COS 103-7 - Low internal CO2 conductance increases carbon isotope discrimination during nighttime CO2 fixation in columnar cacti

Thursday, August 11, 2011: 10:10 AM
18D, Austin Convention Center
David G. Williams, Department of Botany, University of Wyoming, Laramie, WY, Dustin Bronson, Biology, University of Pennsylvania, Philadelphia, Nathan English, School of Earth and Environmental Sciences, James Cook University, Townsville, QLD, Australia, Heath Powers, Earth and Environmental Sciences Division, Los Alamos National Laboratory and Nathan G. McDowell, Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM
Background/Question/Methods

Previous work has shown that carbon isotope composition of spine tissue in long-lived columnar cacti records short- and long-term changes in precipitation and humidity. To better understand this relationship, we investigated determinants of photosynthetic 13C discrimination in three columnar cactus species, one from the Altiplano of South America (Trichocereus atacamensis) and two from the Sonoran Desert of North America (Carnegiea gigantea [saguaro] and Stenocereus thurberi [organ pipe cactus]). Diffusion of CO2 from sub-stomatal air spaces to sites of PEPC carboxylation during the nighttime phase of the CAM cycle was evaluated using concomitant photosynthetic gas exchange and on-line 13C discrimination measurements.

Results/Conclusions

The three constitutive CAM species grown and measured in a glasshouse environment had very similar maximum rates of nighttime net CO2 uptake (ranging between 2.8 to 4.8 μmol CO2 m-2 s-1) and stomatal conductance to water vapour (0.02 to 0.03 mol m-2 s-1), but the timing of maximum CO2 exchange during this initial CAM carboxylation step differed. Air from a customized gas exchange chamber system was routed simultaneously through an infrared gas analyser photosynthesis system and a tuneable diode laser (TDL), allowing for measurements of instantaneous photosynthetic 13C discrimination. A simple 13C discrimination model for nighttime CO2 uptake in CAM assuming infinitely large internal conductance underestimated the 13C discrimination by 1.4 to 2.5 per mil among the three species. Very low internal conductance values of 0.025 to 0.035 mol m-2 s-1 bar-1 were necessary to account for the difference between observed and modelled 13C discrimination. These internal CO2 conductance values are some of the lowest reported to date for vascular plants. The anatomical and physiological determinants of such low internal conductances and their environmental controls remain unresolved. However, the interpretation of carbon isotope ratio values of biomass (including spine tissue) in columnar cacti should take into account this important limitation to 13C discrimination, especially if it is found to vary predictably with environmental changes or physiological conditions during growth.

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