Pollination Increases Gibberellin Levels in Developing Ovaries of Seeded Varieties of Citrus

Abstract

Reproductive and vegetative tissues of the seeded Pineapple cultivars of sweet orange (Citrus sinensis L.) contained the following C-13 hydroxylated gibberellins (CAs): CA,,, GA, , , CA,,, CA,,, CA,, CA,,, and CA,, as well as CA,,, 3-epi-CA1, and several uncharacterized CAs. l h e inclusion of 3-epi-CA, as an endogenous substance was based on measurements of the isomerization rates of previously added [2H,]CA,. Pollination enhanced amounts of CA,,, CA,,, CA,,, and GA, in developing ovaries. Levels of CA, increased from 5.0 to 9.5 ng/g dry weight during anthesis and were reduced thereafter. l h e amount of GA in mature pollen was very low. Emasculation reduced C A levels and caused a rapid 100% ovary abscission. This effect was partially counteracted by either pollination or application of GA,. In pollinated ovaries, repeated paclobutrazol applications decreased the amount of C A and increased ovary abscission, although the pattern of continuous decline was different from the sudden abscission induced by emasculation. l h e above results indicate that, in citrus, pollination increases CA levels and reduces ovary abscission and that the presence of exogenous CA, in unpollinated ovaries also suppresses abscission. Evidence is also presented that pollination and CAs do not, as is generally assumed, suppress ovary abscission through the reactivation of cell division.

Physiology of citrus fruiting

“…Growth arrest of unpollinated ovaries is mostly due to a failure in the re-activation of cell division as is seen in D.J. IGLESIAS et al the pollinated fruitlet (Ben-Cheikh et al, 1997). Thus, fruit set in this cultivar is strictly dependent on pollination and fertilization.…”

“…If the flower is not pollinated, the development of the gynoecium arrests, the whole flower senesces and eventually abscises. In the seeded sweet orange cultivars of 'Pineapple' for example, lack of fertilization will inevitably lead to abscission of the ovary since all emasculated and non-pollinated flowers arrested growth and fell shortly after anthesis (Ben-Cheikh et al, 1997). Growth arrest of unpollinated ovaries is mostly due to a failure in the re-activation of cell division as is seen in D.J.…”

“…Interestingly, the presence of leaves increases GA 1 levels (Ben-Cheihk and Talon, unpublished results) and the chances of setting (Lovatt et al, 1984). In seeded cultivars, pollination stimulates hormonal synthesis and, therefore, increases GA 1 levels in developing ovaries (Ben-Cheikh et al, 1997). It has also been shown that in seeded varieties exogenous GA arrested fruit drop of non-pollinated ovaries.…”

“…Likewise, pollination increases GA levels and reduces fruit abscission in seeded varieties, whereas emasculation reduces the GA content and increases abscission. Once more, exogenous GAs suppress fruit abscission of emasculated ovaries (Ben-Cheikh et al, 1997).…”

“…Gibberellins: Gibberellins are thought to be pivotal effectors responsible for the ovary-fruit transition Ben-Cheikh et al, 1997). Gibberellins activate cell division and cell enlargement processes in vegetative organs (Talon et al, 1991) and therefore are generally associated with the initiation of growth (Talon and Zeevaart, 1992).…”