AN ULTRASTRUCTURAL-STUDY OF POLLEN DEVELOPMENT IN TOMATO (LYCOPERSICON-ESCULENTUM) .2. POLLEN MATURATION

The maturation of tomato pollen grains encompassed several ultrastruct ural changes. The generative cell separated from the intine and was fr ee in the cytoplasm of the vegetative cell. This process coincided wit h the appearance of starch in plastids and the division of elongated m itochondria. This stage was followed by a second phase of vacuolation in the vegetative cell cytoplasm. Starch was still abundant at this st age, as were mitochondria, endoplasmic reticulum (ER), and ribosomes. Lipid droplets were the prominent feature of mature pollen grains. Eac h droplet was surrounded by rough ER (RER), suggesting the role of RER in lipid accumulation and mobilization. Long stretches of ER were pre sent at early stages of maturation, and stacks of up to 50 strands of RER were abundant in mature pollen. The plastids in mature pollen were devoid of starch and had few internal membranes. Mitochondria were ab undant and spherical with parallel cristae. In many cases, the cytopla sm at the periphery of the mature pollen grain was dense, forming a di stinct zone, and contained only ER. The generative cell cytoplasm had mitochondria, ER, and actin-like filaments but no plastids. The pollen wall at maturity had a lamellated foot layer, a lightly sculptured te ctum, and broad intine. The intine was layered in the region of the po llen aperture.