JXB Advance Access originally published online on December 13, 2004
Journal of Experimental Botany 2005 56(411):469-482; doi:10.1093/jxb/eri055
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RESEARCH PAPER |
Physiological and ecological significance of sunflecks for dipterocarp seedlings
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
* Present address and to whom correspondence should be sent: Department of Plant Biology, University of Illinois, Urbana, IL 61801, USA. Fax: +1 217 244 7563. E-mail: leakey{at}life.uiuc.edu
Irradiance is highly dynamic in many plant canopies. Photosynthesis during sunflecks provides 10–90% of daily carbon gain. The survivorship of tree seedlings in the deeply shaded understorey of tropical rain forests is limited by their ability to maintain a positive carbon balance. Dipterocarp seedlings from the SE Asian rain forest were used as a model system to test novel aspects of the physiological and ecological significance of sunflecks. First, understorey seedlings experienced leaf temperatures up to 38 °C in association with sunflecks. Under controlled environment conditions, the inhibition of carbon gain at 38 °C, compared with 28 °C, was significantly greater during a sequence of sunflecks (–59%), than under uniform irradiance (–40%), providing the same total photosynthetic photon flux density (PPFD). Second, the relative enhancement effects of elevated [CO2] were greater under sunflecks (growth +60%, carbon gain +89%), compared with uniform irradiance (growth +25%, carbon gain +59%), supplying the same daily PPFD. Third, seedling growth rates in the forest understorey were 4-fold greater under a dynamic irradiance treatment characterized by long flecks, compared with a regime of short flecks. Therefore, stresses associated with dynamic irradiance may constrain photosynthetic carbon gain. Additionally, seedling photosynthesis and growth may be more responsive to interactions with abiotic factors, including future changes in climate, than previously estimated. The sensitivity of seedling growth to varying patterns of dynamic irradiance, and the increased likelihood of species-specific responses through interactions with environmental factors, indicates the potential for sunflecks to influence regeneration processes, and hence forest structure and composition.
Key words: Dipterocarpaceae, dynamic photosynthesis, elevated CO2, forest understorey, heat stress, photorespiration, photosynthetic induction, seedling regeneration, Shorea leprosula, stomatal conductance
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