Effects Of Light Quality And Photoperiod On The Growth Of Lettuce And Their Applications

LED（Light-Emitting Diode）,as a new type of semiconductor light source,has the advantages of light weight,small volume,long service life,low energy consumption,safe,fast response and so on.As a cold light source,at the same time,the LED can irradiate plants at a short distance,which can effectively improve space utilization,as well as the yield and makes possible the application of the low energy consumption artificial light source in agriculture.Light is not only the energy source and starter for higher plant photosynthesis and nutrition,but also the important signal for biotic activities in adjusting photomorphogenesis,photoperiod response and inner circadian rhythms and so on.Plants sence light mainly through the pigments of phytochrome and cryptochrome to adapt to the changing light environment.Illumination time and light quality are the important factors that light affect plant growth as well as the development and morphogenesis.Many plants’growth process is regulated by light quality and light cycle,different treatment in light cycle and light quality can obviously change the growth processes of lettuce（Lactuca sativa L.）.This research use LED as the light source,treating the hydroponic lettuce with thin layer nutrient film technique by different combinations of illumination time intervals and wavelengths of light（red,green and blue）.Our purpose is to investigate the effects of different illumination time intervals and different wavelengths of light on physiological and biochemical properties of lettuce by using different illumination time and light quality to process the lettuce,and at the same time to strive to find a best suitable combination of red,green and blue light quality and interval of illumination time for lettuce growth in order to apply it in an optically controlled greenhouse plant factory in the future and better serve the social production.The main results are as follows:1.The effects of different illumination time intervals on lettuce growth,physiological and biochemical characteristics（A）show that the yield and quality of lettuce treated by A1(photosynthetic photon flux density,PPFD is 200μmol m^-2 s^-1,illumination time interval is in 24 h,on 10 min,off 10 min)are between B1(the PPFD is 156 μmol m^-2 s^-1,illumination time interval is in 24 h,on 13 min,off 7 min)and C1(the PPFD is 100 μmol m^-2 s^-1,illumination time interval is on in all 24 h);B1 is advantageous to increase the content of vitamin C in lettuce;The processing C1 can significantly promote the growth of lettuce,increasing production.At the same time,C1 is beneficial in improving the photosynthetic pigment content in lettuce,which not only reduces the content of MDA,as well as improves the resistance of lettuce,but also promotes the synthesis of the content of soluble sugar and soluble protein,as well as the quality of the lettuce.C1 can significantly promote the growth of lettuce and improve its quality.2.The results of different illumination time intervals on lettuce growth,physiological and biochemical characteristics（A）reveal that processing A2(the PPFD is 200 μmol m^-2 s^-1,illumination time interval is in 24 h,on 6h,off 6 h)will increase the content of chlorophyll b and promote soluble sugar content in lettuce.The yield and quality of processing B2(the PPFD is 200μmol m^-2 s^-1,illumination time interval is in 24 h,on 12 h,off 12 h)are between A2 and C2(the PPFD is 200μmol m^-2 s^-1,illumination time interval is in 24 h,on 15.6 h,off 8.4 h);C2,which can significantly promote the growth of lettuce and increase production,can significantly increase the content of chlorophyll a,total chlorophylls,and carotenoid.At the same time,processing C2 is beneficial in increasing the content of soluble protein and vitamin C in lettuce.The highest content processing C2 is the malondialdehyde,but the resistance is weak.This demonstrates that processing C2 is beneficial in increasing the yield of lettuce and improving the quality.3.The investigation of different ratios of light wavelengths on the growth of plant indicates that under the condition of the same PPFD’s,by adding green light in different ratios to red and blue lights,it can significantly promote the external growth of lettuce,especially the growth of leaves and stems.Under the condition of the same PPFD’s,red light is advantageous for the growth of lettuce and blue light inhibits the hypocotyl elongation.For the same wavelength and the ratio of red and blue under the same processing,the higher PPFD’s,the more conducive to the growth of lettuce.Adding green to red and blue proportion has no any influence to the synthesis of chlorophyll b and carotenoid,but it is inconducive to the cumulation of the contents of chlorophyll a,total chlorophyll,soluble sugar and soluble protein.However it can increase vitamin C and at the same time promote the synthesis of MDA,which can reduce the resistance of lettuce.The contents of soluble sugar and soluble protein in lettuce are enhanced under the red light.Blue light can promote chlorophyll a and total chlorophyll content,increase the synthesis of vitamin C in lettuce,enhance the anti-reversibility of lettuce,but is inconducive to the increasing of MDA content.Processing 5(R:G:B is 2:2:6 and PPFD is 150μmol m^-2 s^-1)is the most conducive to improve the yield and quality of lettuce.