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

摘要

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.