Pollen Morphological Variations in Hybrids of Hibiscus rosasinensis L.: Implications for Palyno-taxonomy

Abstract

Human-induced cross-pollination is widely done in Hibiscus rosa-sinensis L. (family Malvaceae), an ornamental perennial plant, for its beautiful flower colors. Pollen morphometrical study of 15 hybrids were studied using light microscopy (LM), confocal laser scanning microscopy (CLSM) and field emission scanning electron microscopy (FESEM) to document the ultrastructure of pollen and to facilitate palynological studies. The shape of all the grains is radially symmetrical, sub-spheroidal and apolar measuring from 35 to 70 μm in size. However, variations in pollen shape from spheroidal, prolate spheroidal and sub-prolate have been observed. The aperture in all the specimens is pantoporate which is typical of its family. The pollen surface is echinate (grains having spines) in all the hybrids. The details of exine ornamentation, spine morphological variations and size using FESEM and CLSM majorly show high variability in all the 15 hybrids. The exine thickness varied between 1 to 4 μm with length varying from 5 to 10 μm. The bifurcated and branched echinae, spinules were observed in Hibiscus hybrids. The pollen size significantly correlates with the length of the spine and the color of the flower obtained after cross-pollination. The principal component analysis (PCA) shows significant variability in polar axis/polar length, equatorial diameter, pore length, pore width, spine length, spine width and spine base, which identifies morphological characters within the hybrids. We infer that size, shape, and spine length in pollen distinguishes Hibiscus and its hybrids from other genus in the family. All the studied hybrids show high variability in spine index and branching pattern. The study provides information to strengthen taxonomic and palynological studies and in understanding the pollen morphometrical variability in natural and man-made plant hybrids.