Effects Of Red-blue LED Light Supply Modes On Growth And Quality Of Purple Leaf Lettuce

Plant factory with artificial light is an efficient agricultural system to realize continuous annual production of crops through high-precision environmental control in facility.Plant factory can achieve high yield and mass production of hydroponic vegetables,but at the same time,the problem of low quality of vegetables is prominent,such as the low content of ascorbic acid,phenolic compounds and other antioxidants in vegetables,as well as excessive accumulation of nitrate,which limits the development of plant factory industrialization.For plant factories with artificial light,increasing effective light is the most direct and effective technical means to promote plant photosynthesis as well as improve plant yield and nutritional quality.The performance of artificial light source and light supply strategy have a great impact on crop production and economic benefits.Therefore,based on the importance of red and blue light in plant growth and development,this study proposes to innovate the way of increasing effective light in the artificial light plant factory,optimize the irradiation mode of high light irradiation and continuous light,and study its influence on the active oxygen content,antioxidant enzyme activity and antioxidant substance content of hydroponic lettuce,so as to provide reference for the improvement of energy efficiency and the wide application of plant factory in the future.The main research contents and conclusions are as follows:（1）The growth and antioxidant system of purple leaf lettuce were investigated in purple leaf lettuce exposed to different high light(500μmol·m^-2·s^-1,16/8 h,4R:1B)duration in the middle of red-blue light period(150μmol·m^-2·s^-1,4R:1B)for 0 h（CK）,0.5 h（HL0.5）,1 h（HL1）,2 h（HL2）and 4h（HL4）.The results showed that the shoot fresh weight of lettuce first increased and then decreased with the prolongation of high light duration.The fresh weight,leaf area and dry weight of lettuce under HL1 were the highest,followed by that under HL2.Besides,HL4 induced higher secondary metabolites content,but caused stress on plants,as indicated by higher malondialdehyde content and lower photosystem II maximum photosynthetic efficiency,and substantially decreased biomass production.HL0.5 also increased the content of ascorbic acid（As A）and phenolic compounds in lettuce,which was not subjected to oxidative stress.This may be because the short-term fluctuation of light intensity increased the content of reactive oxygen species in lettuce and induced the accumulation of antioxidant substances.The results showed that it was an effective way to regulate the growth and quality of lettuce to use appropriate high light duration in plant factory.（2）The growth and antioxidant system of purple leaf lettuce were investigated in purple leaf lettuce exposed to constant red-blue LED light(NL,170μmol·m^-2·s^-1,16/8 h,4R:1B)and four fluctuating light treatments,in which high(500μmol·m^-2·s^-1)and low(150μmol·m^-2·s^-1)light（4R:1B）were alternately irradiated for 1 time（A1）,3 times（A3）,6 times（A6）,and 12 times（A12）respectively.The results showed that the yield of lettuce could not be increased by alternating high and low light,on the contrary,compared with the control,A6 treatment significantly reduced the shoot and increased the root shoot ratio of lettuce.Compared with the control,A1 and A12 treatments significantly reduced the anthocyanin content of lettuce,A3 treatment significantly reduced the flavonoid and total phenol content of lettuce,and A1 treatment significantly reduced the As A content of lettuce.However,the content of malonaldehyde was not significantly affected by alternate strong light irradiation.Alternating relatively high and low intensity light could not increase the yield or antioxidant content of lettuce,and the nutritional quality of lettuce under constant light irradiation was better than that under fluctuating light irradiation.（3）The growth and antioxidant system of purple leaf lettuce were investigated in purple leaf lettuce exposed to the different supply modes of continuous light.Two conventional red-blue LED light treatments(200μmol·m^-2·s^-1,4R:1B)were set,with the photoperiod of 16/8 h and 24/0 h respectively.In addition,alternating red and blue light treatments and temporally overlapped red and blue light treatments were set both under continuous light conditions with the intensity of 200μmol·m^-2·s^-1.The results showed that continuous light significantly increased the yield of lettuce,and the alternating red and blue light treatments had the most obvious effect on the shoot biomass of lettuce.The alternating and temporally overlapped light treatments significantly increased the content of hydrogen peroxide,superoxide anion and malondialdehyde,but did not cause serious photo oxidative stress,and increased the activity of antioxidant enzymes and the contents of anthocyanin and total phenols,however,the nitrate content in lettuce leaves was also increased by dynamic light.Generally speaking,the effect of the treatment in which red and blue light are alternately irradiated twice was the best.