Metabolic responses to boron toxicity in different citrus cultivars

Climate change is causing crops, such as citrus, to growth under adverse environmental conditions, where lack of water leads to the use of water from non-conventional sources for irrigation. However, the use of this type of water can cause boron toxicity problems, especially if it comes from marine desalination plants, in sensitive crops such as citrus. To face this reality, it is necessary to know which physiological and biochemical processes are negatively affected as a consequence of boron excess in order to design agronomic strategies to alleviate this problem. For this reason, tools such as omics sciences and, within these, the study of primary metabolites can help to understand how the crop is affected by this stress. In this context, a study was carried out to classify the relative tolerance to boron stress of different citrus cultivars, and a metabolomic study, using the nuclear magnetic resonance (NMR) technique, to determine the metabolic profiles of these cultivars. Seventeen citrus cultivars were used: 3 commercial lemon cultivars (‘Satsuma Owari’, ‘Satsuma Iwasaki’, ‘Clementina Hernandina’, ‘Murcot’, ‘Clemenules’, ‘Nova’ and ‘Ortanique’), and 7 orange trees (‘Washington Navel’, ‘Midknight’, ‘Valencia Late’, ‘Lane Late’, ‘Navelina’, ‘Sanguinelli’ and ‘Navelate’). These plants were irrigated with water containing a boron concentration of 5 mg L^‑1. The most relevant results were: i) in citrus crop there were different levels of tolerance to boron toxicity stress strongly influenced by the cultivar; ii) the tolerance level to boron toxicity, by citrus crop, is not directly related to the concentration of boron accumulated in its leaves; iii) boron toxicity stress affected the main pathways of primary metabolism in citrus and a generalised response was found in the pathways involving the metabolites acetate, citrate, malate, glutamic acid, proline, GABA and alanine; which significantly reduced their concentration as a result of this stress.