Key Laboratory of Biodiversity and Biogeography, Kunming Institute of Botany, Chinese Academy of Sciences, 132 Lanhei Road, Kunming, 650201, Yunnan, China.
Oecologia. 2013 Jun;172(2):359-70. doi: 10.1007/s00442-012-2518-2. Epub 2012 Nov 4.
Specialized bracts are thought to be important for the successful reproduction of some plants and are regarded as adaptations to diverse driving forces. However, few empirical studies have quantified the adaptive significance of bracts within a cost-benefit framework. We explored the adaptive significance of large and showy bracts for reproduction in Rheum nobile, a giant herb endemic to the high Himalayas. We examined whether the bracts enhance reproductive success during flowering and seed development. Bracts increased flower and fruit temperature on sunny days, greatly decreased the intensity of ultraviolet-B (UV-B) radiation reaching flowers and fruits, and prevented pollen grains being washed away by rain. Experiments indicated that high temperature could promote pollen germination, while pollen grains exposed to rain and UV-B radiation at ambient levels were seriously damaged. Furthermore, bract removal decreased the number of pollinators visiting flowers. When bracts were removed before or after flowering, fecundity and progeny quality were adversely affected, but seed predation by larvae of pollinators decreased. A cost-benefit analysis demonstrated that the cost of bracts, i.e., increased seed predation, is modest. Our results suggest that the bracts of R. nobile promote pollen germination, protect pollen grains from rain and intense UV-B radiation, enhance pollinator visitation during flowering, and facilitate the development of fertilized ovules during seed development. We conclude that multifunctional bracts of R. nobile are an effective adaptive strategy in alpine environments and might have been selected for because of abiotic environmental conditions as well as for enhancing pollination success.
特化的苞片被认为对一些植物的成功繁殖很重要，被认为是对多种驱动力的适应。然而，很少有实证研究在成本效益框架内量化苞片的适应意义。我们探讨了大而艳丽的苞片对高山大黄（Rheum nobile）繁殖的适应意义，高山大黄是一种大型高山特有草本植物。我们研究了苞片是否在开花和种子发育过程中提高了繁殖成功率。苞片增加了晴天花朵和果实的温度，大大减少了到达花朵和果实的紫外线-B（UV-B）辐射强度，并防止花粉粒被雨水冲走。实验表明，高温可以促进花粉萌发，而在环境水平下暴露于雨水和 UV-B 辐射的花粉粒则受到严重损害。此外，苞片去除减少了访问花朵的传粉者数量。当在开花前后去除苞片时，繁殖力和后代质量受到不利影响，但传粉者幼虫对种子的捕食减少。成本效益分析表明，苞片的成本，即增加的种子捕食，适度。我们的结果表明，高山大黄的苞片促进花粉萌发，保护花粉粒免受雨水和强烈的 UV-B 辐射，在开花期间增加传粉者访问量，并促进受精胚珠在种子发育过程中的发育。我们得出结论，高山大黄的多功能苞片是高山环境中的一种有效适应策略，可能是由于非生物环境条件以及增强授粉成功而被选择的。
