Ca2+ efflux is negatively correlated with apple firmness

Abstract

Calcium plays an important role in the regulation of fruit firmness, but the mechanism behind this remains poorly understood. In this study, we investigated the firmness of the fruit pulp in the peduncle, calyx, and equatorial parts of Fuji apples. The pulp protoplasts of the three parts were obtained by enzymatic hydrolysis. The patch clamp technique was used to measure the Ca2+ current on the protoplast membranes of the three parts. The firmness was significantly different among the three fruit parts. The equatorial part, which had the lowest firmness, had the highest Ca2+ efflux, while the lowest Ca2+ efflux was measured near the calyx, which had the highest firmness. Supplementation of potassium fertilizer to apple trees promoted fruit firmness and reduced Ca2+ efflux from pulp cells. Spraying 10 mg/L auxin directly onto young fruit increased the firmness and reduced cell Ca2+ efflux. These results indicated that Ca2+ efflux is correlated with fruit firmness. Because fruit softening is a natural phenomenon during fruit ripening, we conclude that the reduction in cellular Ca2+ efflux can maintain cell viability and increase the firmness of apple fruit pulp.