High annual productivity of certain agaves and cacti under cultivation

Seasonal changes in the Crassulacean Acid Metabolism (CAM) activity and growth characteristics of Opuntia humifusa Raf. were examined under midwest climatic conditions. Twenty-four hour studies were done at monthly intervals for two years, with diurnal changes in transpiration, gas exchange, and titratable acidity monitored under natural conditions. CAM activity was observed only from April to September, but occurred regardless of changes in temperature or precipitation. The maximum rate of dark CO2 uptake occurred in May, while greatest acid fluctuations coincided with flowering, new growth, and high tissue water content in June. In spite of conditions favorable for CAM, acidification and rate of dark CO2 assimilation decreased progressively through September as stem water content dropped and shoot production ceased. No CAM was evident during sub-zero winter months and tissue water content decreased to only 65% (Ψ<-20 bars). Winter survival of plants in test plots was found to be affected by the amount of water received the preceding growing season; the driest group showing the lowest mortality rate.Although spring and autumn were considered periods likely to exhibit CAM, it was not observed during those months just prior to or immediately following winter (Oct. and March). Acid fluctuations were minimal with CO2 being taken up during the day and released at night, indicating some degree of flexibility in the CAM activity of Opuntia humifusa.