DOUBLE FERTILIZATION IN BARLEY AND THE CYTOLOGICAL EXPLANATION FOR HAPLOID EMBRYO FORMATION, EMBRYOLESS CARYOPSES, AND OVULE ABORTION

The progeny of an induced barley mutant produced a relatively high percentage of caryopses containing haploid embryos. Some caryopses had a full complement of endosperm, but lack an embryo, and still other ovules abort at an early stage. An attempt was made to investigate the embryological basis of these unusual phenomena. Since the majority of the ovules studied showed normal double fertilization, a built-in control was provided and this process is described. The pollen tube delivers the 2 male gametes to the embryo sac by 1st growing through the micropyle of the inner integument, then between nucellar cells before discharging into the degenerate synergid. Double fertilization is effected after the sperm cells exit the degenerate synergid and enter the intercellular space between the egg apparatus and the central cell. The lack of a cytoplasmic sheath around the sperm nucleus within the egg, the absence of any identifiable male organelles in the egg-zygote cytoplasm, and the presence of a cytoplasmic sheath in the intercellular space outside the zygote at the point where the sperm nucleus had entered, suggest that no male cytoplasm enters the egg during the process of syngamy. Haploid embryos arise from the unfertilized egg. In these ovules, triple fusion of the polar nuclei and 1 sperm occurs normally, but the other sperm fails to enter the egg even though it reaches the intercellular space just outside the female gamete. Embryoless caryopses appear to result from a similar condition, i.e., the lack of syngamy accompanied by triploid endosperm formation. In this case, the egg does not continue to develop into an embryo. The absence of polar nuclei or the failure of triple fusion, even though the egg may be fertilized, results in the ultimate abortion of some ovules.