Penetration of sweet cherry skin by 45Ca-salts: pathways and factors

Calcium is beneficial to sweet cherry physiology. The objective was to investigate factors affecting uptake of Ca into mature sweet cherry fruit through their skins. Penetration of Ca-45-salts was monitored using whole fruit or excised fruit skins mounted in diffusion cells. Penetration of (CaCl2)-Ca-45 into intact fruit and through excised skins increased with time. Sealing the pedicel/fruit junction decreased penetration, but sealing the stylar scar had no effect. There was little difference in permeances of the fruit skin to (CaCl2)-Ca-45, Ca-45(NO3)(2), Ca-45-formate, Ca-45-acetate, Ca-45-lactate or Ca-45-propionate. Only Ca-45-heptagluconate penetrated at a slower rate. Increasing temperature markedly increased Ca-penetration. Penetration was most rapid at 35 degrees C, intermediate at 22 degrees C and slowest at 12 degrees C. Increasing relative humidity (RH) from 0, 28, 75 to 100% increased penetration of (CaCl2)-Ca-45, but penetration of Ca-45-formate was restricted to 100% RH. Increasing the RH from 50 to 100% at 96 h after droplet application had no effect on penetration of (CaCl2)-Ca-45, but increased penetration of Ca-45-formate. The results reveal that: (1) the fruit/pedicel junction is a site of preferential Ca-uptake and (2) Ca-penetration is limited by the mobility of the Ca ion in the dried-down droplet residue when the point of deliquescence of the applied salt exceeds the ambient RH.