Spectral dependence of flavonol and betacyanin accumulation in Mesembryanthemum crystallinum under enhanced ultraviolet radiation

Mesembryanthemum crystallinum L. (Aizoaceae) is a drought‐ and salt‐tolerant halophyte that is able to endure harsh environmental conditions. Upon irradiation with high light irradiance (1200–1500 µmol m^?2 s^?1) it displays a rapid cell‐specific accumulation of plant secondary metabolites in the upper leaf epidermis; a phenomenon that is not detectable with salt or drought treatment. The accumulation of these compounds, the betacyanins and acylated flavonol glycosides, increases if the plants are exposed to polychromatic radiation with a progressively decreasing short‐wave cut‐off in the ultraviolet range. The response is localized in the epidermal bladder cells on the tips of young leaves and epidermal layers of fully expanded leaves. It is demonstrated that the accumulation of flavonols and betacyanins can be described by a weakly sigmoid dose function in combination with an exponential decrease of the response function of the plant with increasing wavelength.     

The Activity of the Peroxidase System in the Course of Stress-Induced CAM Development

The activity of the peroxidase system in Mesembryanthemum crystallinum L. plants in relation to the shift from C₃ to CAM photosynthesis was studied. In detached leaves of the fourth and fifth stories treated with NaCl (0.3 M), a rapid (after 30 min) transient induction of the ionically bound peroxidase (the first maximum) was observed followed by a second weak increase in the enzyme activity (90 min after salt treatment). In the leaves of intact plants, which received a longer treatment with NaCl, a two-phase change in the enzyme activity was also observed. It was most pronounced at the early stages of the NaCl-induced plant shift from C₃ to CAM photosynthesis. In this case, in both detached and intact leaves of juvenile plants, the activity of soluble peroxidase was at a low steady-state level. The situation changed dramatically when M. crystallinum plants transitioned to the reproductive developmental phase and photosynthesis switched from C₃ to CAM. The time dependence of the activities of both peroxidase types, the soluble ones in particular, was characterized by marked diurnal oscillations (light–dark), which coincided with the fluctuations of the total titratable acidity. In this case, the activity of the soluble enzyme was several orders of magnitude higher than the activity of the ionically bound peroxidase, even though the optimum pH for both isoforms was similar (pH 5.0). Three acid isoforms of soluble peroxidases, which operated more actively when the cytoplasm had a higher acidity, were distinguished by isoelectrofocusing. Their activity increased under salinity. Alkaline and neutral components were predominant in more than 30 molecular forms of the soluble peroxidase detected. We concluded that the operation of the peroxidase system changed substantially when plants shifted from the juvenile to the reproductive state and switched from C₃ to CAM photosynthesis: the activity of stress-induced ionically bound peroxidase was drastically inhibited with a concurrent increase in the activity of soluble peroxidase and a change in the spectrum of its molecular forms.     

Adaptation of the tonoplast V-type H+-ATPase of Mesembryanthemum crystallinum to salt stress, C3–CAM transition and plant age

Plants of the facultative halophyte and CAM species Mesembryanthemum crystallinum L. (Aizoaceae) were stressed for 8 d with 400 mol m⁻³ NaCl in the root medium. NaCl was then removed from the substratum, and the plants were watered again with NaCl-free solution. A second set of plants was maintained as controls. A small degree of CAM, as indicated by day-night changes in malate levels, was expressed during ageing of the plants. Salinity-stress-dependent CAM induction was reversible by the removal of salt, as indicated by similar Δ malate levels in previously salt-stressed plants and in non-stressed plants on day 19 of the experiment. Tonoplast vesicles were isolated from leaves during the time-course of stress application, stress removal and ageing. Parameters of the tonoplast H⁺-ATPase were correlated to the application of salinity, the expression of CAM and ageing. It was concluded, first, that a pronounced increase in the amount of tonoplast H⁺-ATPase is related to salinity per se and a smaller increase to ageing; secondly, that there is an increase in the specific activity of the enzyme related to ageing; thirdly, that the induction of two new polypeptides with molecular masses of 32 and 28 kDa is correlated in time with the expression of CAM, and, fourthly, that the two new polypeptides are part of the tonoplast H⁺-ATPase holoenzyme.     

Photoautotrophic Cell Suspension Cultures from Mesembryanthemum crystallinum and their Response to Salt Stress

For the first time, photoautotrophic cell suspension cultures of Mesembryanthemum crystallinum have been established. The cells are growing in a sugar-free culture medium in the presence of 2 % (v/v) CO₂ as the sole carbon source. A 16 h light photoperiod is applied. Increase in fresh and dry weight during a 21 days growth cycle was more than 3-fold. Treatment of the cells with 200 mM NaCl from day 10 to day 21 of subculture stimulated cell culture growth, enhanced CO₂ fixation and elicited an increase in the extractable activities of enzymes related to CO₂ fixation (RubisCO; PEP carboxylase) and malic acid metabolism (NAD / NADP dependent malic enzyme and malic acid dehydrogenase). The cells performed osmotic adjustment to high salinity by uptake of K⁺, Na⁺, Cl^? and formation of proline as well as by a reduction in cell size. Although sugar and starch content of the cells changed during light/dark transition, a CAM-related diurnal fluctuation of malic acid was not observed.