Effects Of Low Light Stress On The Growth And Nitrate Accumulation Of Lettuce In Greenhouse

Using lettuce as materials,effects of different irradiance,illumination time,light qualities and their interaction on the growth and nitrate accumulation were investigated in the purpose of contributing to the improvement of cultivation technique of vegetables in a artificial simulated greenhouse with low light condition. The results were showed as follows:(1)The effects of different irradiance,i.e.75,150,225,450 and 550μmol/m².s on the dry weight,root diameter,SPAD,net photosynthesis rate,nitrate content and nitrate reductase activity were studied.The results showed that the most suitable irradiance for both growth and low nitrate content of lettuce is 450μmol/m².s among all the treatment.(2)The effects of different illumination time,i.e.8 h,12 h,16 h,20 h and 24 h illumination time on the dry weight,root diameter,SPAD,net photosynthesis rate, nitrate content and nitrate reductase activity were studied.As for the dry weight,the highest yield was achieved in 24h illumination time treatment which is infeasible in practical production,the second high yield was got in 12h illumination time which is 1.878g.In 20h illumination time treatment,the nitrate reductase activity is 11.371μg NO₂^-/g.h,which is much higher than that in other treatment,while its nitrate content in leaves was lower than that in 8h illumination time treatment.In the condition of 250μmol/m².s,8h illumination time was needed for the growth of lettuce,when the illumination time was longer than 12h,the nitrate content would increase and the dry weight also would drop.As a whole,12h illumination time treatment was the most suitable condition for the high yield and low nitrate content in lettuce under 250μmol/ m².s.The illumination time should be longer than 12h when the irradiance was lower than 250μmol/m².s.(3)The effects of different light quality,.blue light,red light and white light on the dry weight,root diameter,SPAD,net photosynthesis rate,nitrate content and nitrate reductase activity were studied.The results indicated that SPAD,root diameter and dry weight under red light treatment were much higher than that in red and white light treatment.But under red light treatment,the net photosynthesis rate and nitrate reductase activity is higher than that under other light quality,the nitrate reductase activity is 1.000g NO₂^-/g.h,which is much higher than that in other treatment.In practical production,red light can be applied to reduce the nitrate content.(4)The interaction effects of tow light stress,illumination time and light quality on the dry weight,root diameter,SPAD,net photosynthesis rate,nitrate content and nitrate reductase activity were studied.The results indicated that the effects of irradiance and illumination time on the SPAD and root diameter varied according to the light quality.Also the interaction effects the matter distribution of aerial parts and root.Despite the irradiance,the highest nitrate reductase activity and lowest nitrate content was obtained in blue light and 20 h illumination time treatment.As for photosynthesis rate,the highest value was reached in blue light treatment despite the irradiance and illumination time.But in interaction treatment,the photochemical effects decreased with the illumination time.From all the factors,in low light stress, the better growth and low nitrate content were achieved in blue and 20 h treatment.(5)In conclusion,the growth and nitrate content was influenced by irradiance, light quality and illumination time significantly which providence the evidence and theory for the light regulation in vegetable commercial production in greenhouse.