Abstract:

Aims: As one of the most prominent highly specialized morphologies evolved in alpine subnival plants in Himalayan- Hengduan Mountains, Southwestern China, ‘greenhouse plant’ is characterized by the huge and semi-translucent bracts that cover inflorescence. However, unlike some typical ‘greenhouse plants’, such as Rheum nobile and R. alexandrae (Polygonaceae) and Saussurea obvallata (Asteraceae) that have their inflorescence covered by bracts throughout the entire growing season, some other plant species, such as S. velutina studied here has their inflorescence covered by bracts in the early part of the growing season only, while in the majority of the growing season, this plant chooses to extrude its inflorescence outside the cover of bracts till fruit maturation. The adaptive strategy behind such an alteration in morphology remained largely unknown.
Methods: In this study we purposely investigated the thermal situation of the inflorescence in S. velutina after it has extruded from the bract covering. Given these bracts gradually changed their colors from green-yellow to deep pink-purple during this period and they are still attached to the inflorescence, we also assessed the optical properties of the deep pink-purple bracts and explored their potential role on attracting pollinators by assessing the visual model of the bracts to bumblebee (Bombus) as compared with bedrock in its natural habitat and the green-colored leaves. In addition, the influence of the deep pink-purple bracts on seed productions was investigated by gently striping these bracts off.
Results: Our results showed that the temperature of inflorescence remained on average 3-5 K (Calvin temperature index, used to indicate the differences of temperature) higher than adjacent air temperature at the same height of the plant individuals (15-20 cm above ground) during both daytime and nighttime, with the greatest deviation (> 20 K) reaching at sunny-days. Under all prevailing weather conditions (sunny, cloudy, rainy), the dark and solid inflorescence exhibited a pronounced heating efficiency (> 40%) and the buffering capacity of which reached at nearly 100 min as compared with adjacent air temperature. However, although the deep pink-purple bracts could still exhibit selective filtering (24.4%-36.0%) of red and infrared wavelength range that were attributed to warming effects, the artificial treatment on inflorescence did not significantly reduce the seed quantity and quality compared with controlled inflorescence with bracts kept their natural morphology. The color distance from pink-purple bracts to green leaves (0.30 ± 0.02 CH) was significantly higher than the color distance of Bombus terrestris for recognizing one object from the background. suggesting a function of pollinator attraction of pink-purple bracts and call for some more in situ observations and experiments (e.g. examinations on the impact of bracts on pollination visit rates) to further support this conclusion.
Conclusion: The warming and buffering effects of the inflorescence of S. velutina are explained by its dark and solid structure. Such a highly specialized morphology is an adaptive strategy of subnival plant against the cool climate during the majority of summer monsoon and short period of growing season in their habitat at exceptionally high elevations.

Key words: Himalaya-Hengduan Mountains, alpine subnival plants, highly specialized morphology, ecological adaptive strategy, Saussurea velutina