Effects of foliage spray of phosphites on ripening of kiwifruit ‘Hayward’

Abstract

Phosphites (Phi) are inducers of plant defense responses, although also other effects were reported. In kiwifruit (Actinidia deliciosa 'Hayward'), we demonstrated that Phi-treated fruit were more tolerant to gray mold development caused by Botrytis cinerea. The mechanisms are not yet well elucidated. The objective of this work was to evaluate the effect of Phi (30% P2O5, 20% K2O), 0.3% (v/v) on 'Hayward' fruit maturity and quality indices, by application at different physiological stages of the fruit development: T1) one application at bloom; T2) six weekly Phi applications at the exponential phase of fruit growth; T3) combination of treatments 1 and 2; T4) control (without Phi). Kiwifruit of all treatments were harvested after physiological maturity (at least 6.2% total soluble solids content, SSC) and were analyzed at harvest in terms of firmness (N), SSC (%), titratable acidity (%) and color (CIELab). Also fruits were evaluated after 4 months of cold storage and shelf-life (7 days at 20°C). Ethylene production and respiration rate were determined at 20°C in fruit of all treatments. The experiment was conducted during two consecutive years. The results show that the Phi had an effect on firmness, variation between years, but no effects on other maturity indices. Firmness of Phi-treated fruits was higher than control ones at shelf-life, essentially when Phi was applied at the exponential phase of fruit growth. This change in the softening rate was associated with the ethylene production that was also affected by some Phi treatments. Phi treatments (T2 and T3) significantly affected the ethylene and respiration rates, reducing the values at the climacteric peak. This result of Phi on the softening of 'Hayward' kiwifruit in this study may have been mediated through salicylic acid that affects perception or biosynthesis of ethylene.