Palynological data on the genus Cornus (Cornaceae)

Abstract

Genus Cornus L. (Cornaceae) includes, according to various sources, from 4 to about 60 species, growing mainly in temperate regions of the northern hemisphere. Cornus mas L. is the most widely used species in the genus, from which numerous semi-cultivated and cultivated forms and sorts were obtained. The wild C. mas is of Western Asian origin, distributed in the mountains of the Caucasus, Asia Minor, and Southern Europe, and the genotype of the species shows reliable polymorphism. The purpose of this study was to present a detailed palynomorphological characteristic of the species C. mas of the Armenian flora using light microscopy (LM) and scanning electron microscopy (SEM), to clarify the disagreements noted in the literature regarding to the pollen aperture type for the species C. mas, as well as to conduct a comparative palynomorphological analysis of the genus Cornus as a whole (based on the literary data). The data obtained are consistent with the opinion of a number of authors about the presence in the species C. mas of the quite rare compound pollen aperture type. This compound aperture type is formed by a combination of the colpus with a peculiar endoaperture complex, consisting of a small oblong os and paracolpi, located on both sides of the colpi. The os small, elliptical, reaching the paracolpi, forming together butterfly-like or H-shape endexine thinnings. In our opinion, both the os and the paracolpi arose due to the thinning or absence of the internal layers of the exine. Exine 1.3–1.5 μm, columellae separate, thick, capitate. Exine ornamentation verrucate (LM); exine ornamentation densely verrucate with rare perforations, verrucae varying in size (SEM). An extensive analysis of the literature data on pollen of 42 species of the genus Cornus revealed the significant differences in the characteristics of the type of apertures not only at the interspecific, but also at the intraspecific level. In terms of size, pollen grains are predominantly large or of medium size, rarely small. The large pollen grains were noted for C. alba L., C. alsophila W. W. Sm., C. alternifolia L.f., C. amomum Mill., C. asperifolia Michx., C. controversa Hemsl., C. coreana Wangerin, C. drummondii C. A. Mey., C. excelsa Kunth, C. glabrata Benth., C. hemsleyi C. K. Schneid. & Wangerin, C. monbeigii Hemsl., C. paucinervis Hance, C. poliophylla C. K. Schneid. & Wangerin, C. rugosa Lam., C. sanguinea L., C. sericea L., C. stolonifera Michx., C. walteri Wangerin, C. wilsoniana Wangerin. Medium pollen size is typical for C. boliviana J. F. Macbr., C. brachypoda C. A. Mey., C. canadensis L., C. capitata Wall., C. chinensis Wangerin, C. controversa, C. disciflora Moc. & Sessé ex DC., C. floccosa Wangerin, C. florida Hook., C. foemina Mill., C. hongkongensis Hemsl., C. kousa Buerger, C. longipetiolata Hay., C. macrophylla Wall., C. mas L., C. nuttallii Audubon, C. oblonga Wall., C. peruviana J. F. Macbr., C. sericea, C. sessilis Torr., C. suecica L., C. ulotricha C. K. Schneid. & Wangerin, C. unalaschkensis Ledeb. The largest pollen grains were noted for the species C. monbergii (P x E = 81.0 x 48.5 μm) and the smallest ones for C. officinalis (P x E = 20.0 x 21.0 μm). Exine ornamentation as a rule is represented by a number of primitive types, namely, scabrate, rugulate, verrucate, granulate, echinate, etc., sometimes with perforations.