Effects Of Different Proportions Of Red And Blue Light On The Growth And Physiological Characteristics Of Mulberry Seedlings

Light is the key environmental factor regulating plant growth and development,in recent years,with the development of light emitting diodes(LEDs)technology,the use of LEDs in controlling light quality and promoting the growth of medicinal plants and horticultural crops has been focused due to advantages of low energy consumption,accurate wavelength and so on.Mulberry(Morusalba L.)is a kind of woody plant with higher economic value,with the development of facility agriculture,the farmers in the saline-alkali soil area of Heilongjiang province cultivate and breed mulberry seedlings in the greenhouse in winter.The problems of lack of light and changes of light quality in protected cultivation affect the growth of mulberry seedlings.Hence,it is necessary to explore the mechanism of response of growth and development of mulberry seedlings to the combination of red and blue light.The LEDs as light sources were used in this study with 6 treatments including red light(0%B),blue light(100%B)and combinations of red and blue light(15%B,20%B,30%B,50%B),white light as control,and the effects of different proportions of red and blue light on the growth,photosynthetic activity,carbon-nitrogen metabolism,chlorophyll biosynthesis,endogenous hormone content,reactive oxygen species(ROS)scavenging system and leaf anatomical structure of mulberry seedlings were studied to provide reference for practical application of red and blue LEDs,in addition,the feasibility of application of red and blue LEDs in mulberry seedling breeding was also studied.Main results were as follows:1.Red light could promote stem elongation and leaf expansion of mulberry seedlings,but caused the decrease of net photosynthetic rate(Pn),photochemical activity of photosystem II(PSII),microelements(Mn,Cu and Zn)content,capability of leaf carbon-nitrogen metabolism and root system activity;although the supplemental blue light(B)inhibited stem elongation and leaf expansion,yet the supplemental blue light reversed or alleviated these negative effects of red light.The supplemental blue light promoted the increase of leaf mass per unit area(LMA),Pn,photochemical activity of PSII,total N content per unit leaf area(Narea),chlorophyll content per unit leaf area,microelements(Mn,Cu and Zn)content,carbon-nitrogen metabolism related key enzymes activity,and root system activity.Compared with white light treatment,combinations of red and blue light treatment and blue light treatment,the decrease of Pn under red light was mainly due to the decrease of photochemical activity of PSII,chlorophyll content,stomatal conductance(Gs),Rubisco content,and Rubisco activity.In general,the growth and physiological characteristics of mulberry plants grown under the combinations of red and blue light were similar to that of plants grown under white light(CK).These results indicated that the certain proportions of red and blue light(15%B and 20%B)could alleviate the adverse effects of monochromatic red and blue LEDs light on growth and development of mulberry seedlings.Adding blue light to red light played important roles in maintaining the photochemical activity of PSII in leaves of mulberry seedlings and driving photosynthesis in leaves of mulberry seedlings,the supplemental blue light also could promote carbon-nitrogen metabolism and accumulation of microelements in leaves of mulberry seedlings.2.Red light was not conducive to the biosynthesis and accumulation of chlorophyll in leaves of mulberry seedlings,and contents of chlorophyll and chlorophyll biosynthesis precursors under blue light and the combination of red and blue light were relatively higher,blue light and the combination of red and blue light promoted the process of chlorophyll biosynthesis,and might affect transformation process of chlorophyll a to chlorophyll b,resulting in the increase of chlorophyll content and chlorophyll a/b ratio.3.The increase of blue light percentage(0%-100%)could cause the gradual decrease of gibberellin(GA3)content in stems and leaves of mulberry seedlings,while red light and blue light regulated GA3 content and the sensitivity of stem to GA3 which resulted in enhancing effect of red light and inhibiting effect of blue light on elongation of stem,changes of the proportion of red and blue light had minor effects on abscisic acid(ABA)content in stems and leaves.4.Ascorbic acid(AsA)content,dehydroascorbic acid(DHA)content,total ascorbate pool and antioxidant enzymes activity of mulberry leaves under the treatments of red light,blue light and the combination of red and blue light were higher than mulberry leaves under the treatment of white light control.In general,the increase of blue light percentage(15-50%)was more beneficial to the increase of antioxidant capacity of mulberry leaves,and reduced peroxidation damage of ROS to membrane lipids,but the blue light(100%B)caused relative increase of accumulation of ROS and malondialdehyde(MDA)content in mulberry leaves.5.Different proportions of red and blue light could significantly affect the morphology of epidermal cells and stomata in mulberry leaves,red light increased the stomatal size,while the supplemental blue light was beneficial to the increase of stomatal density,which might have an impact on Gs.Compared with red light treatment,treatments of blue light and the combination of red and blue light enhanced leaf thickness,Palisade tissue thickness and the thickness of grana-thylakoid stack.Blue light and combination of red and blue light were beneficial to the development of leaves and grana lamellaes in chloroplasts.6.Different proportions of red and blue light treatments were performed in stages of seed germination and seedling growth of mulberry,affecting the growth performance of mulberry seedlings in the natural environment after transplanting,the growth performance of mulberry seedlings that germinated and grew under the combination of red and blue light(50%B)was better than that of seedlings treated with red light(0%B),the combinations of red and blue light(15%B,20%B and 30%B)or blue light(100%B),and similar to that of seedlings treated with white light,the above results indicated that,the certain proportion of red and blue light(50%B)treatment could alleviate the adverse effects of monochromatic red light treatment on growth of mulberry seedlings in the natural environment after transplanting,the combination of red and blue light was suitable for the breeding of mulberry seedlings in greenhouse.In conclude,red light treatment decreased photosynthetic activity and capability of carbon-nitrogen metabolism in leaves of mulberry seedlings,inhibited accumulation of microelements and biosynthesis of chlorophyll in leaves of mulberry seedlings,blocked development of leaf structure and chloroplast,suggesting that red light was not suitable for growth of mulberry seedlings.The combinations of red and blue light treatment and blue light treatment enhanced photosynthetic activity and capability of carbon-nitrogen metabolism in leaves of mulberry seedlings,promoted accumulation of microelements,biosynthesis of chlorophyll and development of leaf structure and chloroplast,improved antioxidant enzymes activity in mulberry leaves.However,blue light treatment deceased gibberellin(GA3)content in stems and leaves of mulberry seedlings and sensitivity of stem to GA3,severely inhibited leaf expansion and stem elongation.The growth performance of mulberry seedlings cultivated with the combination of red and blue light after transplanting was better.Comprehensive analysis indicated that compared with red light treatment and blue light treatment,the certain proportions of red and blue light was more suitable for growth of mulberry seedlings.