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 ⁴⁵Ca-salts was monitored using whole fruit or excised fruit skins mounted in diffusion cells. Penetration of ⁴⁵CaCl₂ 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 ⁴⁵CaCl₂, ⁴⁵Ca(NO₃)₂, ⁴⁵Ca-formate, ⁴⁵Ca-acetate, ⁴⁵Ca-lactate or ⁴⁵Ca-propionate. Only ⁴⁵Ca-heptagluconate penetrated at a slower rate. Increasing temperature markedly increased Ca-penetration. Penetration was most rapid at 35 °C, intermediate at 22 °C and slowest at 12 °C. Increasing relative humidity (RH) from 0, 28, 75 to 100% increased penetration of ⁴⁵CaCl₂, but penetration of ⁴⁵Ca-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 ⁴⁵CaCl₂, but increased penetration of ⁴⁵Ca-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.