The inhibition of photosynthesis under water deficit conditions is more severe in flecked than uniform irradiance in rice (Oryza sativa) plants

Water deficit is considered the major environmental factor limiting leaf photosynthesis, and the physiological basis for decreased photosynthesis under water deficit has been intensively studied with steady irradiance. Leaves within a canopy experience a highly variable light environment in magnitude and time, but the effect of water deficit on photosynthesis in fluctuating irradiance is not well understood. Two rice cultivars with different drought tolerance, Champa and Yangliangyou 6 (YLY6), were hydroponically grown under well-watered, 15% (m/v) and 20% PEG (polyethylene glycol, 6000 Da) induced water deficit conditions. The inhibition of steady-state photosynthesis in Champa is more severe than YLY6. The maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport capacity (Jmax) were decreased under 20% PEG treatment in Champa, whereas less or no effect was observed in YLY6. The induction state (IS%, which indicates photosynthesis capacity after exposure of low-light period) of both leaf photosynthetic rate (A) and stomatal conductance (gs) was highly correlated, and was significantly decreased under water deficit conditions in both cultivars. Water deficit had no significant effect on the time required to reach 50 or 90% of the maximum photosynthetic rate (T50%,A and T90%,A) after exposure to high-light level, but significantly led to a greater decrease in photosynthetic rate in the low-light period under flecked irradiance (Amin-fleck) relative to photosynthetic rate in the same light intensity of continuously low-light period (Ainitial). The lower IS% of A and more severe decrease in Amin-fleck relative to Ainitial will lead to a more severe decrease in integrated CO2 fixation under water deficit in flecked compared with uniform irradiance.