Drought tolerance in citrus trees is enhanced by rootstock-dependent changes in root growth and carbohydrate availability

• Sweet orange scions grafted on Rangpur and Swingle rootstocks under water deficit.
• The growth of plants on Rangpur lime was not affected by water shortage.
• Plants on Rangpur increased root growth and maintained the total carbohydrate pool.
• Plant carbohydrate partitioning under water deficit was rootstock-dependent.
• Young leaves and branches have similar importance as sources of carbohydrates.

Valencia orange scions grafted on Rangpur lime or Swingle citrumelo were grown under water deficit to evaluate how those rootstocks modulate the non-structural carbohydrate (NSC) availability and the drought tolerance of citrus trees. Additionally, the importance of young mature leaves as possible sources of carbon in citrus trees was studied. Herein, young mature leaves are those ones fully expanded and developed during water deficit. After 30 days under water limiting conditions, plant growth, leaf water status, photosynthetic rate and carbohydrate availability in old mature and young mature leaves, branches and roots were evaluated. Water deficit reduced the leaf water potential and caused diffusive limitation of photosynthesis in both rootstocks. Drought-induced decrease in total NSC content occurred only in plants grafted on Swingle. While plant growth on Swingle citrumelo was severely reduced by water deficit, plants grafted on Rangpur did not exhibited impairment of dry matter accumulation. The lower sensitivity of plant growth on Rangpur lime was associated with the enhanced root growth, the maintenance of the total carbohydrate pool and to a large shift in the carbohydrate partitioning, with the roots accumulating carbohydrates under water deficit. Regarding the young mature leaves, they exhibited higher photosynthetic rates than the old mature leaves after 30 days of treatment, regardless of the water conditions. As possible sources of carbohydrates, young mature leaves have equal importance as compared to branches under well-watered conditions.