COS 93-8 - Does experimental warming reduce CO2-induced photosynthetic down-regulation in grassland species?

Thursday, August 5, 2010: 4:00 PM
330, David L Lawrence Convention Center
Emma L. Sage1, David G. Williams2, Elise Pendall3, Daniel R. LeCain4 and Jack Morgan4, (1)Program in Ecology and Department of Botany, University of Wyoming, Laramie, WY, (2)Department of Botany, University of Wyoming, Laramie, WY, (3)Botany, University of Wyoming, Laramie, WY, (4)Rangeland Resources Research Unit, USDA-ARS, Fort Collins, CO
Background/Question/Methods: Photosynthetic down-regulation in plants exposed to elevated [CO2] has been observed in several ecosystem types worldwide. However, the interactive effect of climate warming on CO2-induced down-regulation is uncertain. Understanding the pattern and extent of down-regulation associated with rising [CO2] is essential because it ultimately will determine the potential terrestrial feedback to global change. We measured several leaf-level photosynthetic properties, including light-saturated CO2 assimilation rate (Asat), in the C3 perennial grass Pascopyrum smithii after three years of exposure to elevated [CO2] and warming in the Prairie Heating and CO2 Enrichment (PHACE) experiment in southeastern Wyoming, USA. This factorial experiment combines FACE technology (ambient and elevated [600 ppm] CO2 concentration) and experimental warming with infrared heaters (1.5°C daytime, 3°C nighttime) to evaluate direct, indirect and interactive effects of global change factors on semi-arid grassland structure and function. Here “control” treatments refer to both ambient temperature and [CO2].

Results/Conclusions: Warming by itself did not influence Asat. During the peak growing season in June, 2009, Asat averaged 19.8 (SE = 0.4) µmol m-2 s-1 in the warmed treatment plots and 18.4 (SE = 1.2) µmol m-2 s-1 in control plots. However, Asat in the elevated [CO2] treatment (12.4 [SE = 1.3] µmol m-2 s-1) was 33 and 37 % lower than in control and warmed treatment plots, respectively. Plants exposed to combined elevated [CO2] and warming had only slightly lower Asat values (17.4 [SE = 0.9] µmol m-2 s-1) compared to plants in warmed only or control treatments. These patterns of Asat were similar to those observed earlier and later in the growing season, and differences among treatments in water relations could not account for differences in Asat; plants in elevated [CO2] treatments had the highest leaf water potential values. We hypothesize that reduced Asat under elevated [CO2] is driven by non-stomatal photosynthetic limitations, and that warming nearly eliminated this down-regulation. These results will be presented in addition to an analysis of the biochemical properties explaining photosynthetic responses. The potential consequences of warming-induced photosynthetic stimulation are complex, and may ultimately depend on plant nitrogen use and nitrogen availability.

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