Keywords

5.1 Origin and History

Lychee (Litchi chinensis Sonn.) is one of the precious and economically important fruit crops of the world. The species belongs to the family Sapindaceae, which comprises about 2000 species of tropical and subtropical trees, shrubs and vines that have been classified into 140 genera (Chapman 1984). Lychee has been shown to possess variable diploid chromosome numbers where 2n = 28, 30, 32. The variation in chromosome number is thought to be because the modern species had more than one wild progenitor. Lychee cultivation was reported since 1500 BC by the people of Malayan descent and has been growing for thousands of years in southern Guangdong province of China. The first reference to this fruit is available in the literature of the Han dynasty (140 BC to 86 BC). It is sure that lychee is native of South China, but, according to Blume, Cochin-China and the Philippine islands are the lands of its origin (Popenoe 1920). It is also reported to have originated in China’s Kwangtung and Fukien provinces and have been cultivated in China for about forty centuries (Ochse et al. 1961). A monograph written by Tsai Hsiang in 1059 AD is considered to be the first publication in the world devoted to this fruit. However, according to Walter T. Swingle, the first published work of fruit culture was written by a Chinese scholar in 1056 AD, on the varieties of lychee.

From China it reached Burma (Myanmar) by the end of the seventeenth century and was introduced in India about 100 years later. Lychee reached Madagascar and Mauritius around 1870 and was introduced in Hawaii in 1873 by a Chinese trader. It arrived in Florida, from India, between 1870 and 1880 and was introduced in California in 1897. Lychee was reported to be brought to Australia by Chinese migrants in 1954 and arrived in Israel sometime between 1930 and 1940. Presently, lychee is grown in Central and South America, parts of Africa and throughout Asia. China, India, South Africa, Australia, Mauritius, Madagascar and Thailand are now the major lychee-producing countries in the world.

5.2 Production

Lychee plantation requires a warm subtropical to tropical climate (Rivera-Lo’pez et al. 1999). Besides China and India, lychee fruit is also grown as a commercial crop in subtropical Asia, Hawaii, Israel, Mexico, Australia and South Africa (Jiang et al. 2001). India is the second largest producer of lychee after China with an annual production of 428,900 metric tons from 56,200 ha (Fig. 5.1). Lychee is mostly grown in Eastern India, and Bihar state alone contributes to 74% of Indian lychee production (Fig. 5.2). As lychee is an introduced fruit crop, it has great potential of yield in India. Figure 5.3 reveals a trend in the year-wise expansion in the area under lychee cultivation which has increased from 58,100 to 84,200 ha in 1991–1992 to 2013–2014 with a similar trend in the production from 355,900 MT to 585,300 MT in the last decades.

Fig. 5.1
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World lychee production

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Fig. 5.2
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Map showing lychee-producing states in India

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Fig. 5.3
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Production of lychee in India (Indian horticulture database 2014)

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5.3 Classification of Major Lychee Cultivars

The lychee (Lychee chinensis Sonn.) is a non-climacteric fruit of Southeast Asian origin (Nakasone and Paull 1998). It is covered by a pink or red leather-like pericarp due to the presence of anthocyanins (Lee and Wicker 1991; Rivera-Lo’pez et al. 1999) as shown in Fig. 5.4. When distinguishing the cultivars, the shape of fruit skin segments and protuberances are reliable and stable genetic characteristics (Fig. 5.5). Fruit size, shape and taste are also variables but are influenced by other than genetic factors. Chinese researchers report that the shape of the skin segments and protuberances are more reliable characteristics than fruit size, shape or taste, to identify cultivars. The lychee cultivars vary greatly in vegetative flushing patterns, flush colour and flowering ability. The leaf of the Rose Scented is boat shaped, while China has a distinctive twist along the length curved upwards from the midrib and down along its length. Small leaflets in Bedana are oval shaped. The fruit shape of lychee is very distinguishing. The round shape of Bedana is distinguished from oblong shape of China or Shahi. The fruit is smooth and pulp is even or uneven. The apex of the fruit can be round, obtuse, blunt as in Shahi or pointed as in China (Fig. 5.5).

Fig. 5.4
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Lychee tree with ripened fruits

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Fig. 5.5
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Lychee fruit characteristics

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The varieties can also be distinguished depending on the colour of new flush and season of flowering. Shahi produces very light-coloured flush, while China has pinkish flush. Bedana produces bright red or copper-coloured flush and short compact panicles. The fruit colour varies in different varieties and is also influenced by growing conditions. Skin thickness depends on cultivars. Bedana and China have very thick skin, whereas Rose Scented and Shahi have thin skin. Skin surface at maturity also varies being smooth, swelling and sharp pointed. Protuberances of pericarp (skin) can be smooth as in Bedana or sharply pointed as in China. The presence and absence of seed as well as structure and size of seeds also vary from cultivar to cultivar, but it is also influenced by the environmental conditions. In Rose Scented and Bedana, a high proportion of chicken-tongued seeds is observed, while China has bold seeds.

The harvest season lasts 5–10 weeks for a range of cultivars in any one location. Lychee cultivars can be broadly classified as early, mid or late maturing, although the order varies from year to year, depending on seasonal conditions. Table 5.1 enlists the lychee cultivars grown in different countries. According to the Indian Horticulture Database (2014), Indian lychee is exported to China (87 MT), Thailand (19 MT) and Mauritius (1MT).

Table 5.1 Major lychee cultivars grown in different countries
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5.4 Botanical Description

Tree

Lychee is a long-lived, evergreen tree up to 30 m tall in old specimens, with a short stocky trunk. In some cultivars, the branches are crooked or twisting and spreading forming a crown broader than high, while in other cultivars, the branches are fairly straight and upright forming a compact, rounded crown. The varieties can be identified by using tree characteristics. However, they change with weather, soil and culture. Differences in the tree size and shape, length and spread of branches are commonly seen. For example, Brewster is vigorous and erect, with very wide strong crotch angles; Tai So is vigorous, with a spreading habit and sharp crotch angles, while Wai Chee is slow, compact and dome shaped.

Leaves

The leaves are alternative and compound, with two to five leaflets. The leaflets are oblong and 5–15 cm long. The foliage comprises 2–4 pairs of leaflets, which are 3–6 in. long, coriaceous, elliptic-oblong to lanceolate, shortly acute, glabrous and shining above and glaucescent beneath. The new flushes are a distinctive red brown when immature and light to dark green as they mature.

The leaf characteristics include leaf size, shape and colour, e.g. Tai So has large, glossy, dark green leaflets that have an upward curl from the midrib to be almost canoe shaped. Bengal has large leaflets, mid-green in colour with a distinctive twist along their length. Haak Yip has dark, glossy green leaflets that are long, narrow pointed and slightly curled at the tip. Wai Chee leaflets are small, oval shaped and curve upwards from the midrib and down along their length. The new flush of growth is red in Wai Chee and Kwai May Pink and green bronze in Tai So.

Flower

The inflorescences are many branched panicles, each with one or more leaves and up to 3000 flowers, and form 5–80 fruits at harvest. The flowers are small, yellowish white, functionally male or female and apetalous. Functionally, male flowers have 6–10 stamens. There are usually two stages of male flowering overlapping with the female cycle: a true male flower first and then a functionally male flower that opens towards the end of the flowering period. The second male flower has a rudimentary bicarpellate pistil. This is absent in the first stage. Functionally, female flowers have 6–10 staminodes and a functional, bicarpellate pistil (Fig. 5.2). The last stage of male flowering generally supplies most of the pollen used to fertilize the female flowers. The ovary is bilobed, compressed and silky; usually only one lobe develops into a fruit. The stigma is bilobed. According to their hermaphrodite nature, flowers are classified into three classes, viz. type I, type II and type III. Type I and type III flowers are male, while type II flowers function as a fruit-bearing female. Most of the flowers are of type III and only 20 % of the flowers are of fruit producing type II females.

Fruit

Fruits are highly variable, depending on the cultivar (Fig. 5.3). They can be round, ovoid or heart shaped and from 2.0 to 3.5 cm in diameter. The skin can be smooth or rough with distinct protuberances, thick or thin and pink red, bright red or purple red. The flesh or aril is an outgrowth of the outer cells of the seed coat (outer integument), and in good cultivars may comprise 80% of fruit weight. The aril is generally translucent white, juicy or firm and sweet and aromatic in better cultivars. Many cultivars can be distinguished by their flavour and aroma. The fruit contains a single dark brown seed 6–12 mm wide and 10–23 mm long. Some cultivars have a high proportion of aborted seeds and thus a high flesh recovery. They are popular in the marketplace, especially in Asia. There are a few cultivars that produce nearly seedless fruit, although the fruit usually weighs less than 10 g. The fruit shape of some cultivars is very distinctive. The round fruit of Kwai May Pink distinguishes it from the egg shape of Tai So or the heart shape of Haak Yip. The shoulders of the fruit can be smooth or flat as in Wai Chee and Kwai May Pink or uneven as in Souey Tung and Bengal. The apex or tip of the fruit can be round as in Kwai May Pink and Wai Chee, obtuse or blunt as in Souey Tung and Brewster or pointed as in Bengal. The fruit colours are bright red (Bengal), dull red (Wai Chee), purple red (Haak Yip) or pink red (Brewster). The skin can be thick as in Wai Chee, Bengal and Kwai May Pink or thin as in Haak Yip and Souey Tung. Skin segments at full maturity can be smooth (Haak Yip), swelling (Wai Chee) or sharp pointed (Kwai May Red). Similarly, the protuberances on each segment can be smooth as in Haak Yip, sharp pointed as in Kwai May Red and Bengal or hairlike and sharp as in Tai So. The presence or absence of an obvious suture line can distinguish some cultivars such as Haak Yip and Souey Tung.

The texture, juiciness, taste and aroma of the flesh can aid description, although experience is needed to make clear distinctions. For example, Wai Chee is watery, Kwai May Red is firm, Kwai May Pink is spicy, and Bengal is very sweet. The proportion of small or shrivelled seeds is important but varies with season and orchard. Cultivars with a high proportion of chicken-tongue seeds are favoured. Salathiel produces nearly always fruit with small seeds, while Bengal, Souey Tung, Haak Yip and Wai Chee produce hardly any. Other varieties such as Tai So and Kwai May Pink vary. Description to major lychee cultivars grown in different countries is shown in Table 5.2.

Table 5.2 Description of the lychee cultivars/breeding lines
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5.5 Nutritional Composition

Lychee fruit is classified as drupe and has a large seed, white translucent edible aril (flesh) and thin, tough, corky pericarp (skin). The pericarp of the mature fruit varies from pink red to plum, depending on the cultivar, while the aril is succulent, translucent cream or white and with sweet citrus flavour. Lychee is mainly consumed fresh, but different products like canned lychee, squash, cordial, syrup, jam, jelly and juice are also manufactured and marketed. It can be dried or dehydrated (lychee nuts) or used in ice creams and sorbets (Hui, 2008; Salunke and Desai 1984). Dried lychee is very popular among the Chinese. In China, various other products such as pickles and wine are also made from lychee. The food value of lychee mainly lies in its sugar content which varies from variety to variety. Depending upon the variety and climate, the fruit contains 60% juice, 8% rag, 19% seed and 13% skin. Apart from proteins, fats, carbohydrates, minerals, fibrous matter, calcium, phosphorus, iron and carotene, the fruit is also rich in vitamin B1, riboflavin and vitamin C (Table 5.3). Lychee is also an excellent source of anti-oxidants which protects the body from harmful free radicals. Lychee flesh is loaded with nutritional and functional compounds. According to the data released by USDA, 100 g of lychee flesh contains 16.5 g sugars, 276 kilojoule energy, 0.83 g protein, 0.44 g fat, 0.44 g ash, 1.3 g edible fibre, 5 mg Ca, 0.31 mg Fe, 10 mg Mg, 31 mg P, 171 mg K, 1 mg Na, 0.07 mg Zn, 71.5 mg vitamin C, 0.011 mg thiamin, 0.065 mg riboflavin, 0.603 mg niacin, 0.1 mg vitamin B6, 14 mg folate, 0.07 mg vitamin E, 0.007 mg tryptophan, 0.041 mg lysine and 0.009 mg methionine. Taking 100 g lychee flesh satisfies 2–4% of the daily requirement for P, K, Mg, Fe, Zn and Mn and 22% for Cu (Wall 2006). Apart from nutritional value, lychee flesh improves digestion and blood circulation, moistens skin and alleviates symptom of anaemia (Chi et al. 2005). However, active compounds and mechanisms of these functions are unknown. Anti-oxidant activity of lychee flesh is well documented. In addition to vitamin C and E, lychee flesh contains anti-oxidant polysaccharides (Wu et al. 2004) and flavonoids including procyanidin A2 and leucocyanidin (Rooyen and Redelinghuys 1983). Polysaccharides in lychee flesh are effective to eradicate O2 and reported to significantly suppress lipid peroxidation in rat liver (Wu et al. 2004). However, based on Chinese traditional medicine, lychee flesh is a typical ‘heating’ food. Excessive taking causes ‘heating symptoms’ including sore and swell in the throat, boils in the mouth, tongue and face and tonsillitis. ‘Heating effect’ of lychee, which is only shown in some individuals, is not understood in terms of mechanism and effective compounds.

Table 5.3 Nutritive value per 100 g of lychee fruit (Litchi chinensis)
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5.6 Phytochemistry and Functional Activities

Nowadays, HPLC (high-performance liquid chromatography) and HPTLC (high-performance thin-layer chromatography) have become regular analytical techniques due to their efficiency in quantitation of analytes at micro- or even nanogram levels and cost-effectiveness. Leaf, root, seed, fruit and pericarp extracts of various lychee varieties have been subjected to HPLC and HPTLC followed by pharmacological analyses. The recent reports reveal a total of 50 bioactive compounds from different parts of the lychee plants (Table 5.4). These compounds have been categorized under flavonoids, glycosides, phenolic aldehyde, monoterpenes, anthocyanins amino acid, phenolic compounds and fatty acids (Fig. 5.6).

Table 5.4 List of bioactive compounds reported in Litchi chinensis
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Fig. 5.6
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Pharmacological activities of lychee

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5.6.1 Functional Activities and Compounds in Lychee Skin

Anti-oxidant

Traditionally, lychee skin is useful to prevent the ‘heating effect’ from taking litchi flesh, but the mechanism is still unknown. However, the anti-oxidant activity of litchi skin is well defined (Guo et al. 2003a, b; Surinut et al. 2005). Lychee skin contains free-radical scavenging compounds like ascorbic acid, glutathione, carotenoids, polysaccharides (Huang and Wu 2006; Yang et al. 2006) as well as rich phenolic substances including flavonoids (flavonols and anthocyanins) and phenolic acids (Li and Jiang 2007). Zhang et al. (2000) found epicatechin, procyanidin B2, epigallocatechin and procyanidin B4 are among the major flavonoids in fruit skin of ‘Huaizhi’. Analysis conducted by Sarini-Manchado et al. (2000) showed that polymerized tannins (procyanidins) were the most abundant (0.4% fresh weight) in ‘Guiwei’ skin, followed by epicatechin (0.17%), procyanidin A2 (0.07%), anthocyanins (0.04%) and flavanols (0.04%). Two flavonoids in lychee skin especially anthocyanins and procyanidins contribute to the major part of its anti-oxidant activity (Luximon-Ramma et al. 2003). Zhao et al. (2006) found procyanidin B2 was stronger in scavenging hydroxyl free radical and superoxgen anion than procyanidin B4 and epicatechin, while epicatechin is more active in eradicating DPPH than the other two flavonoids.

Lychee fruit development is accompanied by changes in chemical compositions including phenolic substances (Huang and Wu 2006). Hence, anti-oxidant activity in the skin at different maturity differs. Zheng et al. (2003) found skin of immature fruit had a much stronger anti-oxidant activity than that of mature fruit. Cultivars also differ in quantity and quality of phenolics including flavonoids, so do their anti-oxidant activities.

Anti-cancer Activity

Lychee skin is rich in insoluble fibre (40% dry weight), which prevents rectum cancer, diabetes and haemorrhoids (Li et al. 2006). Wang et al. (2006a) reported that water soluble alcohol extract from lychee skin significantly inhibited growth of human hepatoma cells in vitro and that feeding mice carrying liver cancer with lychee skin extract suppressed cancer development. In both cases, the anti-cancer effect was dosage and time dependent. Dosages at 0.14, 0.3 and 0.6 g/kg/day led to a cancer inhibiting rate of 17.3, 30.8 and 44.0%, respectively. Wang et al. (2006b) further found lychee skin extract also effective to suppress breast cancer. They found lychee skin extract caused changes in gene expression pattern, induced programmed cell death and suppressed multiplication in cancers cells (Wang et al. 2006a, b). However, active ingredients in lychee skin were not indicated in their studies. A study made by Zhao et al. (2007) indicated that flavonoids (epicatechin and procyanidin B2) were effective to suppress human breast cancer cells and human lung fibroblast (HLF), although their toxicity to cancer cells was lower than that of paclitaxel.

5.6.2 Functional Activities and Compounds in Lychee Seed

Chemical Composition of Lychee Seed

Lychee kernel contains starch (40.7%), crude fibre (24.5%), proteins (4.93%) and minerals including Mg (0.28%), Ca (0.21%) and P (0.11%). Fatty acids include 12% palmitic acid, 27% linoleic acid, 11% linoleic acid and 42% cyclopropanoic fatty acids (CPFA), among which dihydrosterculic acid accounts for 37%, cis-7,8-methylenehexadecanoic acid 4%, cis-5,6-methylenetetradecanoic acid 0.4% and cis-3,4-methylenedodecanoic acid 0.1% (Ding, 1999). Lychee seed contains also phenolic acids and flavonoids such as methyl 5-O-p-coumaroylquinate, protocatechuic acid, cyanidin-3-O-glu, cyanidin-3-O-rut and malvidin-3-acetyl-O-gluoenin (Ding 1999). Twenty-one amino acids were detected by Huang and Chen (2007), among which four were unknown. There were reports indicating lychee seed contains a special amino acid, α-methylenecyclopropylglycine (Ding 1999). Volatile compounds including ketones, aldehydes, esters, alcohols, enes and terpenoids with unknown functions were also detected in lychee seed (Ding 1999; Le and Fu 2001; Chen et al. 2005; Guo and Pan 2006). Tu et al. (2006) found sterol derivatives, while Yang et al. (2004) measured crude saponin in lychee seeds.

5.6.2.1 Functional Activities and Pharmacological Studies

Dried lychee seed is characterized by traditional Chinese medicine as slightly bitter, warming, qi flow promoting, cold driving, painkilling and liver and kidney tonifying (Tian 2005). There is abundant information on the health-promoting and medical functions of lychee seed, including anti-oxidant, anti-cancer, anti-virus, controlling diabetes and reducing blood lips.

Anti-oxidant Activity

As mentioned above, lychee seed contains flavonoids, which contribute to the anti-oxidant activity of the seed. Water and ethanol extracts from lychee seed were found to reduce the damage caused by free radicals and promoted SOD activity in alloxan monohydrate (ALX)-treated mice (Pan et al. 1999).

Anti-cancer Effect

Xiao et al. (2007) and Wang et al. (2007) reported in the same year that water extract of lychee seed or lychee seed pellets were effective to suppress tumour and hepatoma. Water extract of lychee seed at dosage of 62.5 kg/kg.d obtained 30% suppression on hepatoma tumour in mice (Wang et al. 2007). Xiao et al. (2007) found that extract from lychee seed inhibited the formation of telomere in hepatoma cells and thus their cell division.

Reducing Blood Sugars and Lipids

There have been reports about the effects of lychee seed in reducing blood sugars and lipids and in promoting the function of the liver (Wu et al. 1991; Zheng et al. 1998; Pan et al. 1999; Guo et al. 2003a, b). Results obtained by Wu et al. (1991), Zheng et al. (1998) and Pan et al. (1999) showed that the water extracts of lychee seeds reduced blood sugar in rat suffering from diabetes induced by ALX and that the effect was similar to that of the anti-diabetic drug, biguanides. And it was found that lychee seed extract was safer and the effect lasted longer (over 1 week) than biguanides (Zheng et al. 1998). Pan et al. (1999) suggested that lychee seed extract reduced blood sugar because it inhibited glucose uptake by blood capillary but promoted glucose uptake in ambient tissues. Guo et al. (2003a, b) also found that lychee seed extract alleviated sugar metabolism disorder and improved sensitivity to insulin in rat suffering from insulin-resistant type 2 diabetes (T2DM) induced by streptomycin and therefore reduced blood sugar. There is little information about active antidiabetes and lipid-reducing substances in lychee seed. Ning et al. (1996) attributed the abundant unsaturated fatty acids to blood lipid-reducing effect of lychee seed. Some authors suggested that α-methylenecyclopropylglycine in lychee seed was effective to reduce the blood sugar and glycogen in liver in mice treated with ALX (Huang 1994), while others believed antidiabetes activity was related to saponins (Guo et al. 2003a, b; Yang and Liang 2004).

Anti-virus Effects

There have been not a few reports about the anti-virus effects of lychee seed extracts, which were effective to hepatitis B virus (Zheng and Zheng 1992; Li 1997; Pan et al. 2000; Xu et al. 2004; Xiao et al. 2005; Jiang et al. 2008), respiratory syncytial virus (RSV) (Liang et al. 2006), influenza virus (Luo et al. 2006) and SARS coronavirus (Gong et al. 2008). Zheng and Zheng (1992) found that lychee seed was the second most effective to control hepatitis B among 1000 tested herbal medicines. Pan et al. (2000) found direct inhibition on in vitro expression of HBsAg and HBV-DNA. Yang et al. (2001) showed that both water and alcohol (50%) extracts from lychee seeds were effective to inhibit HBsAg and HbeAg, but water extract was more effective on HbeAg, while alcohol extract was more effective on HBsAg. Xu et al. (2004) examined six extraction fractions of lychee seed, all showing strong effects in inhibiting the expression of HBsAg and HbeAg in Hep G 2.2.15 cell line, with an inhibiting rate reaching 90.9 and 84.3% on HBsAg and HbeAg, respectively. Most authors attribute anti-virus effect of lychee seed extracts to its flavonoids (Luo et al. 2006; Liang et al. 2006; Gong et al. 2008), while Jiang et al. (2008) suggested that saponins in lychee seed were the effective component.

Other Functions

Lychee seed is especially effective to cure haemorrhoids (Deng 2006).

5.7 Postharvest Strategies

The lychee fruit is highly prized, especially in Asia, and is a valuable international commodity. It is, however, also very perishable. This limits marketing in many countries without good storage facilities. The perishable nature of lychee (Litchi chinensis Sonn.) poses a serious problem in its transportation and marketing also. Lychee is delicate, so minimal handling is preferred. Ideally, fruit should be shipped on the day of harvest. The fruit must also be marketed and consumed quickly. Research into the best handling practice for lychee is still in its infancy, and no accepted protocol exists. It is likely to begin with some form of antifungal treatment in the orchard prior to harvest. The harvested fruit would be initially placed in a coolroom to remove the field heat and then sorted on a roller conveyor in the packhouse. The optimum temperature for storage of lychee is approximately 5 °C (Huang and Wang 1990), although fruit stored at 10 °C can last almost as well (Olesen and Wiltshire 2000), with less risk of condensation in the pack. A modified atmosphere of 3–5% O2 and 3–5% CO2 was mentioned earlier, but other mixtures, and gases such as nitrous oxide (Qadir 2001), may also be used.

The shelf life of lychee at ambient temperature (26 ± 2 °C) is less than 72 h. Postharvest losses of lychee are estimated to be 20–30% of the harvested fruit and could reach as high as 50% (Jiang et al. 2001). As fruits start deteriorating quickly upon plucking, they are graded, packed in boxes with green leaves as cushioning materials and immediately routed to wholesale and retail markets (Shi et al. 2001). Besides postharvest decay, pericarp browning is another problem limiting market value of lychee. Much work has been done on the roles of pigments, plant growth regulators and other factors responsible for pericarp browning (Zhang and Quantick 1997). Optimizing suitable temperature and chemicals to inhibit or delay pericarp browning during postharvest is necessary (Paull and Chen 1987, Jiang and Fu 1999). It has been reported that pericarp browning of harvested lychee is due to a rapid degradation of anthocyanidin by polyphenol oxidase (PPO) and peroxidase (POD) (Akamine 1960; Chen and Wang 1989; Lee and Wicker 1991). Dehydration also contributes to pericarp browning (Scott et al. 1982; Underhill and Simons 1993) and leads to 40% decrease in water content after 48 h storage at 25 °C, 60% relative humidity (Underhill and Critchley 1994). Postharvest decay also occurs due to bacteria, yeast and fungi.

Various techniques to reduce browning, control postharvest decay and extend storage life of lychee fruit include sulphur fumigation, fungicide dips, application of plant growth substances, waxes and chitosan coating, use of microbial antagonists (e.g. Bacillus subtilis), irradiation and heat treatments (Table 5.5). Of these, only sulphur fumigation and fungicide dips have been used commercially (Jiang et al. 2003). Alternative procedures to sulphur dioxide (SO2) fumigation of lychee fruits have been proposed. These include team treatment (Kaiser et al. 1995) or hot benomyl dipping (Scott et al. 1982), but so far, no method has been widely accepted or established commercially (Lichtera et al. 2000). SO2-fumigated fruits absorb 30–65% of applied SO2. In recent years, there has been an increasing concern about sulphur residues in fruit, particularly when some consumers are sensitive to sulphites. A maximum residue limit of 10 ppm sulphur is set in Europe, Australia and Japan, while in the USA, sulphur is only registered for postharvest use on grape (Paull et al. 1995). Similarly, a range of fungicides has been evaluated for disease control in lychee fruit, including benomyl, thiabendazole, iprodione and prochloraz (Huang and Scott 1985; Scott et al. 1982; Wong et al. 1991). Among these fungicides, benomyl is known to have a strong and broad spectrum of fungicidal activities and has been shown to be effective for control of lychee fruit decay, but it is no longer registered as a postharvest chemical in many countries due to potential oncogenic risks (National Research Council 1987). Irradiation of fruit is considered to reduce browning and postharvest losses. Storage temperature of 2–5 °C is considered to extend the shelf life. Uses of perforated polythene bags (0.2% ventilation) and storage at 3 °C have also been reported to increase shelf life.

Table 5.5 Overview of strategy used for postharvest management of lychee fruit
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Ascorbic acid treatment of lychee fruit has been reported to increase the anti-oxidation capacity, and chitosan coating inhibits dehydration and microbial attack. Recently, a novel strategy of the combinatorial use of both the aforesaid treatments has been proposed. It includes treatment of the harvested fruits with 1.0% ascorbic acid (w/v) and 40 nmol/l chitosan solution (Sun et al. 2010).

5.8 Lychee Diseases

There are a few diseases affecting leaves, flowers and fruit and some others causing tree deaths or decline. However, no major disease currently limits production in the region. Brown blight (Peronophythora Lycheei) infects leaves, panicles and fruit in China and Thailand but can be controlled with metalaxyl. Anthracnose (Colletotrichum gloeosporoides) and similar diseases also attack fruit in China, India and Australia. Parasitic algae and nematodes affect some orchards but can be readily controlled with available chemicals. Various organisms have been associated with tree deaths or decline in Asia and Australia, although their pathogenicity is yet to be proven.

Regardless of where lychee is grown, several insect groups attack the flowers, fruit, leaves and branches. Lepidopterous fruit borers are generally the most important pests affecting production. Other important species include various leaf- and flower-eating caterpillars and beetles, bark borers, scales, leaf mites, fruit-sucking bugs, fruit-piercing moths and fruit flies (Table 5.6).

Table 5.6 Diseases of Lychee chinensis caused by various classes of organisms
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5.9 Lychee Biotechnology

Since lychee is a cross-pollinated plant, it is highly heterozygous, and the progeny is not true to the parental type. Conventional vegetative propagation methods currently being used, air layering or marcottage, are slow and inefficient (Chapman 1984). Hence, in vitro techniques have potential use in lychee propagation for the large-scale cloning of elite plants. However, lychee has so far proven to be a difficult material for propagation using in vitro culture. Attempts to regenerate plants from explants derived from mature trees have failed to give satisfactory results (Kantharajah et al. 1989). The biotechnological research on lychee, including tissue culture, anther culture, protoplast culture and lychee biopharming, are still in infancy but progressing (Table 5.7). Lychees are now widely grown in tropical and subtropical regions of the world. However, irregular and poor yields are commonly reported, and there is considerable scope for improving fruit quality and marketability through biotechnology (Menzel 1983; Galan Saúco and Menini 1989).

Table 5.7 Tissue culture reports on Lychee chinensis
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5.10 Conclusions

Lychee is a wonder-fruit with a blend of taste and medicinal value. Lychee postharvest research has been ongoing since the 1940s, and there has been a significant growth in the past 10 years. Most of the scientific papers relating to lychee have been published since 1985. Due to its increasing demand, the area under cultivation has increased manifold. But, there are several factors which have cumulatively hampered its production. These include irregular flowering and fruit-set, frost and wind damage, narrow cultivar base, poor growing techniques, high incidence of insect pests, short production season, variable yields, poor-quality fruit, short harvest season, lack of planting material and growing technology, high cost of planting material, lack of irrigation, lack of technical information for new growers, susceptibility of fruits to browning and rotting, short shelf life of fruits, inappropriate pruning, harvesting and postharvest management, high cost of fertilizers, inefficient marketing system and inadequate industry research and extension. The following suggestions can ensure a marked improvement in lychee production, storage and supply strategies.

  • Lychee has a very narrow genetic base, which needs to be widened through selection of genotypes from the existing population. Target-oriented programmes must be launched so that germplasm is conserved and used. In this context of a network programme, ‘operation lychee production’ (OLP) should be initiated, and exchange of information and cultivars among countries should be encouraged. Starting of this network programme would boost lychee production and ensure livelihood security of the people.

  • It should also be possible to apply in vitro mutation induction and selection procedures to address certain fungal diseases that affect specific lychee cultivars.

  • Genetic transformation could possibly be utilized to develop the preferred ‘chicken-tongue’ seeds, using the pistillate gene from Arabidopsis thaliana, which directs seedlessness.

  • Protoplast technology could be harnessed to produce somatic hybrids between haploids and diploids, so as to develop seedless triploids.

  • Suitable cultivars are needed for various climatic conditions. It is also essential to develop promising lines/hybrids, which have larger fruit size, small/chicken-tongued seeds and tolerance to pericarp splitting.

  • Suitable agro-techniques particularly for production and consumption management, postharvest technology and effective marketing need due attention.

  • A systematic approach for the description of cultivars is needed. Thus, a lychee descriptor needs to be developed for herbaria and agriculture.

  • There is need to develop propagation technology for faster multiplication of quality plants.

  • The development of nutrition management to maintain tree health and encourage successful flowering and fruiting quality in sustainable manner requires attention.

  • For efficient fertilizer use, monitoring through leaf analysis should be encouraged.

  • Integrated management of nutrient and water with efficient monitoring mechanisms would improve both production quantity and quality.

  • Through effective recycling of residues coupled with organic manure, it is possible to improve soil health. Thus, there is an immense potential for organic production of lychee through integrated pest management (IPM) to improve productivity and reduce the cost of production.

  • The infrastructure for postharvest management requires input for timely marketing and to reduce the storage losses.