Pollination increases gibberellin levels in developing ovaries of seeded varieties of citrus

Reproductive and vegetative tissues of the seeded Pineapple cultivars of sweet orange (Citrus sinensis L.) contained the following C-13 hydroxylated gibberellins (GAs): GA(53), GA(17), GA(19), GA(20), GA(1), GA(29), and GA(8), as well as GA(97), 3-epi-GA(1), and several uncharacterized GAs. The inclusion of 3-epi-GA(1) as an endogenous substance was based on measurements of the isomerization rates of previously added [H-2(2)]GA(1). Pollination enhanced amounts of GA(19), GA(20), GA(29), and GA(8) in developing ovaries. Levels of GA(1) increased from 5.0 to 9.5 ng/g dry weight during anthesis and were reduced thereafter. The amount of GA in mature pollen was very low. Emasculation reduced GA levels and caused a rapid 100% ovary abscission. This effect was partially counteracted by either pollination or application of GA(3). In pollinated ovaries, repeated paclobutrazol applications decreased the amount of GA and increased ovary abscission, although the pattern of continuous decline was different from the sudden abscission induced by emasculation. The above results indicate that, in citrus, pollination increases GA levels and reduces ovary abscission and that the presence of exogenous GA(3) in unpollinated ovaries also suppresses abscission. Evidence is also presented that pollination and GAs do not, as is generally assumed, suppress ovary abscission through the reactivation of cell division.