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

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. Wellwatered 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 activity, 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.Various species of succulent plants have been shown to shift their photosynthetic carbon metabolism from C32 to CAM due to water stress (8,12,16,23). This shift, which was initially studied as a response to salinity and, hence, thought to be a direct salt effect (23), was later shown to occur also as a response to water stress (14,16,17,19,22 MATERIALS AND METHODSPlant Material. Portulacaria afra (L.) Jacq. (Portulacaceae) plants were propagated by cuttings from a parent plant, rooted in vermiculite, and replanted in 1-gal pots in a standard peat loam soil mix in a greenhouse. Plants were watered 3 times per week or as needed to maintain the soil moist to touch and were fertilized twice a month with half-strength Hoagland solution (10). Greenhouse temperature was kept at an average of 18°C during the night and 29°C during the day. All leaves used were fully expanded and mature. All plants were healthy, mediumsized plants when used (ca. 0.50 m tall).Exposure of Plants to Various Levels of CO2 and Water Stress. Plants were exposed to the following conditions for a period of 16 d: (a) well-watered, ambient C02; (b) well-watered, 40 ppm CO2 during the day; (c) well-watered, 950 ppm CO2 during the day; (d) water-stressed, normal C02; (e) water-stressed, 40 ppm CO2 during the d...

Induction of Crassulacean acid metabolism by water limitation

“…Indeed, the occurrence of CAM in aquatic plants suggests that low [CO 2 ] may have been an important driving force for the evolution of CAM (Griffiths 1988(Griffiths , 1989Keeley 1996Keeley , 1998Raven & Spicer 1996). Despite this, exposing well-watered facultative CAM species such as M. crystallinum and Portulacaria afra to low ambient [CO 2 ] has no apparent influence on CAM induction (Winter 1979;Huerta & Ting 1988). Likewise, the exposure of detached K. blossfeldiana leaves to CO 2 free air or saturating CO 2 (5%) had no effect on CAM induction for either water deficit or ABA treatments (Cushman et al 2000a).…”

Activities of carboxylating enzymes in the CAM species Opuntia ficus-indica grown under current and elevated CO2 concentrations

“…Such increases can affect photosynthesis and plant yield (Cure and Acock 1986;Bowes 1991). Nearly all such studies have been on C3 or Ca plants (Sage et al 1989;Bowes 1993), whereas information on Crassulacean acid metabolism (CAM) species is limited and contradictory (Osmond and Bj6rkman 1975;Idso et al 1986;Szarek et al 1987;Huerta and Ting 1988). Photosynthesis and hence productivity of C3 plants can be increased 50% or more at doubled CO2 levels, reflecting the kinetic properties of ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco; Bowes 1991;Badger 1992).…”

CAM Photosynthesis in Bromeliads and Agaves: What Can We Learn from These Plants?

“…However, divergent results about the influence of elevated CO 2 in CAM plants have been reported during the last decades [147][148][149][150][151][152][153], demonstrating that the enrichment of CO 2 into the atmosphere can trigger complex responses in CAM plants. For example, elevated CO 2 had no effect on diel CO 2 uptake by Kalanchoë daigremontiana [147] nor on nighttime CO 2 uptake in Clusia uvitana and Portulacaria afra [154,155], whereas in several other CAM species more significant impacts of elevated CO 2 on the daytime, nighttime and/or diel carbon acquisition have been reported [148,150,151,153]. Changes in morphology, anatomy and biochemistry driven by modifications in atmospheric CO 2 concentration have also been observed in some CAM plants, commonly associated with concomitant alterations in their growth rates and biomass accumulation.…”