Articles
GIBBERELLIN SIGNALS ORIGINATING FROM APPLE FRUIT AND THEIR POSSIBLE INVOLVEMENT IN FLOWER INDUCTION
Little is known about the signal, which is coming from the fruit or the shoot- tip, and inhibits the transformation of buds into flower buds.
Since exogenous GA3-application causes a reduction in FI, it is possible that gibberellins, coming from the fruit and/or the shoot tips, represent the signal in question.
The biennial-bearing cv.
Elstar and the non-biennial-bearing cv.
Golden Delicious were sprayed with the two gibberellins GA3 and GA4 5 weeks after full bloom (Li et al., 1995). In the following year, the number of flowers was ascertained.
In addition, fruit exudate samples were taken at regular intervals from bloom-time for analyzing their GA contents via Radioimmunoassay.
Flower counting showed that the two gibberellins have different effects.
While GA3 inhibits FI, GA4 has little or no inhibitory effect on FI (Tromp, 1982; Sedgley, 1990). The analysis of the fruit exudates from control trees showed that GA4/7 is exported in a peak with a temporal delay after the peak for GA3 export.
Differences in the GA concentrations between Elstar and Golden Delicious could not be found.
A hint for a GA signal is the fact that during the critical flower bud initiation period (4–6 weeks after full bloom), "high" GA concentrations (200–450 pg./fruit) can be found, which thereafter declined nearly to 0.
A hint against a GA signal is the fact that the biennial-bearing type Elstar, as well as the hardly biennial-bearing type Golden Delicious, is not distinguishable in their GA export.