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 O-2-independent alternative electron flux, and increased O-2-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 O-2-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.