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.