INTRA-FRUIT SEED ABORTION IN A WIND DISPERSED TREE, DALBERGIA-SISSOO ROXB - PROXIMATE MECHANISMS

Dalbergia sissoo, a tropical tree with wind-dispersed pods, exhibits a highly positively skewed distribution of seeds per pod with predomina ntly only one of the four or five ovules maturing into seed. The abort ion cannot be attributed to lack of pollen or resources. This study ex amines the hypothesis that the abortion is due to an intense rivalry a mong the developing sibs to gain dispersal advantage. Aqueous extract of the dominant embryos at the stigmatic end that generally develop to maturity significantly inhibited the uptake of labelled sucrose by th e young developing (subject) embryos in an in vitro assay system. Extr acts of tissues such as subordinate (peduncular embryos), unfertilized ovules and pod coat did not cause such inhibition. Aqueous diffusate of dominant embryos also inhibited the uptake of labelled sucrose by s ubject embryos. The chemical substance responsible for the inhibition appears to be heat-stable and non-proteinaceous. HPLC analysis indicat ed the presence of two retention time peaks, different from that of st andard indole acetic acid, but with considerable overlap. We hypothesi ze that the compound could be an indole derivative. We propose that th e stigmatic embryos have a head start due to earlier fertilization and produce a chemical that either directly (by metabolically killing) or indirectly (by preventing the uptake of assimilates) kills the proxim ally placed peduncular embryos.