Temperature Factor Affecting the Abscission Response of Mature Fruit and Leaves to CMN-Pyrazole and Ethephon in `Hamlin' Oranges

in Journal of the American Society for Horticultural Science
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  • 1 Horticultural Sciences Department, University of Florida, Institute for Food and Agricultural Sciences, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850

The effect of temperature on the ability of 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) and ethephon to induce ethylene evolution and abscission of mature fruit and leaves was determined using 3-year-old potted `Hamlin' orange [Citrus sinensis (L.) Osb.] trees in environment-controlled growth rooms in seasons 2001-02 and 2002-03. Ethylene evolution and abscission of CMNP or ethephon-treated fruit and ethephon-treated leaves were highly temperature dependent. Fruit detachment force (FDF) and fruit ethylene evolution were not affected by application of ethephon at 200 mg·L-1 or CMNP at 200 mg·L-1 when air temperature was 10 °C for ethephon treatment or ≤15.6 °C for CMNP treatment. However, ethylene evolution of CMNP or ethephon-treated fruit increased sharply, and FDF decreased drastically as temperature increased from 10 to 26.7 °C for ethephon treatment or from 15.6 to 26.7 °C for CMNP treatment. Several 10 hour day/14 hour night temperature regimes were explored to determine the effect of varying daily and nightly temperatures on efficacy and ethylene evolution. At least 3 days of exposure to 21/10 °C were required for CMNP to effectively loosen fruit, whereas only one day of exposure to 26.7/15.6 °C was enough to induce similar changes. At 21/10 °C, CMNP significantly reduced FDF to<25 N and markedly enhanced fruit ethylene evolution, regardless of interruption by 1 day of low temperature at 10/10 °C in the first 5 d after application. Ethephon had no significant effect on leaf ethylene evolution and leaf abscission when temperature was 10 °C, but caused a marked increase in both leaf ethylene evolution and leaf abscission as temperature increased from 10 to 26.7 °C. CMNP did not stimulate leaf ethylene evolution and leaf abscission regardless of temperature. Chemical names used: 5-chloro-3-methyl-4-nitro-1 H-Pyrazole (CMNP); 2-chloroethylphosphonic acid (ethephon).