Regulation Mechanisms Of Ascorbate Metabolism In Lettuce By Continuous Light Of Red And Blue LEDs

As one of the most important antioxidants,ascorbic acid(AsA)plays an important role in maintenancing the normal growth of plants and the health of human.However,due to the relatively low light level in protected horticulture,AsA shortage of horticultural vegetables,especially of those hydroponic leafy vegetables in plant factory is very prominent.Continuous light increases effective light by maximizing light duration,and is an effective way to increase the yield and quality of vegetables,but also a potential photooxidation stress.The LED light source has accurate spectrum and can be regulated as required,which enriches the connotation and regulation mode of continuous light.In order to investigate the effect of red and blue LED continuous light on AsA content in hydroponic lettuce and the corresponding metabolic mechanism,the response site of AsA metabolism to continuous light was considered as the breakthrough point.The effect of red and blue continuous light on the AsA content and key enzymes(L-galactose-1,4-lactone synthase GalLDH,ascorbate catalase APX,monodehydro-ascorbate reductase MDHAR,dehydroascorbate reductase DHAR,and glutathione reductase GR)of AsA metabolism pathway of lettuce and their relationship with duration,light intensity and light quality of continuous light were studied to reveal response mechanism of lettuce AsA metabolism to continuous light.In addition,the tolerance characteristics of lettuce to the duration,light intensity and light quality of continuous light were revealed by investigating the response of lettuce growth and oxidative stress to continuous light.These will provid scientific basis for the establishment of continuous light stratege which could improve yield and quality of hydroponic lettuce.The main results and conclusions are as follows:(1)Dynamic changes in growth,ascorbate pool and its metabolism,as well as oxidative stress were monitored during 15 days in lettuce plants exposed to continuous light(CL,24/0 h,200 ?mol·m-2·s-1)and normal photoperiod(Control,16/8 h,200 ?mol·m-2·s-1).The results showed that CL treatment could increase biomass and dry matter content of lettuce within 15 days,but had no significant effect on leaf area.CL significantly promoted the accumulation of AsA and DHA by upregulating the activity of GalLDH and APX,and also upregulated the activity of MDHAR to promote the reduction of MDHA to AsA at the later stage of experiment.Although the content of H2O2 increased significantly under CL,it did not increase constantly.Moreover,the fluorescence image of the photosystem II maximum photosynthetic efficiency(Fv/Fm)also showed that lettuce leaves were not injured by CL,indicating that lettuce could withstand 15 days' continuous light.(2)Dynamic changes in growth,ascorbate pool and its metabolism,as well as oxidative stress were monitored during 12 days in lettuce plants exposed to continuous light of different intensities:100?mol·m-2·s-1),200?mol·m-2·s-1,300 ?mol·m-2·s-1.The results showed that the higher the light intensity was,the higher the biomass,dry weight ratio,root shoot ratio and specific leaf weight were,but the leaf area was not affected.In general,AsA content and activity of key enzymes involved in AsA metabolism were positively correlated with light intensity under continuous light.It is suggested that high light intensity promotes AsA accumulation by stimulating AsA synthesis and regeneration under continuous light.In addition,the content of H2O2 and MDA alos increased with light intensity.Fv/Fm of 200 and 300?mol·m-2·s-1 treatments decreased significantly on the 6th day,but returned to the normal level on the 12th day.The results show that short-term continuous light of high intensity induced reversible photoinhibition.However,100?mol·m-2·s-1 treatment did not cause any photooxidative stress,indicating that the continuous light stress was caused by excessive light energy.(3)Dynamic changes in growth,ascorbate pool and its metabolism,as well as oxidative stress were monitored during 12 days in lettuce plants exposed to continuous light of different red:blue light ratio:3R:1B,1R:1B,and 1R:3B.The results showed that the growth parameters,including biomass,root shoot ratio and leaf area,of lettuce were positively correlated with red light ratio,while AsA content was positively correlated with blue light ratio.1R:3B treatment increased the activity of enzymes involved in AsA regeneration(APX,MDHAR,DHAR and GR)on the 6th to 12th day,thus promoting the accumulation of AsA,while the activity of GalLDH did not respond significantly to light quality.H2O2 content also increased with the increase of blue light ratio.Under continuous light,the ratio of red:blue light had no significant effect on Fv/Fm of lettuce.Fv/Fm of all treatments was higher in the later stage than the earlier stage.(4)Circadian rhythm of ascorbate pool and its metabolism,as well as oxidative stress were monitored during 72 h in lettuce plants exposed to continuous light with different light intensity at nignt time:NL(0?mol·m-2·s-1),LL(20 ?mol·m-2·s-1),CL(200 ?mol·m-2·s-1).The results showed that LL treatment had no significant effect on biomass,but decreased the root shoot ratio of lettuce.AsA content was significantly increased by CL within 8 hours,and then remained stable.During 72 hours,CL significantly increased the activity of other enzymes except DHAR at several time points.There was no significant difference of AsA content between NL and LL,and both of them had same circadian rhythm.Under CL treatment,Fv/Fm decreased significantly within 8 hours,and then remained stable.The response of AsA content and its metabolism of lettuce to LL indicated that light signal did not participate in the regulation of continuous light on AsA.