Ultrastructure of Early Stages of Cotton Fiber Differentiation

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Abstract
The fine structure of the outer epidermal layer of cotton [Gossypium hirsutum] ovules was examined from 16 days'' preanthesis until the day of anthesis when cotton fibers first started to elongate. Epidermal cells were morphologically similar at earlier times, and at 24 h preanthesis numerous small vacuoles in the cells possessed an electron-dense material identified as phenolic substances by cytochemical and extraction techniques. Differentiating primordial fiber cells could be detected at 16 h preanthesis by cell and nuclear enlargement, a reduction in amount of electron-dense phenolic substances in cytoplasmic vacuoles and an increase in cytoplasmic density. Nonfiber cells retained the electron-dense substance within the vacuoles, but the phenolic material was dispersed from the vacuoles of fiber primordia at 16 h preanthesis, making the latter cells electron dense or dark. Only dark cells with most, but not necessarily all, of the phenolic material dispersed from their vacuoles became cotton fibers, suggesting the possibility that phenolic-type compounds were involved in the initiation of cotton fiber differentiation. At anthesis, the nucleus of the dark fiber cell was enlarged and contained a single enlarged nucleolus. Ribosomes and rough endoplasmic reticulum (RER) appeared more numerous in differentiating fiber cells than in adjacent nondifferentiating epidermal cells, suggesting a greater capacity for protein synthesis in the fiber cells.