Abstract: 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.

Key words: Nitrogen form, Photosynthetic electron allocation, Alternative electron flux, Nitrate reductase

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