Minor Physiological Response to Elevated CO2 by the CAM Plant Agave vilmoriniana

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Abstract
One-year-old plants of the CAM leaf succulent Agave vilmoriniana Berger were grown outdoors at Riverside, California. Potted plants were acclimated to CO2-enrichment (about 750 microliters per liter) by growth for 2 weeks in an open-top polyethylene chamber. Control plants were grown nearby where the ambient CO2 concentration was about 370 microliters per liter. When the plants were well watered, CO2-induced differences in stomatal conductances and CO2 assimilation rates over the entire 24-hour period were not large. There was a large nocturnal acidification in both CO2 treatments and insignificant differences in leaf chlorophyll content. Well watered plants maintained water potentials of -0.3 to -0.4 megapascals. When other plants were allowed to dry to water potentials of -1.2 to -1.7 megapascals, stomatal conductances and CO2 uptake rates were reduced in magnitude, with the biggest difference in Phase IV photosynthesis. The minor nocturnal response to CO2 by this species is interpreted to indicate saturated, or nearly saturated, phosphoenolpyruvate carboxylase activity at current atmospheric CO2 concentrations. CO2-enhanced diurnal activity of ribulose bisphosphate carboxylase activity remains a possibility.