Differences In Water Potential Research Articles

Pretreated cheese whey wastewater management by agricultural reuse: Chemical characterization and response of tomato plants Lycopersicon esculentum Mill. under salinity conditions

The agricultural reuse of pretreated industrial wastewater resulting from cheese manufacture is shown as a suitable option for its disposal and management. This alternative presents attractive advantages from the economic and pollution control viewpoints. Pretreated cheese whey wastewater (CWW) has high contents of biodegradable organic matter, salinity and nutrients, which are essential development factors for plants with moderate to elevated salinity tolerance. Five different pretreated CWW treatments (1.75 to 10.02dSm−1) have been applied in the tomato plant growth. Fresh water was used as a control run (average salinity level=1.44dSm−1). Chemical characterization and indicator ratios of the leaves, stems and roots were monitored. The sodium and potassium leaf concentrations increased linearly with the salinity level in both cultivars, Roma and Rio Grande. Similar results were found in the stem sodium content. However, the toxic sodium accumulations in the cv. Roma exceeded the values obtained in the cv. Rio Grande. In this last situation, K and Ca uptake, absorption, transport and accumulation capacities were presented as tolerance mechanisms for the osmotic potential regulation of the tissues and for the ion neutralization. Consequently, Na/Ca and Na/K ratios presented lower values in the cv. Rio Grande. Na/Ca ratio increased linearly with the salinity level in leaves and stems, regardless of the cultivar. Regarding the Na/K ratio, the values demonstrated competition phenomena between the ions for the cv. Rio Grande. Despite the high chloride content of the CWW, no significant differences were observed for this nutrient in the leaves and stems. Thus, no nitrogen deficiency was demonstrated by the interaction NO3−/Cl−. Nitrogen also contributes to maintain the water potential difference between the tissues and the soil. Na, P, Cl and N radicular concentrations were maximized for high salinity levels (≥2.22dSm−1) of the pretreated CWW.

Influence of fruit maturity in the susceptibility of Navelina oranges to develop postharvest non-chilling peel pitting

Peel pitting is a disorder occurring mostly during postharvest storage at non-chilling temperatures in different varieties of citrus fruit and consists in collapse of flavedo and albedo tissues that may affect oil glands. It has been demonstrated that during postharvest, sharp variations in water potential of cells from flavedo and albedo are sufficient to provoke fractures in cell walls from external albedo resulting in tissue collapse. However, morphology and composition of cells and cell walls in flavedo and albedo varies during fruit maturation and this may affect water flow through the different fruit peel layers and susceptibility of fruit to develop peel pitting. In this paper, we have studied the influence of the stage of maturation in the susceptibility of Navelina orange to develop peel pitting. Except in mature-green fruit, peel pitting increased with maturation after transferring fruit from 45% to 95% relative humidity and was also more severe as more dehydrated was the tissue before transference. Also, differences in water potential of fruit maintained at 45 or 95% relative humidity increased as fruit matured, suggesting that tissue reduces the ability of water adjustment during maturation. In this sense, only mature-green fruit flavedo was able to recover water potential when transferred from 45 to 95% relative humidity. Ethylene production upon transfer from low to high relative humidity increased only in mature tissue and was rapid and transient, and before initial symptoms of peel pitting. Flavedo and albedo water potential (ψw) was substantially reduced during fruit maturation. As lower was the ψw of freshly harvested fruit, minor variations were observed by changes in the storage relative humidity and higher the induced damage. Therefore, the increasing susceptibility of Navelina fruits to develop peel pitting with fruit maturation may be related to a reduced ability to regulate peel evapotranspiration and osmotic adjustment during postharvest storage.

Permafrost‐driven differences in habitat quality determine plant response to gall‐inducing mite herbivory

Summary Canada's northern boreal forests are undergoing rapid change associated with warming trends, with permafrost thaw leading to increased forest stress and compositional changes in plant communities. Within this context, the influence of commonly occurring, resident natural enemies on plant species remains poorly understood. One of the dominant deciduous shrub genera, Betula, is abundant in northern forest–wetland landscapes, and is highly susceptible to leaf galling by mites, but the impacts of these specialist arthropod herbivores on plant‐level physiology remain unknown. We examined the impacts of mite galling on a suite of ecophysiological traits in Betula glandulosa, a shrubby species commonly found on permafrost plateaus, bogs and fens in a boreal wetland–forest mosaic, and on two congeners, B. occidentalis and B. neoalaskana, found only on plateaus. B. glandulosa leaves on plateaus showed marked declines in photosynthetic capacity [Amax], stomatal conductance [gs], and transpiration [E] in response to galling. However, B. glandulosa leaves in bogs and fens did not respond similarly, implying strong habitat‐related differences in response to galling herbivory. The impacts on plateaus were reinforced by similar responses of B. occidentalis and B. neoalaskana leaves to galling, and confirmed that on plateaus, galling appears to have significant negative effects on photosynthetic uptake. Moreover, carbon uptake in infested leaves showed photosynthetic declines even at low galling intensity. Interestingly, neighbouring nongalled leaves adjacent to galled ones showed similar declines in gas‐exchange rates in all the three species found on plateaus. However, reduced stomatal conductance and transpiration in galled and neighbour leaves did not affect whole plant water status, as we found no differences in either midday or diurnal stem water potentials between galled and gall‐free stems in any of the species. Synthesis: Based on these findings, we suggest that the impacts of mite galling are variable, and are likely to be most pronounced in habitats and on individuals that are already exposed to abiotic stresses such as the cooler, but more variable soil temperatures experienced by individuals on permafrost plateaus. Furthermore, galling can potentially contribute to localized thaw processes in these environments via impacts on plant functioning.

HOW DO SEEDS SENSE THEIR ENVIRONMENT? SOME BIOCHEMICAL ASPECTS OF THE SENSING OF WATER POTENTIAL, LIGHT AND TEMPERATURE

ABSTRACT The importance of sensing the environment for seeds is analysed with special attention to three environmental factors—water, light and temperature. Firstly, the possible ways in which differences in water potential of different parts of an imbibing seed might affect metabolism are examined. The necessity of correlating metabolism in different parts of the seed during imbibition is stressed. Secondly, light sensing via phytochrome is analysed in terms of possible steps in the reaction sequence subsequent to the primary photoreaction. Some apparent discrepancies in the light sensing mechanism are considered. Finally, sensing of temperature, the most erratic and unpredictable environmental signal, is analysed in terms of mediation by changes in either membrane properties or membrane metabolism and in terms of possible effects on the growth potential of the embryo. During imbibition and early germination, different parts of the seed become hydrated to different extents. As a result, metabolic events ...

Use of plant water relations to assess forage quality and growth for two cultivars of Napier grass (Pennisetum purpureum) subjected to different levels of soil water supply and temperature regimes

Napier grass (Pennisetum purpureum Schumach.) is an important fodder and relatively drought-tolerant crop in tropical and subtropical regions, especially in developing countries. For this and other species, tools are needed for identifying drought-tolerant cultivars to aid selection for semi-arid environments. We determined tissue water status, carbon assimilation, biomass yield and forage quality for Napier grass cvv. Bana and Atherton grown in bins and subjected to three soil-water supply levels (100, 50 or 25% of field capacity) in glasshouses set at either low (15−25°C) or high (25−35°C) temperature regimes, over three growing cycles. Our aim was to explore whether differences in leaf water potential (LWP) and carbon assimilation rates could be reliable indicators of the relative yield potential and forage quality of the two cultivars in environments prone to water and heat stresses. At the low soil-water supply of 25% and low temperature, Bana had lower (more negative) LWP and relative water content (RWC) than Atherton, while at 50% and 100% soil-water supply, Bana had a higher tissue water status. Under the high temperature regime, Bana had consistently more positive LWP and RWC than Atherton, but the differences were not significant. The two cultivars had a similar CO2 assimilation rate (A) and there were no significant differences in the total dry matter yields over the three growing cycles. Water-use efficiency for above-ground biomass (kg ha–1 mm–1) was similar for both cultivars and was 28.5–35.1 under the low temperature regime and 16.9–22.9 under the high temperature regime. Neutral detergent fibre (NDF) was often higher for Bana at low water supply and low temperature than for Atherton, but the trend was reversed under the high temperature regime. Digestibility was generally improved under water-stressed conditions, and there was a positive correlation between NDF and both LWP and RWC measured at midday, but only under the low temperature regime. We conclude that LWP, RWC and A, alone or together, are inadequate for selecting cultivars for dry and hot environments, because cultivars may differ in other mechanistic responses to water stress and high temperatures.

Virtual Special Issue (VSI) on whole‐plant water transport

Virtual Special Issue (VSI) on whole‐plant water transport

Stem and whole-plant hydraulics in olive (Olea europaea) and kiwifruit (Actinidia deliciosa)

A field study and an experiment under controlled conditions using pressure-flux relationships were conducted to compare the stem and whole-plant conductance in olive (Olea europaea) and kiwifruit (Actinidia deliciosa) species. Anatomical observations were also made on one-year-old stem to determine the conductive area of vessels (A ves) and the total xylem area (A xyl). Results show that A ves of kiwifruit twigs was ~2.5-fold of that in olive twigs, and the hydraulically weighted mean diameter was up to threefold that of the olive ones. One-year-old olive twigs had lower hydraulic conductivity (k) than the kiwifruit, while values of leaf-specific conductivity (i.e. k normalised per unit leaf area) were higher than the kiwifruit (i.e. ~49 and 29 × 10−6 kg m−1 s−1 MPa−1, respectively). In the field experiment, the flux of sap (heat balance method) and differences in water potential through the soil–plant system (ΔP) were used for both species to calculate the whole-plant conductance that was normalised per unit leaf area (leaf-specific whole-plant conductance, K plant,LA). Values of K plant,LA are attributable to the combined effect of the ΔP and anatomical features of conduits. Olive species showed a larger ΔP (2.4 MPa at midday) than the kiwifruit (0.5 MPa) which contributed to lower K plant,LA in Olea than the Actinidia plants. This information, combined with vessel density data, contributes to explain differences amidst olive and kiwifruit species, in terms of susceptibility to some drought-related hydraulic impairments induced by the Mediterranean environment.

WEED CONTROL WITH A COVER CROP (NEONOTONIA WIGHTII) IN MANDARIN ORCHARDS IN GUADELOUPE (FWI)

ISHS VI International Symposium on Banana: XXVIII International Horticultural Congress on Science and Horticulture for People (IHC2010): International Symposium on Citrus, Bananas and other Tropical Fruits under Subtropical Conditions WEED CONTROL WITH A COVER CROP (NEONOTONIA WIGHTII) IN MANDARIN ORCHARDS IN GUADELOUPE (FWI)

Variation in water potential, hydraulic characteristics and water source use in montane Douglas-fir and lodgepole pine trees in southwestern Alberta and consequences for seasonal changes in photosynthetic capacity

Tree species response to climate change-induced shifts in the hydrological cycle depends on many physiological traits, particularly variation in water relations characteristics. We evaluated differences in shoot water potential, vulnerability of branches to reductions in hydraulic conductivity, and water source use between Pinus contorta Dougl. ex Loud. var. latifolia Engelm. (lodgepole pine) and Pseudotsuga menziesii (Mirb.) Franco (interior Douglas-fir), and determined the consequences for seasonal changes in photosynthetic capacity. The Douglas-fir site had soil with greater depth, finer texture and higher organic matter content than soil at the lodgepole pine site, all factors that increased the storage of soil moisture. While the measured xylem vulnerability curves were quite similar for the two species, Douglas-fir had lower average midday shoot water potentials than did lodgepole pine. This implied that lodgepole pine exhibited stronger stomatal control of transpiration than Douglas-fir, which helped to reduce the magnitude of the water potential gradient required to access water from drying soil. Stable hydrogen isotope measurements indicated that Douglas-fir increased the use of groundwater during mid-summer when precipitation inputs were low, while lodgepole pine did not. There was a greater reduction of photosynthetic carbon gain in lodgepole pine compared with Douglas-fir when the two tree species were exposed to seasonal declines in soil water content. The contrasting patterns of seasonal variation in photosynthetic capacity observed for the two species were a combined result of differences in soil characteristics at the separate sites and the inherent physiological differences between the species.

Soil water content criteria for peach trees water stress detection during the postharvest period

Abstract Irrigation scheduling based on soil water moisture sensors requires that the soil water status be maintained within a range that is optimal for plant growth. The objective of this work was to evaluate whether soil water content dynamics, measured by multi-sensor capacitance probes, could be used to determine indices of a drying soil to detect the commencement of water stress in a peach tree orchard. For this, an experiment was carried out in a drip irrigated mature peach tree orchard in Murcia (Spain). During the postharvest period well irrigated trees (control treatment) were compared with two water stress treatments consisting of a drying cycle applied for one month in two ways: withholding irrigation (Rapid Stress) and progressively reducing irrigation (Gradual Stress). The soil water content (SWC) was measured continuously using multi-sensor capacitance probes. The beginning of plant water stress was identified by the first significant difference in midday stem water potential (Ψstem) between stressed and control trees. The ‘breaking point’ (the transition between a relatively rapid reduction of SWC in the drying soil to a slower rate) as well as the stabilization of the SWC-derived indices coincided with appearance of a water stress level as severe as to reduce plant water uptake, as judged from the Ψstem reduction. The results suggested that a lower SWC limit could be established for irrigation management in early maturing peach trees using capacitance probes at 90% of the field capacity value during the postharvest period.

A micropump based on water potential difference in plants

In land plants, water vapor diffuses into the air through the stomata. The loss of water vapor creates a water potential difference between the leaf and the soil, which draws the water upward. Quantitatively, the water potential difference is 1–2 MPa which can support a water column of 100–200 m. Here we present the design and operation of a biomimetic micropump. The micropump is mainly composed of a 48-μm thick metal screen plate with a group of 102-μm diameter micropores and an agarose gel sheet with nanopores of 100 nm diameter. The micropores in the screen plate imitate the stomata to regulate the flow rate of the micropump. The agarose gel sheet is used to imitate the mesophyll cells around the stomata. The lost of water from the nanopores in the gel sheet can generate a water potential difference (more than 30 kPa) which can drive solution flow in a microfluidic chip. Results have shown that a precise flow rate of 4–8 nl/min can be obtained by using this micropump, and its ultra-high flow rate is 113–126 nl/min. The advantages of this biomimetic micropump include adjustable flow rate, simple structure and low fabrication cost. It can be used as a “plug and play” fluid-driven unit in microfluidic chips without any external power sources or equipments.

Nutrient effects on diurnal variation and magnitude of hydraulic lift in winter wheat

We have investigated hydraulic lift by winter wheat in response to four fertilizer treatments—nitrogen (N), phosphorus (P), nitrogen and phosphorus (NP) and control (CK, no fertilizer)—in a greenhouse experiment, in which root systems of wheat plants were split between a drier, upper layer and a wetter, lower layer. The soil volumetric water content (Øv) was measured at 2 h intervals in the upper layer by time domain reflectometry (TDR). Under the N, NP and CK treatments the fluctuations in this parameter were maximal during the blooming stage, amounting to 0.0038, 0.0127 and 0.0100 m3 water m−3 soil, respectively, but under the P treatment it peaked at 0.0116 m3 water m−3 soil during the grain-filling stage. Increases in the Øv of the upper layer occurred from 22:00 to 04:00 (6.7, 11.5, 13.5 and 7.5% under the N, P, NP and CK treatments, respectively), during the blooming stage. The wheat roots under the NP treatment showed the highest level of hydraulic lift across the entire growth period (0.3762 m3m−3); 1.36, 3.49 and 1.02 times higher than under the CK, N and P treatments, respectively. Hydraulic lift was shown to be driven by the soil water potential difference between the upper and lower layers using controls in which the soil moisture content of the two layers was the same.

Trees never rest: the multiple facets of hydraulic redistribution

AbstractThe upward movement of water due to transpiration stops when soil water potential (Ψs) drops below leaf water potential (ΨL). Under these circumstances, water can move in any direction in the plant‐soil continuum through the passive conduits of roots and stems towards the lowest Ψs. This is generally termed as hydraulic redistribution (HR), but the positioning and orientation of the driving water potential gradient may vary. Any experimental method that can measure bi‐directional and low flows in the sapwood of roots and stems will be suitable to detect HR. Using one approach for measuring sap flow (the heat field deformation technique, HFD) in several forest species and sites across Europe, we were able to provide evidence on different types of HR: vertical hydraulic redistribution (VHR), horizontal hydraulic redistribution (HHR), foliar uptake (FU) and tissue dehydration (TD). VHR is the vertical water movement through roots in response to water potential differences between deep and topsoil, either hydraulic lift or hydraulic descent. HHR is the lateral water movement through roots in response to horizontal water potential gradients, namely under localised irrigation. FU is the water movement from crown to soil through stems when the crown is wetted by foggy weather. TD is the downward movement of water in stems or roots from above‐ground tree tissues to soil under prolonged drought or frost. Results from direct sap flow measurements indicated the vectoral and widespread nature of HR, a phenomenon of paramount importance for overall physiology and ecohydrology. Copyright © 2010 John Wiley & Sons, Ltd.

SODA: A new method of in-scene atmospheric water vapor estimation and post-flight spectral recalibration for hyperspectral sensors: Application to the HyMap sensor at two locations

A new method of estimating per-pixel atmospheric column water vapor (ACWV) and potential differences in the reported band center wavelengths of the HyMap sensor has been developed. The new method uses variations of a second order derivative algorithm (SODA) to assess the impact of atmospheric residual features on calculated surface reflectance spectra after atmospheric compensation. The SODA method provides an alternative to the current band ratio techniques of ACWV estimation and also allows the same form of algorithm to be used for the estimation of possible band shifts. A comparison of in-situ measured surface reflectance at two field sites in Western Australia demonstrates improvement in the resulting spectra when post-flight updates are made to the reported HyMap band center wavelengths and applied during the atmospheric compensation process. The same SODA methodology was varied to estimate the ACWV on a per-pixel basis and found to significantly reduce the appearance of the underlying surface structure on the resulting ACWV images as well as improve the overall accuracy of the estimation. The ACWV estimated from the HyMap imagery at the two field sites was found to agree with in-situ atmospheric ACWV measurements to within 2% and represented a two fold increase in accuracy over a 3 band ratio Continuum Interpolated Band Ratio (CIBR) technique of ACWV estimation.

The Cocoon of the Fossorial FrogCyclorana australisFunctions Primarily as a Barrier to Water Exchange with the Substrate

Studies of evaporative water loss using streams of dry air in the laboratory have demonstrated reduced rates in various taxa of cocooned frogs. However, because the cocoon is formed in subterranean burrows with humid microclimates and no air flow, loss of water by evaporation is likely to be negligible. In contrast, although potentially important, the influence of the cocoon on water exchange with the soil surface has not been characterized. In dry soils, there is a sizable water potential gradient between the frog and the soil; hence, we hypothesized that cocoons would play a role in reducing liquid water loss to dry substrates. Individuals of the burrowing frog Cyclorana australis (Hylidae: Pelodryadinae) were induced to form cocoons in the laboratory. On semisolid agar-solute substrates across a range of water potentials, the hygroscopic cocoon absorbed small but similar amounts of moisture. With the cocoon removed, the frogs gained or lost water, depending on the direction of the frog-substrate water potential difference. Plasma osmolality of cocooned frogs was significantly higher than in hydrated frogs. Because cocooned frogs did not exchange significant amounts of water at either high (wet) or low (dry) substrate water potentials, we conclude that the cocoon of fossorial frogs acts as a physical barrier that breaks the continuity between frog and substrate. We contend that the primary function of the cocoon is to prevent liquid water loss to drying clay and loam soils, rather than to prevent subterranean evaporative water loss.

Physiological performance and xylem water isotopic composition underlie gender-specific responses in the dioecious shrub Corema album

Gender-specific requirements of reproduction in dioecious species can lead to different physiological responses in male and female plants, made in relation to environmental constraints, and influencing growth, survival and population structure. Gender-related physiological differences and seasonal responses, indicating the existence of compensatory mechanisms of reproduction, were examined during a drought year in the dioecious shrub species Corema album. To integrate aboveground and belowground physiological responses, chlorophyll fluorescence, leaf gas exchange, water potential and xylem water isotopic composition were monitored throughout the diurnal cycle and annual phenological sequence of the species. Sampling was carried out in Doñana Natural Park (SW Spain) in Mediterranean-type climate conditions. The gender which bore greater reproductive effort showed higher physiological stress. Intersexual differences in leaf water potential were interpreted as arising from each gender's maximum reproductive allocation; lower values were found during flowering in males and during fruit production in females. Cold temperatures during winter fostered photoinhibitory responses that were most evident in male individuals, as a response to their relatively higher investment in reproduction during flowering. Net assimilation rate was not influenced by reproductive status; however, females tended to show higher values of this parameter at midday. The integrated analysis of photosynthetic variables and water relations indicated a gender effect in the physiological response at midday. The oxygen isotopic composition of xylem water showed a lack of dependence on the water table during the drought period, and indicated intersexual differences in water catchment. Females reached deeper soil layers, suggesting mechanisms compensating for their higher reproductive effort, and giving new evidence of physiological gender dimorphism in the belowground responses of a woody species.

Plant growth and yield responses in olive (Olea europaea) to different irrigation levels in an arid region of Argentina

Over the last two decades, a significant increase in intensively managed olive orchards has occurred in the northwest of Argentina where climatic conditions differ greatly from the Mediterranean Basin. Annual amounts of applied irrigation are generally high due to low rainfall, access to deep ground water, and little information about water use by the crop in the region. The objectives of this study were to: (1) assess the responses of plant growth, yield components, and several physiological parameters to five different irrigation levels and (2) determine an optimum crop coefficient (Kc) for the entire growing season considering both fruit yield and vegetative growth. Five irrigation treatments (Kc = 0.50, 0.70, 0.85, 1.0, 1.15) were employed from late winter to the fall over 2 years in a 6-year-old cv. �Manzanilla fina� olive orchard. Tree canopy volume was approximately 15 m3 with a leaf area of about 40 m2 at the beginning of the experiment. During much of each year, the volumetric soil water content was lower in the Kc = 0.50 treatment than in the other irrigation levels evaluated (Kc = 0.85 and 1.15). Although differences in midday stem water potential (?s) were not always apparent between treatments during the first year, there were lower ?s values in Kc = 0.50 and 0.70 relative to the higher irrigation levels during the second year. Shoot elongation in Kc = 0.50 was about 50% of that in Kc = 1.0 and 1.15 during both years leading to significant differences in the increase of tree canopy volume by the end of the first year. Fruit yield was similar among irrigation levels the first year, but yield reached a maximum value the second year between Kc = 0.70 and 0.85 above which no increase was apparent. The somewhat lower fruit yield values in Kc = 0.50 and 0.70 were associated with decreased fruit number rather than reductions in individual fruit weight. The water productivity on a yield basis (fruit yield per mm of applied irrigation) decreased as irrigation increased in the second year, while similar calculations based on trunk cross-sectional area growth indicated that vegetative growth was proportional to the amount of irrigation. This suggests that the warm climate of northwest Argentina (28° S) can induce excessive vegetative growth when very high irrigation levels are applied. A Kc value of approximately 0.70 over the course of the growing season should be sufficient to maintain both fruit yield and vegetative growth at adequate levels. An evaluation of regulated deficit irrigation strategies for table olives in this region could be beneficial to further reduce irrigation

Grapevine and Soil Water Relations with Nodding Needlegrass (Nassella cernua), a California Native Grass, as a Cover Crop

Nodding needlegrass [Nassella cernua (Stebbins & R.M. Love) Barkworth], a California native perennial grass, was tested for its effects on grapevine and soil–water relations in a drip-irrigated vineyard in Parlier, CA. Vine water status and in-row and between-row soil moisture (at 0.3 m, 0.6 m, 0.9 m, 1.2 m, and 1.5 m) were monitored semiweekly from June to September. There was no overall significant difference in leaf water potential between treatments. In-row soil moisture was lowest at depths of 0.6 m to 0.9 m within the nodding needlegrass treatment but was lowest from 0.3 m to 0.9 m within the clean cultivation treatment. Compared with clean cultivation, nodding needlegrass in-row soil moisture was significantly higher at depths of 0.3 m and 0.6. m and did not differ at depths of 0.9 m and 1.2 m. In contrast, in-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at 1.5 m. Between-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at every depth. Combining in-row and between-row data, overall vineyard soil moisture was slightly lower, by 1.2% points, in the nodding needlegrass treatment compared with clean cultivation. There was no interaction between treatment and depth for between-row soil moisture, indicating that the vines used little water from the between-row area. The lack of difference between treatments in the rate of soil moisture depletion over the season indicates that nodding needlegrass used little water during the summer. Based on these results, nodding needlegrass appears to be suitable as a permanent cover crop in California drip-irrigated vineyards where competition for summer water is a concern.

Eggs under Pressure: Components of Water Potential of Chameleon Eggs during Incubation

Water exchange of squamate eggs is driven by the difference between the water potentials of eggs and of their nest environment. While osmotic potential is generally assumed to dominate the net water potential of eggs, resistance of the eggshell to stretching also affects egg water potential. We therefore determined osmotic potentials and pressure potentials (mechanical pressure) of eggs of the veiled chameleon Chamaeleo calyptratus over the course of incubation. Because embryos are diapausing gastrulae when eggs are laid and diapause persists several months, the water potential of eggs can be evaluated before it is influenced by the developing embryo. Water uptake during the first 2 wk of incubation was rapid as a result of the large difference between the total water potential of the egg (-848 kPa) and that of its incubation substrate. After about 2 wk, water potential of the egg stabilized at -460 kPa. By day 80 of incubation, the developing embryo and allantois affected water exchange of the egg. The allantoic fluid was initially very dilute, but its osmotic potential decreased to about -200 kPa by the end of incubation. Pressure potential of the egg averaged 25 kPa, with no systematic trend during incubation. The pressure potential exerted by the eggshell reduced the difference between the water potential of the egg and the water potential of the environment, that is, the ability of eggs to take up water. At the time of oviposition, this effect was relatively small, producing a 4%-6% reduction in water potential difference. Once the yolk osmotic potential stabilized, however, the reduction was 12% or more. This observation means that the dynamics of water uptake by squamate eggs cannot be fully understood without consideration of the pressure that is exerted on the contents of eggs by their shells.

The influence of arbuscular mycorrhizal colonization on soil–root hydraulic conductance in Agrostis stolonifera L. under two water regimes

The hypothesis that mycorrhizal colonization improves the soil-root conductance in plants was experimentally tested in a growth chamber using pot cultures of Agrostis stolonifera L. colonized by Glomus intraradices. Plants were grown in 50-l pots filled with autoclaved sand/silt soil (1:1), with and without the mycorrhizal fungus. Within the mycorrhizal treatment, half of the pots remained well watered, while the other half was subjected to a progressive water deficit. Soil water potential (estimated as plant water potential measured at the end of the dark period), xylem water potential measured at the tiller base, transpiration rate, and soil water content were monitored throughout the experiment. Soil-root hydraulic conductance was estimated as the ratio between the instantaneous transpiration rate and the soil and xylem water potential difference. To obtain cultures with similar nutritional status, the P in the modified Hoagland's nutrient solution was withheld from the inoculated pots and applied only once a month. Even though there were no differences on growth or nutrient status for the mycorrhizal treatments, water transport was enhanced by the inoculum presence. Transpiration rate was maintained at lower xylem water potential values in the presence of mycorrhizae. The analysis of the relationship between soil-root hydraulic resistance and soil water content showed that mycorrhizal colonization increased soil-root hydraulic conductance as the soil dried. For these growing conditions, this effect was ascribed to the range of 6-10%.