Study On The Classification Of Wintersweet Cultivars By Pollen Morphology

Wintersweet[Chimonanthus praecox (L.)Link], a deciduous shrub of Chimonanthus L., belonging to Calycanthaceae, having a cultivation history of more than one thousand years, is Chinese native tree species and world famous ornamental garden species. Pollen morphology is an important part related to the population reproduction, generally it's influenced less by the environment, and takes a great deal of information about evolvement and possesses strong genetic stability. Therefore, the study on pollen morphology of the flowering plants is widely used in plants classification and relationship identification, etc.Pollen morphologies of 33 different wintersweet cultivars in Henan were observed under SEM. Through the study on pollen's shape, size, exine ornamentation, germinator and so on, by mathematical analysis method, this paper is devoted to establish the quantitative indicators of wintersweet pollen morphology, and probe the relationships between pollen morphology and hereditary characters, cultivar evolution and classification.It is hoped to provide basic evidence for the wintersweet's classification, identification, genetic breeding, and so on. The results of the experiment are as follows:1 Pollen morphological characters of wintersweet cultivars. The wintersweet pollen is iso-polar, bilateral symmetry and mediansize; according to Erdtman's NPC classification system, it's type N2P4C5; the pollen exists in the form of simple grain, and mostly prolate, rarely perprolate;the polar view is elliptic or suborbicular; long-equatorial (germinal furrows in the middle) view elliptic or long-elliptic; short-equatorial (germinal furrows in two sides) view subelliptic, rectangle or subrectangle. The pollen of tested wintersweet cultivars all have 2-colporate, which spread over the two sides of the short-equatorial view; the difference among the widths of colporates is significant, varied from 1.59μm to 13.39μm; the length is almost up to the two poles; some cultivars' colporate have bulges of various degree near the equator, and some cultivars have orbicular granules near the bulge.The exine sculpture of wintersweet pollen divide into four types: verrucous, vermicule, perforated and cave-shaped; the shape of most apertures in exine sculpture is irregular round; and the size, distribution density and shape of aperture are different among different cultivars. All the tested wintersweet cultivars of four kinds of exine sculpture have a variable amount of apertures; the size of aperture varied from185.71nm to 454.44nm, and the width of net-ridge 522.17nm to 856.25nm. So there's no obvious relevance between the size of aperture and the width of net-ridge.2 Results of principal component analysis show that the important pollen morphologic indexes in classifying wintersweet cultivars are: style of exine sculpture, the width of net-ridge,short-equatorial view, polar axis length, long-equator axis length, size of aperture, and so on, which indicates that exine sculpture features, pollen shape and pollen size could be taken as reference basis in the wintersweet cultiviars classification using pollen morphological characters.3 Results of cluster analysis show: At genetic distance 5.295,33 tested wintersweet cultivars could be divided into two major groups. The first major group contains 18 cultivars, and the main overall character of the pollen appears as: the polar view is elliptic and long-equatorial (germinal furrows in the middle) view long-elliptic, except for cultivar 'zhuxin'; the second major group contains 15 cultivars, and the polar view suborbicular; long-equatorial (germinal furrows in the middle) view elliptic. At genetic distance 4.051,the first major group could be divided into main types, and the 5 cultivars in the first main type is obviously larger than the 13 cultivars in the second main type in width of net-ridge, width of colporate and length of short-equator axis. At genetic distance 2.807,the second main type could be divided into two type, namely type2 and type3, and the wintersweet cultivars in type3 is bigger than in type2 both in the pollen size and in P/E value; the second main group could be divided into 2 types, namely type4 and type5, and the main difference in the pollen is mainly shown in the width of net-ridge and size of aperture, in which the wintersweet cultivars in type4 are smaller.This clustered results is not in conformity with the morphological classification, which indicates that the pollen morphologic characters of wintersweet cultivars is not directly correlated with the external morphologic basis for classification. Consequently, the author holds that the function of the difference among pollen morphologic characters in the classification and identification of wintersweet cultivars needs further verified. This study hold no brief for classifying and identifying wintersweet cultivars by the pollen morphologic characters alone, but suggests combining with external morphology, cytology, isozyme analysis and DNA markers, and synthetically analyzing, to get a more creditable conclusion.Besides, pollens of tested cultivars presented varietal specificities in the size, polar view, colporate width and external sculpture, which indicates that there's abundant intraspecific genetic diversity, therefore, it's possible to innovate and enrich the germplasm resource of wintersweet through cross breeding, tissue culture and genetic transformation.