PARENT SESSION
Oral Session #52: Plant Ecology: Light and Carbon Allocation.
Presiding: T. Day
Wednesday, August 7. 8:00 AM to 11:30 AM. Gila Meeting Room, TCC.

Common-garden studies of adaptive divergence in photosynthetic traits along a sun-shade gradient in the Hawaiian lobeliads.

Givnish, Thomas^*,1, Montgomery, Rebecca¹, ¹ University of Wisconsin, Madison, WI

ABSTRACT- From a single colonist, the Hawaiian lobeliads have invaded habitats ranging from open alpine bogs to densely shaded rain-forest interiors. As part of our investigation of the evolution of photosynthetic adaptations to sun vs. shade in this group, we had previously shown that 11 species - representing each of the major Hawaiian lineages - showed among-species differences in several static and dynamic photosynthetic parameters in the field that appeared to be adaptive, in the sense that those differences were in the direction expected based on economic theory. However, the extent to which these differences were genetic - and hence part of an actual radiation - was not clear. We therefore studied the photosynthetic responses and growth of species representing a wide range of natural light regimes in a common experimental greenhouse under four different light treatments (6.5, 14, 33, and 80% full sunlight; R:FR ratio 0.4 to 1.0). Maximum photosynthetic rates ranged from 0.07 - 0.30 mol g^-1 s^-1 and differed between species and across light treatments. Across light treatments, species mean Amax was positively related to light habitat of origin, but species differed in their degree of plasticity. Species found in a broader range of natural light regimes showed greater plasticity in response to light in the common-garden experiments. Other traits associated with leaf assimilation (dark respiration rates, instantaneous compensation points and SLA) shifted with light treatment in the expected directions, but showed little difference among species. These data suggest that the Hawaiian lobeliads have indeed diverged genetically in photosynthetic traits; differences among taxa observed in the field have both genetic and environmental components, but a substantial proportion of those appear to be genetic. Furthermore, shifts in physiology among taxa appear adaptive. Even under controlled conditions species show phenotypic responses that would tend to maximize C gain in their habitat of origin.

KEY WORDS: adaptive radiation, photosynthetic light response, lobeliads, Hawaii