Germination of Corylus avellana L. (Hazel) pollen: hydration and the function of the oncus

As reported by Schoch-Bodmer fifty years ago, the onci play an important part in accommodating volume changes in the 3-porate pollen of Corylus avellana L. during cycles of dehydration and rehydration. The onci, which are rich in pectins judging from their cytochemical properties, possess a remarkable fine-structure, characterised by densely packed, convoluted lamellae. Staining responses suggest that the protein content of the body of the oncus is low, although esterase activity is distributed throughout. During germination a granular sporopollenin seal at the preferred aperture is disrupted by the gelatinisation of underlying pectins, and the pollen tube emerges as an outgrowth from the inner cellulosic layer of the inline. This process is preceded by the release of proteins from each aperture site. The source of this outflow is a protein-rich umbonate cap, 1-3 /mi in thickness, lying beneath the pore over the body of the oncus and extending under the encompassing exine annulus. The nexine is absent from the annulus, so that the apertural proteins are continuous with the interbacular material. This implies that in this pollen type there are no clearly separate “gametophytic” and “sporophytic” domains. As in species of Compositae and Cruciferae with a similar sporophytic type of self-incompatibility (SI) system, incompatible pollen of C. avellana is inhibited on the stigma surface, the stigma papillae responding rapidly to the presence of the incompatible pollen by synthesis of internal callose. This suggests that the 5-factors are held in the pollen wall, and it is possible that they form one component of the poral proteins. In the light of recent reports of sporophytic proteins held at the apertural sites in various angiosperm pollens, the possibility is considered that the poral proteins of C. avellana are also of sporophytic origin, at least in part. Should they contain tapetum-derived 5-factors a ready explanation of the sporophytic control of the SI system is at hand.