Citrus Fruit Abscission Induced by Methyl-jasmonate

ADDITIONAL INDEX WORDS. Citrus sinensis, 'Hamlin' orange, 'Valencia' orange, mechanical harvest, ethylene ABSTRACT. Methyl jasmonate (MJ) was tested as a potential abscission chemical to enhance mechanical harvest of 'Hamlin' and 'Valenica' orange ( Citrus sinensis (L.) Osb.). In field experiments, a solution of 1, 5, 10, 20, or 100 m M MJ was applied either as a stem wrap to individual fruit or as a spray to entire trees or canopy sectors. Solutions of 10, 20, and 100 mM MJ resulted in significant and consistent reduction of fruit detachment force and caused fruit drop within 7 to 10 days. Fruit loosening was preceded by an increase in the internal ethylene concentration of fruit similar to that of other experimental abscission compounds. While concentrations of 10 m M and less caused no or negligible phytotox- icity, solutions exceeding 10 m M MJ induced unacceptable levels of leaf abscission. ological maturity (sugar to acid ratio of >13). Before applications, the ground under each tree was cleared to monitor fruit and leaf drop during the course of the experiments. Detachment force (FDF) of mature fruit was measured using a digital force gauge (Force Five, Wagner Instruments, Greenwich, Conn.). Fruit were clipped with 3 cm of stem, inserted into the gauge, and the stem pulled parallel to the fruit axis until it separated at the abscission zone. In experiments where only a limited number of marked fruit were treated, the weight (in N) of abscised fruit was included in the calculation of mean FDF values. In all experiments, Kinetic (Setre Chem. Co., Memphis, Tenn.) was included in all solutions as an adjuvant at 0.125% to allow dispersion of MJ.