Potassium availability modulates magnesium and calcium accumulation during ‘Salustiana’ (Citrus sinensis) orange fruit development

Citrus fruit quality for fresh consumption largely depends on nutrient availability, with potassium (K), magnesium (Mg), and calcium (Ca) playing a central role. However, the uptake of these nutrients is influenced by the presence of the others, often resulting in antagonistic interactions. To investigate whether K affects Mg and Ca accumulation in citrus fruits, a three-year field trial was conducted using ‘Salustiana’ orange trees. Plants were annually fertilized with high (216 kg.ha−1 K2O), medium (108 kg.ha−1 K2O), or low (54 kg.ha−1 K2O) K doses. Leaf samples were collected annually, and fruit flavedo was sampled monthly from spring until the commercial harvest in June. High K application increased leaf and flavedo K concentration ([K]), accompanied by a decrease in [Mg] and [Ca]. In high-K treatments, the flavedo exhibited a pronounced [K] peak during summer compared to medium and low treatments, concomitantly with a reduction in the Mg and Ca accumulation. These findings confirm that K-induced antagonism on Mg and Ca occur in citrus fruit tissues and demonstrate that the substantial K influx during summer impaired Mg and Ca uptake. Despite these alterations in nutrient composition, fruit quality parameters, including peel thickness, external fruit color, firmness, soluble solids content, and acidity of ‘Salustiana’ oranges, remained unaltered. Above all, this work reports that the well-known K antagonism on Mg and Ca it also occurs in a specific tissue such as the flavedo of ‘Salustiana’ orange.