Physiological factors affecting response of mature 'Valencia' orange fruitto CMN-pyrazole. I. Effects of young fruit, shoot, and root growth

Influence of young fruit, shoot, and root growth on response of mature 'Val encia' oranges [Citrus sinensis (L,) Osbeck] to the abscission chemical CMN -pyrazole was examined in 1999 and 2000, CMN-pyrazole dramatically increase d ethylene production in mature fruit and reduced the fruit detachment forc e (FDF), except during a period of reduced response to CMN-pyrazole in earl y May when spring vegetative growth, young fruit of the following year's cr op, and mature fruit were all on the trees. Removal of spring flushes, whic h included spring vegetative shoots and leafy and leafless inflorescences, prevented any young fruit and shoot growth, but did not inhibit root growth . However, trunk girdling in combination with removal of spring flushes not only prevented growth of young fruit and shoots but also inhibited root gr owth, During the responsive period, there were no differences in either eth ylene production or FDF of CMN-pyrazole-treated mature oranges between 1) t he nonmanipulated trees and those manipulated by either 2) removal of sprin g flushes alone, or 3) in combination with trunk girdling. However, during the less responsive period, ethylene production in CMN-pyrazole-treated mat ure oranges was significantly lower while the FDF was higher in nonmanipula ted trees than in trees treated by either removal of spring flushes alone, or in combination with trunk girdling, There was no difference in either fr uit ethylene production of FDF between trees manipulated by (2) removal of spring flushes alone, and (3) removal of spring flushes in combination with trunk girdling regardless of CMN-pyrazole application. Shoot growth termin ated at least 2 weeks before the onset of the less responsive period. Remov al of young fruit increased response of mature fruit to CMN-pyrazole during the less responsive period. This suggests that hormones from rapidly growi ng young fruit may be responsible for the occurrence of the less responsive period. Chemical name used: 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyr azole).