Comparison of Drought Stress Responses in Large- and Small-Rooted Rice Lines: Physiological, Anatomical, and Hormonal Changes

Drought stress severely restricts crop yields. Changes in root morphology and function are critical to rice performance under drought conditions, affecting water uptake efficiency, hormone regulation, and nutrient absorption. This study explores the responses of two rice lines, IZ036 with a small root system and IZ144 with a large root system, to drought stress (soil water potential of – 30 ± 5 kPa). The results showed that drought stress significantly inhibited the growth and yield of both rice lines by 35.6–58.1%. Under drought stress, the root-to-shoot ratio of IZ144 increased by 17.8–68.0%, while IZ036 decreased by 10.2–59.1%. While IZ036 experienced a significant reduction in leaf water potential under drought stress, no such impact was observed in IZ144. Both varieties exhibited altered tissue anatomy under drought stress, including, reduced leaf vascular size, increased proportion of vascular bundles in root cross-section, and changes in root thickness. Notably, IZ036 displayed cell and vessel shrinkage and leaf deformation. In response to drought stress, both rice lines exhibited elevated concentrations of auxin, salicylic acid and abscisic acid (ABA) in leaves and increased ethylene and gibberellin (GA) in roots. Notably, IZ144 had significantly higher ABA, cytokinin (CTK), GA, and auxin levels in leaves and CTK in roots than IZ036. Overall, our findings highlight the superior drought tolerance of IZ144 over IZ036, as evidenced by enhanced physiological and anatomical performances and more effective hormone distribution in leaves and roots, indicating the importance of root size in determining drought stress resilience in rice.