Citrus plants exude proline and phytohormones under abiotic stress conditions.

KEY MESSAGE: This article describes the root exudation of proline and phytohormones in citrus and their involvement in salt- and heat-stress responses. Plants are constantly releasing several compounds to the rhizosphere through their roots, including primary and secondary metabolites. Root exudation can be affected by growth conditions, including pH, nutrient availability, soil salinity, or temperature. In vitro-cultured plants of two citrus genotypes with contrasting tolerance to salt- and heat-stress conditions were used as plant material. Proline and phytohormone contents in root exudates from plants subjected to salt or high-temperature conditions were evaluated. In addition, tissue damage and lipid peroxidation together with endogenous levels of chloride, proline, and phytohormones were determined in roots and shoots. Proline was released in larger quantities to the rhizosphere when plants were subjected to salt or heat stress. In each stress condition, the concentration of this amino acid was higher in the exudates obtained from plants tolerant to this particular stress condition. On the other hand, root exudation of phytohormones salicylic acid, indole acetic acid, abscisic acid, and jasmonic acid generally increased under both adverse conditions. Results confirm a phytohormone exudation in citrus plants, which had not been described previously and can have an important role in the rhizosphere communication. Moreover, stress conditions and the different tolerance of each genotype to the particular stress significantly modify the exudation pattern both quantitatively and qualitatively.