Vineyard calcium sprays induce changes in grape berry skin, firmness, cell wall composition and expression of cell wall-related genes.

Having a central role in cell wall pectin cross-linking, calcium has been increasingly used as supplement to promote fruit firmness and extended shelf-life. However, the molecular rearrangements associated to increased fruit robustness are still a matter of debate. In this study, mechanical, histochemical and molecular assays were conducted to understand the mechanisms underlying the effects of Ca in fruit physical properties. In a two-year field trial, grapevines were sprayed with exogenous CaCl2 throughout the fruiting season. Results showed an increase in berry Ca concentration at harvest, associated to increased fruit consistency and skin resistance. Scanning electron microscopy showed that fruits from Ca-treated plants had smoother skin surfaces than control fruits, and that microcracks encircling the lenticels were less prominent. Histochemistry assays suggested higher deposition of pectin-like material in skin cell walls in grapes from Ca-treated vines, but no evident modifications in cellulose content were observed. Accordingly, the expression of cellulose synthase family gene CesA3 was not affected by exogenous Ca, while polygalacturonase-encoding genes PG1 and PG2 were downregulated, together with EXP6 belonging to expansin family, and CER9 and CYP15 involved in cuticle biosynthesis. These results suggested that Ca acts by inhibiting pectin degradation and cell wall loosening, while remodeling cuticle structure.