Hawaiian lobeliads have radiated into habitats from open alpine bogs to densely shaded rainforest interiors, and show corresponding adaptations in steady-state photosynthetic light responses and associated leaf traits. Shaded environments are not uniformly dark, however, but punctuated by sunflecks that carry most of the photosynthetically active light that strikes plants. We asked whether lobeliads have diversified in their dynamic photosynthetic light responses and how dynamic responses influence daily leaf carbon gain. We quantified gas exchange and dynamic light regimes under field conditions for ten species representing each major Hawaiian sublineage. Species in shadier habitats experienced shorter and less numerous sunflecks: average sunfleck length varied from 1.4 ± 1.7 min for Cyanea floribunda in shaded forest understories to 31.2 ± 2.1 min for Trematolobelia kauaiensis on open ridges. As expected, the rate of photosynthetic induction increased significantly toward shadier sites, with assimilation after 60 s rising from ca. 30% of fully induced rates in species from open environments to 60% in those from densely shaded habitats. Uninduced light use efficiency-actual photosynthesis versus that expected under steady-state conditions-increased from 10 to 70% across the same gradient. In silico transplants-modeling daily carbon gain using one species' photosynthetic light response in its own and other species' dynamic light regimes-demonstrated the potential adaptive nature of species differences: understory Cyanea pilosa in its light regimes outperformed gap-dwelling Clermontia parviflora, while Clermontia in its light regimes outperformed Cyanea. The apparent crossover in daily photosynthesis occurred at about the same photon flux density where dominance shifts from Cyanea to Clermontia in the field. Our results further support our hypothesis that the lobeliads have diversified physiologically across light environments in Hawaiian ecosystems and that those shifts appear to maximize the carbon gain of each species in its own environment.
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Acknowledgements The authors gratefully acknowledge the research support provided by grant IBN-9904366 from the Ecological and Evolutionary Physiology Program of the US National Science Foundation. We thank Hawai‘i Volcanoes National Park, the National Tropical Botanical Garden, the Koke‘e Natural History Museum, Limahuli Garden and the University of Hawai‘i Agricultural Experiment Station in Volcano for important logistical assistance. Ken Wood, Linda Pratt, and Katie Cassel provided invaluable aid in locating populations of several rare species. We are deeply grateful to Wayne Souza of Kaua‘i State Parks, Edwin Pettys of Kaua‘i Division of Forestry and Wildlife, David Foote and Linda Pratt of the US Geological Survey Biological Resources Division, Betsy Gagné, Randy Kennedy, Bill Stormont, and Jon GiVen of Hawai‘i Natural Areas Reserves, Susan Cordell and Julie Denslow of the US Department of Agriculture Forest Service and Sterling Keeley of the University of Hawai‘i for their support in helping us obtain research permits and access to research facilities. Mahalo to Aubrey Kelly, Leilani Durand, Nicole Kuamo‘o and Ken Wood for help and camaraderie in the Weld and for their commitment in helping to achieve the objectives of this research. We thank Elke Naumburg for sharing the dynamic model, and both Elke Naum-burg and Ned Fetcher for advice in model implementation. We thank Megan Kranz-McGuire for her work on the dynamic model simulations and Vlad Bluvstein for his programming assistance in the sun-Xeck analysis. These experiments comply with current laws of the USA.
- Dynamic response of photosynthesis
- Light heterogeneity
- Light use efficiency