Developmental Anatomy of the Avocado Stigma Papilla Cells and Their Secretion

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Abstract
The development of the avocado stigma was observed by EM and light microscopy from .apprx. 3 wk prior to anthesis to flower opening. The stigma papilla cells grew from 50 to 250 .mu.m long. At the earliest stages the cells had many features of the mature cells, including extensive smooth endoplasmic reticulum and dictyosomes. Plastid clusters appeared at 3 days and wall thickenings containing lipid at 2 days prior to anthesis. Some secretion containing carbohydrate and protein was present at all stages, but secretion of most of the carbohydrate and all of the lipid occurred during the 2 days prior to anthesis. The lipid bodies apparently were surrounded by carbohydrate, as stained by the thiosemicarbazide-silver proteinate method, prior to passing through the wall thickenings by eccrine secretion. This method also stained the secretion, cell wall, dictyosomes and plastids. Serial sectioning showed that most of the plastid clusters were arranged with the large starch-containing heads on the periphery and the long tails pointing toward the center. Many plastids were large and complex, but the clusters were composed mainly of discrete organelles. The plastids probably have a role in the lipid secretion of the papilla cells, which passes through the wall, along with some carbohydrate, by eccrine secretion via specialized areas of thickened cell wall.