Euphorbia pulcherrima (poinsettia)

Abstract

Starch grain morphology in laticifer amyloplasts of Euphorbia pulcherrima Willd. (poinsettia) was examined for evidence of starch metabolism in vegetative and flowering plants. Laticifer starch grains in vegetative plants were rod shaped with lengths ranging from 3 to 60 ,um. Average grain size was significantly larger in stems than leaves, and in older than younger tissues. Starch grain length frequency was unimodal and approximated a normal probability distribution in stems, but was skewed positively toward smaller grains in leaves. Frequency distributions were shifted toward larger grains in older tissues. Under short-day photoperiod (flowering) conditions, round starch grains formed in latex of stems, and the average length of rod-shaped grains decreased in latex of stems and leaves. Round grains did not occur in laticifers of leaves or bracts. Round starch grains often occurred in aggregates of two or more subunits. Changes in size and shape of latex starch grains indicate that amyloplasts in fully differentiated laticifers metabolize starch. Identification of metabolically active amyloplasts in differentiated laticifers suggests that the function of these organelles may involve starch mobilization under certain physiological conditions. STARCH GRAINS IN amyloplasts of the nonarticulated laticifer of Euphorbia differ from those in other cells in their complex and varied morphologies, unusual structural properties, and apparent nonutilization (Mahlberg, 1975; Biesboer and Mahlberg, 1978, 1981; Spilatro and Mahlberg, 1985). Laticifer starch increases in quantity in leaves throughout their expansionary phase of growth, and then remains stable in mature leaves (Spilatro and Mahlberg, 1986). Although amylase activity was detected in latex of several Euphorbia species, laticifer starch was not mobilized in plants subjected to an extended dark period or during repro