Effects of Light-Emitting Diode (LED) Red and Blue Light on the Growth and Photosynthetic Characteristics of Momordica charantia L

With andromonoecious Momordica charantia L. (bitter gourd) as material, three light qualities (50 μmol·m·s) including white LED light (WL), blue monochromatic light (B, 465 nm), and red monochromatic light (R, 650 nm) were carried out to investigate their effects on seed germination, physiological and biochemical parameters, sex differentiation and photosynthetic characteristics of bitter gourd. The results showed that compared to the WL treatment, the R treatment significantly promoted seed germination, seedling height elongation and soluble sugar content, the B treatment significantly increased seedling stem diameter, reducing sugar content and soluble protein content, the R and B treatments both significantly reduced sucrose content, but their POD activity showed no significant difference. Compared with the R treatment, the B treatment significantly increased the total female flower number and female flower nod ratio in 30 nods of main stems. The study of photosynthetic characteristics found that the R and B treatments could effectively increase the stomatal conductance (GS) of leaves, significantly improved the net photosynthetic rate (Pn) compared to the WL treatment, and the effect of the B treatment was better. Compared to the R and WL treatments, the B treatment increased the maximum photosynthetic rate (Pmax), apparent quantum efficiency (AQE) and light saturation point (LSP), and reduced the dark respiration rate (Rd) and light compensation point (LCP) of the leaves. Fit light response curves showed that the adaptability and utilization of weak light in bitter gourd were middle or below, but it showed higher adaptability and utilization of strong light. Thus, it suggests that Momordica charantia is a typical sun plan with lower Rd. In summary, it is concluded that blue light has a positive effect on the seed germination, seedling growth, sex differentiation and improving the photosynthetic performance, and this will lay the foundation for artificially regulating optimum photosynthesis usHow to cite this paper: Wang, G.L., Chen, Y.Z., Fan, H.Y. and Huang, P. (2021) Effects of Light-Emitting Diode (LED) Red and Blue Light on the Growth and Photosynthetic Characteristics of Momordica charantia L. Journal of Agricultural Chemistry and Environment, 9, 1-15. https://doi.org/10.4236/jacen.2021.101001 Received: September 27, 2020 Accepted: December 11, 2020 Published: December 14, 2020 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Light is a vital environmental factor regulating the growth, morphogenesis, photosynthesis, metabolism, and gene expression of plants (XU, K, et al., 2005) [1]. Plants can use light as a signal to optimize growth and development for the ambient light conditions during their whole life cycle through sensing the quality, quantity, direction and duration of the incident light, then use it (Alfred, B, 1998) [2]. Many invents, such as seed germination, seedling development and induction of flowering are affected by light (Smith, T W, et al., 1986;Chory, J, et al., 1996;Chory, J, 1997) [3] [4] [5]. Generally, specific wavelengths are used in practice to optimize leaf photosynthesis and crop yield. Because in plants photosynthesis, the absorption spectrum of photosynthetic pigments mainly focuses on blue (400 -500 nm) and red light spectrum (600 -700 nm), blue and red light are the most effectively utilized wavelengths.
Physiological and biochemical approaches have given a broad foundation for the understanding of how blue and red light influence plant growth and development. Blue light strongly influences plant growth and development, such as the growth of the stem, the cotyledons and the leaves, stomatal opening, photosynthesis, flowering, and gene expression (Liscum, E, and Hangarter, R P, 1994; Short, T W, and Briggs, W R, 1994; Jenkins, G I, et al., 1995; Briggs, W R, and Liscum, E, 1997) [6] [7] [8] [9]. Blue light stimulates "sun-type" characteristics such as high photosynthetic capacity on the chloroplast level. Plants generally exhibit higher photosynthetic characteristics under blue light than under red light (Savvides, A, et al., 2012) [10]. Red light can influence the normal development of photosynthetic apparatus, increase leaf starch accumulation. Various plants showed differential physiological response to light quality changes. Blue light is more essential than red light for maintaining the activities of photosystem II and I and photosynthetic electron transport capacity in cucumber leaves (Miao, Y X, et al., 2016) [11].
Momordica charantiais is a monoecious Cucurbitaceae plant, mainly cultivated as a vegetable for its medicinal and nutritional properties in tropical and subtropical Asia. It is used as a traditional medicine because it contains biologically active chemicals and has various medicinal properties ( This paper was projected to lay foundation for scientific light supplement and inducing sex differentiation in bitter gourd production, accordingly, the study mainly focused on the response of seed germination, physiological and biochemical change, as well as the photosynthetic performance to different LEDs light quality.

Materials
QX001, an andromonoecious bitter gourd variety, comes from the long-term breeding resources of our project group. Seed germination experiments started in February 2015 in the laboratory of LED light biology, Huizhou University.
The environmental conditions were controlled as below, the temperature persisted in about 25˚C through day and night, the humidity was 60% -80%.
Screened full bitter gourd seeds were cleaned and soaked in cold water for 2 hours in advance, then transferred to 55˚C -60˚C hot water soaking for 15 min with constant stirring, until all seeds cooled down to room temperature naturally, soaked them in clean water for 12 hours again. Lastly, all seeds were grouped and partitioned into culture dishes with a tile filter paper at the bottom, 25 grains sowed on a culture dish and 100 grains for a group.
Intensity test: at 25˚C ± 1˚C, dark treatment as the control, light intensity experiments set two treatments including 150 μmol·m −2 ·s −1 and 50 μmol·m −2 ·s −1 separately for seed germination. After 5 days, the intensity of 50 μmol·m −2 ·s −1 was selected in next study because seed germination rate was the highest under it. All seeds were sowed at 25˚C in LED illumination chamber, three treatments included white LED light treatment (WL), blue LED light treatment (465 nm, B) and red LED light treatment (650 nm, R), exposure time from 8:00 to 20:00.
All treatments were transplanted to seedling tray and cultured to two-leafone-heart stage after germination, then the seedlings with the uniform growth speed were transplanted into the experimental field. Each treatment set three experimental plots, 30 strains were planted in each plot with area of 15 m 2 , and the general cultivation and management technical was applied in all plots. When the seedlings grew to six, twelve, and eighteen-leaf-one-heart, tested materials with the same growth speed were selected randomly from each plot for determining physiological and biochemical parameters as well as photosynthetic parameters. Left strains in field were in use for sex differentiation statistics until their nods above 30 in main stem.

Determination of Morphological Parameters
When bitter gourd seedling grew to two-leaf-one-heart, the ruler with precision of 0.001 m was used to measure the plant height, stem diameter was measured with vernier caliper with precision of 0.01 mm (measured position below the first leaf 1 cm near the base of the plant). For each treatment, 5 strains were selected and 3 times were repeated.

Determination of Physiological and Biochemical Parameters
Taking seedling apical bud and the first leaf below it as material, total soluble sugar content was determined by anthrone colorimetric determination method, 3,5-dinitrosalicylic acid colorimetric method for the determination of glucose content, Coomassie brilliant blue G250 staining method for the determination of soluble protein content, and guaiacol method for the determination of POD activity (Li, H S, 2007) [15].

Determination of Photosynthetic Parameters
Photosynthetic parameters were measured with LI-6400XT portable photosynthesis system from American company LI-COR when the seedling grew to the stage of six, twelve, and eighteen-leaf-one-heart, 5 strains, grew well with the same growth speed, without plant diseases and pests, were randomly selected from each plot. At 8:30 in a sunny morning, mature function leaves at the same position were clipped in the transparent leaf chamber of LI-6400XT for about 2 -3 min, then began to record data until CO 2 concentration in the sample chamber persisted stable. Determined parameters included net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO 2 concentration (Ci), and transpiration rate (Tr).

Determination of Light Response Curve
LI-6400XT portable photosynthesis system was continually used to determine the light response curve of bitter gourd. The 5 strains like above were randomly selected in each plot, the 18 th in vivo mature functional leaf under normal growth situation was selected to determine light response curve in a sunny morning from 8:30 to 11:30. LED 6400-02B red and blue light source leaf chamber was used to measure instantaneous apparatus photosynthetic rate (IAPR) of attached leaf. In the process of determination, we switched LI-6400XT automatic "light-curve 2" function on, selected LED red and blue photosynthetic photo flux density (

Statistic Analysis
Microsoft Office 2013 Excel software was used for data processing and mapping, The theoretical formula of the non-rectangular hyperbola model is as follows: In this formula, A represents Pn, φ represents AQE, A max is P max , Q is the photosynthetic active radiation (PAR), k is the angle of light response curves, Rday is dark respiration rate.

Seed Germination and Growth Parameters
The results in Table 1 showed that different LED light treatments influenced seed germination. Germination rate and germination potential of seeds varied with different LED treatments. The R treatment significantly promoted the seed germination, the germination rate and germination potential of the seeds increased by 5.71% and 5.73% compared to the WL treatment. Compared to the WL and R treatments, the B treatment significantly reduced the seed germination rate and increased germination potential, and the germination rate decreased by 6.80% and the germination potential increased by 12.40% compared to the WL treatment separately. This suggested that red light promoted the seed germination, while blue light is conducive to the improvement of germination potential in bitter gourd.
Different LED light treatments affected the growth of bitter gourd seedlings.
The R treatment significantly promoted seedling elongation compared to the WL and B treatments, and increased by 33.51% compared to the WL treatment.
The B treatment significantly increased the stem diameter by 22.49% compared to the WL treatment, but there was no difference between the WL and R treatment. This implied that blue light was conductive to breed strong and dwarf bitter gourd seedlings.

Soluble Sugar Content
The change of soluble sugar content of apical buds during the period of six, twelve and eighteen-leaf-one-heart was shown in Figure 1(a). The results showed that along with the increase of seedling age, soluble sugar content presented an increase trend. The R treatment significantly increased soluble sugar content compared to the WL and R treatment, and increased by 51.11%, 34.82% and 79.55% respectively during the stage of six, twelve and eighteen-leaf-one-heart compared with the WL treatment. While in the B treatment, soluble sugar content decreased  significantly by 31.11%, 31.03% and 6.82% respectively during three periods compared with the WL treatment.

Sucrose Content
The change of sucrose content of apical bud during the period of six, twelve and eighteen-leaf-one-heart was shown in Figure 1(b

Reducing Sugar Content
The change of reducing sugar content of apical bud during three periods was shown in Figure 1(c)

Soluble Protein Content
The change of soluble protein content of apical bud during the period of six, twelve and eighteen-leaf-one-heart was shown in Figure 1(d

Peroxidase (POD) Activity
The change of POD activity of apical bud during the period of six, twelve and eighteen-leaf-one-heart was shown in Figure 1(e). Both red and the B treatment showed no significant difference in the POD activity of apical bud in bitter gourd among three treatments during three periods.

Sex Differentiation Statistics in Field
Statistical results of sex differentiation of different treatments were shown in Table 2. Results found that three light qualities had no significant influence on first female flower node of tested plants, the B treatment significantly increased the total female flower quantities and female flower node ratio of 30 nodes in main stem compared to the R treatment, but both showed no significant difference from the WL treatment. Figure 2 shows the fluctuation trend of Pn, Gs, Ci, and Tr was similar during and 21.05% compared to the WL treatment during six-leaf-one-heart period, but it showed no difference among three treatments during the later twelve and eighteen-leaf-one-heart periods. Ci and Tr showed no differences among three treatments during three periods.

Light Response Curve of Bitter Gourd
Light response curve was fitted with the non-linear hyperbolic model (  Table 3).
Previous study found that the maximum value of plant AQE varies from 0.08 to 0.125, but in natural environment, its value is much smaller than the theoretical upper limit and varies from 0.03 to 0.07 in well grown plants (Qiu, G W, 1992) [18]. From Table 3, we knew that the AQE of bitter gourd varied from 0.063 to 0.071, the average was 0.066, which suggested that bitter gourd had higher light energy utilization rate. The B treatment significantly increased the AQE compared with the WL and the R treatment, it implied that blue light could

Monochromatic Light Positively Affects Seed Germination and Seedling Growth
Light can not only provide energy to green plants for growth, but also regulate their morphological development, and these two functions coordinate each other. Some events of growth and development, such as seed germination, seedling

Blue and Red Light Induces the Differential Physiological and Biochemical Response and Sex Differentiation of Bitter Melon
Previous study showed that monochromatic light has a significant impact on the growth morphology, physiological and biochemical parameters during seedlings  Lastly, fit Light response curve of bitter gourd indicated that AQE, P max , Rd, LCP and LSP value all followed the plant physiological law. Based on these parameters, it is concluded that not only can Momordica charantia make use of weak light at a high level, but also had strong adaptability to high light intensity, it suggests that Momordica charantia is a typical sun plant. Moreover, the Rd of Momordica charantia is much low, thus may we say Momordica charantia have high efficiency for solar energy utilization.

Conclusions
1) Red light treatment promoted the seed germination, kept the germination potential at a high level, as well as promoted seedling elongation and leaf expansion. Blue light treatment reduced the seed germination rate, but significantly increased seed germination potential, and strongly affected the growth of the stem and flowering.
2) Physiological and biochemical parameters of Momordica charantia response to blue and red light differently. Soluble sugar content of apical buds significantly increased under red light treatment, whereas blue light treatment promoted reducing sugar and soluble protein content increased significantly, but POD activity of three treatments showed no significant difference. Blue light is more efficient to regulate the photosynthetic parameters of bitter melon and improves photosynthetic performance. Moreover, Blue light could increase female flower quantity and nod ratio compared with red light, to some extent, it is in accordance with their physiological and biochemical response to different light quality.
3) Under blue light treatment, related photosynthetic parameters of light response curve involving Pn max , AQE, LCP were also significantly higher than that of the R and WL treatments, whereas Rd and LCP were lower. Related results indicated that Momordica charantia is a typical sun plant, the Rd of Momordica charantia is much low, and may we say Momordica charantia have high efficiency for solar energy utilization.