Pathways and factors in calcium uptake through the skins of strawberry fruit

High levels of fruit calcium (Ca) in strawberry are associated with low levels of surface disorders, including water soaking. Because of the dysfunctional xylem in developing strawberry, fruit Ca can only be increased by spray application. The objectives were to quantify ⁴⁵Ca uptake through the fruit surface from drying spray droplets. Uptake of ⁴⁵CaCl₂ was initially rapid but slowed down after droplet drying. There was no difference in Ca uptake through the rim between achene depressions and that through the achene depressions. The uptake of ⁴⁵CaCl₂ increased with fruit development and with increasing spray concentration. Uptake was highest for CaCl₂ and Ca(NO₃)₂, but was lower for organic salts. The surfactants Glucopon (0.01%) and Triton X-100 (0.1%) increased Ca uptake. Increased temperature and increased relative humidity increased Ca uptake. Incubating ripe strawberries for 1 h in deionized water increased microcracking. Despite the increased microcracking ⁴⁵CaCl₂ uptake did not increase. Spray applications of 30 mM in the greenhouse increased fruit Ca content and decreased susceptibility to water soaking. Our study establishes that strawberry fruit take up Ca very readily from surface sprays of either CaCl₂ or Ca(NO₃)₂. Increases in fruit Ca decrease susceptibility to water soaking.