Effects of nitrogen form on growth, CO2 assimilation, chlorophyll fluorescence, and photosynthetic electron allocation in cucumber and rice plants*

Cucumber and rice plants with varying ammonium (NH4 +) sensitivities were used to examine the effects of different nitrogen (N) sources on gas exchange, chlorophyll (Chl) fluorescence quenching, and photosynthetic electron allocation. Compared to nitrate (NO3 −)-grown plants, cucumber plants grown under NH4 +-nutrition showed decreased plant growth, net photosynthetic rate, stomatal conductance, intercellular carbon dioxide (CO2) level, transpiration rate, maximum photochemical efficiency of photosystem II, and O2-independent alternative electron flux, and increased O2-dependent alternative electron flux. However, the N source had little effect on gas exchange, Chl a fluorescence parameters, and photosynthetic electron allocation in rice plants, except that NH4 +-grown plants had a higher O2-independent alternative electron flux than NO3 −-grown plants. NO3 − reduction activity was rarely detected in leaves of NH4 +-grown cucumber plants, but was high in NH4 +-grown rice plants. These results demonstrate that significant amounts of photosynthetic electron transport were coupled to NO3 − assimilation, an effect more significant in NO3 −-grown plants than in NH4 +-grown plants. Meanwhile, NH4 +-tolerant plants exhibited a higher demand for the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) for NO3 − reduction, regardless of the N form supplied, while NH4 +-sensitive plants had a high water-water cycle activity when NH4 + was supplied as the sole N source