Salt Requirement for Crassulacean Acid Metabolism in the Annual Succulent, Mesembryanthemum crystallinum

Abstract

In experiments with the facultative Crassulacean acid metabolism (CAM) species, Mesembryanthemum crystailinum, only plants which received high levels of inorganic salts fixed substantial amounts of CO2 by the CAM pathway. Equivalent osmolarities of polyethylene glycol 6000 did not yield any CAM fixation. Plant water potential and turgor pressure had no detectable influence on the amount of CAM fixation. These observations rule out the possibility that the inorganic ions were acting as osmotic agents.Carbon dioxide and water exchange analysis showed that when water supply was not limiting, salt-deprived plants sustained higher reductive pentose phosphate cycle carbon fixation rates than salt-treated plants. Under water stress conditions, salt-deprived plants using only the reductive pentose phosphate cycle pathway assimilated less carbon and were less efficient in their water use than salt-treated plants using predominately the CAM pathway. These results support the hypothesis that the ability to use the CAM pathway reduces the capacity for reductive pentose phosphate cycle fixation but permits higher productivity in water-limited environments.In plants exhibiting the CAM pathway of carbon fixation, the relative amounts of CAM and C33 used for external CO2 assimilation may vary with water availability (1,10,15,27), day-night temperature differentials (11, 26), or photoperiod (16). Generally, conditions which increase water stress favor CAM because the water use efficiency of CAM is greater than that of C3 (17).Mesembryanthemum crystallinum shifts from primarily C3 fixation to primarily CAM fixation after a few days' exposure to high salt concentrations (23).This effect of high salt has been viewed as a result of increased osmotic stress (25). This species accumulates very large quantities of salt even when given unlimited access to nonsaline water (20, 23). This accumulation suggests a metabolic requirement for high levels of inorganic ions. In the following study, M. crystallinum was unable to sustain significant CAM fixation when salt levels in the growth medium were restricted. Moreover, the salt requirement for CAM provided an opportunity for comparing within a single population the relative merits of C3 and CAM fixation. (8). In nutrient solution culture, high levels of water stress are difficult to achieve without resorting to osmoticums. For these reasons, the selected growth medium consisted of equal parts potting soil and sand and the selected method for producing water stress was water deprivation. MATERIALSThe plants were grown in an environmental chamber with 12-h day-night cycle, 25 C days and 15 C nights, 12 C dew point day and night, and a light intensity of 46 nE cm-2 s-' PAR. These growth conditions are typical of M. crystallinum's natural habitat and were found to favor CAM (3).Watering and Experimental Treatments. The protocol for watering the plants ( Fig. 1) was that all plants received identical treatment but for a relatively short period. The initial 2-week drought period was to in...

High productivity and photosynthetic flexibility in a CAM plant

“…of M. crystallinum was also very high with a maximum of 0.17 g/g/day and seasonal averages of 0.071 and 0.047 for Age Classes 1 and 2, respectively. Accounting for the changes in dry weight attributable to salt (Bloom, 1979) and using the conversion factor between CO2 fixed and saccharides formed which is given in Sestak et al (1971), the maximum R.G.R. implies a carbon fixation rate of 1.3 nMoles CO2 cm-2 s-1.…”

Succulence and CAM relationships in Aeonium genus

“…three samples in similar position on the plant, were taken from each species for the following determinations: pH measurements at 0.01 unit, rates of dark CO2 uptake in confined environment through variations of the NaHCO3 5 x 10 -s N buffer system [24] in 300cm 3 flasks and acid titrations to a pH 7.00 end point [9,31] with 0.01 N NaOH (data expressed as /~eg/g fresh weight). The pH end point used is halfway between the most habitual in the bibliography [27,3,5,14,13,26,33,22]. The mean values (n = 6) of the three samples are presented in this paper, except for regressions calculations where all the values were used in order to obtain more degrees of freedom (n = 18).…”

Diurnal Ion Fluctuations in the Mesophyll Tissue of the Crassulacean Acid Metabolism Plant Mesembryanthemum crystallinum

“…This species requires high accumulations of K4 or Na4 before achieving any significant CO2 fixation by the CAM pathway (2). Salt deprivation inhibits CAM specifically rather than metabolism in general.…”