D. J. von Willert

Availability of water controls Crassulacean acid metabolism in succulents of the Richtersveld (Namib desert, South Africa)

Features of Crassulacean acid metabolism (CAM) were studied in a variety of different succulents in response to climatic conditions between March 1977 and October 1983 in the southern Namib desert (Richtersveld). A screening in 1977 and 1978 revealed that nearly all investigated succulents performed a CAM, but overnight accumulation of malate declined gradually with decreasing soil water potential, tissue osmotic potential, and leaf water content. This was further substantiated by an extended period of insufficient rainfall in 1979 and 1980 which damaged the evergreen CAM succulents between 80 and 100%. In most of the species still living, neither CO2-gas exchange nor diurnal acid fluctuation, indicative of CAM, could be detected unless an abundant rainfall restored both CAM features. Plants persisted in a stage of latent life.Water supply is one necessary prerequisite for CAM in the Richtersveld. But even well-watered plants with CAM were sensitive to short-term water stress caused by high water-vapour partialpressure deficit (VPD) in the night, which reduced or prevented CO2 uptake and resulted in a linear relation between overnight accumulated malate and VPD. The results do not support the opinion that, for the Namib succulents, CAM is an adaptive mechanism to water stress since long-term and short-term water stress stopped nocturnal malate synthesis, but instead lead to the conclusion that nocuturnal CO2 fixation is only performed when the water status of the plant can be improved simultaneously.

Oxygen and carbon dioxide exchanges in crassulacean-acid-metabolism-plants.

The 24 h O2 uptake and release together with the CO2 balance have been measured in two CAM plants, one a non-succulent Sempervivum grandifolium, the other a succulent Prenia sladeniana. The O2 uptake was estimated by the use of 18O2. It was found that the mean hourly O2 uptake in the light was 7 times that in the dark for Sempervivum and 5 times that for Prenia, after correction for the lightdark temperature difference. It was estimated that oxygen uptake in the light was 2.4 times greater than oxygen release (=net photosynthesis) in Sempervivum and 1.4 times greater in Prenia. In both plants there was a positive carbon balance over the 24 h period under the experimental conditions. It was estimated that malate formed during the night could, if completely oxidized to CO2 and water, account for 74% of the light phase O2 uptake in Sempervivum. In Prenia the O2 uptake was more than sufficient to account for a full oxidation of malate.

The effect of leaf age and salt stress on malate accumulation and phosphoenolpyruvate carboxylase activity in Mesembryanthemum crystallinum

Abstract Prior to treatment with 300 mM NaCl Mesembryanthemum crystallinum leaves of different ages have only small differences in phosphoenolpyruvate (PEP) carboxylase activity and do not accumulate malate at night, indicating that they behave like C3-plants. After 12 days of NaCl treatment the mature leaves exhibit a 20-fold increase in PEP carboxylase activity and a significant accumulation of malate during the night. These features are characteristics of crassulacean acid metabolism (CAM) plants. The PEP carboxylase activity in the youngest leaf did not increase until the leaf reached a certain stage of development. Since the youngest leaves always showed the lowest PEP carboxylase activity and never accumulated malate at night they do not contribute to a CAM. The results provide evidence for the existence of two regulatory processes in Mesembryanthemum crystallinum both causing a change from C3 to CAM. One process is under environmental control (NaCl concentration) and occurs only in mature leaves. The second process is an ontogenetic response and is associated with maturation.

Occurrence and changes of proline content in plants in the southern Namib desert in relations to increasing and decreasing drought

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 activity and malate inhibition/stimulation of phosphoenolpyruvate-carboxylase in crassulacean-acid-metabolism plants in their natural environment

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.

CO2 gas exchange and transpiration of Welwitschia mirabilis Hook. fil. in the central Namib desert

SummaryThe diurnal course of CO2 gas exchange, 14CO2 incorporation, malate and citrate content, and traspiration of Welwitschia mirabilis were measured in one of its natural habitats, the Welwitschia-Vlakte in the central Namib desert (Namibia), in order to decide which CO2 fixation pathway is used by this gymnosperm.The CO2 gas exchange of Welwitschia is that of a C3 plant under arid conditions. Younger leaf parts show a two-peaked pattern of photosynthetic CO2 uptake whereas in older parts the morning peak is followed by net CO2 release during the rest of the day. The maximum rates of net photosynthesis decrease from 3.4 μmol m-2 s-1 in 1-year-old parts to 1 μmol m-2 s-1 in 7-year-old parts. No net CO2 uptake was detected during the night. The diurnal CO2 balance indicates that the old leaf parts live at the expense of the younger ones. Irrigation of Welwitschia plants resulted in an increased CO2 uptake throughout the light period with maximum rate of 4.1 μmol m-2 s-1. 14CO2 was only incorporated during the day.The water loss of Welwitschia by transpiration is considerable, reaching a peak value of 1.9 mmol m-2 s-1 around noon. Leaf conductance corresponds with the twopeaked pattern of CO2 uptake.Although there is no sign of a crassulacean acid metabolism in Welwitschia the leaf contains rather high amounts of malate (up to 200 μmol g-1 dry matter) and citrate (up to 250 μmol g-1 dry matter), which depend on leaf age but do not show any significant day-night oscillation.In spite of all this the δ13C values are in the range of-17.77 to-19.64‰. Possible reasons for such a high 13C content in a C3 plant are discussed.

CO2 exchange of CAM exhibiting suceulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO2 exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditiens of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO2 and led to CO2 release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO2 uptake while the recovery of the noctural CO2 uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO2 uptake considerably. Even in well watered plants noctural CO2 uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.

Ökophysiologische Untersuchungen an Pflanzen der Namib-Wüste

The southern Namib desert has a vegetation cover of mainly succulent plants in which species of the Mesembryanthemaceae are predominant. Climatically this area is characterized by hot and dry days, and cool and humid nights with episodic rainfalls only in winter. In this environment a great number of species perform a crassulaceaen acid metabolism (CAM). The responses of these plants to water stress as well as the regulation of CAM in the natural habitat are described and discussed.

Ecophysiologic investigations in the family of the Mesembryanthemaceae

SummaryOvernight accumulation of malate, citrate, and isocitrate in a large number of species of Mesembryanthemaceae grown under identical environmental conditions was studied. Of the 27 species investigated, 24 showed malate accumulation, which in 3 cases was accompanied by considerable overnight accumulation of citrate. In the leaves of the same plants, the Na+, K+, Cl-, SO42-, and PO43- contents were determined. Although the plants were not exposed to substrates of high NaCl content, they exhibited extraordinarily high levels of Na+ and Cl-. All plants accumulated, much more Na+ than K+. No readily discernible correlation between the amount of any particular ion and the extend of CAM was found. It is concluded that halophilism and CAM are widespread phenomena in the family of Mesembryanthemaceae that possess ecologic significance.

Light Modulation of the Activity of the PEP-Carboxylase in CAM-Plants in the Mesembryanthemaceae

Summary The effect of light on the activity of PEPC has been studied in members of the Mesembryanthemaceae both in a growth chamber and in the natural environment. It was found that light together with malate could start to inhibit PEPC during the second part of the night. In the natural environment decreased activity is found after plants come into bright sunlight. Almost complete inactivation occurs after 90 min of illumination. The results are discussed in relation to the control of CO2 fixation pathways in CAM plants together with the possible role of conformational changes in PEPC.