Environmental responses and productivity of the CAM plant, Agave tequilana

Agave tequilana (Weber), whose stem is commercially harvested for the production of the distilled alcoholic beverage tequila, exhibited Crassulacean acid metabolism (CAM), as do other agaves. About 87% of net CO2 uptake occurred at night and was accompanied by an increase in tissue acidity. Such nocturnal acid accumulation was maximal for day/night air temperatures of 30°C/15°C, approximately the mean annual values at the field site in Tequila, Jalisco, Mexico. Nocturnal acid accumulation increased with the total daily photosynthetically active radiation (PAR) incident on the leaves, reaching 90% of maximum at 22 mol PAR m−2. Drought reduced nocturnal acid accumulation by 50% after 7 days and by 90% after 30 days. For the 1-year study period, 93% of the annual rainfall of 1082 mm occurred from June to September, leading to a water index (nocturnal acid accumulation relative to that under wet conditions) averaging 0.95 for June through November. For this 6-month period, the temperature index averaged 0.89 and the PAR index averaged 0.44, indicating that PAR was then the main environmental factor limiting nocturnal acid accumulation by A. tequilana. The product of these three indices was termed the environmental productivity index (EPI); when EPI was >0.2 generally 4 or more leaves unfolded monthly from the central spike of the rosette, but when EPI was <0.1 usually less than half as many leaves unfolded. Based on the plant fraction made up of leaves (55–60% by dry weight) and the leaf dry weight as a function of leaf length, productivity could be determined per unit ground area. For plants initially 1-, 3-, and 6-years-old, the dry weight productivity was 2.49, 2.24, and 2.11 kg m−2y−1, respectively. This is higher than for any other CAM plant so far reported and is in the range of 2–3 kg m−2y−1 found for many C3 and C4 crops.