Impacts of Pre- and Postbloom Sprays of Tryptophan on Calcium Distribution within ‘Red Jonaprince’ Apple Trees and on Fruit Quality

Abstract

The aim of this study was to examine the effects of tryptophan (L-TRP) sprays on calcium (Ca) distribution within mature ‘Red Jonaprince’ apple (Malus domestica Borkh.) trees and on fruit quality. Trees were sprayed with L-TRP before flowering (at the green and pink bud stages and when 5% to 10% of flowers were open), after flowering (at petal fall and 14 and 28 days later), and both before and after flowering. In each spray treatment, 50 g of L-TRP per ha was applied. Other trees were sprayed with calcium chloride (CaCl2) six times during the growing season at rates ranging from 6 to 9 kg⋅ha−1. Plants that were not sprayed with L-TRP or CaCl2 served as controls. The results found that the studied spray treatments did not affect yield, mean fruit weight, apple skin russeting and blush, seed set, mean seed weight, or acidity of fruit at harvest. Prebloom L-TRP sprays enhanced the concentrations of both free indole-3-acetic acid (IAA) in fruitlets (by ≈230% compared with those of the control plants) and Ca in fruitlets and fruit (on average by 18% compared with the control combination) but did not affect the leaf Ca concentrations. Apples from trees sprayed with L-TRP before flowering had lower starch index (SI) values at harvest than those of the control plants. Postbloom L-TRP sprays increased leaf Ca concentration, but had no effect on apple Ca concentration or fruit quality at harvest. Combined pre- and postbloom L-TRP sprays did not improve the effectiveness of this amino acid. Overall, preharvest CaCl2 sprays increased leaf and fruit Ca concentrations and decreased fruit SI. Apples sprayed with CaCl2 had lower soluble solids concentrations (SSCs) and were firmer than fruit from control trees. Prebloom L-TRP sprays are effective in improving apple Ca concentration, at least for triploid varieties that have small seed numbers.