Agronomic and Metabolic Responses of Citrus clementina to Long-Term Irrigation with Saline Reclaimed Water as Abiotic Factor

The Panel on Climate Change has predicted an intensification of drought and heat waves. The aim of this study was to determine the physiological response of mandarin trees in a semi-arid area to the effects of a long period of irrigation with saline reclaimed water (RW) and freshwater (FW) in terms of leaf mineral constitution, free amino acids and phytohormone balance, and their influence on yield and fruit quality. Results showed that higher foliar levels of Cl^-_, B, Li⁺, and Br^- were found in the RW treatment. In addition, fruit quality (juice content, soluble solid content, titratable acid, and maturity index) and yield (fruit weight and diameter) parameters and growth canopy were negatively affected by irrigation with RW. Regardless of the treatments, L-alanine (Ala) and proline were the most abundant amino acids, with Ala being described as a majority for the first time in the literature. Concretely, in FW, the total amino acid content was twice as high as the concentration in RW (51,359.46 and 23,833.31 ng g^-1, respectively). The most abundant hormones were 1-Aminocyclopropane-1-carboxylic acid and <i>trans</i>-zeatin in both treatments. The saline stress response would be reflected in the higher concentration of salicylic and abscisic acids in the leaves of RW trees. In view of the high correlations found in a simplified correlation matrix of (i) Ala with the canopy growth and (ii) the salicylic acid (SA) with most of the evaluated agrometabolic parameters, it can be concluded that the exogenous application of the Ala and SA would increase tree size and could mitigate the effects of salt stress, respectively. However, these treatments could be completed with the external application of ACC since this phytohormone presents the lowest parameter during treatment with RW.