Pyrroline‐5‐carboxylate (P‐5‐C) reductase (EC 1.5.1.2) was extracted from cell suspension cultures, which proved to be very suitable for investigation of proline accumulation. Proline accumulation in cell suspensions of M. nodiflorum L., as well as P‐5‐C reductase activity and substrate affinity, increased with progressive adaptation to NaCl stress. In vitro NaCl treatment inhibited enzyme activity and decreased substrate affinity, independent of pretreatment of the cells. NaCl concentrations below 100 mM did not inhibit enzyme activity of adapted cells. High substrate concentrations counteracted in vitro NaCl inhibition (up to 200 mM). Cycloheximide inhibited the increase of P‐5‐C reductase activity, as well as proline accumulation, after NaCl treatment, indicating stress‐induced de novo synthesis of the enzyme. The different reactions of P‐5‐C reductase upon salt treatment are discussed with respect to its possible role in the regulation of proline accumulation.

No abstract

Over a period of seven years (1977-1983) the proline content and its responses to climatic changes were investigated in plants - especially Mesembryanthemaceae - in the southern Namib Desert (South Africa). Among 95 species in 26 families, 61 had detectable amounts of proline. In several of these species the proline content increased considerably in years with insufficient rainfall but decreased when the rainfall was abundant again. When individuals of the same species were grown at different sites, water availability in the soil determined their proline content. Many of the investigated species showed a clear diurnal fluctuation in their proline content with a remarkable proline accumulation during times of highest evaporative demand. In general, the higher the proline content the more pronounced were the changes, indicating that in these species-predominantly annual plants - proline was most probably involved in drought tolerance. The observation that proline accumulation and degradation reacted sensitively to changing climatic conditions over many years confirmed the correlation of proline synthesis to increasing water stress as postulated by the results of laboratory experiments with Mesembryanthemaceae.

The effect of environmental conditions, temperature, relative humidity, and light, together with the regulation of PEPC (phosphoenolpyruvate-carboxylase) activity by malate and pH on CAM (crassulacean acid metabolism), was studied in members of the Mesembryanthemaceae in their natural environment, the southern Namib desert. It was found that during a 24 h period the characteristics of PEPC change. Before sunrise the activity is higher when measured at pH 7 than 8. With bright sunlight the activity measured at pH 7 drops to 20% of its pre-sunrise value, the activity only recovers gradually after malate disappearance and stays constant throughout the night. When measured at pH 8, PEPC shows an opposite behavior, i.e., activity increases in bright sunlight and declines as the pH 7 activity increases. A day-night oscillation in the capacity of malate to stimulate or inhibit PEPC was found. During the day malate inhibits about 90% of the PEPC activity at both pH 7 and 8. After sunset there is a sudden decrease in this inhibition and, at pH 8, malate stimulates the activity by 50%. At pH 7 the stimulation was less.Both stomatal conductance and malate formation were found to increase only when the relative humidity at night rose to 80%. Changes in the properties of the PEPC coincided with the exposure to bright sunlight and changes in leaf temperature. The importance of these metabolic and environmental controls on the regulation of CAM in the Mesembryanthemaceae will be discussed.

No abstract

No abstract

No abstract

No abstract