Photosynthetic responses of Larrea tridentata to seasonal temperature extremes under elevated CO2

Elevated CO2 potentially decreases the effects of temperature stress on photosynthesis. Under both freezing and high temperatures previous studies have shown that elevated CO2 can particularly enhance photosynthetic rates, although results from freezing studies are more variable. Here we show gas exchange responses of Larrea tridentata to elevated CO2 over a 6-yr. period when temperature stress events may have had a significant effect on photosynthesis in the field. Nighttime freezing air temperatures decreased subsequent daytime photosynthetic rates, stomatal conductance, and the maximum yield of PSII similarly under ambient and elevated CO2. Further, we found no statistically significant relationship between leaf temperature and photosynthetic enhancement. Overall, the degree of photosynthetic enhancement under elevated CO2 was directly proportional to the response of stomatal conductance to CO2. Thus, elevated CO2 does not significantly affect apparent physiological responses of Larrea to temperature extremes. However, because of the tight relationship between stomatal conductance and photosynthetic enhancement, potential climate change effects on stomatal conductance will significantly influence Larrea performance in the future.