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Endogenous Gibberellins in Developing Apple Seeds in Relation to Alternate Bearing

Tu, Yicheng, M.S., Purdue University, December, 2000. Endogenous Gibberellins in Developing Apple Seeds in Relation to Biennial Bearing. Major Professor: Peter M. Hirst. It is believed that Gibberellins (GA) produced in apple seeds act as an inhibitory signal to flower induction in the neighboring buds. As an exception, GA4 is not inhibitory and sometimes found to be promotive to apple flowering. The alternate bearing of apple cultivar is thus associated with the activity of endogenous GAs in the seeds of that specific cultivar. To elucidate the impact of GAs on flowering habit of biennial and nonbiennial apple cultivars, freeze-dried embryos of ‘Fuji’ (biennial, on-year) and ‘Gala’ (non-biennial) were thoroughly scanned for GAs by GC-MS. Altogether, 17 different kinds of GAs were identified in ‘Gala’ and ‘Fuji’. They are: GA1, GA3, GA4, GA7, GA20, GA31, GA34, GA35, GA44, GA45, GA50, GA54, GA61, GA63, GA68, GA80 and GA88. In addition, we also traced 6 GA-like mass spectra that do not match any published GA mass spectrum reference. The possible structures of these GA-like compounds were also proposed. More types of GAs were found in ‘Fuji’ embryo samples than in those of ‘Gala’. ‘Fuji’ sample contained significant amount of GA88, which didn’t appear in ‘Gala’ samples. This suggests higher GA activity in ‘Fuji’ considering most GAs identified are biologically active. However, quantitative analysis of GA1, GA3, GA4, GA7, and GA20 for both cultivars in 1998 (‘on-year’ for ‘Fuji’) and 1999 (‘off-year’ for ‘Fuji’) indicated a central role of GA4, a flower-promoting gibberellin, in control of apple flowering. We did not find higher GA1, GA3 and GA7 activity in ‘Fuji’ seeds in 1998. In fact, ‘Gala’ even produced more GA7 than ‘Fuji’ at certain time of the 1998 growing season. The GA4 concentration in ‘Gala’ was constantly higher in ‘Gala’ than in ‘Fuji’ in 1998. We thus propose that regular bearing cultivars such as ‘Gala’ can use GA4 to