Effects of water stress on photosynthesis, chlorophyll fluorescence and photoinhibition in wheat plants

Effects of water stress on photosynthesis, PS II photochemistry and photoinhibition were investigated in wheat plants (Tritium aestivum L.). To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (180 μmol m^-2 s^-1). When water stress developed gradually, net CO₂ assimilation rate and leaf stomatal conductance decreased significantly. However, water stress had no effects on the PS II photochemistry in dark-adapted leaves. There were no significant changes in the maximal efficiency of PS II photochemistry and no apparent damages in PS II reaction centre, its oxidising and acceptor sides, or its antennae system. However, PS II photochemistry in light-adapted leaves was modified in water-stressed plants. This was shown by the decrease in the efficiency of excitation energy capture by open PS II reaction centres and the quantum yield of PS II electron transport and a significant increase in non-photochemical quenching. In addition, water stress increased the susceptibility to photoinhibition. The extent of photoinhibition became more pronounced as water stress increased. It was found that water-stressed plants exhibited a much greater accumulation of the Q(B)-non-reducing PS II reaction centres and a smaller increase in non-photochemical quenching during photoinhibition. Such changes might be responsible for the increased susceptibility to photoinhibition.