| The sub-nival belt (alpine scree) is located at altitudes between alpine meadows and the alpine nival zone, which is the highest land ecosystem. Because of the harsh climatic conditions (including low atmospheric pressure, a short growing season, low temperature, strong UV-B radiation and low pollinator activity), vegetation in the sub-nival belt is found at low densities, and scattered groups of plants are predominant. Because of the special topography and Indian and Pacific Ocean monsoon circumfluence, the sub-nival belt of the Hengduan Mountain hosts remarkable biodiversity (including48families,168genera and942seed plant species). The environmental factors have resulted in plants evolving rather unique adaptations to live in this region. Among several alpine plants having highly specialized adaptative characters, four types (greenhouse plants, downy plants, cushion plants and cryptic colouration plants) are the most outstanding. The adaptive evolution of plants has attracted tremendous interests of biologists in this region. Reproduction is the key process of the species evolution. The diversity of sexual seytem, special life form, floral character, pollination mechanism, and seed germination that involved in this process, are always the focus of research. This dissertation tries to explore the reproductive adaptation strategies of alpine plants by using the pattern of plant sexual systems in the sub-nival belt of the Hengduan Mountains and the adaptation strategies of Eriophyton wallichii (downy plant) as examples. The main research contents and results are as follows:1. Associations between plant sexual systems and other traits in the sub-nival belt of the Hengduan MountainsThis study analyzed the associations in this region, and compared the pattern of sexual systems with that of other floras, and also reviewed the breeding system of36alpine species. Information on plant sexual systems and these traits were mainly based on published information, herbarium collections, and field observations. The results showed that there was a high proportion of hermaphroditic species and a low proportion of dioecious and monoecious species. Hermaphroditism was strongly associated with showy floral displays, specialist entomophily, and perennial forbs. Dioecy was strongly associated with inconspicuous, pale-colored flowers, anemophily, and shrubs, whereas monoecy was strongly associated with inconspicuous, pale-colored flowers, generalist entomophily, and perennial grasses. Although no association was found between sexual systems and fruit type, plants with dry fruits (98.1%) were predominant in our study region. With respect to the plant breeding systems in the alpine region of southwest China, we predicted that pollination service might be particularly low; most of the plants lacked enough outcrossing pollinators. Our data revealed that97.1%of the hermaphroditic species were self-compatible, and88.2%of them showed autonomous or facilitated selfing. The plant sexual systems identified might serve to optimize the probability of pollination and thus ensure reproductive success.2. Pollination ecology of Eriophyton wallichiiThe results showed that this species bloom from early-July to late-August. Flowers were protogyny (incomplete dichogamy) and lasted5.14±0.14d. The pollen viability and stigma receptivity lasted a long time. Combined characters of herkogamy and incomplete dichogamy indicated this species favoured outcrossing, and pollinators were needed for complete pollination, however, all of the examined individuals in both populations were self-compatible. Pollination limitations were not found in this species. Bombus sp. was the sole visitors or pollinators, and they transferred pollen within the single flower and individual, which finally resulted in "facilitated selfing". The combination of self-and cross-pollination, as well as the prolonged pollen viability and stigma receptivity, may be the main reason for the survivorship of Eriophyton wallichii.3. The ecological significance of pubescence in Eriophyton wallichiiThis study selected the pubescence on the leaf epidermis as the research object, exploring the ecological significance of pubescence of E. wallichii. The experimental study showed that (1) The stomata was outward and distributed on both leaf surfaces; palisade tissue was underdeveloped, and spongy tissue had a lot of aerenchyma.(2) Leaf trichome was concave and consisted of3-13cells. Corolla and calyx had a similar trichome structure of leaf.(3) Densely trichomes absorbed a lot of incident radiation absorption and acted as a heat trap.(4) The contact angle (8) and retention angle of the adaxial surface of leaves were180.0±0.0°and19.6±0.9°, which indicated dense trichomes impart good water repellency. The structure of stomata and aerenchyma can absorb and store more photosynthesis gas, which may be the adaptation to the low atmospheric pressure in alpine region. Leaf pubescence can result in higher leaf temperatures relative to air temperatures on sunny day, and it may accelerate the growth of plants in the short, cool and wet growing season.4. The ecological significance of overlapping leaves in Eriophyton wallichiiThe overlapping leaves cover flowers, and it seems logical that they protect interior flowers from the damage of rain, UV-B and low temperatures. This study selected the overlapping leaves of E. wallichii as the research object, exploring the the selection pressure of the overlapping leaves covering flowers. Pollen germination experiments indicated water seriously decreased pollen germinability, but corolla structure themselves can protect pollen from rain. Pollen grains exposed to UV-B at ambient levels were seriously damaged, but the remaining UV-B beneath corollas did not affect pollen germinability. When exposed to intense solar radiation, the extreme high temperature of flowers (35℃or even higher) can inhibit pollen germination. When the leaves were artificially lifted up, the seed production or seed vigor reduced significantly in two populations. The overlapping leaves increased the interior temperature of flowers and fruits to an optimal level on sunny hours, but prevented them from overheating when transient intense solar radiation occurs. The study concludes that overheating also might interact with other factors governing the evolution of extrafloral structures in alpine environments.5. Ecology of seed germiantion in three endemic species:Eriophyton wallichii, Lamiophlomis rotate and Marmoritis complanatum (Lamiaceae)This three species distribute in the same area, however, Eriophyton wallichii, Lamiophlomis rotate only exist in alpine scree, and Marmoritis complanatum only exists in alpine meadow. In this study, the seed germination characteristics of the three species were investigated. Germination was studied for fresh seeds, for seeds subjected to cold stratification, darkness, and different GA3concentration and temperatures. In Eriophyton wallichii and Lamiophlomis rotate, GA3treatments enhanced the final seed germination percentages, but not enhanced the seed germination of Marmoritis complanatum. Dry after-ripened can increase seed germination of Lamiophlomis rotate and Marmoritis complanatum. Darkness can decrease seed germination of Lamiophlomis rotate, but not decrease the other two species. According to these features, seeds of Lamiophlomis rotate and Marmoritis complanatum had non-deep physiological dormancy, Eriophyton wallichii with intermediate physiological dormancy. In Eriophyton wallichii seeds, cold stratification cannot break dormancy, but the buried treatment in the field can break dormancy. Once the dormancy was broken, the non-dormancy seeds of the three species can germinate in a wide temperature range (5-30℃), however, the optimum germination temperature were different in different species. In conclusion, these three Lamiaceae species have different germination behaviours, indicating that their seed germination characteristics are adaptions to their microhabitats and are species-specific. |
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