Crassulacean Acid Metabolism

Abstract

Crassulacean acid metabolism (CAM) is a carbon dioxide acquisition, carbon dioxide transient storage and carbon dioxide concentrating mechanism of plants based on organic acid synthesis. In this variant of photosynthesis carbon dioxide can be fixed nocturnally in the dark and is used during the day for assimilation in the light. This has arisen polyphyletically many times during evolution. It is an ecophysiological adaptation that allows carbon dioxide acquisition with exceptionally economic use of water. It 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. 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.

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation

“…Griffiths et al, 2002; Wyka & Lüttge, 2003) and K. fedtschenkoi (Borland et al, 2009) but also with studies on other CAM species such as Mesembryanthemum crystallinum (Dodd et al, 2003), Clusia minor (Grams & 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 (Borland et al, 2011; Lüttge, 2001; Winter, 2019). Notably, a robust nocturnal CAM profile was maintained for both species, without applying a day‐night difference in temperature.…”

“…It is therefore not surprising that a similar gas exchange profile is shown in review papers on CAM plants (Borland et al, 2011;Lüttge, 2001;Winter, 2019). Notably, a robust nocturnal CAM profile was maintained for both species, without applying a day-night difference in temperature.…”

“…The nocturnal period in which stomata are open is referred to as phase I. During this phase, phosphoenolpyruvate carboxylase (PEPC) catalyzes atmospheric or respiratory CO 2 (as HCO 3 − ) to bind with phosphoenolpyruvate (PEP), which is stored in the vacuole as a C 4 acid (Lüttge, 2001). The refixation period during the day, when stomata are closed, is referred to as phase III.…”

“…) to bind with phosphoenolpyruvate (PEP), which is stored in the vacuole as a C 4 acid (Lüttge, 2001). The refixation period during the day, when stomata are closed, is referred to as phase III.…”

CAM‐physiology and carbon gain of the orchid Phalaenopsis in response to light intensity, light integral and CO2

“…Despite PEP‐C having a high affinity for CO 2 (Lüttge, 2001), our results clearly show that a CO 2 concentration well‐above atmospheric levels strongly increased the diel net CO 2 ‐uptake (25–31%; Table 4). As the CO 2 ‐uptake also increased proportionally during phase I, our results support the conclusion that neither the storage capacity for malate in the vacuole nor the supply of PEP as substrate for CO 2 ‐fixation into malate via PEPC, were limiting factors for an increased carbon gain at elevated CO 2 concentration under the conditions used in this study.…”