Water and solute transport along developing maize roots

Hydraulic and osmotic properties were measured along developing maize (Zea mays L.) roots at distances between 15 and 465 mm from the root tip to quantify the effects of changes in root structure on the radial and longitudinal movement of water and solutes (ions). Root development generated regions of different hydraulic and osmotic properties. Close to the root tip, passive solute permeability (root permeability coefficient, P_sr) was high and selectivity (root reflection coefficient, _sr) low, indicative of an imperfect semipermeable root structure. Within the apical 100–150 mm, P_sr decreased by an order of magnitude and _sr increased significantly. Root hydraulic conductivity (Lp_r) depended on the nature of the force (hydrostatic and osmotic). Osmotic Lp_r was smaller by an order of magnitude than hydrostatic Lp_r and decreased with increasing distance from the root tip. Throughout the root, responses in turgor of cortical cells and late metaxylem to step changes in xylem pressure applied to the base of excised roots were measured at high spatial resolution. The resulting profiles of radial and longitudinal propagation of pressure showed that the endodermis had become the major hydraulic barrier in older parts of the root, i.e. at distances from the apex ä 150 mm. Other than at the endodermis, no significant radial hydraulic resistance could be detected. The results permit a detailed analysis of the root's composite structure which is important for its function in collecting and translocating water and nutrients.Abbreviations and Symbols CPP cell pressure probe - IT root segments with intact tips; - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_app hydrostatic pressure applied to cut end of root - P_c cell turgor - P_{c, cor} turgor of cortical cell - P_c,xyl turgor of late metaxylem vessel - P_ro stationary root pressure - P_r0,seal stationary root pressure of sealed root segment - P_sr solute permeability coefficient of root - RPP root pressure probe - TR root segments with tip removed - _sr reflection coefficient of root Dedicated to Professor Andreas Sievers on the occasion of his retirement     

Observations on the maintenance and measurement of soil water in simple pot experiments and its effects on seed-borne Fusarium culmorum seedling blight of winter wheat

A simple method for maintaining and measuring soil water and the relationship between soil water and seed-borne Fusarium culmorum seedling blight of wheat was investigated under controlled environmental conditions to develop reproducible assay conditions for epidemiological studies and the testing of fungicide seed treatments. For reproducibility, soil matric potential (ψ_m) was used to define soil water, and a range of watering regimes were tested. Treatments were watered to either a maximum ψ_m or to maintain a mean ψ_m in order to establish which parameter best described the effect that soil water had on the incidence of disease symptoms. The severity of disease symptoms was closely related to the mean soil water status and not the maximum ψ_m value. Watering intervals, ranging from three times a day to once every three days, did not affect this relationship. The percentage of seedlings showing symptoms after emergence (i.e. localised or extensive necrotic lesions) was inversely related to the mean ψ_m. With increasing soil dryness (mean ψ_m from -0.14 MPa to -0.17 MPa) the percentage of seedlings with post-emergence symptoms decreased from 50% to 20%. However, as the mean ψ_m changed from -0.14 MPa to -0.17 MPa the percentage of seedlings dying before emergence increased from 25% to 55% in direct proportion to ψ_m. Overall the incidence of infection as indicated by the total number of seedlings showing symptoms either before or after emergence remained relatively constant, and was not significantly related to mean ψ_m.     

Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.     

Water relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis (Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae)

At Mono Lake, California, we investigated field water relations, leaf and xylem chemistry, and gas exchange for two shrub species that commonly co-occur on marginally saline soils, and have similar life histories and rooting patterns. Both species had highest root length densities close to the surface and have large tap roots that probably reach ground water at 3.4-5.0 m on the study site. The species differed greatly in leaf water relations and leaf chemistry. Sarcobatus vermiculatus had a seasonal minimum predawn xylem pressure potential (ψ_pd) of -2.7 MPa and a midday potential (ψ_md) of -4.1 MPa. These were significantly lower than for Chrysothamnus nauseosus, which had a minimum ψ_pd of -1.0 MPa and ψ_md of -2.2 MPa. Sarcobatus had leaf Na of up to 9.1 % and K up to 2.7 % of dry mass, and these were significantly higher than for Chrysothamnus which had seasonal maxima of 0.4% leaf Na and 2.4 % leaf K. The molar ratios of leaf K/Na, Ca/Na, and Mg/Na were substantially lower for Sarcobatus than for Chrysothamnus. Xylem ionic contents indicated that both species excluded some Na at the root, but that Chrysothamnus was excluding much more than Sarcobatus. The higher Na content of Sarcobatus leaves was associated with greater leaf succulence, lower calculated osmotic potential, and lower xylem pressure potentials. Despite large differences in water relations and leaf chemistry, these species maintained similar diurnal patterns and rates of photosynthesis and stomatal conductance to water vapor diffusion. Sarcobatus ψ_pd may not reflect soil moisture availability due to root osmotic and hydraulic properties.     

Removal of Cr(VI) from ground water by Saccharomyces cerevisiae

Chromium can be removed from ground water by the unicellular yeast, Saccharomyces cerevisiae. Local ground water maintains chromium as CrO₄ ^2- because of bicarbonate buffering and pH and E _h conditions (8.2 and +343 mV, respectively). In laboratory studies, we used commercially available, nonpathogenic S. cerevisiae to remove hexavalent chromium [Cr(VI)] from ground water. The influence of parameters such as temperature, pH, and glucose concentration on Cr(VI) removal by yeast were also examined. S. cerevisiae removed Cr(VI) under aerobic and anaerobic conditions, with a slightly greater rate occurring under anaerobic conditions. Our kinetic studies reveal a reaction rate (V_max) of 0.227 mg h^-1 (g dry wt biomass)^-1 and a Michaelis constant (Km) of 145 mg/l in natural ground water using mature S. cerevisiae cultures. We found a rapid (within 2 minutes) initial removal of Cr(VI) with freshly hydrated cells [55–67 mg h^-1 (g dry wt biomass)^-1] followed by a much slower uptake [0.6–1.1 mg h^-1 (g dry wt biomass)^-1] that diminished with time. A materials-balance for a batch reactor over 24 hours resulted in an overall shift in redox potential from +321 to +90 mV, an increase in the bicarbonate concentration (150–3400 mg/l) and a decrease in the Cr(VI) concentration in the effluent (1.9-0 mg/l).     

Transfer of heavy metals from sediment to fish,and their biliary excretion

Uptake and biliary excretion of metals were studied in rainbow trout, Oncorhynchus mykiss, exposed through spiked sediment to a mixture of seven heavy metals. Metal concentrations and toxicity of bile and blood plasma were used as indicators of exposure. Among the seven metals (Cd, Cr, Cu, Hg, Ni, Pb, and Zn) only three (Cu, Hg, and Pb) were concentrated in the bile (bile-plasma ratio >1). Bile-plasma ratios in the rainbow trout were similar to those found in rats for Cu and Hg. Daphnia magna bioassays were used to determine toxicity of bile and blood plasma in the same trout. Toxicity of bile and blood plasma increased after treatment with acid. An analysis of variance (ANOVA) showed that toxicity of bile and blood plasma to D. magna in metal-exposed trout was significantly correlated with (1) bile and blood plasma test concentration, (2) acid treatment of bile and blood plasma (hydrolysis of metal-plasma and metal-bile complexes) and (3) sediment concentration of metals during exposure of trout. In order to significantly detect the magnitude of the exposure to a xenobiotic the biomarker must respond in a dose- or time-dependent manner. Therefore, the potential use of bile toxicity as a biomarker of heavy metal exposure in fish is probably limited by the low bioconcentration of many of these toxicants in bile.     

Collembolan water relations and environmental change in the maritime Antarctic

The water status of the collembolan Cryptopygtus antarcticus (Willem) was investigated from April 1984 to December 1987 at Signy Island, maritime Antarctic, by monthly field sampling to determine body water content. Water content, expressed either as the weight of water per unit dry weight or as a proportion of fresh weight, exhibited both a seasonal cycle and an upward trend over the 44-month study, both of which were highly significant. On an annual basis, body water content was at a minimum (1.21 g g^?1) in July and maximal (1.98 g g^?1) in September, whilst over the entire study water contents increased from 1.3 to 2.0 g g^?1 (or 57-66% of fresh weight) calculated from the fitted linear regression line. Field water contents were below those found for this species in culture (2.9-5.9 g g^?1). Individual C. antarcticus survived experimental loss of 20% of their body water with a resultant significant rise in haemolymph osmolarity from 285 to 397 mOsm L^?1 and there was no evidence of osmoregulation under the experimental conditions of 20 °C and 35% relative humidity. The cuticular permeability (mean conductance) of individual Collembola in dry air increased exponentially with temperature over the range D-45 °C (Q₁₀= 2.0) showing no control of water loss. The physiological response of C. antarcticus suggests that it experiences water stress in its maritime Antarctic habitats with significant seasonal variations of body water content, which correlate with annual cycles of water availability. It is concluded that the significant rise in its mean body water content over the 44-month field study was associated with increased glacial ablation due to higher levels of irradiation and windspeed making available more liquid water. Analyses of climate records for Signy Island from 1947 to 1990 showed that mean monthly air temperature rose by 0.93 °C over this period and by 2.29 °C during the 1980s, both statistically significant increases. Mean monthly windspeeds also increased significantly during 1970–90, and it is suggested that this parameter is the primary climatic driving force behind the increase in glacial ablation during the last two decades. The field water status of species such as C. antarcticus may reflect changes in the patterns of atmospheric circulation, associated with the circumpolar vortex, through increased ozone depletion due to increased tropospheric concentrations of halocarbons.     

Detection of mining impacts on aquatic macroinvertebrate communities: Results of a disturbance experiment and the design of a multivariate BACIP monitoring programme at Coronation Hill,Northern Territory

Abstract Multivariate methods for the detection of impacts on communities are: (i) explored in the context of a disturbance experiment at a site of previous uranium mining in Kakadu National Park, Northern Territory; and (ii) applied to the design of a monitoring programme for a proposed mining development at nearby Coronation Hill. The disturbance experiment in Rockhole Mine Creek used a gravity-fed redirection of polluted water from the mine adit to create a point-source pollutant. Weekly samples of the macroinvertebrate communities were taken both before and after disturbance at the control and‘impact’ site, as part of a multivariate BACIP (Before-After-Control-Impact-Pairs) design. The results of the experiment show that the multivariate approach provided high sensitivity in detecting impacts on the community, with little difference in sensitivity between species and family level analyses. A simple procedure for deleting those taxa that show poor discrimination between pristine and presently disturbed areas further increased the sensitivity of the tests. In addition to the BACIP analyses, robust ordination of the community data suggested a time/treatment interaction effect. Application of the multivariate BACIP approach also was explored using baseline data for benthic communities from the South Alligator River near Coronation Hill. The method provides high statistical power for monitoring; a relatively small change in community dissimilarity would be detectable in a programme with 5 years of observations before and after commencement of mining. A robust ordination of the yearly samples for the control and potential-impact sites showed that, although samples varied along identifiable gradients over time, the two areas changed in parallel so that the difference between the areas was approximately constant over years. This result demonstrates that the multivariate BACIP approach can provide relative constancy of pre-impact difference values, a key requirement of BACIP designs that is difficult to satisfy using abundance data for a single species.     

Relationships between food resources, foraging patterns, and reproductive success in the water pipit, Anthus sp. spinoletta

A basic but rarely tested assumption in optimal foraging theoryis that positive relationships exist between the foraging patternof an animal, its short-term benefits in feeding, and its long-termfitness. We present evidence for these relationships for a centralplace foraging situation. We studied the foraging behavior ofadult water pipits (Anthus sp. spinoletta) feeding nestlingsin an Alpine habitat near Davos, Switzerland, with the followingresults: (1) searching effort decreases with increasing distancefrom the nest, (2) the amount of prey and the proportion oflarge items brought to the nest increases with increasing foragingdistance, (3) water pipits do not forage according to habitatavailability, but prefer vegetation types with the highest fooddensity (mainly grass and herbs) and avoid those with the lowest,and (4) this selectivity is only expressed when the birds foragemore than 50 m from the nest, i.e., usually outside the territory.Among the several potential interpretations of these results,the most parsimonious is that foraging decisions are based onprofitability, i.e., on the net energy gain per time unit. Additionally,we found that food conditions translate into fitness: the numberof fledglings per nest is related positively to the averageprey biomass at the foraging place and negatively to the averagedistance between the foraging place and the nest. Maximum economicdistances, which were predicted from this food-fitness relationship,agreed well with the actual foraging distances observed. Thissuggests a dose connection between foraging decisions and fitness.In addition to the theoretical issues, some conservation issuesare also briefly discussed.     

Growing a stratified,cornified primary culture of rat keratinocytes with epidermis-like water permeation barrier function

Summary The culture of cutaneous keratinocytes grown on a Puropore nylon microporous membrane at the air-liquid interface has been shown to be similar to the epidermis in a number of molecular and morphologic characteristics but to exhibit a significantly greater degree of tritiated water permeation. Various culture conditions have been altered in an effort to improve the water barrier properties. A Kp value in the range of 5.5±1.6×10⁻³ has been obtained for 79% of the culturea) by plating 0.9×10⁶ viable basal cells on a piece (13-mm diameter) of membrane for 7 days of submerged growth,b) by placing two membranes on two stacked glass fiber filters (47-mm extra-thick) in a culture dish (60 mm) for 14 days of growth at the air-liquid interface,c) by replacing the growth medium, i.e., 1 ml of complete minimum essential medium (CMEM) every 24 h after lifting,d) by using 10% fetal bovine serum (FBS) in the CMEM during the submerged culture period and 15% FBS in the CMEM during the lifted culture period, ande) by adding a dialysis membrane on top and a Puropore nylon membrane below the culture when the cultures were inserted in the permeation cell for testing. The percentage of cultures with this value for Kp can be increased to 90% if only cultures with yellow, smooth, and shiny surfaces are tested. This system should be useful as a replacement for skin in testing the cutaneous permeation of some chemicals. To whom correspondence should be addressed at 1528 Public Health, The University of Michigan, Ann Arbor, Michigan 48109-2029.