Abstract:

Aims Saussurea involucrata is not only a rare and protected species, but also an important traditional Chinese medicinal plant. The species is an herbaceous monocarpic perennial that grows in the alpine zone. Our objective was to investigate floral biology of this species in a high-altitude area of the Tianshan Mountains and to gain an understanding of how its floral biology is adapted to the alpine environment.
Methods The research was conducted at the Tianshan Glaciological Station of the Chinese Academy of Sciences (43°06′ N, 86°49′ E, 3 540 m a.s.l.) in Xinjiang. We selected an undisturbed population of plants and measured and recorded floral characteristics during flower opening. Also, the dynamics of nectar secretion were monitored by collecting nectar with a syringe and determining sugar concentration with a pocket refractometer. Temperature and relative humidity both inside and outside the involucral bracts were measured with a portable hygrothermograph, and number of seeds produced by plants was counted in the field.
Important findings The inflorescence is a compound capitulum composed of many capitula on a short stem, and it is surrounded by large green involucral bracts. About 26 of the uppermost capitula contained 137 ± 34 fertile florets each, but the lower capitula in the axils of the bracts had sterile florets. The maximum temperature and relative humidity differences in one day between inside and outside the bracts were 7.2 °C and 54.2%, respectively. Opening of florets on a single plant lasted about 45-55 days and for the population about 65-75 days. The duration of peak flowering was ca. 35 days, and the mean number of flowering florets per plant each day was 201. Flowering phenology did not differ between years (p > 0.05). Longevity of a floret from open corolla to a wilted stigma was 4-6 days. Herkogamy, protandry and secondary pollen presentation were conspicuous during floret opening, thus avoiding self-pollination and interference between female and male function. Both nectar volume and sugar concentration of a single floret reached a maximum value at 12:00 (solar time) within 1 day. Over a 3-day period of consecutive nectar secretion, nectar volume gradually increased, while sugar concentration decreased. In the natural habitat, mean percent seed set per capitulum and total seed number per plant were 91.7% ± 4.2% and 3 326.4 ± 28.7, respectively. These characteristics are adaptations to the extreme alpine environment of the Tianshan Mountains.

Key words: alpine plant, Saussurea involucrata, floral biology, secondary pollen presentation, reproductive strategy