The Irreversible C₃ to CAM Shift in Well‐watered and Salt‐stressed Plants of Mesembryanthemum crystallinum is under Strict Ontogenetic Control

Abstract

The shift from C3 to CAM was investigated as a function of both leaf and plant age in well‐watered and salt‐stressed (300 mM NaCl solution) plants of Mesembryanthemum crystallinum. Initiation of a night‐time accumulation of malic acid, the decisive criterion of CAM, was followed in plants that were continuously stressed at different points in their life cycle. The deinducibility of CAM was examined after the release from stress by extensively rinsing the potting soil with de‐mineralized water. Our results show that in M. crystallinum CAM is under strict developmental control, since CAM appeared only when a certain stage of development of the whole plant was reached. CAM was not present in any plant before this threshold, which was the same in salt‐stressed as well as in well‐watered plants. The metabolic shift coincided with the change from the seedling to the juvenile growth phase, and not with that from vegetative to reproductive growth, represented by the start of branching. The latter is timed to the end of extension growth. In well‐watered plants, after this decisive point in development, a weak nighttime accumulation of malic acid could be measured (≅ 0.05 mol kgDW−1) in the oldest, mature leaves but not in young, developing ones. This “CAM capacity” gradually increased up to 0.2 mol kgDE−1 with further plant ageing. Leaf senescence, characterized by wilting and yellowing, diminished the CAM activity. In mature leaves salt stress drastically enhanced the magnitude of diurnal fluctuation in malic acid content. Removal of salt stress did not deinduce CAM activity, but diminished the amplitude of malic acid oscillations to some extent in those plants which had been stressed from early in their life cycle. In these plants, salt stress delayed plant development and growth thus retarding the life cycle. Well‐watered plants, for example, branched about three weeks earlier than those that had been stressed continuously from one week after germination. After removal of stress a quasi‐preserved earlier developmental stage in relation to the control plants determined the weaker CAM expression.

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C3-CAM species

“…The observation that weak CAM can develop in M. crystallinum grown in non-saline conditions has led to the suggestion that the occurrence of CAM in M. crystallinum is an inevitable developmental process that is accelerated by stresses such as salinity (Cushman et al 1990;Herppich et al 1992;Adams et al 1998;Lü ttge 2004). However, in this study, M. crystallinum grown for up to 63 days of age in the absence of added NaCl, by which time the plants had five side branches that supported ca.…”

“…The induction of CAM in M. crystallinum is not salt-specific, drought and other treatments that reduce water availability can also elicit CAM, and some CAM activity may even occur in mature to old plants in the absence of a stress treatment. The latter observation has led some authors to conclude that the switch from C 3 photosynthesis to CAM is primarily a developmental phenomenon, and that salinity merely accelerates an inevitable, essentially irreversible ontogenetic process (von Willert and Kramer 1972;Herppich et al 1992;Adams et al 1998;Cushman and Borland 2002;Lu¨ttge 2004). However, growth of a halophyte under non-saline conditions may eventually become stressful because the plant will lack the salts required for adequate osmotic adjustment and turgor maintenance.…”

Induction of Crassulacean acid metabolism by water limitation

“…Leaf age also appears to be important, as illustrated by M. crystallinum plants under salinity or drought stress, which undergo nocturnal acidification when a critical developmental stage (about 4-5 weeks of age) is reached ( Fig. 1; Herppich et al 1992).…”

Growth and development of Mesembryanthemum crystallinum (Aizoaceae)

“…The gene encoding PPDK (pyruvate orthophosphate dikinase) shows little induction in response to stress, but increases steadily during development (Fisslthaler et al, 1995). In summary, irrespective of the developmental disposition that leads to CAM (Herppich, Herppich & Von Willert, 1991), stress is an accelerator of the transition necessary to make the complete CAM-syndrome manifest and, as well, is an inducer of the transition from adult to reproductive growth.…”