Moderate alternate wetting and drying irrigation enhances drought-resistance abilities by improving structural mesophyll conductance of water-saving and drought-resistant rice under severe drought.

Water-saving and drought-resistant rice (WDR) coupled with alternate wetting and drying irrigation (AWDI) possesses a high photosynthetic potential due to higher mesophyll conductance (g m ) under drought conditions. However, the physiological and structural contributions to the g m of leaves and their mechanisms in WDR under AWDI are still unclear. In this study, WDR (Hanyou 73) and drought-sensitive rice (Huiliangyou 898) were selected as materials. Three irrigation patterns were established from transplanting to the heading stage, including conventional flooding irrigation (W1), moderate AWDI (W2), and severe AWDI (W3). A severe drought with a soil water potential of -50 kPa was applied for a week at the heading stage across all treatments and cultivars. The results revealed that severe drought reduced gas exchange parameters and g m but enhanced antioxidant enzyme activities and malondialdehyde content in the three treatments and both cultivars. The maximal photosynthetic rate (A max ) of HY73 in the W2 treatment was greater than that in the other combinations of cultivars and irrigation patterns. The contribution of leaf structure (54%) to g m (g m -S, structural g m ) was higher than that of leaf physiology (46%) to g m (g m -P, physiological g m ) in the W2 treatment of Hanyou 73. Additionally, g m -S was significantly and linearly positively correlated with g m under severe drought. Moreover, both the initial and apparent quantum efficiencies were significantly and positively with g m in rice plants (p < 0.05). These results suggest that the improvements in photosynthesis and yield in the WDR combined with moderate AWDI can mainly be attributed to the enhancement of g m -S under severe drought conditions. Quantum efficiency may be a potential factor in regulating photosynthesis by cooperating with the g m of rice plants under severe drought conditions.