Effects Of Different Light Quality And Ratios Of LED On The Physiological Basis And Nutritional Quality Of Lettuce

The precise regulation of light environment is an important technology in determining the yield,quality,and production efficiency of crops in a facility.Leaf lettuce is a widely planted vegetable crop in greenhouses worldwide.It is a rich source of proteins,vitamins,cellulose,and other nutrients that play important roles in human nutrition and diet.This study used hydroponic and full LED light conditions in investigating the effects of different monochromatic light qualities and red–blue?RB?combined lights on the growth,quality,and nitrogen metabolism of leaf lettuce to explore the different light quality regulation mechanisms of a leaf lettuce's growth and quality.In terms of whether the RB combined light completely satisfied the optimal growth and development of vegetables,This study investigates the effects of RB?control?,red–blue–purple?RBP??red:blue:purple=16:4:5?,red–blue–green?RBG??red:blue:green=16:4:5?,and red–blue–yellow?RBY??red:blue:green=16:4:5?combined lights on the morphological,photosynthetic physiological,and ecological indexes,nutritional quality,and antioxidant capacity of leaf lettuce.At the same time,the transcriptome analysis of lettuce leaves under different LED combination light sources was carried out to examine the differentially expressed genes and key enzyme activities of nitrogen metabolism in lettuce with different light quality combinations.The main results and research conclusions are as follows:1.During the growth of lettuce,using the LED light source of eight different light treatments?white light?control,W?,purple light?P?,blue light?B?,red light?R?,green light?G?,yellow light?Y?,red-blue light in a 9:1 ratio?9R/1B?,and red-blue light in a 4:1 ratio?4R/1B??with light intensity of 200?mol m^-2s^-1.The results showed that compared with the white light,the purple light,blue light,red light,and the red-blue light combination could increase the activities of the nitrogen?N?metabolism-related enzymes such as nitrate reductase?NR?,nitrite reductase?NiR?,glutamine synthetase?GS?,glutamine synthetase?GOGAT?,and glutamate dehydrogenase?GDH?,promote N assimilation and improve the aboveground biomass accumulation in lettuce.Particularly under the 4R/1B light source,the biomass,soluble protein,VC,and total amino acid content were rather high in lettuce,which indicated that the 4R/1B light source could better effectively improve the nutritional quality and promote the growth of lettuce.2.In the late growth stage of leaf lettuce,four different combinations,namely,RB?control,R:B=4:1?,RBP,RBG,and RBY with light intensity of 200?mol m^-2 s^-1,were treated,and the following results were obtained:RBG greatly increased the growth of leaf lettuce and stem elongation.RBY promoted the growth of leaf lettuce,but RBP indexes showed a decreasing trend.RBG and RBY treatments increased the chlorophyll and carotenoid contents of leaf lettuce.Fv/Fm,?PSII,and Pn were relatively high under RBG treatment,thereby resulting in large biomass.RBY treatment increased the blade Gs and added the Ci and Tr,which were beneficial for improving the utilization of light energy.The appropriate addition of green and yellow lights based on the red and blue lights is beneficial to the growth of leaf lettuce,thereby improving its photosynthetic performance and yield.3.In the late growth stage of leaf lettuce,four different combinations,namely,RB?control,R:B=4:1?,RBP,RBG,and RBY with light intensity of 200?mol m^-22 s^-1,were treated,and the following results were obtained:RBP can increase the content of soluble protein,VC,and anthocyanin;reduce nitrate content;and greatly improve the total phenol and flavonoid contents to enhance the antioxidant capacity of leaf lettuce.RBG can increase the content of soluble sugar,soluble protein,and VC;greatly reduce nitrate content;and promote the total phenolic and antioxidant capacity of leaf lettuce.RBY nearly has no positive effect,except for the increase in soluble sugar and VC contents.This phenomenon indicates that appropriate amount of purple or green light can be added on the basis of red and blue light to improve the quality and antioxidant capacity of the leaf lettuce.4.In the late growth stage of leaf lettuce,after 3d of treating the four different combinations,namely,RB?control,R:B=4:1?,RBP,RBG,and RBY with light intensity of 200?mol m^-2 s^-1,the transcriptome sequencing analysis and key enzyme activities of nitrogen metabolism were performed,and 74.43 Gb data were obtained.The transcriptome was analyzed by using a series of bioinformatic methods,such as NR,GO,and KEGG pathway annotations.The nitrogen metabolism pathway was analyzed on the basis of these methods.A total of 14,9,and 8 remarkable differential genes were found in the RBP,RBG,and RBY nitrogen metabolic pathways compared with RB.RBP and RBG treatment could greatly improve the activities of nitrogen metabolism enzymes,such as NR,NiR,and GS.The addition of yellow light inhibits the activities of the aforementioned enzymes.Under different treatments,the expression trend of nitrogen metabolism enzymes was consistent with the activity of nitrogen metabolism enzymes,indicating that the light quality could regulate the gene expression of nitrogen metabolism-related enzymes at the transcriptional level,thereby affecting the activities of related enzymes and the nitrogen metabolism process of leaf lettuce.The comprehensive regulation of RBG and RBP on the key enzyme activities of nitrogen metabolism and the expression of related genes may be important reasons for reducing the nitrate content and improving the quality of leaf lettuce.