Crassulacean Acid Metabolism

Abstract

Crassulacean acid metabolism (CAM) is a carbon dioxide acquisition, transient storage and concentrating mechanism of plants based on organic acid synthesis. Amongst 350 000 species of vascular plants, 21 000 species perform CAM. In this variant of photosynthesis, carbon dioxide can be fixed nocturnally in the dark and is stored in the form of organic acids from which it is remobilised during the day for assimilation in the light. This has arisen polyphyletically during evolution. It is an ecophysiological adaptation that allows carbon dioxide acquisition with exceptionally economic use of water. CAM allows acclimation to a variety of interacting stresses. It is an adaptation for survival and not for high productivity. Nevertheless, performance of CAM species on drought‐prone marginal lands makes them potential alternative crop plants and biofuel plants. CAM is regulated in a natural night/day rhythm, but can also oscillate freely under the control of circadian biological clocks. Key Concepts Metabolic mechanisms concentrating carbon dioxide internally in plants are important for photosynthesis at the low external carbon dioxide concentration in the present atmosphere. Massive nocturnal CO 2 uptake, fixation and storage in the form of organic acids result in inorganic carbon acquisition with high water‐use efficiency (WUE). Rearranged internal management of available metabolic housekeeping functions can generate new metabolic options of adaptive value. Generation of isoforms of housekeeping enzymes can facilitate polyphyletic evolution of new metabolic pathways. Circadian oscillations can be controlled by different biological clocks, which are feedback related to each other and present in many copies. Flexibility in the expression of metabolic pathway variants facilitates adaptation under variable stress situations. Stress adaptation in plants is for survival and mostly not for high productivity. Resistance to drought stress and high water‐use efficiency supports the quest for new crop plants including biofuel crops to be grown on marginal land.

Adaptations of Chloroplastic Metabolism in Halophytic Plants

“…On the other hand, photoinhibition, as long as reversible during the night, is a protective process which limits the extent of excitation of PSII antennae (Lüttge 2000). A reversible midday photoinhibition has been noted frequently in halophytic species (A. centralasiatica-Qiu et al 2003, Artemisia anethifolia Weber ex Stechm.…”

From diel photosynthesis to crop growth in the crassulacean acid metabolism (CAM) orchid Phalaenopsis

“…Griffiths et al, 2002;Wyka and Lüttge, 2003) and K. fedstchenkoi (Borland et al, 2009) but also with studies on other CAM species such as Mesembryanthemum crystallinum (Dodd et al, 2003) Clusia minor (Grams and Thiel, 2002) and Guzmania lingulata (Maxwell et al, 1999). It is therefore not surprising that a similar gas exchange profile is shown in review papers on CAM plants (Lüttge, 2001, Borland et al, 2011, Winter, 2019.…”

“…While PEPC protein abundance does not change over a diel cycle, its activity is regulated via phosphorylation by phosphoenolpyruvate carboxylase kinase (PPCK), which activates PEPC and renders it less sensitive to inhibition by malate (Borland and Taybi, 2004). In its phosphorylated state, PEPC has a six times higher affinity for CO2 than Rubisco (Lüttge, 2001). Therefore, regulation of carboxylases is essential for CAM to function.…”

“…Photosynthesis in CAM plants is best described by four phases within a diel cycle (Osmond, 1978 (Osmond et al, 1996, Lüttge, 2001. End of day re-opening of stomata marks the start of transitional phase IV, mainly due to the depletion of C4 acid and drawdown of available internal CO2 (Ci).…”

“…The nocturnal period in which stomata are open is referred to as phase I. During this phase, phosphoenolpyruvate carboxylase (PEPC) catalyses atmospheric or respiratory CO2 (as HCO3 -) to bind with phosphoenolpyruvate (PEP), which is stored in the vacuole as a C4 acid (Lüttge, 2001). The refixation period during the day, when stomata are closed, is referred to as phase III.…”