The Ontogeny of Rubber Formation in Guayule, Parthenium argentatum Gray

Rubber formation in stems of guayule, Parthenium argentatum Gray, first occurs in the cytoplasm of epithelial cells surrounding the resin ducts and eventually in the cytoplasm of adjacent parenchyma cells. With age, rubber droplets appear in the vacuoles of both cell types At maturity, rubber droplets increase in frequency and size, and most of the droplets occur in the vacuole. The rubber particles assume different shapes in each compartment-irregular or globoid in the cytoplasm, spherical in the vacuole. Guayule epithelial cells contain numerous plastids, mitochondria, microbodies, and Golgi bodies but do not contain lutoids The epithelial cells also are rich in smooth endoplasmic reticulum, which is often associated with complex tubular membrane structures that may be involved in the production and secretion of terpenoid substances found in the resin ducts