LONG-TERM EFFECTS OF A DOUBLED ATMOSPHERIC CO2 CONCENTRATION ON THE CAM SPECIES AGAVE DESERTI

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 mu mo l CO2 mol(-1) air and gas exchange was measured under various environm ental conditions. Doubling the CO2 concentration increased daily net C O2 uptake by 49% throughout the 17 months and decreased daily transpir ation by 24%, leading to a 110% increase in water-use efficiency, Unde r the doubled CO2 concentration, the activity of ribulose-1,5-bisphosp hate carboxylase/oxygenase (Rubisco) was 11% lower, phosphoenolpyruvat e carboxylase was 34% lower, and the activated:total ratio for Rubisco was 25% greater than under the current CO2 concentration. Less leaf e picuticular 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 enhance ment of daily net CO2 uptake by doubling the CO2 concentration increas ed 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 degrees C, More leaves, each with a greater surface area, were produced per plan t under the doubled CO2 concentration. The combination of increased to tal leaf surface area and increased daily net CO2 uptake led to an 88% stimulation of dry mass accumulation under the doubled CO2 concentrat ion, A rising atmospheric CO2 concentration, together with accompanyin g changes in temperature, precipitation, and PPF, should increase grow th and productivity of native populations of A. deserti.