Revisiting Citrus Rootstocks Polyploidy as a Means to Improve Drought Resilience: Sometimes Less Is More.

Polyploid varieties have been suggested as an alternative approach to promote drought tolerance in citrus crops. In this study, we compared the responses of diploid and tetraploid Sunki 'Tropical' rootstocks to water deficit when grafted onto 'Valencia' sweet orange trees and subjected to water withholding in isolation or competition experiments under potted conditions. Our results revealed that, when grown in isolation, tetraploid rootstocks took longer to show drought symptoms, but this advantage disappeared when grown in competition under the same soil moisture conditions. The differences in drought responses were mainly associated with variations in endogenous leaf levels of abscisic acid (ABA), hydrogen peroxide (H₂O₂) and carbohydrates among treatments. Overall, tetraploids were more affected by drought in individual experiments, showing higher H₂O₂ production, and in competition experiments, rapidly increasing ABA production to regulate stomatal closure and reduce water loss through transpiration. Therefore, our results highlight the crucial importance of evaluating diploid and tetraploid rootstocks under the same soil moisture conditions to better simulate field conditions, providing important insights to improve selection strategies for more resilient citrus rootstocks. Summary statement: This study investigates the drought responses of diploid and tetraploid Sunki 'Tropical' rootstocks grafted onto sweet oranges 'Valencia'. Tetraploids initially showed delayed drought symptoms when considered in isolation; however, this advantage was lost under competitive conditions. Variations in abscisic acid (ABA), hydrogen peroxide (H₂O₂) and carbohydrates played a crucial role in these responses. Notably, although tetraploids exhibited an initial advantage, they displayed greater sensitivity to drought under conditions that more closely mimic field environments. These findings challenge the widely accepted notion that tetraploids are inherently more drought‐tolerant than diploids, indicating the need for more detailed and realistic evaluations to improve drought resilience in citrus rootstocks. [ABSTRACT FROM AUTHOR]