Effects of different NO₃^– concentration on photosynthetic characteristics in cucumber seedling

Cucumber plants grew in nutrient solution under different NO3– concentration for 7 d at hydroponic system. The changes of photosynthesis rate and related parameters in cucumber seedling leaves were studied in order to reveal the response mechanism of the photosynthetic functions of plants to NO3– stress in cucumber seedling. The results indicated that when available NO3– concentration was low (14 to 98 mmol/L NO3–), the net photosynthesis rate (Pn), light saturation point (LSP), carboxylation efficiency (CE), number of starch grains and absolute increment of leaf area were increased by proper supplement of NO3–. Supplement of NO3– at appropriate range improved the accumulation of photosynthates and the growth of plants. With further increase of NO3- concentration (98 mmol/L NO3–) in the medium, the pigment contents in cucumber seedling leaves were lower; the apparent quantum yield (AQY ), Pn, LSP, CE all significantly decreased compared to control. When NO3– concentration increased to as high as 182 mmol/L, the ultrastructure of chloroplasts were damaged，and the number of grana in cells and number of lamellae in granas , the number of starch grains in cells as well as starch grain size all decreased. Leaf area was reduced and dry weight was significantly lower than that of control plants. Consequently, the capabilities of cucumber seedlings using light and CO2 were lower than control. Factors caused reduction of Pn was non-stomatal limitation because mesophyll cell were damaged.