Long-term effects of a doubled atmospheric CO2 concentration on the CAM species Agave deserti

Abstract

To examine the effects of a doubled atmospheric CO2 concentration and other aspects of global climate change on a common CAM species native to the Sonoran Desert, Agave deserti was grown under 370 and 750 μmol CO2 mol-1 air and gas exchange was measured under various environmental conditions. Doubling the CO2 concentration increased daily net CO2 uptake by 49% throughout the 17 months and decreased daily transpiration by 24%, leading to a 110% increase in water-use efficiency. Under the doubled CO2 concentration, the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was 11% lower, phosphoenolpyruvate carboxylase was 34% lower, and the activated:total ratio for Rubisco was 25% greater than under the current CO2 concentration. Less leaf epicuticular wax occurred on plants under the doubled CO2 concentration, which decreased the reflectance of photosynthetic photon flux (PPF); the chlorophyll content per unit leaf area was also less. The enhancement of daily net CO2 uptake by doubling the CO2 concentration increased when the PPF was decreased below 25 mol m-2 d-1, when water was withheld, and when day/night temperatures were below 17/12 °C. More leaves, each with a greater surface area, were produced per plant under the doubled CO2 concentration. The combination of increased total leaf surface area and increased daily net CO2 uptake led to an 88% stimulation of dry mass accumulation under the doubled CO2 concentration. A rising atmospheric CO2 concentration, together with accompanying changes in temperature, precipitation, and PPF, should increase growth and productivity of native populations of A, deserti.