Mild alternate wetting and drying irrigation improves stomatal and mesophyll conductance of WDR under drought in association with anatomical and hormonal traits.

Alternate wetting and drying irrigation can improve the drought resistance of water-saving and drought-resistant rice (WDR). This response is associated with higher stomatal conductance (gs) and mesophyll conductance (gm), which corresponds to elevated net photosynthesis under drought stress. However, how anatomical and hormonal adjustments contribute to the responses of gs and gm under drought stress in WDR exposed to alternate wetting and drying irrigation remains largely unclear. A pot experiment was conducted under alternate wetting and drying irrigation using cultivars Hanyou73 (HY73, WDR genotype) and Huanghuazhan (HHZ, non-drought-resistant genotype). At heading, severe drought was imposed for seven consecutive days. The results indicated that mild alternate wetting and drying produced the highest grain yield among the three irrigation regimes under drought across both years, especially in cultivar HY73. Meanwhile, the highest net photosynthetic rate was mainly associated with high gs, elevated theoretical maximum stomatal conductance on the adaxial and abaxial leaf surfaces and enhanced gm in mild alternate wetting and drying irrigation coupled with cultivar HY73 under well-watered and drought conditions compared with the other combinations. Correspondingly, the combination consistently improved stomatal morphology, cellular structure, chloroplast ultrastructure and hormone contents in well-watered and drought conditions. In general, mild alternate wetting and drying irrigation minimized the drought-induced changes in gas exchange parameters, structural characteristics and endogenous hormone contents among the three irrigation regimes compared with well-watered conditions in both cultivars. Correlation analysis revealed that stomatal aperture on the adaxial surface was closely and significantly correlated with cell wall thickness, chloroplast surface area and cell surface area in drought. Moreover, both IAA and CTK showed significant correlations with the above-mentioned structural parameters. Our study provides reliable insights into photosynthetic physiology by integrating anatomical and hormonal responses linked to drought adaptation in WDR under alternate wetting and drying irrigation.