Comparative study of the auxins 2,4-D, fluroxypyr, dicamba, MCPA and hydrogen sulphide to inhibit postharvest calyx senescence and maintain internal quality of Valencia oranges

Abstract

ABSTRACT The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is sometimes applied commercially to delay calyx senescence, decay and maintain citrus fruit internal quality parameters, however, there is a need to find alternative treatments to reduce calyx senescence and maintain fruit quality during storage. In this study, the effectiveness of pre-storage dips of different auxin formulations, 2-(4-amino-3,5-dichloro-6-fluoropyridin-2yl) oxyacetic acid (fluroxypyr), 3,6-dichloro-2-methoxybenzoic acid (dicamba) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), were assessed against the standard 2,4-D treatment using Valencia oranges at concentrations of 0.2 and 1 mM. Hydrogen sulphide (H2S) a gaseous signalling compound known to delay senescence in postharvest produce was also assessed by fumigating Valencia oranges at 100 µL L−1. During four weeks storage at 20°C, fluroxypyr produced the greatest reduction in calyx abscission, calyx browning and fruit decay and down regulation in endogenous ethylene production, respiration rate and ethanol formation, relative to other treatments. Fluroxypyr dip at 1 mM was most effective, with the 0.2 mM concentration still superior to 2,4-D. MCPA showed only modest activity, while dicamba was ineffective. H2S fumigation significantly reduced calyx deterioration and delayed the loss of internal quality factors. Therefore fluroxypyr or H2S, as a non-auxin treatment, have the potential to replace 2,4-D for commercial use.