| Codonopsis clematidea (Schrenk) C. B. Clarke., a perennial herbaceous plant that grows mainly in the middle zone on northern slope of Tianshan mountains, is characterized secondary pollen presentation(SPP). The breeding system and pollination biology were studied in the field and statistical analyze, and that its adaptive strategies to the upland environment were analyzed. Our aims were to bring to light the type, process and the ecological adaptative significance of SPP mechanism in plants. The main results were as follows:(1)The flowering stage of C. clematidea flowers was about 6 days which were divided into totally different male and female phase. It had the character of protandrous. From the dehiscing of the anthers until the spreading of the stigma lobes, a flower was considered male phase. It was divided once more into dehiscing period of anthers and releasing period of pollen. The former lasted approximately 6 h, anthers dehisced after anthers drawing close with stigma, and pollen was discharged and deposited immediately onto the area of the back of stigma lobes. Subsequently, the upper parts of filaments withered and curled away. Thus, pollen of the area was presented to potential visitors. The latter lasted about 72 h. In the period, stylar hairs invaginated successively and pollen of the area was released and transferred by pollinators. The course was the process of SPP. The present mechanism of this phase was deposition mechanism, and that the present type was sub-terminal stylar presenters. The female phase started from the stigma lobes spreading to the stigma withering. The duration was about 48 h. The expanding stigma lobes received the cross pollen taken by pollinators.(2)The pollen viability remained above 70% in the male phase in C. clematidea flowers. The pollen viability was above 90% from the beginning of dehiscencing of anthers to the pollen relaesed for 24 h, and this was the period of the highest pollen viability. The stigmas was always receptive in the female phase, but from the beginning of female pahse to the 24 h of female pahse was the period of the highest stigma receptivity. Hand-polination experiments verified that the species can not be spontaneous autogamy and self-incompatibility. The fruit set and seed set of artificial cross-pollination and natural pollination were above 65%. Both percentages of fruit set and seed set between natural pollination and artificial xenogamy were not significantly different(P>0.05). It showed that this species is the typical breeding system of xenogamy and that the fruit set and seed set have not pollen limitation. The characters of dichogamy and herkogamy avoid effectively self-pollen interference.(3)Bombus silvarum, Dolichovespula sylvestris and B. paradoxus were effective pollinators of C. clematidea flowers. Their visiting frequencies were 2.10±0.29,1.68±0.37 and 1.32±0.28 per flower per hour, respectively and the pollination efficiency were 4.77%,2.61% and 2.68%, respectively. Among the three pollinators, B. silvarum had the highest visiting frequency and pollination efficiency. Thus, it was the most effective pollinator. The fastigium of visiting is not only consistent with the highest pollen viability, but also closely related to secretory dynamic of nectar(r=0.865, P<0.05). The fastigium of visiting in female phase accorded with the period of the highest stigma receptivity, favouring increasing of pollination efficiency for xenogamy.(4)The flowers with the pendulous and campanulate corollas effectively protect pollen viability, stigma receptivity and nectar from rain water. The rain water may impact on pollen viability and stigma receptivity and also wash nectar. The longer flower stages extend period of pollen presentation in male phase and duration of stigma receptivity in the female phase, thereby increasing the chances of pollinators visiting and advancing the pollination efficiency. Dichogamy, herkogamy and SPP mechanism in C. clematidea offer the guarantees for avoiding self-pollen interference and promoting xenogamy.
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