Boron reduces the damage of copper toxicity to <i>Citrus sinensis</i> cell wall structure and function via maintaining copper, calcium, magnesium, potassium and phosphorus homeostasis in subcellular fractions

Commercial citrus trees are predominantly grown in acidic soils with low boron (B) and high copper (Cu) concentrations. There are limited data on how B-Cu treatments affect the concentrations and distributions of nutrients in leaf and root subcellular fractions. Citrus sinensis seedlings were exposed to 2.5 (B2.5) or 25 (B25) μM H3BO3 × 0.5 (Cu0.5) or 350 (Cu350) μM CuCl2 for 24 weeks. Thereafter, we assayed the concentrations of Cu, calcium, magnesium, potassium and phosphorus in leaf and root cell wall (CW) fraction, organelle fraction and soluble fraction, as well as the expression levels of genes related to their homeostasis. B25 reduced Cu350-induced damage of CW structure and function via alleviating Cu350-induced increment in the Cu concentration and decrements in the calcium, magnesium, potassium and phosphorus concentrations, as well as Cu350-induced alterations of their distributions in root and leaf subcellular fractions, thereby promoting seedling growth. Also, leaves and roots of B2.5-treated seedlings displayed some adaptive responses to Cu350. Cu350 increased the distribution of Cu in CW fraction to prevent it from entering more sensitive targets, and the distributions of calcium, magnesium and potassium in CW fraction to maintain CW structure and function. However, Cu350 decreased the distribution of phosphorus in CW fraction, but increased the release of phosphate from organic-phosphate compounds and the conversion of pyrophosphate into phosphate to maintain phosphate homeostasis. Therefore, the study provided novel evidence for B alleviating Cu toxicity in citrus via maintaining the Cu, calcium, magnesium, potassium and phosphorus homeostasis in subcellular fractions, and a scientific basis for the rational application of calcium, magnesium, potassium and phosphorus fertilizers in woody crops (citrus) to prevent Cu toxicity.