse effects of drought. The drought management strategies and low-cost soil moisture conservation practices using in situ biowaste is recommended for each agroclimatic zone where coconut is grown (Naresh Kumar 2004; Naresh Kumar and Rajagopal 2005).

2.9.1 Water Management Through Irrigation Scheduling and Soil Moisture Conservation

The frequency and amount of irrigation influence the water relations and DM production of coconut palms (Rajagopal et al. 1989; Kasturi Bai et al. 1997). Summer irrigation (Nelliat and Padmaja 1978) and soil moisture conservation practices like husk burial in basins, leaf mulching, Gliricidia culture and application of compost and farm waste in basins increase nut yield in coconut (Rajagopal et al. 2000b; Rajagopal and Naresh Kumar 2001). The studies on the extent of influence of irrigation on coconut palms grown on sandy and laterite soils with different levels of irrigation through drip [in sandy, 66, 100 and 133 % of open pan evaporation (Eo); in laterite, 33, 66 and 100 % of Eo and basin (100 % of Eo)] indicated that the source parameters, viz., photosynthetic rate, Ψ _leaf and photosynthetic II (PS II) efficiency, varied with the irrigation level and soil type. Palms receiving irrigation also showed marked improvement in pistillate flower production and their retention to produce higher nut yields. Three types of physiological conditions in source and sink relationships were observed in the palms based on the type of irrigation treatment they were subjected to (Naresh Kumar et al. 2002a). The drip irrigation provided conditions for better physiological efficiency of source and sink for high WUE and yield. WUE was found to increase at field, plant and leaflet level (Naresh Kumar et al. 2002a).

The net photosynthetic rates were higher in the palms grown in laterite soils compared to those grown in sandy soils, and irrigation significantly increased the Pn rates and stomatal conductance. The chlorophyll fluorescence, PS II efficiency parameter Fv/Fm, an indicator of extent of physiological stress in leaf, has been found to be higher in irrigated palms compared to the rainfed palms (Naresh Kumar et al. 2002a). The pistillate flower production and setting percentage normally increase with irrigation. The percentage of nut retention is an indicator of efficiency of conversion of pistillate flowers into mature nuts. Percent increase in nut yield in irrigated palms over rainfed palms was higher in sandy soils compared to laterite soils (Naresh Kumar et al. 2002a). The consequent increase in yield could be related to the increases in source (the Pn rates) and sink (pistillate flower production) efficiency under irrigation.

The physiological conditions of source and sink in palms grown under different systems of irrigation are defined (Table 15.11) (Naresh Kumar et al. 2002a). The drip irrigation is a system where not only the available water is used to the optimum with negligible losses but also because of the presence of dry zones in the root system possibly acting as the stomatal regulation system to provide optimal physiological efficiency for higher WUE and better yields. Drip irrigation increases the WUE not only at a field level but at a plant and leaf level also. From the study, it is indicated that even in basin irrigation, by applying water in such a way that the dry pockets are created in the root system, it may be possible to increase WUE with high yields (Naresh Kumar et al. 2000) (Photo 15.7).

Photo 15.7

The plantations with drip irrigation (left) survived the drought (1998–2002) while those without timely irrigation failed to cope (right) with drought in Pollachi area of Tamil Nadu, India

Table 15.11 Summary of physiological conditions of source and sink as influenced by the type of irrigation

Soil moisture conservation also improved the yields by improving the source-sink efficiencies (Naresh Kumar et al. 2003, 2006). Results of a multilocation experiment indicated that the soil moisture conservation practices helped to retain and make soil moisture available for longer periods during summer (Photos 15.8 and 15.9). This helped to maintain higher photosynthetic rates and water use efficiency thus helping improve pistillate flower production, nut retention and yield (Table 15.12; Naresh Kumar et al. 2003). The prolonged soil moisture availability due to soil moisture conservation treatments helped to increase the photosynthetic source number and efficiency as also the sink capacity and efficiency (Naresh Kumar et al. 2002b, 2006).

Table 15.12 Effect of drought management practices on nut yield at different centres (mean for 5 years)

Photo 15.8

Soil moisture conservation practices in coconut. Polyphenol depeleted coirpith was used. Generally, one year Sun and rain exposed coirpith is phenol depleted and can be used for mulching

Photo 15.9

Soil moisture conservation practices in coconut using coconut husk

3 Areca Nut

Unlike scientific information on coconut, the information on areca nut is very meagre as far as the abiotic stress tolerance is concerned. The cultivation of areca nut is mostly confined to 28° north and south of the equator. In general, areca nut is mainly grown in low altitudes. This to some extent depends on the latitude. In the northeast region of India (Assam and West Bengal), the major areca nut-growing area is in plains, since at higher elevations the winter temperature would be too extreme for the crop. Although areca palms grow at an altitude of up to 1000 m above MSL, at higher altitudes, the quality of the fruits will not be good. In the high-altitude areas, the endosperm of the fruit does not develop sufficient hardiness. It is also reported that high altitudes affect the germination of seeds and quality of marketable dry kernel. The percentage of germination of nut and the proportion of dry weight of kernel to whole fruits are less at altitudes above 850 m than in the lower altitudes.

The yield potential of areca palm mainly depends on climate. Generally more than 50 % of variations in yield are due to climatic differences. Though the areca nut is grown under different agroclimatic conditions, it is very sensitive to extreme climatic conditions. The most important climatic factors that influence the growth and development are altitude, relative humidity and rainfall. Temperature also influences the crop growth and yield to some extent. Regression analysis of weather variables of 12 years indicated that areca nut yield is influenced by relative humidity, evaporation and rainfall.

Areca palms grow well within the temperature range of 14–36 °C. However, the crop is being grown in temperatures ranging from 5 to 40 °C. But extreme temperatures and wide diurnal variations are not conducive for the healthy growth of the palms. Heavy damage to foliage and death of palms is reported when the minimum temperature was below −2.8 °C. Even temperature around 5 °C with low humidity causes severe foliage damage. Though areca nut flourishes well in tracts of heavy rainfall, it is grown in areas with wide variations in annual rainfall ranging from 750 mm to more than 4,500 mm. In low-rainfall areas, the palms are irrigated. Very high or low relative humidity is not conducive for growth and development of areca nut. Relative humidity directly influences the water relations of palm and indirectly affects leaf growth, photosynthesis, pollen dispersal, occurrence of diseases and finally economic yield. Relative humidity has considerable influence on evapotranspiration and hence on the water requirement of areca nut. Areca nut plantations also are affected by drought stress causing a loss ranging from 10 to 50 % depending on the duration and severity of stress. The loss due to drought was more severe in nontraditional areas of areca nut cultivation than in traditionally grown areas. Once affected, palms require 2–3 years to recover to normal yield condition. In severe cases, deaths of palms are reported from parts of Karnataka, India. Similarly, areca nut is affected by the high light intensities not only palms at seedling stage but also the grownup palms. Palm trunks get scorching damage due to high light intensities.

3.1 Management of Abiotic Stresses in Areca Nut Plantations

As in the case of coconut, providing drip irrigation, soil moisture conservation and raising the drought-tolerant areca nut varieties are the commonly recommended strategy for overcoming the drought stress in areca nut. To overcome the high light intensity, the seedlings are provided shades, and the trunks are covered with dry leaves to avoid direct incidence of sunlight (Photo 15.10).

Photo 15.10

Trunk protection against direct sun light in arecanut

4 Constraints and Opportunities

Abiotic stresses constrain the productivity of plantation crops. Drought is a major constraint for coconut productivity in entire coconut-growing areas at a global level. Realization of impact of drought on coconut yield forced increased attention towards this problem. A methodical research approach led to understanding the drought tolerance mechanism in coconut. So far, conventional breeding strategies were applied for development of drought-tolerant varieties and hybrids. However, this takes a lot of time and testing for yield stability under stressful conditions is time consuming. Lack of large-scale regeneration techniques handicapped the genetic engineering approach to impart drought tolerance in high-yielding cultivars. Hence it is very much important to globally co-ordinate the breeding for drought tolerance programmes as studies indicated that hybrids with talls as parents can perform better under water stress conditions. It is essential to conserve the natural desirable gene pools present in the in situ palms in farmers’ fields before they become extinct. These materials are highly valuable for crop improvement programmes. Comprehensive molecular markers need to be developed for rapid screening of coconut germplasm for drought tolerance at a global level. Further, it is of importance to characterize the nature and intensity of drought in different coconut-growing areas in order to develop suitable drought management strategies. More importantly, available technologies need to be demonstrated and disseminated to the farmers for improving productivity in not only coconut but also areca nut.

5 Conclusions and Future Thrust

To sum up, a coconut responds to the stressful environments at morphological, anatomical, physiological and biochemical levels. Generally, drought coincides with high temperature and high light intensity stresses making the problem more complex. However, it is also important to note that the adaptive strategies of plants for different abiotic stresses are overlapped in most of the cases as the nature of abiotic stresses themselves. Further studies should be focused to develop the molecular markers linked to desirable traits and to understand the inheritance patterns of these traits. The genotypes with desirable traits for tolerance to stress conditions can be used in breeding strategies for abiotic stress tolerance in future crop improvement programmes. Identification and characterization of field-tolerant palms to abiotic stresses will be helpful in exploiting the natural tolerance to abiotic stresses. More basic research efforts are needed for the areca nut as is done for the coconut.

The results obtained so far indicate that variation exists among the tall(s), dwarf(s) and hybrids for drought-tolerant traits. Generally, tall(s) and hybrids with tall(s) as mother palms have higher drought tolerance compared to dwarf(s) and hybrids with dwarf(s) as mother palms. The heterosis for drought-tolerant traits can be exploited for breeding for drought tolerance. Further, in situ tolerant palms need to be identified and used in breeding programmes. Observations delineated the mechanism for drought tolerance (Schematic Diagram 15.1) and breeding for drought tolerance (Schematic Diagram 15.2) based on morphological, anatomical, physiological and biochemical traits. Germplasm should be screened for targeted traits under the targeted environments varying in time and space. In perennials, the stability of such tolerance over a period of time is the key factor for realizing stable yields even during stress years. Special emphasis should be given for field tolerance and in situ tolerant plants. The plants which can withstand the natural occurrence of drought and other stresses and still produce good yield are of premium in nature for their possessing desirable gene pool. Early vigour and high revival capacity are very significant factors that should take pivotal place. Selecting such genotypes should be based only on field stress. These experiments can be extrapolated to other germplasm sources, which were not studied so far, and for making a cross combination of potential success to come out with cultivars/hybrids with high drought tolerance and stable yield.

Schematic Diagram 15.1

Adaptive strategies of coconut palm under stressful conditions

Schematic Diagram 15.2

Screening for drought tolerance in coconut