Grafting improves tolerance to combined drought and heat stresses by modifying metabolism in citrus scion

Mediterranean basin and other citrus-growing areas, such as Florida or California, are among the most vulnerable regions to the impacts of global warming. Therefore, citrus will be likely subjected to increasing periods of water scarcity combined with high temperatures that will impair plant growth, development and yield. In citrus industry, grafting is used to improve varietal characteristics, such as abiotic stress tolerance. Previous research showed that under drought and heat stress conditions, Carrizo citrange ( Poncirus trifoliata × Citrus sinensis ) is a better rootstock than Cleopatra mandarin ( Citrus reshni ) because it induces a higher antioxidant activity on the scion, reducing oxidative damage and increasing plant tolerance. Here, it is shown that metabolic reconfiguration, including changes in carbohydrate and amino acid fluxes, are key responses for plant acclimation to stress conditions. Moreover, the importance of the rootstock on scion metabolic and hormonal responses to drought and heat stress combination has been addressed by using reciprocal grafting between Carrizo and Cleopatra genotypes. Thus, Carrizo as a rootstock improves the metabolic and hormonal response of Cleopatra scions to the stress combination by inducing the accumulation of protective compounds such as raffinose, galactinol and salicylic acid. In turn, Cleopatra as a rootstock reduces levels of raffinose, galactinol, proline, phenylalanine and tryptophan in Carrizo scions, which impairs plant tolerance to the stress combination. Our findings show the effect of the rootstock on scion metabolic response to stress combination and remark the importance of the rootstock in citrus plants exposed to harsh environmental conditions. • Drought and heat stress combination caused in citrus a specific metabolic response. • Rootstock influences metabolic responses of scion to abiotic stresses. • Proline is a key compound in citrus tolerance to drought and heat combination. • Carrizo rootstock improves metabolic response of scion to stress combination.