Water use and physiological responses of new citrus rootstocks under drought stress conditions

• Physiological responses to water deficit varied widely among genotypes. • B11R5T36 combines rapid stomatal closure with RWC stability. • US802 and US942 achieve water efficiency via reduced transpiration. • Seedling responses may not directly translate to grafted trees. Water stress is becoming an important limiting factor in crop production, especially in arid and semi-arid regions. Since rootstocks may modulate the performance of grafted trees under drought and flooding conditions, rootstock selection is essential for sustainable citrus production. We evaluated the response of new citrus rootstocks under water scarcity conditions. A pot experiment was conducted under controlled environmental conditions, involving (i) water stress treatments (0%, 50% and 100% ETc) and (ii) citrus rootstock (B11R3T27, B11R5T36, B11R5T64, US802 and US942, Cleopatra mandarin and Swingle citrumelo were included as a reference). Stomatal conductance, relative water content (RWC), chlorophyll content, biomass and visual symptoms of stress were assessed. Results revealed plant growth inhibition under drought conditions and reduced stomatal conductance in all studied rootstocks. Stomatal conductance was correlated with visual symptoms of stress, indicating that stomatal closure appeared as the main avoidance mechanism. The response time to water deficit varied among rootstocks and was associated with the water extraction rate, which depended on leaf biomass and transpiration. A rapid response contributed to the maintenance of the RWC in leaves in B11R5T36, but not in B11R3T27. Reduced leaf biomass in B11R5T64 allowed the maintenance of RWC despite the high stomatal conductance, whereas reduced transpiration compensated for high leaf biomass in US942 and US802. Our results indicated that B11R5T36, US802 and US942 exhibited the most effective adaptive responses to water scarcity among all the rootstocks studied, and show promise for use in water-limited orchards. These findings may help citrus growers in selecting rootstocks with potential adaptability to water scarcity conditions.