Effects of prebloom sprays of tryptophan and zinc on calcium nutrition, yielding and fruit quality of ‘Elstar’ apple trees

Abstract

This study examined the impacts of tryptophan (L-TRP) sprays with or without of zinc (Zn) on calcium (Ca) nutrition, yielding and fruit quality of ‘Elstar’ apple (Malus domestica Borkh.) trees. The experiment was conducted in 2014–2016 on mature apple trees grafted on M.9 EMLA rootstock. Trees were prebloom-sprayed with L-TRP with/without Zn three times during the growing season (at the green and pink bud stages and at bloom beginning), at rates of 50 g of L-TRP ha−1 and 100 g of Zn ha−1 per spray. Other trees were sprayed with calcium chloride (CaCl2) five times during the growing season at rates ranging from 6 to 9 kg ha−1. Trees that were not sprayed with L-TRP, Zn or Ca served as controls. The studied spray treatments did not affect fruit yield, mean fruit weight, apple coloring, soluble solid concentration of fruit, seed set and mean seed weight. In two of the three study years, L-TRP sprays with and without Zn were able to increase fruit Ca concentration by 16% on average. The effectiveness of L-TRP sprays in improving fruit Ca concentration was comparable to L-TRP sprays with the addition of Zn. Sprays of L-TRP with/without Zn did not affect postbloom leaf Ca concentrations. Only combined sprays of L-TRP with Zn increased fruit Zn concentrations, although they caused apple skin russeting. Preharvest CaCl2 sprays raised Ca concentrations in leaves and fruit, on average by 22% and 39%, respectively. In two of the three study years, CaCl2-sprayed apples had increased flesh firmness. To improve apple Ca concentration, prebloom L-TRP sprays without the addition of Zn are justified as a supplement to preharvest Ca sprays, at least under conditions of reduced seed set and optimal Zn nutrition.