OVULE DEVELOPMENT IN DIPLOID RED CLOVER

Abstract

In red clover only one ovule of the two in each ovary regularly develops into a seed. Results of two experiments designed to reveal the fate of the ovules initially present provide a basis for comparing some features of ovule development in high- and low-seed-yielding plants. High-yielding plants produced embryo sacs in over 70% of ovules whereas low-yielding ones produced them in about 40%. The ability to produce embryo sacs seems to be genetically determined but strongly influenced by environment and occasionally by meiotic irregularities. Development started in about 85% of the fertile ovules in the high-yielding plants but in only about 60% of them in low-yielding plants. Apparently there was slow or limited growth of pollen tubes in the latter plants. Nearly 55% of the ovules developing just after fertilization matured into seeds in the high-yielding plants but only about 25% in the low ones. The higher frequency of ovule collapse in low-yielding plants seems to be a consequence of the delayed fertilization followed by retarded growth of the embryos and endosperms.Although a few more ovules did start to develop in out-crosses than in sib-crosses, the more vigorous growth of the endosperm seems quite important in accounting for the superior seed yields in the out-crosses. Also a few more ovules started to develop when the pollen came from a high-yielding plant instead of a low-yielding one and this initial advantage seems to lead to improved seed production.It is suggested that low-seed-producing plants may have certain genes that act like weak self-incompatibility alleles by inhibiting pollen tube growth, or, that their genetic constitutions are such that they cannot produce enough embryo sacs and/or support the growth of pollen tubes and ovules.