ABA-induced Calcium Uptake in 'Honeycrisp' Apple [(Malus domestica (Borkh.)] Determined Using a Stable Calcium Isotope Tracer

Calcium (Ca) is a critical nutrient in plants that contributes to the structural integrity of cell walls, acts as a counter-cation for anion exchanges and is a critical messaging receptor for plant responses to environmental cues. In deciduous tree fruit, Ca plays a key role in quality.  Localized deficiencies in Ca affect storability and susceptibility to storage problems. The pre-harvest conditions that contribute to the development of these Ca imbalances in fruit are poorly understood. Here, we sought to test the use of stable Ca isotopes as a tracer to explore Ca uptake and distribution in apple. In two separate experiments, ‘Gala’ and ‘Honeycrisp’ apple trees were selected for uniformity of size and caliper. Trees were grown for 60 days in a greenhouse at ambient conditions at the Tree Fruit Research and Extension Center in Wenatchee, WA. After 60 days of growth, abscisic acid (ABA) was applied at 500 mg L-1 to half of the trees and distilled water was applied to the other half as a control. In the first experiment, 2 mg of 97 atom% ⁴⁴Ca tracer was applied to each 10 L pot. In the second experiment in 2015, 20 mg of ⁴⁴Ca was used. ABA application reduced stomatal conductance and photosynthetic rates. In association with the changes in stomatal conductance, there was an immediate 40% reduction of water-use that slowly increased for 20 days until transpiration was close to that of the control. Differences in water-use were confirmed using both gravimetrically and using a Li-COR 6400XT gas exchange system. Total leaf Ca concentration and ⁴⁴Ca tracer uptake measurements were used to look at changes in Ca uptake. Differences in transpiration were expected to result in reduced Ca uptake. However, the opposite occurred. An increase in Ca uptake was observed in in plants treated with ABA. There was between 5 and 10% more calcium in leaves in trees treated with ABA compared to the control and this corresponded to an increase in the amount of ⁴⁴Ca tracer in the plant. This was consistent across both cultivars. Here, we demonstrate an uncoupling of transpiration and Ca uptake when ABA-induced stomatal closure occurred. More work is needed to identify the mechanisms by which Ca uptake was affected. This work highlighted a new approach to look at Ca uptake and distribution in horticultural crops that frequently suffer from Ca -related disorders.