Development of Cucurbita pepo pollen: ultrastructure and histochemistry of the sporoderm

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Abstract
Sporoderm development is described in Cucurbita pepo, a species characterized by large spinous pollen grains covered by pollenkitt; i.e. the degeneration product of the tapetum. During the tetrad stage, separate types of primexine are observed for microspines and macrospines. At maturity these spines are affected differently by acetolysis: microspines contain polysaccharides as well as sporopollenin, and macrospine contain only sporopollenin. Unlike other species investigated so far, the intine begins to form at the early microspore stage and is very complex in the pore area where it forms a Zwischenkörper. The pore is covered by a protein layer of sporophytic origin and by the operculum. Just before anther opening, the pore protrudes, exposing part of the poral intine. Mature pollen has a thin callosic wall. The findings are discussed in the light of the fact that Cucurbita pepo pollen does not dehydrate prior to anthesis. Key words: Cucurbita pepo, pollen, sporoderm, exine, intine, sporophytic proteins.