Enhanced Visualization of the Fine Structure of the Stigmatic Surface of Citrus using Pre-fixation Washes

Abstract

<jats:p>Stigma characteristics and morphology can be useful in taxonomic and phylogenetic studies, indicate relationships in stigma function and receptivity, and be valuable in evaluating pollen–stigma interactions. Problematic is that in some taxa, copious stigmatic exudate can obscure the fine structural details of the stigmatic surface. Such is the case for <jats:italic>Citrus,</jats:italic> which has a wet stigma type on which abundant exudate inundates surface papillae. The components of stigmatic surface compounds are highly heterogeneous and include carbohydrates, proteins, lipids, glycoproteins, and phenolic compounds. This study evaluated the efficacy of several pre-fixation wash treatments on removing surface exudate to visualize the underlying stigmatic surface. Wash treatments included various buffer solutions, surfactants, dilute acids/bases, and solvents. Stigmas prepared using conventional fixation methods in glutaraldehyde had considerable accumulations of reticulate surface deposits with stigmatic cells obscured. Pre-fixation washes containing solvents such as methanol, chloroform, and ethanol left accumulations of incompletely removed exudate and crystalline deposits. Alkaline water washes produced a crust-like deposit on stigma surfaces. Buffer washes left residues of plaque-like deposits with perforated areas. In contrast, excellent removal of stigmatic exudate was obtained with a pre-fixation wash composed of 0.2 M Tris buffer, pH 7.2, containing 0.2% Triton X-100 surfactant and allowed clear imaging of the stigma and surface papillae morphology. A central sinus and radially arranged openings on the stigmatic surface were clearly visible and shown for the first time using scanning electron microscopy (SEM).</jats:p>

Ultrastructural Evidence Elucidates the Mode of Action of Sulfur in Preventing Pollen Tube Development in Stigma of Citrus cv. Nadorcott and Other Horticultural Species

“…The Nadorcott stigma ultrastructure is similar to that previously described for other citrus [32,33,78]. Nadorcott mandarin presents a wet-type stigma with abundant exudate that uniformly covers the upper papillae, which also occurs in wet-type stigma species [27,28].…”

Simultaneous movement of style and stamen set during anthesis in Etlingera elatior (Zingiberaceae)

“…Histochemical and biochemical analyses of the stigma exudate in Angiosperms have demonstrated a heterogenous composition, including lipids, polysaccharides, and proteins (Wetzstein & Law 2012). The presence of exudate in the region of the stigma has been reported for other Zingiberaceae species such as Caulokaempferia coenobialis (Wang et al 2005) and Roscoea debilis (Fan & Li 2012) and is considered to facilitate pollen dispersal.…”