ULTRASTRUCTURAL EVIDENCE FOR SECRETORY PHASES IN THE LIFE HISTORY OF STIGMATIC HAIRS OF COTTON: A DRY TYPE STIGMA

Stigmatic hairs of the cotton flower were studied through their developmental stages up to anthesis. Stigmatic hairs invariably develop from a densely straining band of epidermal cells opposite the transmitting tissue cells. At anthesis, these are single cell structures measuring up to 300 ,um long. At the 5-mm stage of stylar length (7-10 days before anthesis), some stigmatic hair cells begin to accumulate an osmiophilic substance between the plasmalemma and the cell wall, possibly synthesized in the endoplasmic reticulum. This material is apparently never secreted outside the cell wall. Immediately following this secretory phase in some stigmatic hair cells a second secretory phase starts. A dense osmiophilic substance, different in appearance from the previous phase, accumulates in the vacuoles of each hair cell. Concomitantly, dimorphism develops in the cytoplasmic densities of stigmatic hair. Some stigmatic hair cytoplasm appears very dense and shows signs of degeneration while other cytoplasm appears normal. A third secretory phase, which begins at anthesis, occurs in the normal hair cells. This phase is characterized by enhanced activity in the cytoplasm of the endoplasmic reticulum and Golgi apparatus. Large vesicles containing granular material are seen fusing with the plasmalemma. Coincident with this activity there is dissolution of the middle layers of the cell wall and the cuticle is ruptured at various points. The dense osmiophilic substance that had accumulated in the vacuole breaks down into fine granular material. Significance of these changes is discussed in relation to the pollen germination mechanism on the dry type stigma of cotton. THE STIGMA of cotton is classified as dry because it lacks any liquid exudate at anthesis. In plants with wet stigmas (e.g., Petunia, Lilium, etc.) the relationship of stigmatic exudate and pollen germination is fairly well understood (Konar and Linskens, 1966a, b; Rosen and Thomas, 1970; Dashek, Thomas and Rosen, 1971; Dumas, 1976). In comparison, the dry type stigma has not yielded any obvious morphological characteristics (Jensen and Fisher, 1969) that can be related to the understanding of pollen germination on such stigmas. Therefore a developmental study is of interest to record pertinent changes which will be of help in focussing attention to the cell structures possibly involved in successful pollen germination. MATERIALS AND METHODS-Growth conditions-Cotton (Gossypium hirsutum) plants were grown under greenhouse conditions where light and temperature were partially controlled. They were illuminated with fluorescent tubes so as to provide a 16 hr light and 8 hr dark cycle. A minimum temperature 1 Received for publication 3 May 1980; revision accepted 14 August 1980. This investigation was supported in part by NSF grant PEM 79-11018. of 21 C was maintained. Year round flowering was obtained under these conditions. Bud growth, stylar growth and floweringFrom the time the bud is visible, it takes 2530 days for flowering to occur. The variability is introduced by the time of the year and the vigor and age of the plant. Bud maturation follows a definite pattern of growth (Fig. 1). Stylar growth inside the bud follows a pattern similar to bud growth, therefore when bud length is plotted against stylar length it shows a linear relationship (Fig. 2). This relationship was used to select the stylar developmental stage by measuring bud length. Fixation and embedding-Stigmatic portions of the style were dissected in 4% glutaraldehyde in 0.1 M cacodylate buffer at pH 6.8. Tissue segments 1-2 mm in length were fixed for 1.5-2 hr at room temperature, washed briefly with buffer, and post-fixed in OSO4 for 1 hr. Dehydration was carried out with 2,2'-dimethoxypropane (Chin, Falk and Stocking, 1977), with a final change of 100% acetone. Dehydrated tissue was taken through a series of Spurr's low viscosity resin (Spurr, 1969). Longitudinal sections of the stigmatic hair were cut with a diamond knife on a MT-I ultramicrotome (Du Pont Company, Newton, Conn., U.S.A.). Thick sections were stained