Physiological and molecular responses of individual and combined drought and heat stress on early anthesis of maize (Zea mays L.)

Climate change has intensified and co-occurrence of drought and heat events, becoming a major threat to the crops. The reproductive stage of maize is very sensitive to combined drought and heat (D + H) stress. Thus, this study investigated the impact of combined D + H stress on phenotypic and molecular responses in maize. Ten contrasting sets of maize inbreds were subjected to combined D + H stress in a Field-based Crop Phenotyping Facility. Results revealed that combined D + H stresses caused a significant effect on morphological, physiological, and yield-related traits. The inbred CAH 192 showed the highest pollen grains per anther (1181.00) with the highest seed set (8.67 g), whereas UASBM 25 showed the lowest pollen grains per anther (577.33) with zero seed set under combined D + H stress conditions. Based on the phenotypic performance, contrasting inbreds (CAH 192 and UASBM 25) were selected for gene expression study in the anther tissues. Moreover, gene expression patterns revealed the genes responsible for drought and heat tolerance were significantly overexpressed in tolerant inbred (CAH 192). Interestingly, the observed phenotypic variations and gene expression analysis revealed that different sets of physiological traits and stress-responsive genes are involved in drought, heat, and combined D + H stress tolerance. This study reveals that maize response to stress combinations was unique with rapid readjustment at physiological and molecular levels. Overall, phenotypic changes under D + H stress showed substantial adaptive plasticity in maize, which warrants further investigations at the molecular level.