Effect of Defoliation on Blossom-end Rot Incidence and Calcium Transport into Fruit of Tomato Cultivars under Moderate Water Stress

The translocation of calcium (Ca) within the tomato plant and the causes of Ca deficiency, a factor associated with blossom-end rot (BER) in fruit, are still a matter of conjecture. The objective of this study was to determine the effect of defoliation on BER incidence and Ca transport into different size tomato fruit cultivars. Four experiments were conducted. The start and end dates for each experiment were; 14 March-2 May, 22 July-23 August, 30 August-7 October 2017, and 20 May-25 June 2018, for experiments 1, 2, 3, and 4, respectively. Five tomato cultivars including one large ('Momotaro fight (MF)', >= 200 g), three medium ('Lui60 (L60)', 'Flo cook (TC)', and 'Cindy sweet (CS)', 30-80 g), and one small ((Pepe (PP)', <= 20 g) fruit cultivars, respectively, were grown under moderate water stress controlled by a combination of root zone restriction and solar mediated fertigation. Leaf area of plants was reduced by 20-30% by removing alternate leaflets on all leaves. Defoliation significantly reduced BER in all experiments. In experiment 4, no BER was observed in defoliated plants of L60 and PP, and in MF and TC, BER incidence decreased to a quarter of the control. Defoliation increased the fruit growth rate (FGR) in experiment 1, in which the temperature was the lowest, by a ratio of 1.42 and by 1.39 in experiment 4, in which the radiation was strongest and day length longest. Defoliation increased the rate of daily Ca transport into fruit (CTR) in MF, L60, TC, CS, and PP by average ratios of 1.64, 1.55, 1.35, 1.30, and 1.13, respectively. The increase in CTR in defoliated plants was highest in experiment 4 with a ratio of 1.68 followed by 1.37, 1.33, and 1.28 in experiments 1, 3, and 2, respectively. Defoliation increased both FGR and CTR and there were significant linear relationships between them. However, the degree of increase was larger in CTR than that in FGR, especially in the BER-sensitive large fruit cultivar MF, and defoliation increased the total Ca concentration in fruit accordingly. We conclude that under moderate water stress by root zone restriction and certain other BER inductive conditions, defoliation could be a promising approach to reduce BER incidence by improving Ca nutrition in susceptible large fruit cultivars.