E. Brinckmann

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.

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.

Leaf temperatures and energy balance ofWelwitschia mirabilis in its natural habitat

SummaryWelwitschia mirabilis is a perennial desert plant with extremely large leaves (0.5–1.0 m broad, 1–2 m long). Leaf temperatures were measured in the field and the energy budget was calculated. The portions of the leaf which were kept above the ground had leaf temperatures which were only 4–6°C above air temperature. In the leaf portions which were in contact with the ground leaf temperatures were 6–12°C above air temperature (absolute maximum 51°C). The important feature in the energy budget ofWelwitschia mirabilis is its high reflectivity (38% of the global radiation). Only about 56% of the global radiation is absorbed by the thick leathery leaves. The energy loss due to convection is of the same order of magnitude as the reflection and it is abouy the same in the portions of leaf on and above the ground. The difference in leaf temperatures found in these portions is due to the loss of thermal radiation from the section of leaf above the ground to the cooler ground which is shaded by the leaf. The provision of a heat sink due to the large area of shade cast by these large leaves is of significance to the existence ofWelwitschia mirabilis in its arid habitats.

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.

Responses of Crassulacean Acid Metabolism (CAM) to Increasing and Decreasing Water Stress in Plants in the Southern Namib Desert

CAM features in a variety of different succulents in response to climatic changes between March 1977 and April 1981 were studied in the southern Namib desert (Richtersveld). In 1977 and 1978 all investigated succulents performed a CAM. After an extended period of insufficient rainfall which damaged the succulents between 80 and 100% most species did not show a measurable CO2 gas exchange or diurnal acid fluctuation, indicative of CAM. But both features were restored quickly after an abundant rainfall in March 1981.

Water loss and malate fluctuations during the day for plants in the southern Namib desert

SummaryThe daily course of transpiration for 12 different plants growing in the southern Namib desert was investigated. Sclerophyllous species with C3 photosynthesis were the most effective water savers followed by succulents exhibiting CAM, while C3 pathway-succulents lose as much water as malacophyllous species. The different species showed either one or two peaked patterns of both transpiration and leaf conductance. With C3 plants the most common pattern is a single morning peak in leaf conductance followed by decreases in conductance over the remainder on the day. With CAM succulents leaf conductance is high in the morning, shows a pronounced midday depression and increases in the early afternoon when the malate pool is depleted, but malate consumption did not start earlier than 4 h after dawn. Seven of nine investigated C3 plants had rather high contents of malate, up to 180 μmol·g-1 dry matter. In these plants the malate content increased with increasing leaf conductance and disappeared when leaf conductance declined in the evening, indicating that malate was synthesized during photosynthesis.

The effect of different growing conditions on water relations parameters of leaf epidermal cells of Tradescantia virginiana L.

SummaryTradescantia virginiana L. plants were cultivated under contrasting conditions of temperature, humidity, light quality and intensity, and nutrient status in order to investigate the effect of growth conditions on the water relations parameters of the leaf epidermal cells. Turgor pressure (P), volumetric elastic modulus (ɛ), half-time of water potential equilibration (T1/2), hydraulic conductivity (Lp) were measured with the miniaturized pressure probe in single cells of the upper and lower epidermis of leaves. Turgor differed (range: 0.1 bar to 7.2 bar) between treatments with lowest values under warm and humid conditions and additional supply of fertilizer, and highest values under conditions of low air humidity and low nutrient supply. The volumetric elastic modulus changed by 2 orders of magnitude (range: 3.0 bar to 350 bar, 158 cells), but ɛ was only affected by the treatments, in as much as it was dependent on turgor. The turgor dependence of ɛ, measured on intact leaves of T. virginiana, was similar to that for cells of the isolated (peeled) lower epidermis, where ɛ as a function of turgor was linear over the whole range of turgors. This result has implications for the discussion of pressure/volume curves as measured by the pressure bomb where changes in “bulk leaf ɛ” are frequently discussed as “adaptations” to certain treatments. The measurements of the hydraulic conductivity indicate that this parameter varies between treatments (range of means: 2.4×10-6 cm s-1 bar-1 to 13.4×10-6 cm s-1 bar-1). There was a negative correlation for Lp in cells of intact leaves as a function of turgor which was altered by the growing conditions. However, a correlation with turgor could not be found for cells from isolated epidermis or cells from a uniform population of plants. The large variation in Lp from cell to cell observed in the present and in previous studies was accounted for in a study of 100 cells from a uniform population of plants by the propagation of measurement errors in calculating Lp. The results suggest that in T. virginiana cellular water relations are changed mainly by the turgor dependence of ɛ.