Low internal CO2 conductance increases carbon isotope discrimination during nighttime CO2 fixation in columnar cacti

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.