The present study aimed to investigate, under controlled conditions, the physiological attributes of drought tolerance induced by new hybrid citrus rootstocks with demonstrated potential to enhance productive efficiency in sweet orange under water-limiting conditions in the field. Twelve-month-old ‘Valencia’ sweet orange grafted on four new hybrid rootstocks were cultivated in plastic pots filled with 1.5 kg dm⁻³ of soil, under greenhouse conditions, and subjected to different intensities of drought stress defined on the basis of predawn leaf water potential. The results showed that the net rate of photosynthesis, stomatal conductance, transpiration and intercellular carbon concentration did not differentiate between plants maintained under moderate and severe stress conditions, evidencing high water use efficiency. The rootstocks were also able to induce osmotic adjustment, cell wall stiffening, decreased osmotic potential at the turgor loss point (ΨTLP) and stimulation of guaiacol peroxidase (GPX) activity and gene expression in leaves of the drought-stressed plants, allowing the maintenance of cell turgor, oxidative status, carboxylation efficiency and photosynthesis, irrespective of the intensity of drought stress. The rootstocks also showed a constitutively or drought-induced high density of fine roots, contributing to the soil–water uptake. These results revealed various attributes underlying drought tolerance in citrus and provided a valuable reference for developing drought-tolerant citrus rootstocks.
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