Water and solute relations of young roots of Phaseolus coccineus have been measured using the root pressure probe. Biphasic root pressure relaxations were obtained when roots were treated with solutions containing different osmotic test solutes. From the relaxations, the hydraulic conductivity (Lpr), the permeability coefficient (Psr), and the reflection coefficient (σsr) of the roots could be evaluated. Lpr was 1.8 to 8.4 . 10−8 m . s−1 . MPa−1 and Psr (in 10−10 m . s−1): methanol, 27–62; ethanol, 44–73; urea, 5–11; mannitol, 1.5; KCl, 7.1–9.2; NaCl, 2.1; NaNO3, 3.7. The hydraulic conductivity was similar when using osmotic and hydrostatic pressure gradients as driving forces. The hydraulic conductivity of individual root cortex cells (Lp) was by two orders of magnitude larger than Lpr (Lp = 0.3 to 4.7 . 10−6 m . s−1 . MPa−1) which indicated a predominant cell‐to‐cell rather than an apoplasmic transport of water in the Phaseolus root. Except for distances shorter than 20 mm from the root apex, the hydraulic resistance of the roots was limited by the radial movement of water across the root cylinder and not by the hydraulic resistance within the xylem. Reflection coefficients were low: methanol: 0.16–0.34; ethanol: 0.15–0.47; urea: 0.41–0.51; mannitol: 0.68; KCl: 0.43–0.54; NaCl: 0.59; NaNO3: 0.54. The transport coefficients (Lpr, Psr, σsr) have been critically examined for influences of unstirred layers and active transport. The low σsr suggests that the common treatment of the root as a rather perfect osmometer (σsr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σsr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed.

A combined system has been developed in which epidermal cell turgor, leaf water potential, and gas exchange were determined for transpiring leaves of Tradescantia virginiana L. Uniform and stable values of turgor were observed in epidermal cells (stomatal complex cells were not studied) under stable environmental conditions for both upper and lower epidermises. The changes in epidermal cell turgor that were associated with changes in leaf transpiration were larger than the changes in leaf water potential, indicating the presence of transpirationally induced within-leaf water potential gradients. Estimates of 3 to 5 millimoles per square meter per second per megapascal were obtained for the value of within-leaf hydraulic conductivity.Step changes in atmospheric humidity caused rapid changes in epidermal cell turgor with little or no initial change in stomatal conductance, indicating little direct relation between stomatal humidity response and epidermal water status. The significance of within-leaf water potential gradients to measurements of plant water potential and to current hypotheses regarding stomatal response to humidity is discussed.Since its development by Husken et al. (2), the miniaturized pressure probe has for the most part been used to estimate the water relations parameters (hydraulic conductivity and volumetric elastic modulus) of individual plant cells (e.g. 1 1). However, the value of cellular turgor itself is also considered to be a fundamentally important physiological parameter in plants (e.g. 9), particularly for current hypotheses regarding stomatal sensitivity to atmospheric humidity (e.g. as reviewed by MaierMaercker [3]). These hypotheses ascribe a central role to withinleaf water potential gradients and the consequences of these gradients to epidermal and stomatal cell turgor. In situ measurement of epidermal cell turgor is possible on attached leaves of Tradescantia virginiana L. (11), and in principle may be combined with in situ measurements of leaf gas exchange and water potential under contrasting levels of leaf transpiration. This measurement should then provide direct evidence for the occurrence of transpirationally induced within-leaf water potential gradients (10) if the differences in leaf epidermal cell turgor that are associated with differences in transpiration are larger than the corresponding changes in leaf water potential. In addition it should provide information regarding the proposed relationship between epidermal cell turgor and level of stomatal conductance (e.g.8). The following paper reports the development of a combined system for measuring epidermal cell turgor and leaf water relations parameters under controlled environmental conditions. ' Supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 137).Results are also presented concerning within-leaf water potential gradients and relations between epidermal cell turgor and stomatal responses. MATERIALS AND METHODS

No abstract

A succulent is a plant with water storing tissue, but succulence clearly is a quality that can be possessed to a higher or lesser degree. This paper gives a definition of succulence and discusses problems concerning Dells measure of succulence. A new measure, Succulence Quotient, is proposed. It measures the amount of water that a plant can store at the expenditure of one gram of organic matter. We demonstrate this measure on a number of plants from the Namib desert, southern Africa, and compare it with the measured caloric values of the plant tissues.We also discuss life cycles of desert succulents in terms of utilizable and structural biomass and water. We contrast the concepts of growth form, life form and life strategy, and propose, on the basis of life cycle characteristics, an outline classification of life strategies of desert succulents. One detail of this outline classification is further worked out and illustrated for succulents from the Namib desert.

No abstract

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.

Growth and development of dark-grown Arabidopsis thaliana seedlings were studied in microgravity during space shuttle mission STS-84. The major purpose of this project was to determine if there were developmental differences among the four ecotypes studied--Wassilewskija (Ws), Columbia (Col), Landsberg erecta (Ler), and C24--and to evaluate whether particular ecotypes are better suited for spaceflight experimentation compared with others. A secondary goal was to study the growth of three starch-deficient strains of Arabidopsis by extending the observations made in a previously published report. For all strains, seed germination was not affected by microgravity, but seedlings were smaller in the spaceflight samples compared with the ground controls. The starch-deficient strains continued to exhibit vigorous growth until the termination of the experiment at 121 h after imbibition of seeds. However, ethylene effects, i.e., reduced growth and exaggerated hypocotyl hooks, were observed in all strains studied. Nevertheless, the Ler and C24 ecotypes seem to be more suitable for spaceflight research, compared with the other two ecotypes, based on measurements of their relative and absolute growth. This type of information should aid in the design of plant experiments for the International Space Station.

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.