The Rest Period ofRhododendronFlower Buds

Abstract

Rhododendron flower development occurs in three easily defined stages: a pre-rest stage, during which petal growth is mainly by cell elongation; an indeterminate rest period; and an after-rest stage, that begins when the flowers resume growth and ends at anthesis. Early in the pre-rest stage of development, protein bodies and amyloplasts accumulate in the petals. The epidermal cells accumulate only protein bodies and the mesophyll cells accumulate amyloplasts that have a few small protein bodies around the periphery. The subepidermal cells and the cells around the vascular bundles accumulate both large protein bodies and amyloplasts. During the rest period there is a cessation of cell elongation and the reserve protein bodies and amyloplasts remain intact. The protein bodies in all of the cells including those around the amyloplasts are proteolized early in the after-rest stage of development. Digestion of the starch granules occurs when the petals are about one-half their final size. Epidermal-cell expansion during after-rest is relatively uniform; the walls between adjacent epidermal cells remain attached to each other. The mesophyll cells elongate irregularly and the walls of adjacent cells separate giving rise to large intercellular spaces. At anthesis the petal cells consist of a cell wall, a parietal cytoplasm, and a large central vacuole.