EMBRYO SAC DEVELOPMENT AND ENDOGENOUS GIBBERELLINS IN POLLINATED AND UNPOLLINATED OVARIES OF WALNUT (JUGLANS-REGIA)

The effects of pollination on both embryo sac development and gibberel lin (GA) content in walnut (Juglans regia L.) ovaries were investigate d. Microscopic observations on early structural changes in the embryo sac indicated that fertilization occurred shortly after pollination (w ithin 5 days). In unpollinated ovaries, the two polar nuclei also fuse d during this period, leading to a 2n endosperm which started cellular ization (wall formation) about 5 days later. Zygote division in pollin ated ovaries started approximately 9 days after fertilization while th e endosperm at this stage was still in the free-nuclear phase. Gas chr omatography-mass spectrometry (GC-MS) analyses of GAs in pollinated an d unpollinated ovaries demonstrated the presence of all members of the 13-hydroxylation GA pathway (GA(53), GA(44), GA(17), GA(19), GA(20), GA(29), GA(1), and GAs). In addition, GA(4), GA(9), and an unknown mon ohydroxy-GA(12) were also detected. Endogenous GAs were quantified by selected ion monitoring (SIM) from pollination up to 13 days latter. A t the end of this period, pollinated ovaries contained high levels of C-20 GAS whereas C-19 GAs had declined. Gibberellins A(44) and A(19) i ncreased slightly shortly after fertilization, then decreased, and las tly accumulated. Gibberellin A(20) followed the same step-by-step tren d albeit in an opposite manner. Gibberellin A(1) showed a transitory i ncrease immediately after fertilization and decreased gradually subseq uently. The pattern of GA change in unpollinated ovaries differed most ly as follows: (1) within 13 days after the time of pollination GA(53) and GA(44) decreased; (2) GA(19) increased continuously; (3) GA(20) d ecreased also continuously following the opposite pattern to GA(19); a nd (4) the brief rise in GA(1) was higher (2-fold) than in pollinated ovaries, although 13 days after the rime of pollination its levels wer e much lower in unpollinated ones. These results suggest that the sequ ence pollination/fertilization modulates GA(1) levels, first reducing them immediately after fertilization, and subsequently, stimulating mo derate amounts shortly thereafter at the beginning of embryogenesis. I n unpollinated ovaries the lowest GA(1) levels coincided with ovary gr owth arrest, the onset of a rapid and heavy abscission (100%), and an accelerated 2n endosperm cellularization. However, repeated exogenous GA(3) applications did not suppress the growth arrest and abscission.