Effects of Various Levels of CO2 on the Induction of Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq.

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Abstract
In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO2 fixation (C3) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C3- type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO2 concentration and that these changes in CO2 could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Well-watered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal (ca. 330 ppm), and high (950 ppm) CO2 during the day with normal concentrations of CO2 during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO2 uptake, and phosphoenolpyruvate carboxylase activty, remained unaffected by low, normal, or high CO2 treatments. In well-watered plants, however, both low and high ambient concentrations of CO2 tended to reduce organic acid concentrations, low concentrations of CO2 reducing the organic acids more than high CO2. It was concluded that exposing the plants to the CO2 concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism.