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Multifunctional bracts enhance plant fitness during flowering and seed development in Rheum nobile (Polygonaceae), a giant herb endemic to the high Himalayas

Abstract

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.

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Acknowledgments

We thank De-Li Peng, Gao Chen, and Yun-Gang Guo for their support during the experiments. We are also grateful to two anonymous reviewers for their constructive comments to the manuscript. This work was supported by the National Natural Science Foundation of China (40930209, 31061160184, 31100179, 31200184), Hundred Talents Program of the Chinese Academy of Sciences (2011312D11022), the Natural Science Foundation of Yunnan Province (2011FB102), Western Light Talent Culture Project to B. Song and China Postdoctoral Science Foundation funded project (2012T50787).

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Correspondence to Hang Sun.

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B. Song and Z.-Q. Zhang contributed equally to this work.

Communicated by Russell Monson.

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Song, B., Zhang, ZQ., Stöcklin, J. et al. Multifunctional bracts enhance plant fitness during flowering and seed development in Rheum nobile (Polygonaceae), a giant herb endemic to the high Himalayas. Oecologia 172, 359–370 (2013). https://doi.org/10.1007/s00442-012-2518-2

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Keywords

  • Abiotic agent
  • Floral trait
  • Pollen protection
  • Pollinator attraction
  • Reproductive strategy