The authors found that the key elements of plant form and function, analysed at global scale, are largely concentrated into a two-dimensional plane indexed by the size of whole plants and organs on the one hand, and the construction costs for photosynthetic leaf area, on the other.

The global spectrum of plant form and function

Publisher Summary This chapter discusses a search for physiological causes and ecological consequence in reference with the variation in growth rate between higher plants. When grown under optimum conditions, plant species from fertile, productive habitats tend to have inherently higher relative growth rates (RGR) than species from less favorable environments. Under these conditions, fast-growing species produce relatively more leaf area and less root mass, which greatly contributes to their larger carbon gain per unit plant weight. Fast-growing species also have higher respiration rates per unit organ weight, due to demands of a higher RGR and higher rate of nutrient uptake. Fast-growing species have a greater capacity to acquire nutrients, which is likely to be a consequence, rather than the cause, of their higher RGR. This chapter analyses variations in morphological, physiological, chemical, and allocation characteristics underlying variation in RGR, to arrive at an appraisal of its ecological significance. The lower specific leaf area (SLA) of slow-growing species is because of the relatively high concentration of cell-wall material and quantitative secondary compounds, which may protect against detrimental abiotic and biotic factors. This chapter concludes that it is likely that there are trade-offs between growth potential and performance under adverse conditions, however, the current ecophysiological information explaining variation in RGR is too limited to support this contention quantitatively.

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Inherent Variation in Growth Rate Between Higher Plants: A Search for Physiological Causes and Ecological Consequences

Based on SEM examinations of about 5000 species of seed plants, this is a survey of their epidermal surface characters with an aim to application in taxonomy. Surface characters may be grouped into four categories: (1) Cellular arrangement or cellular pattern. (2) Shape of cells (the “primary sculpture” of a surface). (3) Relief of outer cell walls (the “secondary sculpture” superimposed on the primary sculpture), caused mainly by cuticular striations and superficially visible wall inclusions and wall thickenings. (4) Epicuticular secretions (the “tertiary sculpture” superimposed on the secondary sculpture), i.e. mainly waxes and related substances.

The systematic applicability is discussed for each of these structural groups. Epidermal characters are only slightly influenced by environmental conditions. Their high structural diversity provides most valuable criteria for the classification between species and family level. There is also some evidence for their systematic applicability above the family level.

The possible evolutionary–ecological significance of surface sculpturing is discussed briefly. There is evidence that these features may be seen primarily under the aspects of reduced ability of plants to contaminate and as temperature control mechanisms of the surfaces.

Epidermal and seed surface characters of plants: systematic applicability and some evolutionary aspects

Stomatal apertures are the major pathway for the movement of CO2 from the atmosphere into the mesophyll of leaves. The presence of this pathway for the movement of gases also results in water loss from the hydrated surfaces within leaves to the atmosphere. Stomatal aperture appears to be controlled by complex mechanisms which operate to maintain a variable balance between allowing CO2 uptake to proceed, while restricting the loss of water vapor, and preventing leaf desiccation. Recent reviews have examined the physiological bases of stomatal function (Raschke 1979; Allaway and Milthorpe 1976) and stomatal responses to environment (Sheriff 1979; Burrows and Milthorpe 1976; Hall et al. 1976). Analyses which integrated stomatal effects on CO2 exchange, transpiration, and energy balance were developed based upon theory (Cowan 1977), which have led to hypotheses concerning optimal stomatal function (see Chap. 17, this Vol.; Cowan and Farquhar 1977). However, information concerning the simultaneous responses of stomata, water loss, and CO2 assimilation rates has not been reviewed for plants in natural environments.

Stomatal Responses, Water Loss and CO2 Assimilation Rates of Plants in Contrasting Environments

There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.

Plant aquaporins: multifunctional water and solute channels with expanding roles.

Preface 1. The succulent 2. Climate and vegetation of deserts 3. The Namib desert 4. Physiological implications 5. Life strategies of succulents References Index.

Life Strategies of Succulents in Deserts: With Special Reference to the Namib Desert

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.

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

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.

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

Welwitschia 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.

Leaf temperatures and energy balance ofWelwitschia mirabilis in its natural habitat.

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.

E. Brinckmann

Papers

Life Strategies of Succulents in Deserts: With Special Reference to the Namib Desert

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

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

Leaf temperatures and energy balance ofWelwitschia mirabilis in its natural habitat.

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