PARENT SESSION
Oral Session #13: Plant Ecology: Light Relationships. Presiding: J. Rebbeck.
Monday, August 6, 2001. 1:00 PM to 4:30 PM. Hall of Ideas P&Q.

Putting the "adaptive" in adaptive radiation: static photosynthetic light responses in Hawaiian lobeliads.

GIVNISH, THOMAS^1,2, MONTGOMERY, REBECCA^1,2, GOLDSTEIN, GUILLERMO², ^{1 2}

ABSTRACT- The Hawaiian lobeliads have long been considered to be one of the most spectacular cases of adaptive radiation in plants. From a single colonist, they have invaded habitats ranging from open alpine bogs to densely shaded rain-forest interiors, and evolved a striking diversity in leaf morphology. Yet very little is known about their ecology and physiology or - indeed - whether they have undergone an adaptive radiation in photosynthetic traits related to life in sun vs. shade. We have therefore begun intensive studies of species representing each Hawaiian sublineage. In field populations of 11 species, light levels vary from 3.0 to 24.2 mol day^-1 and A_max varies from 3.1 to 13.5 g CO₂ g^-1 s^-1. Across species, variation in SLA, leaf longevity, carboxylation efficiency, and the compensation point, saturation point, and convexity of the static photosynthetic light response correlate significantly with light levels in the expected directions. Surprisingly, neither A_max nor R on a mass basis is significantly related to light when all species are included, but this appears to be an artifact of the substantial allocation to water-storage tissue in leaves of closed related Trematolobelia and Lobelia sect. Galeatella. Instantaneous leaf compensation points range from 2.3 to 22.8 mol m^-2 s^-1 (0.1 - 1.1% full sunlight), far lower than the whole-plant compensation points of 1.4 - 10% sunlight calculated based on photosynthetic parameters, leaf longevity, and allocation to leaf vs. non-leaf tissue. Whole-plant compensation points are much closer to the lower limits of light conditions actually experienced by lobeliads.

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