Ecology and Evolution of Photosynthetic Light Responses in the Hawaiian Lobelioids

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The Hawaiian lobelioids are one of the most spectacular examples of ecological diversification among plants. These woody plants occur in alpine bogs, dry sea cliffs, moist scrub, and tropical rainforest interiors and edges, and display a striking variety in growth form and leaf morphology. Based on the PIs' DNA family tree for this group, these plants appear to have invaded densely shaded habitats from the open, sunny sites they occupied before and shortly after their ancestors colonized Hawaii. The central aim of this project is to understand how plants acquire photosynthetic adaptations to sunny vs. shady conditions. The PIs will attempt to reconstruct changes in photosynthetic physiology, leaf form, biochemistry, and whole-plant growth that occurred when shady habitats were invaded from open sites. Species representing groups of related Hawaiian lobelioids, including species of relatively open habitats and closely related species that have more recently invaded deeper shade, will be studied. Field studies will be conducted to characterize the natural light environments of each study species and to measure their response in leaf form and photosynthesis to light availability under natural conditions. Greenhouse studies will be conducted to determine the extent to which different lobeliads show genetic differences in photosynthetic response, leaf traits, and whole-plant growth under different light regimes. Furthermore, these studies will reveal whether species of different habitats respond to variation in light in ways thought to be advantageous (e.g., do species of sunny habitats have photosynthetic rates and leaf traits that tend to increase growth in brightly lit environments at the cost of relatively poor growth in the shade (and vice versa for species of shaded habitats), and whether understory species - which inhabit a light environment that is patchy and relatively unpredictable - show more ability to alter their photosynthetic form and physiology in response to changed light conditions than species of more open habitats. When combined with information on the evolutionary relationships among the study species, this information will tell us what changes occurred at different stages in the invasion of shaded habitats, what characteristics appear to be adaptive to shady environments, and what characteristics appear to be constrained by the evolutionary history of the different groups.This research will allow the PIs to investigate, for the first time, the rise of photosynthetic adaptations to sun vs. shade in a historical framework. The results should increase our understanding of the ecological constraints on photosynthetic performance in plants generally, and of the habitat requirements for lobelioids specifically, many species of which have become quite rare.