Exochorda: five species or one? : a biosystematic study of the Rosaceous genus Exochorda

The Rosaceous genus Exochorda consisted sofar of five accepted species and is distributed in East Asia and Central Asia. Morphologically the taxa in the Exochorda complex have sirnilar characters, hence the classification in the complex was
questionable. A further problem is the relationship between the genus Exochorda and other genera in Rosaceae. In this dissertation, based on wild material of 22 populations in China, cultivated accessions in the botanical gardens of Wageningen Agricultural University, the Netherlands, and herbarium material, a biosystematic study of this genus is reported. The results are summarized as follows:
Leaf anatomy: The epidermis consists of one layer of cells and the shape of epidermis cells is irregular in outline as seen on both surface. Epidermal cell shape in surface view is quite uniform in Exochorda. Stomata are always confined to the lower leaf surface. An anornocytic pattern is found in all samples.
Wood anatomy: growth rings are distinct, boundaries are marked by differences in vessel diameter between earlywood and latewood, and by rows of fiqtened late wood fibres. The wood is ring-porous to semi-porous. Quantitative anatomical characters express continuous variation- within the "species", wood of Exochorda shows a single wood structural type similar to that of the Prunoideae than to the Spiraeoideae.
Pollen morphology and exine structure show little variation in this genus. Exochorda pollen grains are isopolar, three-colporate, small or medium-sized, grain shape is spheroidal. P = 20.9-30.5 (19.6-36.9) gm, E = 22.1-30.7 (17.9-35.7) μm, A = 5.2-9.5 (3.6-15.5) μm, P/E = 0.88-1.36 (0.81-1.73). Ornamentation is striate. All the samples are closely grouped due to similarities in these characters. These "species" have to be regarded as one species. Compared with pollen morphology of other genera in Rosaceae, Exochorda pollen is similar to that of Neillia, Sorbaria, Spiraea and Prunus.
All the populations are diploid (2n = 16 or 2n = 18), both numbers are frequently in the same root tip preparation. The chromosomes are small, 1-2 μm; there is little difference between different pairs; the structure is basically rnetacentric; symmetry is high; no satellite chromosomes were observed.
The results of embryological investigation in this genus showed that the rneiosis of the microspore mother cells is of the Simultaneous type. The tapetum belongs to the Glandular type. The mature embryo sac is of the Polygonurn type.
Exochorda has a mixed mating system, inbreeding and outbreeding, and>7% of its progeny result from self-pollination. The most important vectors for pollination are honeybees, their relative abundance on the flowers and their intrafloral behaviour make them a major contributor to the reproductive success of Exochorda. Over 30% of the pistils degenerate during the flowering period, resulting in low fruit set. The fruit set varies from 11% to 70%.

From the RAPD results in some Rosaceae, it is concluded that five previous taxa (five species) of Exochorda are not clearly separated. Instead, all Exochorda samples were defined in one group and are distinct from other genera in Rosaceae. These data indicate that the interspecific relationships in Exochorda are not very clear. The RAPD data also disturb the common opinion regarding the traditional subfamilies in Rosaceae. At least some of the four subfamilies as separate taxonomic units in Rosaceae are quite heterogeneous.
The findings in multivariate analysis point to a continuous variation of morphological characters. This analysis supports the opinion that overall differences are small within the Exochorda complex. The multivariate analysis does not support five distinct taxa (species) in the Exochorda complex, but three loosely defined taxa can be recognized. E. korolkowii, E. tianshanica and E. racemosa appear conspecific. E. korolkowii and E. tianshanica are united with E. racemosa and treated as synonyms.
The results obtained in the earlier chapters are used in the revision to delimit taxa. It is concluded that all the taxa. belong to one species. The original five different "species" are reduced to three subspecies, i.e. E. racemosa (Lindl.) Rehder subsp. racemosa, subsp. giraldii (Hesse) F.Y. Gao & Maesen and subsp. serratifolia (S. cont Moore) F.Y. Gao & Maesen.
This genus does not fit in any of the four classical subfamilies in Rosaceae. A tribal status as Exochordeae is more suitable.
Exochorda has a relatively narrow and disjunct distribution limited to East Asia and Central Asia. East Asia (mainly China) is the centre of geographical distribution and diversity. So far only one fossil was found in Fushun, Liaoning province of China, which is indicative, but is not decisive for the place of origin. It is inferred that Exochorda originated in the temperate mountain zone of East Asia, the time of origin dates back to the Early Tertiary. It can be logically deduced that Exochorda originated before the emergence of the Taklamakan Desert. The genus probably once occupied a large area in the northern hemisphere of Asia. A formerly continuously distributed taxon became separated into different areas and has been subjected to divergent evolution there. The disjunction may represent the relict of a former wide and continuous distribution pattern, the intervening areas having been depopulated by the rigours of the climate.