Light quality and quantity also have a drastic effect on the excitation of PS I and PS II, which is directly interlinked with photosynthesis processes . Light spectral quality considerably affects plant shoots and roots regarding their growth and morphology, and their interaction . In the present experiment, the longest RL was recorded from L1 and L9, and the opposite result was found from L2 and L6 . In a previous study, plant height and weight were found better in red light; RL was longer in red and natural light while shorter in white and blue light undergreenhouse conditions . In the present study, the addition of FR light to R and B increased the LL and LW, while decreasing the LN and NN . It was reported that the cannabis plant attained a higher plant height and leaf area in white light compared to the combination of red and blue light , but we did not find any significant differences in those morphological traits under similar light conditions. In a previous study, the addition of FR to R and B decreased the LL and SL of tomato plants while no signifificant effect was observed for RL and LN .
Results indicated that a higher number of leaves and branches will make the hemp plants shorter in size with narrow leaves and vice versa. Importantly, LN and NN were closely associated with the treatments L9, L10, and L11, whereas LL and LW manifested an opposite relationship with them . This may be due to the inflfluence of UV-A, which reduced the leaf area but increased the leaf number and branching. Generally, UV radiation impacts the phytohormone auxin levels higher in leaf regions with high division activity and lower in areas of cell expansion , resulting in a decrease in adaxial pavement cell expansion . Furthermore, compared to white light, PFW, PGR, and NN increased in most LED composition. The results indicate the incapability of monochromatic white light for plant growth and development, as it also resulted in lower TSC and sucrose accumulation in most cases. The PCA analysis indicated a negative relationship between the L2 treatment and NN, PFW, PGR, TSC, and sucrose, which also had a positive correlation with the parameters. An earlier study showed that the Chl a and Chl b contents increased in lettuce, basil, spinach, kale, and pepper under different combinations of R and B .
In our study, the Chl a and b of hemp plants were significantly increased, but the chlorophyll a/b ratio and Car drastically decreased in the L3 treatment compared to L1 . Besides this, Chl a and b significantly increased, and the Chl a/b ratio significantly decreased in all treatments, while the carotenoid concentration was found decreased in the L2, L3, L4, L6, and L9 treatments compared to L1 . Similar results from a previous study described that a high ratio of both R:B and R:B:W manifested higher chlorophyll compared to natural light in Silene capitate Kom. . Each of the plant pigments has been characterized by an absorbance pattern in wavelength called the absorbance spectrum, where the blue and red regions are absorbed strongly by Chl a and b, with less absorbance of other wavelengths . It is also known that the Chl and Car pigments absorb 400– 500 nm and 630–680 nm in the light spectrum in plants with the help of light-harvesting antenna . The positive influence of red and blue light on Chl synthesis in the present study complies with these findings . Furthermore, the accumulation of Car and the Chl a/b ratio were increased, whereas Chl b was decreased significantly under the spectra composed with green light when compared to the red and blue spectra . Since red and blue light are absorbed by photosynthetic pigments more strongly, their influence is predominant in the upper cell layers, while green light can penetrate deeper into leaf tissues and excite the photosystem in the deeper cell layer .
On the other hand, Car are lipid-soluble colored pigments that mostly consist of carotenes and xanthophylls , whose absorbance range extend into the green region , effectively cover the poorest region of chlorophyll absorbance . Thus, the addition of green light along with others in the present study might increase the concentration of Car. The photochemical activity of photosystem II is expressed by Fv/Fm, which characterizes the maximum efficiency of the photochemical activity under PS II , where Fv , Fm, and F0 denotes the maximal variable flfluorescence, maximal fluorescence intensity, and fluorescence intensity at 50 µs, respectively. The photosynthetic fluorescence is a byproduct of the photosynthetic process created by trapping light energy at the reaction center within a photosynthetic membrane and after being used in photochemistry that dissipates along with heat energy . A decreasing trend in Fv/Fm was observed in the treatments L1, L6, L7, and L10 in the present study . A lower Fv/Fm may be a consequence of decreasing Fm, since F0 does not change too much in light stress .Proline is an important compound as its synthesis and catabolism play an important role in the stress adaptation of plants by keeping in balance the redox reaction . Under stress conditions, ROS-mediated regulation, including H2O2, can upregulate P5CS and downregulate PRODH activity in the plant, which can trigger the biosynthesis of proline .