S-ABA-induced changes in root to shoot partitioning of root-applied 44Ca in apple (Malus domestica Borkh.)

Abstract

Using 44Ca as a stable isotope tracer, calcium transport to aboveground portions of apple trees was associated with transpiration rates, but overall calcium uptake was not. Calcium is a critical plant nutrient with important roles in quality, storability of fruit, and resistance to abiotic and biotic stresses. Calcium is largely immobile in the plant, increasing the risk of localized calcium deficiencies. It can be difficult to quantify how changes in transpiration affect calcium uptake and allocation to aboveground organs. Here, the effect of an exogenous abscisic acid (ABA) application on calcium uptake by roots and allocation to leaves of Malus domestica Borkh. cv. Honeycrisp was measured by isotopically labeling the potting media with 44Ca and measuring tracer movement after 30 days. Gravimetric water use and leaf level transpiration were lower in trees that were treated with ABA. ABA application reduced water use by more than 40% immediately after treatment and its effect was sustained. Uptake of 44Ca into the plant was not significantly different between ABA-treated trees and untreated trees, but calcium allocation between roots and shoots was affected. The amount of 44Ca tracer in shoots after 30 days was lower in ABA-treated trees compared to the untreated control and corresponded to a greater proportion of 44Ca in the roots, suggesting that calcium allocation to aboveground parts is dependent on transpiration but calcium uptake by the roots is not.