Recovery from Phytotoxicity after Foliar Application of Fruit-loosening Abscission Compounds to Citrus

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-1 H -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 (Ψ leaf ) 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.