Effect of Red and Blue Lights on Photomorphogenesis in Brassica chinensis
An experiment was conducted to investigate the photomorphogenesis response of the combination of red (R) and blue (B) light in different photoperiod. Brassica chinensis were hydroponically cultured at 22/20 °C (day/night), 65 % relative humidity, 400 ppm CO2 level, and 100 μmol m−2 s−1 photon flux density under RB treatment T1 (12 h light, 12 h dark) and T2 (1 h lights, 15 min dark in a day) inside the control environment room for 30 days (14 days after sowing). The fresh weights (FW) and dry weights (DW) of the plants treated with T2 were higher than plants treated with T1. Blue and red LEDs induced relatively higher growth under pulse photoperiod (1 h light, 15 min dark in a day) than normal photoperiod (12 h light, 12 h dark). More importantly, it induces high growth and photomorphogenesis in control environment
KeywordsStomatal Conductance Transpiration Rate Light Quality Stomatal Frequency Control Environment Room
Red and blue light-emitting diodes (LEDs) are necessary for plant development and physiology. Plant morphogenesis and differentiation of plant tissue and cell were controlled by photoperiod and light quality and quantity . Several studies have shown that the combination of different light resulted in many positive effects on growth, development, nutrition, appearance, and the edible quality of plants [11, 14, 23, 28]. The quality of light which refers to the color or wavelength that reached a plant’s surface was strongly influencing plant growth and development . The major energy sources for photosynthetic CO2 assimilation in plants were from red (R) and blue (B) lights. There are studies that examined photosynthesis of higher plants in the reaction of light spectra. It is well known that R and B ranges have a maximum reaction for light spectra [6, 20]. Previous studies show that LED light has strong effect on several plant growth, such as maize , grape , banana , strawberry , potato [17, 31], Chrysanthemum [1, 13, 21, 22], Withania somnifera , Cymbidium , Eucalyptus , Phalaenopsis orchids , Zantedeschia , Lilium , Spathiphyllum , Rehmannia glutinose [11, 14], and Euphorbia milii .
Past studies have proven the combination of R and B lights in controlled environments as a lighting source for the effective production of many plant species, including Brassica chinensis [3, 9, 15, 22, 37, 40, 45, 46]. LED lights are widely used for plant photomorphogenesis and growth at present. No research is available on the effects of RB lights on the photomorphogenesis in B. chinensis grown in hydroponic system in controlled environment with different photoperiod under low light intensity which is constant at 100 μmol m−2 s−1 photon flux density. In order to apply the findings to B. chinensis quality and production, it is important to investigate the effect of combination of RB with low intensity under different photoperiod when provided as the sole source of light. Therefore, the hypothesis of this study was that plants would grow better under RB LED pulse lighting (continues 1 h lights, 15 min dark in a day) compared to RB LEDs (12 h lights, 12 h dark). The final goal of the research was to develop a year-round and rapid production system for fresh, high-quality, pesticide-free, and economically feasible hydroponic B. chinensis that is produced close to the final retail market.
2 Material and Methods
2.1 Plant Material and Growth Conditions
Seeds of B. chinensis were germinated in sponge cubes (3 × 3 cm) and hydroponically grown for 14 days in an environmentally controlled growth room. The temperature was at constant 20 °C under a light intensity 100 μmol m−2 s−1 photon flux density (PFD) for 12 h under RB lights. Uniform-sized seedlings of B. chinensis at the 3-leaf stage were individually raised in a polystyrene foam cube, then mounted into a Styrofoam plate with 15 holes, and placed in a container (55.5 × 42 × 13 cm) filled with complete nutrient solution in a control environment room. The nutrient solution was renewed every week and adjusted to pH 6 and an electrical conductivity of 2.5 mS cm−1. The air temperature, relative humidity, and CO2 levels for all treatments were respectively maintained throughout the experiment at 22/20 °C (day and night), 65 % and 400 ppm under a light intensity 100 μmol m−2 s−1 PFD.
2.2 Light Treatments
Treatments with T1 (12 h lights,12 h dark) and T2 (1 h lights, 15 min dark in a day) consisted a ratio of 16:4 combination of red (R) and blue (B) light source. The peak emissions of the B (454 nm) and R (660 nm) LEDs closely coincide with the absorption peaks of chlorophylls a and b, and the reported wavelengths are at their respective maximum photosynthetic efficiency . The same light intensity expressed as photosynthetic PFD of 100 μmol m−2 s−1 was measured daily above the plant canopy and maintained by adjusting the distance of the LEDs to the plant canopy. Plants were harvested at 30 days after transplant.
2.3 Plant Growth Measurements
Measurements included plant height (PH), number of leaf, plant fresh weight (FW), plant dry weight (DW), moisture content, and leaf area (LA). PH and leaf number were recorded on the plants in four replicates every week. Measurements included plant fresh weight (FW), plant dry weight (DW), moisture content, and leaf area (LA). Plant tissue samples were dried in a drying oven for 48 h at 65 °C before weighing. The LA (cm2) of every plant was measured by an LA meter (LI-3100, LI-COR).
2.4 Chlorophyll (chl)
2.5 Gas Exchange Measurements
Measurements of net photosynthesis (μmol CO2 m−2 s−1), leaf stomatal conductance (mol H2O m−2 s−1), and transpiration rate (mol H2O m−2 s−1) of 20 different leaves per treatment were monitored using a Portable Photosynthesis System Li-6400XT (LICOR, USA). To assess the trade-off between CO2 uptake and water loss, instantaneous water-use efficiency (WUE) was calculated as ratio between photosynthetic rate and transpiration rate (μmol CO2/μmol H2O). Diurnal measurements of gas exchange were taken from 0900 h to 1500 h on the fifth youngest fully expanded leaf of four plants in each replicate for T1 and T2. Statistical assessment was done on gas exchange parameters at between 1100 and 1200 h, which was presumed to be the diurnal period when photosynthetic rates would be maximal .
2.6 Statistical Analysis
Statistical analyses were conducted with statistical product and service solutions for Windows, version 16.0 (SPSS). All measurements were evaluated for significance by an analysis of variance (ANOVA) followed by the least significant difference (LSD) test at the p < 0.05 level .
3.1 Plant Growth and Morphology and Pigment Contents
Influence of treatment on fresh weight (FW), dry weight (DW), leaf area (LA), chlorophyll a (chl a), chlorophyll b (chl b), and total chlorophyll at 30 days after sowing
Moisture content (%)
Number of leaf
Chl a contents of B. chinensis leaves in both treatments were higher than the respective chl b contents. However, significant differences were observed in pigment contents (chl a, chl b, and total chl) regardless of the photoperiod light treatment (Table 1).
3.1.1 Leaf Photosynthesis Rate
3.1.2 Leaf Transpiration Rate
The diurnal mean leaf transpiration rate of B. chinensis under RB (16:4) LED in different photoperiods is shown in Fig. 1. Similar to photosynthesis, T2 depicted higher transpiration rate when compared to counterparts within the time frame of the study. The result of the statistical comparison between treatment means in 30 days shows that there were significant (p < 0.05) differences among both treatments.
3.1.3 Water-Use Efficiency (WUE)
The diurnal mean leaf WUE of B. chinensis under various levels of photoperiods is shown in Fig. 1. The T1 plants recorded higher WUE values than its T2 counterparts thus implying that photoperiod influenced the WUE of plants substantially. Plants grown under T2 gave lower values of WUE 4.54 compared to T1 with values of 6.79. The result from the comparisons of treatment means for 30 days showed that there were significant (p < 0.05) differences among both treatments.
3.1.4 Leaf Stomatal Conductance
The diurnal mean leaf stomatal conductance of B. chinensis under different treatments is depicted in Fig. 1. All plants under T2 conditions gave a higher value of 0.39 mol H2O m−2 s−1 than those in T1. Regardless of treatments, the stomatal conductance values were relatively higher for T2 plants which also contributed to higher photosynthesis of the plants. This was due to the impact of stomatal opening which maintained photosynthetic efficiency without much considerable change in photoperiod. The outcome of the statistical analysis of treatment means for 30 days in T1 and T2 revealed that there were significant (p < 0.05) differences.
B. chinensis is widely grown in Malaysia, and its production is very important, both economically and commercially. The spectral quality of lights is the constant intensity and quantity of different photoperiods emitted by a light source and perceived by photoreceptors within a plant. Plant yields and quality are the result of interactions of various environmental factors under which plants are grown.
The present study examined the effects of different photoperiod light spectral conditions on the yield and quality of B. chinensis grown under the same environmental conditions. The FW and DW was comparatively greater in plants grown under T2 than under T1. These results indicate that the relationship between FW and DW will slightly affect the moisture contents that give a higher result (Table 1). The greater FW is likely associated with the greater LA achieved under these conditions. The larger leaf allowed greater light interception, which may have led to the significant increase in biomass . The induction of the shade-avoidance syndrome requires the perception of the spectral changes associated with shade, rather than changes in total light quality . The increase in plant height and enhancement of number of leaf as a consequence of pulse light and long photoperiod found in T2. However, there are significant different shown between T1 and T2 according their growth and will be defined as T2 have a greater growth and photomorphogenesis impact due to the additional photoperiod that is given in a pulse condition. From this study, it was shown that long light will affect the growth and light irradiation of a plant that causes the plant’s height reduction and no significant changes in total leaf area when given during the night period .
According to Wang et al. , plant pigments have specific wavelength absorption patterns known as absorption spectra. Biosynthetic wavelengths for the production of plant pigments are referred to as action spectra. Although different photoperiods for both treatments were applied at the same PFD level, plants showed significant different absorption spectra of photosynthetic pigments, chl a, chl b, and total chl (Table 1). Perhaps, the applied PFD level (100 μmol m−2 s−1) had reached certain minimal PFD, which is essential for sufficient synthesis and activity of photosynthetic pigments and electron carrier. It was reported by Ref.  that plants with smaller chl contents seemed to use the chl more efficiently than plants with excessive chl. In this case, chl a, chl b, and total chl in the leaves have statistically different treatments; the chl contents under T2 were the lowest. This indicates that B. chinensis grown under T2 might be using chl more efficiently than grown under T1. Plants grown under both treatments appeared to synthesize more chl a compared to chl b (Table 1); it is due to the wider spectrum for chl a absorption and chl a is the molecule that makes photosynthesis possible .
The photosynthesis rates recorded (Fig. 1) showed that there are significant differences among T1 and T2. Another factor that contributed to the decreased in photosynthesis might be the limited CO2 diffusion into the intercellular spaces of the leaf as a consequence of reduced stomatal conductance . This consequence showed that photosynthesis rate was closely related to the changes in the leaf stomatal conductance. B. chinensis grown under T2 showed that stomatal conductance might cause high photosynthesis, because once the stomata remain open, photosynthesis activities will always occur to produce energy that will make food for plant growth. This indicates that long exposure to light under T2 (1 h light, 15 min dark in a day) will increase the activity of photosynthesis, stomatal conductance, and transpiration rate. In addition, higher stomatal frequency could facilitate CO2 uptake and thus maintain a high photosynthetic activity . Light is the energy source for photosynthetic organisms, and light intensity plays an important role in plant growth. Low light conditions inhibit plant growth and productivity by affecting gas exchange . In our study, although gas exchange at T1 showed a low frequency compared with T2, it shows that 100 μmol m−2 s−1 photon flux density is suitable for their growth when induced health photosynthesis, stomatal conductance, and transpiration. The smaller stomatal frequency could restrain photosynthesis rates by increasing diffusive resistance to CO2 uptake, which might reduce the burden of photosynthetic organs . Stomatal conductance was strongly influenced by light quality, with highest numbers in leaves grown under RB LED light . It can therefore be concluded that light quality has an important permanent effect on gas exchange during leaf development. WUE showed that there are significant differences between T1 and T2 that consist of frequency of photosynthesis over the transpiration (Fig. 1). The transpiration rate was reduced significantly thus resulting in an increased of WUE as water stress induces stomatal closure. This is in agreement with Arndt et al. , which can be attributed to the postponement of the damaging effects caused by water deficit in trees through stomatal control.
As a conclusion, it was shown that the growth of B. chinensis was promoted by RB light-emitting diode, 100 μmol m−2 s−1 photon flux density under pulse photoperiod (1 h light, 15 min dark in a day) in comparison with the normal photoperiod (12 h light, 12 h darks). In this study, we investigate the photomorphogenesis response of the combination of red (R) and blue (B) light in different photoperiods. Based on this study, it appears that the combined RB LEDs induced relatively higher growth under pulse photoperiod (1 h light, 15 min dark in a day) than normal photoperiod (12 h light, 12 h dark). More importantly, it induces high growth and photomorphogenesis in control environment.
This work was supported by the Fundamental Research Grant Scheme (4 F144) and GUP Tier 2 (002 J8) by the Ministry of Higher Education (Malaysia).
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