Regulation of melatonin on chlorophyll fluorescence and nitrate accumulation in lettuce seedlings under excess nitrate stress

Excess nitrate stress has become a threat of environmental issues to crop cultivation in China. While measures of remediation and knowledge of the mechanisms underlying excess nitrate stress are limited. Previous studies reported that melatonin could improve the plant resistance to abiotic stress. Thus, we investigated the role of melatonin on the photosynthetic characteristic involved in nitrate assimilation under the excess nitrate stress in lettuce (Lactuca sativa ). Lettuce plants were subjected to moderate excess nitrate stress (160 mM NO₃⁻) with/without supply of 1.0 μM melatonin. Results showed that adding of melatonin decreased the nitrate content both in leaves and roots. Melatonin also dramatically increased the fresh and dry weights in lettuce seedlings. According to the chlorophyll fluorescence parameters, exogenous melatonin inhibited the reduction inφPSII, Fv'/Fm', qP and rETR caused by excess nitrate stress, while reduced NPQ. In conclusion, exogenous melatonin could decrease the nitrate content in lettuce tissues and enhance the activity of PSII as well as the light energy conversion efficiency for carbon assimilation.