Lettuce Growth,Nutritional Quality,and Energy Use Efficiency As Affected By Different Combinations Of Light Wavelengths

Compared with light intensity and photoperiod,light quality,especially its components of different wavelengths,shows much more complex effects on plant growth,morphology,photosynthesis,secondary metabolite and so on.In the past decade,a large number of studies focused on the influence of different light intensity or ratios of red and blue light on plant growth.However,plant is capable of sensing and responding to light spectrum with a broad range of light wavelengths,from UV(280-400nm)to far-red(720-780nm).More research is needed to deepen our understanding of the impact of combinations of red,blue and less studied wavelengths,such as yellow,green,orange and far-red light.Moreover,the demand of light quality of different growth stage of plants need to be explored deeply.Based on the current research progress,lettuce growth as affected by red-blue light combined with different monochromatic wavelengths and different stage of lettuce growth as affected by different combinations of light wavelengths were studied.To reduce the operation cost of the plant factory with artificial light,improving the energy use efficiency,and to find the optimum combination of light wavelengths for lettuce(Lactuca sativa cv.'Tiberius')growth in a plant factory with artificial light,This research was conducted in a plant factory with artificial light in which the environmental factors,such as temperature,relative humidity,CO2 level and nutrient solution,can be controlled precisely.1)the mixture of red(R,662 nm)and blue(B,447 nm)light with an R/B ratio of 4:1 and light intensity of 200 ?mol m-2·s-1 was set as the control(RB)and four treatments by using the mixture of red and blue light with an R/B ratio of 4:1 and light intensity of 150?mol·m-2·s-1 supplementing with 50?mol·m-2·s-1 green light(G,525 nm,RBG),yellow light(Y,592 nm,RBY),orange light(O,605 nm,RBO)and far-red light(FR,742 nm,RBFR),respectively,were set up in this experiment.Lettuce growth and its morphology,photosynthetic ability,nutritional quality and energy use efficiency were investigated.The results showed that:RBG,RBO and RBFR increased the shoot fresh weight of lettuce by 14.5%,14.5%,and 30.8%,and increased the shoot dry weight of lettuce by 13.3%,24.1%and 29.9%,respectively,compared with those under RB;RBG increased the height,total leaf area,net photosynthetic rate of lettuce and energy use efficiency;RBY had no significant influence on lettuce growth,compared with RB;RBO increased photosynthetic ability of lettuce and energy use efficiency;RBFR increased lettuce yield,but lettuce under RBFR was easy to get tipburn injury.Based on the above results,RBG and RBO can be considered as suitable combinations of light wavelengths for lettuce production in a plant factory with artificial light.2)The total photon flux density of this experiment was maintained at 200 ?mol·m-2·s-1.In two of all treatments,the mixture of red and blue lights with a light intensity of 150 ?mol·m-2·s-1 and a red to blue ratio of 4:1 was supplemented with either green(RBG)or far red(RBFR)light with a light intensity of 50 ?mol·m-2·s-1.The treatment RBFR*G was set when lettuce plants were transferred to RBG for 10 days growth after growing 10 days under RBFR.The mixture of red and blue lights with a light intensity of 200 ?mol·m-2·s-1 and a red to blue ratio of 4:1(RB)was set as the control.Lettuce growth and its morphology,photosynthetic ability,nutritional quality and energy use efficiency were investigated.The results showed that:RBG,RBFR,and RBFR*G increased the total fresh weight of lettuce by 15.9%,33.2%,and 22.7%,and increased the total dry weight of lettuce by 22.6%,20.8%,and 32.1;The plant height and total leaf area of lettuce under RBFR was increased by 66.7%and 43.6%respectively at 10 days after transplanting,compared with those of RB.When transferred to RBG after 10 days growing under RBFR(RBFR*G),the photosynthetic capacity of lettuce was increased,such as Pn,qP,? PSII,ETR,etc.Meanwhile,compared with that of RBG and RBFR,the dry weight of RBFR*G was increased by 7.7%and 6.3%respectively;RBG,RBFR and RBFR*G increased the energy use efficiency of lettuce.During the last 10 days growth of lettuce,compared with those RBG and RBFR,the EUE of RBFR*G was increased by 11.6%and 25.2%,and the RUE was increased by 11.5%and 51.2%respectively;The nutritional quality indexes of lettuce under different treatments were different.Compared with those under RB,RBFR and RBFR*G increased the content of soluble sugar,RBG decreased the content of nitrate of lettuce.Thus,the appropriate combinations of light wavelengths could be selected according to the actual production purpose.Therefore,the growth and energy use efficiency of lettuce can be improved through reasonable design of light quality in different cultivation stages in plant factories with artificial light.In conclusion,different combinations of light wavelengths showed different effects on the growth and development of lettuce.RBG,RBO,and RBFR increased the yield of lettuce,however,lettuce under RBFR was easy to get tip-burn,which affected the quality of lettuce.Therefore,RBG and RBO can be considered as suitable combinations of light wavelengths for lettuce production in plant factory.Moreover,different combinations of light wavelengths showed different effects on the different growth stages of lettuce.RBFR increased the leaf area of lettuce in the early growth stage,to improve the interception of light energy.RBG improved the overall photosynthetic performance of lettuce in the late growth stage.Thus,Plant yield and energy use efficiency under RBFR*G showed significantly higher than those under RBFR and RBG.Therefore,the light quality for lettuce growth can be reasonably designed according to the different stages of lettuce growth.