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.Abbreviations CAM Crassulacean acid metabolism - VPD water vapour pressure deficit Dedicated to Professor H. Ziegler on the occasion of his 60th birthday     

Stem CAM in arborescent succulents

Stem CAM with a peripheral chlorenchyma in stem succulents growing up to arborescent sizes and life forms appears to be a unique evolution as it requires delayed and reduced bark formation and stem stomata. However, stem succulence as a convergent morphotype and with it the stem CAM physiotype evolved polyphyletically in many divergent taxa of the dicotyledonous angiosperms. Controlling water budgets is the main ecophysiological benefit of stem succulence and CAM, where the cooperation of a peripheral photosysnthetically active chlorenchyma and a central water storing hydrenchyma is co-ordinately regulated. Thus, a major factor important for performance of stem CAM succulents at the community level is water or drought. Although this implies fitness under osmotic stress, CAM performing stem succulents are not adapted to salinity and are salt stress avoiders where they occur in saline habitats. Notwithstanding the low overall productivity of CAM plants in general, stem CAM plants can show very high productivity under certain circumstances and may also respond to elevated environmental atmospheric CO₂ concentrations with increased growth.     

Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains

Soil nematodes are fundamentally aquatic animals, requiring water to move, feed, and reproduce. Nonetheless, they are ubiquitous in desert soils because they can enter an anhydrobiotic state that allows them to persist when water is biologically unavailable. In the hyper‐arid Namib Desert of Namibia, rain is rare, but fog routinely moves inland from the coast and supports plant and animal life. Very little is understood about how this fog may affect soil organisms. We investigated the role of fog moisture in the ecology of free‐living, soil nematodes across an 87‐km fog gradient in the gravel plains of the Namib Desert. We found that nematodes emerged from anhydrobiosis and became active during a fog event, suggesting that they can utilize fog moisture to survive. Nematode abundance did not differ significantly across the fog gradient and was similar under shrubs and in interplant spaces. Interplant soils harbor biological soil crusts that may sustain nematode communities. As fog declined along the gradient, nematode diversity increased in interplant soils. In areas where fog is rare, sporadic rainfall events can stimulate the germination and growth of desert ephemerals that may have a lasting effect on nematode diversity. In a 30‐day incubation experiment, nematode abundance increased when soils were amended with water and organic matter. However, these responses were not evident in field samples, which show no correlations among nematode abundance, location in the fog gradient, and soil organic matter content. Soil nematodes are found throughout the Namib Desert gravel plains under a variety of conditions. Although shown to be moisture‐ and organic matter‐limited and able to use moisture from the fog for activity, variation in fog frequency and soil organic matter across this unique ecosystem may be biologically irrelevant to soil nematodes in situ.     

Succulent plants on arid inselbergs

Succulent plants in arid areas are believed to be often associated with high altitudes, except where they form the dominant, zonal vegetation such as in the Succulent Karoo Biome of southern Africa. To test this hypothesis the contribution of perennial succulents to inselberg floras was investigated on twenty-two isolated mountains (inselbergs) at four study sites (Etendeka–Barab: 19°34′ S/13°42′ E; Spitzkoppe: 21°49′ S/15°09′ E; Namibrand: 25°17′ S/15°47′ E; and Sperrgebiet: 27°43′ S/16°04′ E) along a nearly 1000 km bioclimatic gradient in the Namib Desert and adjacent areas, from the Succulent Karoo in the south to the northern Namib, and along altitude gradients.The contribution of succulents to the flora clearly increased with altitude at the two central study sites, thus showing a hump-shaped trend along the bioclimatic gradient. No such correlation was found at the northern and southern end of the bioclimatic gradient. The surrounding (rocky) lowlands likely resulted in a more even distribution of succulents throughout the landscape in the north, while the position in the heartland of the succulent flora in southern Africa at the southern-most site, levelled out potential patterns related to altitude. Nevertheless, even here, a fair number of succulents restricted to mountain habitats occur. This is the case in all other study sites, where succulents restricted to mountain habitats are always present.Some perennial succulents, such as Aridaria noctiflora, Euphorbia gummifera and Tetragonia reduplicata indicate an upward movement with increasing distance from the Succulent Karoo, by growing on plains and lowlands in the Succulent Karoo, but only on inselbergs at study sites north of the Succulent Karoo.Many of the mountain specialist succulents are sought after by plant collectors, such as species of Conophytum, Crassula and Lithops. Protecting and monitoring vulnerable populations as well as growing these in horticulture may help to alleviate the pressure on some of these rare species in nature. Preserving genetic material of isolated populations from mountain tops ex situ is another important conservation measure, and particularly important in the view of anticipated climatic change.     

Leaf Epidermal Hydathodes and the Ecophysiological Consequences of Foliar Water Uptake in Species of Crassula from the Namib Desert in Southern Africa

Abstract: Epidermal hydathodes were found on leaves of 46 of 48 species of Crassula collected from the Namib Desert in southern Africa. The possibility that these structures might allow the absorption of surface water was investigated in 27 species (including subspecies). The presence of hydathodes on leaf epidermi correlated, in most cases, with increases in leaf thickness and enhanced rates of nocturnal, and sometimes diurnal, CO₂ uptake following wetting of the leaves during the night. The precise nature of these responses varied depending on the species. In addition, wetting only the older leaves on the lower portion of the shoot of C. tetragona ssp. acutifolia not only resulted in increased thickness of these leaves, but also effected an increase in leaf thickness and stimulation of CO₂ uptake rates in the distal, younger portion of the shoot that was not wetted. Overall, foliar hydathodes were implicated in the absorption of surface water in many species of Crassula such that the ecophysiology of these desert succulents was positively affected. Although rainfall in the Namib Desert is infrequent, surface wetting of the leaves is a more common occurrence as a result of nighttime dew or fog deposition. Presumably, species with hydathodes benefit directly from this source of moisture. These findings have important implications in understanding a relatively unexplored adaptation of some xerophytes to an extremely arid environment.     

Ecogeographic variation in body size and shape of Cape sparrows (Passer melanurus) in southern Africa

Canonical discriminant analyses were used to assess whether four populations of Cape sparrows varied in body size and shape according to predictions from Bergmann's Rule. In accordance with Bergmann's Rule, birds from two hot, arid Namib desert sites (Namib 1 and Namib 2) were smaller than birds from two cool, mesic Transvaal sites. If heat dissipation through extremities (tarsi) is important to reduce water lost from evaporative cooling, birds under hot conditions in dry environments (Namib 2) should have longer tarsi than those in more humid hot environments (Namib 1). Contrary to this, males at Namib 2 had relatively longer wings hut shorter tarsi than at Namib 1, and female relative tarsus length did not vary between desert sites     

A phylogenetic view of low-level CAM in Pelargonium (Geraniaceae)

Crassulacean acid metabolism (CAM) is common in several plant families and is often associated with succulence. Few studies have examined the occurrence of CAM from a phylogenetic perspective. The genus Pelargonium is promising for such a study because members are characterized by dramatic variation in growth form (including geophytes, shrubs, and stem succulents) and because growth form diversity is expressed to the greatest extent in a monophyletic group comprising 80% of Pelargonium species. This clade, predominantly from the winter rainfall region of southern Africa, likely proliferated in response to Miocene or Pliocene aridification. We present a survey for CAM across Pelargonium, emphasizing the winter rainfall clade. Dawn/dusk fluctuations in titratable acidity were examined in 41 species, with detailed measurements of carbon uptake and stomatal conductance under progressive water stress in four species. No species exhibited obligate CAM. When well-watered, most species exhibited stomatal conductances and acid fluctuations characteristic of C(3) photosynthesis, though some exhibited more pronounced increases in nocturnal acidity, suggesting CAM cycling. In four species examined during dry-down, water stress led to increased nighttime acid levels and decreased daytime stomatal conductance. Ultimately, stomata closed and external carbon uptake ceased, consistent with CAM idling. These results are discussed from the perspective of the evolution of CAM flexibility.     

Water use efficiency of two succulents with contrasting CO2 fixation pathways

Two succulents with similar growth forms but different types of photosynthesis, Cotyledon orbiculata (crassulacean acid metabolism, CAM) and Othonna opima (C₃ pathway), were investigated with respect to the modulation of water use efficiency (WUE) during the transition from the rainy season to subsequent drought. Environmental conditions were simulated in a controlled-environment experiment on the basis of data collected in the habitat of the two species in the southern Namib desert. Experiments included one or more periods of hot bergwind, which frequently occurs in this region. When water was readily available, daily net CO₂ fixation was similar in the two species. This result confirms that the daily CO₂ fixation of CAM plants is as high as that of morphologically similar C₃ plants adapted to the same habitat. As expected, both species reduced CO₂ fixation and water loss through transpiration during simulated hot bergwind periods and their WUE values increased. However, after the second hot bergwind period, nearly identical WUEs were recorded: 41.0 and 40.0 mmol mol^?1 for C. orbiculata and O. opima, respectively. Therefore the statement that a CAM plant is a better ‘water saver’ than a C₃ plant does not necessarily hold for CAM and C₃ plants with similar growth forms growing under the same environmental conditions.     

Quantifying succulence: a rapid, physiologically meaningful metric of plant water storage

Quantification of succulence should ideally convey information about physiological function and yet also be straightforward to measure. While important aspects of succulence and its physiological consequences may be quantified using parameters derived from pressure-volume (P-V) curves, this technique applied to succulent tissues is difficult, time consuming and generally not suitable for large comparative datasets. We performed P-V curves on leaves of 25 taxa from across Caryophyllales and compared the results with direct measures of saturated water content (SWC(meas) ), the ratio of water mass at full saturation to tissue dry mass, for the same taxa. SWC(meas) was significantly related to relative capacitance, the most physiologically relevant parameter describing tissue succulence. We developed a linear model describing SWC(meas) as a function of relative capacitance and leaf volume, which is also supported when accounting for the phylogenetic relationships among taxa. These results indicate that SWC(meas) is a suitable proxy for tissue succulence, and that both cellular properties and variation in gross morphology contribute towards a plant's relative water storage capacity. Quantifying SWC(meas) across many taxa showing variation in tissue succulence will provide a new avenue for exploring the evolutionary dynamics of this important ecological adaptation.     

Water homeostasis in desert-dwelling horses

This study set out to investigate tolerance of the body water pool to short-term water deprivation in horses and, in particular, to assess whether feral horses from the Namib Desert showed tolerance to dehydration superior to Transvaal. Hydration status was compared in six feral horses from the Namib Desert and in six Boerperd farm horses under conditions of normal hydration and after 72 h of dehydration. Under normal hydration, the two groups did not differ significantly in water intake, plasma sodium and potassium concentrations, plasma osmolality, hematocrit, total plasma protein, body water content, or water turnover (ml.kg-0.82.day-1). The Namib horses were significantly smaller (P less than 0.0001) and turned over 5 liters less water per day than the Boerperd during normal hydration and 4 liters less during dehydration. Increases in plasma sodium concentration after 72 h of dehydration were greater (P less than 0.05) in the Namib horses. It was concluded that horses can easily tolerate water deprivation that results in a 12% reductions in body mass. The feral horses of the Namib desert were not significantly different per unit mass from domestic horses with regard to indexes of total body water content under conditions of normal hydration and after 72 h of dehydration. Their smaller size and, hence, lower water turnover might be mechanisms they use for survival in the Namib Desert.     

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

Summary The 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 mol·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.Dedicated to Prof. Dr. O.H. Volk (Würzburg) for his 80th birthday     

准噶尔盆地荒漠植物的叶片水分吸收策略

在干旱地区, 经常出现只能湿润植物地上部分和表层土壤的小量级降雨和凝结水输入, 此量级的水分输入无法通过入渗进入根区土壤而被植物根系吸收利用。最近研究发现, 叶片吸收水分是利用小量级降水和凝结水的主要方式。该研究通过鉴定和观测准噶尔盆地东南部5个自然植物群落中夏季地上部分仍存活的所有51种荒漠植物叶片的单位面积吸水量(LWUC)、叶片含水量增加率(Rw)以及其他7种植物功能性状, 探讨了这些植物功能性状与植物叶片吸收水分能力的关系。研究结果表明: (1)与Rw相比, LWUC是更加适合评价植物叶片吸收水分能力的指标; (2)植物生活型是相对合理的评价荒漠植物叶片吸收水分能力的植物性状; (3)荒漠植物地上(叶片)和地下(根系)部分吸收水分策略存在着权衡; (4)荒漠植物叶片耗水和吸水存在着权衡。总之, 叶片吸收水分策略对于荒漠植物, 特别是一年生草本植物最大限度地利用有限的水资源渡过持续的干旱并完成生活史具有重要的意义。     

Growth form diversity on the Canary Islands

An analysis is given of the growth form composition of the flora of the Canary Islands based on a checklist published by Hansen & Sunding (1985) and the authors' field studies. Results are compared with similar data available from Israel and California. Special attention is given to endemism, the Mediterranean-wides and introduced species, as well as to some special plant features such as succulence, spinescence and fleshy fruits. Long-distance dispersal is stressed as the mechanism by which the Canaries obtained their flora. The growth-form composition of the Canaries is discussed in the context of the prevailing present-day environmental conditions, not in the context of climatic conditions in geological time. Characteristic features are the low percentages of endemic and even of indigenous annuals, the high percentage of chamaephytes (many of whom are endemic), the strong development of arborescence, the richness in leaf succulents, and the paucity of spines.     

Rectifier-like Activities of Roots of Two Desert Succulents

Axial and radial water flows for roots in response to appliedhydrostatic pressure drops, water loss from roots after variousperiods of drying, and development of new roots after rewettingdroughted plants were examined for two sympatric desert succulents.Agave deserti Engelm. and Ferocactus acanthodes (Lemaire) Brittonand Rose. For a 40 kPa hydrostatic pressure drop applied to20 mm long root pieces, radial water flows from the epidermisto the root xylem were 2- to 5-fold greater at the tip thanat midlength and were much less than axial flows along the xylem.Upon drying detached roots in air at 20 °C and a water vapoursaturation deficit of 1.2 kPa (50% relative humidity), radialwater flow decreased more than 10-fold in 3–6 h, and couldrecover to the original level 6 h after rewetting. The rateof water loss from attached roots of plants dried in air at20 °C and a 1.2 kPa saturation deficit decreased about 200-foldin 72 h, which would greatly limit water loss from the plantto a drying soil. At 96 h after rewetting roots of A. desertithat had been exposed to air at 20 °C and a 1.2 kPa saturationdeficit for 120 h, rehydration of existing roots and developmentof new roots contributed about equally to water uptake by thewhole plant. In summary, roots of these desert succulents canreadily take up water from a wet soil but do not lose much waterto a dry soil, thus effectively acting like rectifiers withrespect to plant-soil water movement. Key words: Agave, Cactus, Drought, Root, Water flow, Xylem     

嘉峪关草湖湿地植物功能群组成及其性状对不同生境的响应

植物功能群与生境的关系是生态学研究的重要内容,对于揭示植物的环境适应机制、群落构建机制、预测植被变化等具有重要意义。为研究局域尺度上干旱区湿地、沙漠、戈壁等生态系统功能群的环境适应机制,在嘉峪关草湖国家湿地公园内,基于植物功能性状数据,研究了荒漠区不同生境条件下的功能群组成和功能性状特性。结果表明:随着生境条件的改变,土壤含水量、含盐量、容重、pH值等土壤性质发生显著变化;植物的功能群组成和功能性状会发生适应性的变化,沼泽湿地生境内植被的功能性状以克隆、C₄光合途径、根茎型等特征为主,盐沼湿地以叶片被蜡、叶茎肉质、CS策略等特征为主,沙丘生境以叶片具毛、风播、具增厚或增长的根、一年生、CR策略等特征为主,荒漠戈壁以叶片少、具增厚或增长的根、虫媒、地上芽等特征为主。局域尺度上,生境异质性尤其是土壤条件的异质性是影响植物群落组成和植物功能性状变化的重要环境因子。     

An examination of the bacteriophages and bacteria of the Namib desert

Bacteria and their viruses (called bacteriophages, or phages), have been found in virtually every ecological niche on Earth. Arid regions, including their most extreme form called deserts, represent the single largest ecosystem type on the Earth's terrestrial surface. The Namib desert is believed to be the oldest (80 million years) desert. We report here an initial analysis of bacteriophages isolated from the Namib desert using a combination of electron microscopy and genomic approaches. The virus-like particles observed by electron microscopy revealed 20 seemingly different phage-like morphologies and sizes belonging to the Myoviridae and Siphoviridae families of tailed phages. Pulsed-field gel electrophoresis revealed a majority of phage genomes of 55 approximately 65 kb in length, with genomes of approximately 200, 300, and 350 kb also observable. Sample sequencing of cloned phage DNA fragments revealed that approximately 50% appeared to be of bacterial origin. Of the remaining DNA sequences, approximately 50% displayed no significant match to any sequence in the databases. The majority of the 16S rDNA sequences amplified from DNA extracted from the sand displayed considerable (94 approximately 98%) homology to members of the Firmicutes, and in particular to members of the genus Bacillus, though members of the Bacteroidetes, Planctomycetes, Chloroflexi, and 8-Proteobacteria groups were also observed.     

L’assemblage lithique du site Acheuléen de Namib IV (Namib central,Namibie)

Despite a rich archaeological record, the old prehistory of Namibia, particularly from the Earlier Stone Age, is poorly documented, due to the surface context of the findings, exposed by erosional processes. In this context, human behaviors and responses to environment changes in this region during the Middle Pleistocene remain unclear. In the late 1970's, Myra Shackley surveyed the Central Namib dune desert and discovered lithic artefacts associated to fragmented fossilized bones of antelopes, elephants, zebras and buffalos at Namib IV. She interpreted this locality as an Acheulean butchery site on the shores of a paleolake. The radiometric dating suggested an age of ~347ka whereas biochronology broadly pointed the human occupation to the Mid-Pleistocene Transition (1–0.5 Ma). Thanks to these ages, Namib IV is currently the earliest dated evidence of human presence in the current Namib coastal desert. Contextualized by the existing chronological and paleoenvironmental data, we analyzed the unstudied lithic material recovered at Namib IV. To do so, we applied a qualitative morpho-structural approach to the stone tools and tried to reconstruct the productional schemes (chaînes opératoires). Our results argue for an important economy of the raw material, a spatial fragmentation of stone tool production and an important technical homogeneity of the lithic assemblage despite few possible intrusive elements. In the light of these analyses, we finally discussed the advanced site function of Namib IV as butchery site.     

Soil fungal diversity and assembly along a xeric stress gradient in the central Namib Desert

The Namib Desert of south-western Africa is one of the oldest deserts in the world and possesses unique geographical, biological and climatic features. While research through the last decade has generated a comprehensive survey of the prokaryotic communities in Namib Desert soils, little is yet known about the diversity and function of edaphic fungal communities, and even less of their responses to aridity. In this study, we have characterized soil fungal community diversity across the longitudinal xeric gradient across the Namib desert (for convenience, divided into the western fog zone, the central low-rainfall zone and the eastern high-rainfall zone), using internal transcribed sequence (ITS) metabarcoding. Ascomycota, Basidiomycota and Chytridiomycota consistently dominated the Namib Desert edaphic fungal communities and a core mycobiome composed of only 15 taxa, dominated by members of the class Dothideomycetes (Ascomycota), was identified. However, fungal community structures were significantly different in the fog, low-rainfall and high-rainfall zones. Furthermore, Namib Desert gravel plain fungal community assembly was driven by both deterministic and stochastic processes; the latter dominating in the all three xeric zones. We also present data that suggest that the inland limit of fog penetration represents an ecological barrier to fungal dispersal across the Namib Desert.     

Are drought-resistance promoting bacteria cross-compatible with different plant models?

The association between plant and plant growth promoting bacteria (PGPB) contributes to the successful thriving of plants in extreme environments featured by water shortage. We have recently shown that, with respect to the non-cultivated desert soil, the rhizosphere of pepper plants cultivated under desert farming hosts PGPB communities that are endowed with a large portfolio of PGP traits. Pepper plants exposed to bacterial isolates from plants cultivated under desert farming exhibited a higher tolerance to water shortage, compared with untreated control. This promotion was mediated by a larger root system (up to 40%), stimulated by the bacteria, that enhanced plant ability to uptake water from dry soil. We provide initial evidence that the nature of the interaction can have a limited level of specificity and that PGPB isolates may determine resistance to water stress in plants others than the one of the original isolation. It is apparent that, in relation to plant resistance to water stress, a feature of primary evolutionary importance for all plants, a cross-compatibility between PGPB and different plant models exists at least on a short-term.