Grafting induces hormonal and metabolic responses enhancing citrus scion tolerance to combined abiotic stress

Abstract

Climate change poses a threat for some of the main citrus growing areas, including those in the Mediterranean basin. Climate modelling for these regions forecasts significant temperature rise together with increasingly frequent periods of water scarcity. Such stress combination would imply an important threat to plant growth, development and yield. Grafting has been extensively used as a strategy to confer agronomic desired qualities to commercial varieties, including abiotic stress tolerance. Carrizo citrange (Poncirus trifoliata × Citrus sinensis) has revealed as a better rootstock than Cleopatra mandarin (Citrus reshni) under drought and heat stress conditions, as the former induces higher antioxidant activity on the scion, thus reducing oxidative damage and enhancing plant tolerance. Reciprocally grafted Carrizo citrange and Cleopatra mandarin plants were subjected to both individual and combined drought and heat stress. Results revealed that citrus plants response consisted on a series of hormonal and metabolic readjustments, including changes in carbohydrate and amino acid fluxes, which seem to be key responses in the process of plant acclimation to stress conditions. Moreover, rootstock genotype had a significant effect on scion metabolic and hormonal response to the combination of drought and heat stress. Thus, Carrizo as a rootstock positively influenced the metabolic and hormonal response of Cleopatra scions by inducing the accumulation of protective compounds such as raffinose, galactinol and salicylic acid. In turn, Cleopatra rootstock induced lower levels of raffinose, galactinol, proline, phenylalanine and tryptophan in Carrizo scions, and a reduced tolerance to the stress combination. Taken together, our findings show the effect of the rootstock on scion metabolic response to stress combination and remark the relevance of the rootstock in citrus plants exposed to harsh environmental conditions.