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.

Photoinhibition in the Hawaiian lobeliads across field and common-garden light gradients.

Montgomery, Rebecca^*,1, Givnish, Thomas¹, ¹ University of Wisconsin, Madison, WI

ABSTRACT- While commonly viewed as beneficial, high irradiance places considerable stress on the photosynthetic apparatus. Excess irradiance can damage photosystem II, causing photoinhibition and a reduction in quantum yield. We examined among-species variation in photoinhibition as part of our investigation of the evolution of photosynthetic light responses in the Hawaiian lobeliads, a spectacular example of adaptive radiation. Descended from a single ancestor, species of this group have colonized habitats ranging from shaded montane rain forest to open bogs and sea cliffs. We measured F_v/F_m (the intrinsic efficiency of PSII) of 11 species of lobeliads in their natural habitats, where daily totals of photosynthetically active radiation varied from 3.0 - 24.2 mol day-1, and in which these species show the expected differences in various static and dynamic photosynthetic parameter. We also applied a high light stress (20 minutes at 2000 mol m^-2 s^-1) and tracked recovery of F_v/F_m for 40 minutes. Across the light gradient, species showed no difference in intrinsic F_v/F_m (mean = 0.82 ±0.02), corroborating other studes that show remarkably constant F_v/F_m in the absence of severe stress. Species also did not differ in their ability to recover from the applied light stress; mean recovery was 75± 14%. These data suggest that, despite diversification in other photosynthetic characteristics, all species have retained the ability to recover from high light stress, at least in the short term. However, greenhouse data indicate that long-term exposure to high irradiance can cause chronic photoinhibition in some species. Across four light treatments (6.5, 14, 33, and 80% full sun), all species recovered >80 % of F_v/F_m, but species from intermediate light habitats of origin showed substantial declines in intrinsic efficiency of PSII in the 80% treatment. While there are no significant differences in photoinhibition among species in the field, this appears to be at least partly a result of species native to different light regimes having different levels of xanthophyll pigments adjusted to those regimes.

KEY WORDS: adaptive radiation, photoinhibition, lobeliads, Hawaii