Recovery from phytotoxicity after foliar application of fruit-loosening abscission compounds to citrus

Abstract

The use of abscission compounds to loosen fruit from stems can be accompanied with various levels of phytotoxicity. To determine the effects of a promising abscission compound, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), and ethephon on sweet orange [Citrus sinensis (L.) Osbeck] leaf function, water relations, and young fruit growth, we sprayed CMNP at 0, 200, 500, 1000, or 2000 mg·L-1 or ethephon at 400 or 800 mg·L-1 to fruiting branches of potted and field-grown sweet orange during the 2005-06 harvest season. Both compounds induced abscission of mature fruit and leaves 3 days after application but had little effect on leaf chlorophyll content, water content, and midday leaf water potential (Ψleaf) of remaining leaves. CMNP sprayed at 200 mg·L-1 or either concentration of ethephon did not affect leaf photosystem II efficiency, as indicated by leaf chlorophyll fluorescence (Fv/Fm). High CMNP concentrations (1000 or 2000 mg·L-1) reduced Fv/Fm 1 day after treatment, but Fv/Fm of leaves remaining on sprayed branches gradually recovered to the level of control leaves by 4 days after treatment. Similarly, high concentrations of CMNP and ethephon temporarily reduced net gas exchange of leaves for about 4 days. Young fruit growth also was temporarily inhibited by CMNP concentrations greater than 200 mg·L-1. We conclude that CMNP sprayed at recommended concentrations (200-500 mg·L-1) caused mature fruit abscission with little long-term phytotoxic effect on leaves or young fruit.