Base water potential of Picea abies as a characteristic of the soil water status

Variation in base water potential (Ψ_b, a daily maximum level of plant water potential, which is presumed to correspond to the condition of equilibrium between the soil and plant water potentials) was examined in shoots of Norway spruce trees growing in well-drained and waterlogged soils. The influence of soil water content, air temperature, and vapour pressure deficit of the atmosphere on Ψ_b was studied using the pressure chamber technique. Maximum daily water potentials were not always observable before dawn; some were registered up to two hours later. This tendency being characteristic of trees growing under stress (shade, waterlogging) conditions, increased with declining soil water availability. In trees growing in well-drained soil, Ψ_b depended asymptotically on the available soil water storage (R²=0.73), while the values were slightly influenced by vapour pressure deficit of the atmosphere as well. In trees growing in waterlogged soil, Ψ_b was independent of the soil water storage, but sensitive to the vapour pressure deficit.     

Does pre-dawn water potential reflect conditions of equilibrium in plant and soil water status?

Variation in base water potential (Ψ_b, a daily maximum level of plant water potential, which is presumed to correspond to the equilibrium between soil and plant water potentials) was examined in shoots of Picea abies and Vaccinium myrtillus with respect to soil (available water storage, water potential, temperature) and atmospheric (temperature, relative humidity, vapour pressure deficit) conditions. The available soil water storage (W_tr) accounted for 77% of the dynamics of Ψ_b, while the influence of atmospheric factors became evident under high evaporative demand. Ψ_b was not always observable immediately before dawn, but on 30% of observation days, the recovery continued up to an hour or two after dawn. Full equilibrium between soil and plant water potentials in P. abies in northern conditions is rather improbable by dawn in summer-time, because of the shortness of the dark period and probable night-time transpiration in the case of high atmospheric vapour pressure deficit.     

Field water relations of a compass plant, Lactuca serriola L.

Abstract Field water relations of Lactuca serriola serriola and L. serriola integrifolia were examined. Leaf conductance to water vapour was high early in the morning and declined rapidly during the midday hours. Leaf water potentials decreased to their minima early in the morning and remained low all day. Afternoon recovery of leaf conductance occurred occasionally. Leaf conductance was shown to have a linear response to vapour concentration difference. No differences were seen between L. serriola serriola and L. serriola integrifolia. The pattern of diurnal gas-exchange activity appeared to be complemented by the pattern of intercepted solar irradiance which results from the compass plant leaf orientation observed in L. serriola.     

The Relation of Water Absorption by Wheat Seeds to Water Potential

The progress of water absorption by wheat grains was studiedby supplying water in the vapour phase, at controlled potentials. At a potential of –250 metres of water, the curve forwater uptake against time shows exponential approach to equilibriummoisture content. Living and dead seeds behave similarly untilgermination effects are apparent. Water uptake in the earlystages is probably due to physical rather than physiologicalprocesses. When germination occurs, it causes an exponentialincrease in the rate of water uptake. At higher potentials, up to zero, the uptake curves for deadseeds depart from the simple exponential relationship; in additionto the exponential component, there is a component the rateof which increases with time to a constant rate. The first componentmay represent the physical process of imbibition by the starch,and the second the initiation and progress of starch hydrolysis. A parameter a of the formula derived for the curves is interpretedas representing the diffusivity of water vapour in the seedmaterial, depending upon the physical properties and dimensionsof the seed.     

Aspects of the Water Relations of Ileostylus micranthus (Hook. f.) Tieghem, a New Zealand Mistletoe

Leaf water potentials in the mistletoe, Ileostylus micranthusgrowing outdoors decreased rapidly during the early part ofthe day but remained relatively steady in the early afternoondespite increases in atmospheric vapour pressure deficit (vpd).Minimum water potentials of the mistletoe were relatively constant.They were held at values lower than those of hosts when thelatter maintained high water potentials but approached or evenexceeded those of hosts when they developed low water potentials.In contrast, cut shoots of Ileostylus usually maintained higherwater contents and leaf water potentials than those of its hostswhen both were desiccated separately in the laboratory. Pressure-volumeanalyses indicated that Ileostylus had lower water potentialat full turgor, a lower water potential but higher relativewater content at turgor loss, and a higher bulk modulus of elasticitythan the following four hosts: the native Kunzea ericoides andCoprosmapropinqua, and the introduced Ribes sanguineum and Teline monspessulana.Water potential at turgor loss (_tlp) was strongly correlatedwith the minimum field water potential of both mistletoes andhosts. When _tlpof mistletoe and host is similar (as on Kunzeaand Ribes) field water potentials are also similar, but when_tlpis lower in the mistletoe (as on Coprosma and Teline), thefield water potential of the mistletoe is lower than that ofits host. Consequently, I. micranthus is likely to be more frequenton hosts that maintain high field water potentials than on hoststhat develop low water potentials. Copyright 1999 Annals ofBotany Company Water relations, water potential, osmotic potential, pressure-volume, Ileostylus micranthus , mistletoe, New Zealand.     

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa^-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa^-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa^-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO₂ partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.     

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes

Summary Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ¹³C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ¹³C values in lichens can be explained by a C₃ carboxylation system and the various effects of different limiting processes for photosynthetic CO₂ fixation. If carboxylation is rate limiting, there will be a strong discrimination of¹³CO₂, at high internal CO₂ partial pressure. The resulting very low δ¹³C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO₂ becomes more and more rate limiting and the internal CO₂ pressure decreases, the¹³C content of the photosynthates increases and less negative δ¹³C values results, as are found for blue-green lichens.     

Water relations and growth of rose plants cultured in vitro under various relative humidities

Stomatal malfunctioning is one of the main reasons why plants desiccate when transferred from in vitro to greenhouse conditions. In order to overcome this problem in Rosa hybrida cv. Madame G. Delbard (R) Deladel, two techniques, bottom cooling and water vapour permeable lid, were used. Both methods aimed to increase the vapour pressure gradient between leaf and atmosphere and consequently to improve plant transpiration.The results showed that these techniques increased leaf resistance to dehydration and improved stomatal regulation. Water relations of treated plantlets were similar to those generally observed in hardened plants: lower leaf water and osmotic potentials, and lower leaf water content than in the control ones. Osmotic adjustment occurred in treated plantlets maintaining turgor pressure. Each technique also induced some effects on growth during the rooting phase: with bottom cooling, roots were shorter, with permeable lids, apices were necrosed.These results are discussed in terms of physiological causes and in terms of effect during the following acclimatization.Abbreviations AWC absolute water content - DW dry weight - FW fresh weight     

The relationship between guard cell water potential and the aperture of stomata in Populus

Abstract Previous work with clones of Populus trichocarpa demonstrated that the water vapour conductance of leaves from well-watered cuttings of this species does not decline with loss of turgor from the bulk leaf. In the present study, stomatal responses to water potential in Populus were examined with detached epidermal strips. Stomata in epidermal strips from well-watered plants of P. trichocarpa did not close at low water potentials which led to plasmolysis of the guard cells. In contrast, stomata of P. deltoides and a P. trichocarpa×deltoides hybrid closed when the guard cells lost turgor. A period of water stress preconditioning resulted in modified stomatal responses in P. trichocarpa such that stomata of stressed and re-watered plants nearly closed when guard cell turgor was lost.     

Transpiration rate and intercellular diffusive resistance in the tobacco leaf

The increase in the measured transpiration rate in tobacco leaves due to the experimentally decreased humidity of the bulk air was found to be significantly lower than the theoretical value calculated from the change of water vapour concentration gradients. Boundary layer and stomatal diffusive resistances remained unchanged under experimental conditions with no change of net photosynthetic CO₂ uptake. This suggests an increase in intercellular diffusive resistance with an increase in water vapour concentration gradient which is the driving force of water vapour diffusive part of transpiration flux. The increase can be ascribed to the lengthening of intercellular diffusive pathway as steeper water vapour concentration gradient in intercellular spaces results in an increased evaporating surface of intercellular cells thus moving the effective plane of vaporization in leaf mesophyll further inwards. Due to different and independent changes of concentration gradients for water vapour and CO₂, different length of intercellular diffusive pathways for CO₂ and water vapour may be expected.     

Water balance of over-wintering beetles in relation to strategies for cold tolerance

The vapour pressure of the haemolymph of a supercooled insect is higher than the vapour pressure of the haemolymph of a frozen insect at the same temperature. The aim of the study was to see whether this may affect the water loss of freeze-avoiding and freezetolerant, over-wintering beetles. The rates of water loss were determined on freeze-tolerant Pytho depressus larvae and Upis ceramboides adults. Within each species one group was kept supercooled whereas another group was frozen. All groups were incubated at-5°C. Both species displayed significantly lower rates of water loss when they were frozen than when they were supercooled. Values of respiratory rate and water loss of freeze-avoiding and freeze-tolerant species were compared to corresponding values of desert beetles. The results indicate that the freeze-avoiding species have lower rates of cuticular water loss than freeze-tolerant species. This indicates that the freeze-avoiding species have developed more efficient water-saving mechanisms than freeze-tolerant species. The reason for this may be that the haemolymph in frozen animals will be in vapour pressure equlibrium with the ice in the hibernaculum and thus there is no danger of desiccation during winter. The supercooled insects will have a vapour pressure of the haemolymph that is higher than the vapour pressure of water in the surrounding air and will thus lose water.Abbreviations BW body weight - BW_i initial body weight - BW_t body weight at time t - P vapour pressure difference between the water in the haemolymph and the water in the air - DWL_t dry weight loss at time t - M _w rate of metabolic water production - MF_w mol fraction of water, in the haemolymph - MO₂ rate of oxygen consumption - Osm osmolality of the haemolymph - P _a vapour pressure of water in the air - P _h vapour pressure of water in the haemolymph - P _w vapour pressure of pure water - Q a constant (2,02 1 oxygen per g fat metabolized) relating oxygen consumption to dry weight loss when fat is metabolized - R a constant (1,89 1 oxygen per g water produced) relating metabolic water production to oxygen consumption when fat is metabolized - R _dwl rate of dry weight loss - RH relative humidity of the air - RWC_i initial relative water content measured by weighing - RWC_t relative water content at time t - STP standard temperature and pressure     

Carbon,water and nutrient relations of two mistletoes and their hosts: A hypothesis*

Abstract. The carbon, water and nutrient relations of the xylem parasites Loranthus europaeus and Viscum laxum and their respective hosts. Quercus robur and Pinus sylvestris, were followed throughout clear days in July in order to study water and nutrient interactions in a simple system in which the plant growth depends on the host for its water and nutrients. At similar quantum flux densities, temperatures and vapour pressure deficits, the mistletoes had higher rates of transpiration and lower leaf water potentials than their hosts, but similar rates of CO₂ assimilation. Based on measurements of the nutrient content of the xylem and on seasonal measurements of the biomass and the tissue nutrient content, the present study suggests that the high rates of transpiration may be necessary for the parasites to take up sufficient nitrogen from the xylem of the host for production of biomass (leaves, fruits and stems).     

The Genus Nomocharis

Background: Ziziphus lotus, wild jujube, is a xerophytic shrub of the Rhamnaceae family widely distributed in arid and semi-arid regions of Tunisia, where it occupies most soil types. Phenological patterns of desert plants are strongly affected by the seasonality of water availability and phreatophytes represent a particularly interesting case for studying such relationships.

Aim: This study aims to investigate the relationship between phenological traits and water potential patterns of the wild jujube as a tool for understanding how plants cope with extreme drought.

Methods: Phenophases and predawn (Ψ_pd) and midday (Ψ_md) xylem water potentials of wild jujube were studied monthly (Nov 2007–Oct 2008) at Samaâliate and Oued El Hallouf in southern Tunisia. These sites receive164 mm and 191 mm of annual rainfall, respectively, and differ in slope and soil type.

Results: The Ψ_md decreased progressively and concomitantly with increasing seasonal drought, reaching the lowest values in late summer (down to –3.9 MPa for both sites). Seasonality of Ψ_pd was less pronounced for plants established in Oued El Hallouf (–2.09 MPa) than in Samaâliate (–2.63 MPa) at the end of the dry season. Wild jujube is dormant from October through to March and mature plants flower in May and produce fruits in August.

Conclusions: Our results clearly demonstrate that wild jujube is a drought tolerant species reaching low water potentials during the driest months of summer.     

Diurnal courses and factorial dependencies of leaf conductance and transpiration of differently potassium fertilized and watered field grown barley plants

During the grain filling period we followed diurnal courses in leaf water potential (ψ₁), leaf osmotic potential (ψ_π), transpiration (E), leaf conductance to water vapour transfer (g) and microclimatic parameters in field-grown spring barley (Hordeum distichum L. cv. Gunnar). The barley crop was grown on a coarse textured sandy soil at low (50 kg ha⁻¹) or high (200 kg ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken at full irrigation or under drought. Drought was imposed at the beginning of the grain filling period. Leaf conductance and rate of transpiration were higher in the flag leaf than in the leaves of lower insertion. The rate of transpiration of the awns on a dry weight basis was of similar magnitude to that of the flag leaves. On clear days the rate of transpiration of fully watered barley plants was at a high level during most part of the day. The transpiration only decreased at low light intensities. The rate of transpiration was high despite leaf water potentials falling to rather low values due to high evaporative demands. In water stressed plants transpiration decreased and midday depression of transpiration occurred. Normally, daily accumulated transpirational water loss was lower in high K leaves than in low K leaves and generally the bulk water relations of the leaves were more favourable in high K plants than in low K plants. The factorial dependency of the flag leaf conductances on leaf water potential, light intensity, leaf temperature, and leaf-to-air water vapour concentration difference (ΔW) was analysed from a set of field data. From these data, similar sets of microclimatic conditions were classified, and dependencies of leaf conductance on the various environmental parameters were ascertained. The resulting mathematical functions were combined in an empirical simulation model. The results of the model were tested against other sets of measured data. Deviations between measured and predicted leaf conductance occurred at low light intensities. In the flag leaf, water potentials below-1.6 MPa reduced the stomatal apertures and determined the upper limit of leaf conductance. In leaves of lower insertion level conductances were reduced already at higher leaf water potentials. Leaf conductance was increased hyperbolically as photosynthetic active radiation (PAR) increased from darkness to full light. Leaf conductance as a function of leaf temperature followed an optimum curve which in the model was replaced by two linear regression lines intersecting at the optimum temperature of 23.4°C. Increasing leaf-to-air water vapour concentration difference caused a linear decrease in leaf conductance. Leaf conductances became slightly more reduced by lowered water potentials in the low K plants. Stomatal closure in response to a temperature change away from the optimum was more sensitive in high K plants, and also the decrease in leaf conductance under the influence of lowered ambient humidity proceeded with a higher sensitivity in high K plants. Thus, under conditions which favoured high conductances increase of evaporative demand caused an about 10% larger decrease in leaf conductance in the high K plants than in the low K plants. Stomatal sizes and density in the flag leaves differed between low and high K plants. In plants with partially open stomata, leaf conductance, calculated from stomatal pore dimensions, was up to 10% lower in the high K plants than in the low K plants. A similar reduction in leaf conductance in high K plants was measured porometrically. It was concluded that the beneficial effect of K supply on water use efficiency reported in former studies primarily resulted from altered stomatal sizes and densities.     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

The apparent feed-forward response to vapour pressure deficit of stomata in droughted, field-grown Eucalyptus globulus Labill

This study tested a multiplicative model of stomatal response to environment for drought‐affected trees of Eucalyptus globulus Labill. growing in southern Australia. The model incorporates a feed‐forward response to vapour pressure deficit of ambient air (δe_a) and performed well if evaluated using reduced major axis regression and log‐transformed data. There was strong evidence from gas‐exchange data, leaf water potentials and sapflow measurements of the feed‐forward response by stomata to leaf‐to‐air vapour pressure deficit (δe_l). The response of stomata to δe_l was irreversible. Stomatal conductance and the rate of net photosynthesis were highly correlated and declined, together with the rate of transpiration, throughout the afternoon as δe_a increased despite increasing leaf water potentials. The concentration of CO₂ inside leaves (c_i) increased as stomatal conductance declined indicating increasing non‐stomatal limitations to photosynthesis. The stomatal response to δe_l of E. globulus in the field is best described as an ‘apparent feed‐forward response’ that probably results from both slowly reversible depression of net photosynthesis and abscisic acid accumulation in guard cells. We suggest that the stomatal response to c_i may strengthen the link between photosynthetic capacity and stomatal conductance during leaf drying as a result of either drought or large δ e_l.     

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

Summary The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa^-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa^-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.     

Competition between Hardwood Hammocks and Mangroves

A bstract The boundaries between mangroves and freshwater hammocks in coastal ecotones of South Florida are sharp. Further, previous studies indicate that there is a discontinuity in plant predawn water potentials, with woody plants either showing predawn water potentials reflecting exposure to saline water or exposure to freshwater. This abrupt concurrent change in community type and plant water status suggests that there might be feedback dynamics between vegetation and salinity. A model examining the salinity of the aerated zone of soil overlying a saline body of water, known as the vadose layer, as a function of precipitation, evaporation and plant water uptake is presented here. The model predicts that mixtures of saline and freshwater vegetative species represent unstable states. Depending on the initial vegetation composition, subsequent vegetative change will lead either to patches of mangrove coverage having a high salinity vadose zone or to freshwater hammock coverage having a low salinity vadose zone. Complete or nearly complete coverage by either freshwater or saltwater vegetation represents two stable steady-state points. This model can explain many of the previous observations of vegetation patterns in coastal South Florida as well as observations on the dynamics of vegetation shifts caused by sea level rise and climate change.     

On the water balance ofPhytoseiulus persimilis A.-H. and its ecological significance

The net water vapour exchange ofPhytoseiulus persimilis A.-H. is described. Water loss by transpiration increases progressively with ambient temperature. The transpiration rate is directly proportional to the saturation deficit of the air (15 to 30° C) and at constant temperature linearly dependent on the water vapour activity: m_T=–0.81 a_v+0.91 (for a_v 0.0 to 0.85 at 20°C). Phytoseiulus persimilis is able to absorb water vapour from the unsaturated atmosphere. This occurs above a certain threshold (critical equilibrium activity, CEA), which is a_v=0.9 at 15 to 25°C and increases to a_v=0.935 at 30°C.The environmental humidity conditions influencingP. persimilis on the leaf surface are described. The diurnal water vapour profile within the laminar layer at the leaf surface includes periods with water vapour values high enough for these mites to utilize their water vapour sorption capability and to restore a previously-suffered water deficit. In addition,P. persimilis shows a positive hygrotactic behaviour when in a state of water deficit.The survival time of starvingP. persimilis is at least doubled when a possibility to absorb water vapour is available. The water balance at limited food resources is discussed. With a food supply (one prey mite, containing about 5.5 g water) every 3 days and a water vapour activity of a_v=0.76 (20°C), water balance is achieved and the survival time is maximal (approximately 120 days).     

Determination of Water Stress of Eucalypts in the Field

Abstract

Two methods for determining plant hydrature in the field have been tested on Eucalyptus viminalis Labill. and E. dalrympleana Maiden one-year-old seedlings. The parameters selected are the physico-chemical properties of sap and the diffusion pressure deficit of leaves. Before extracting sap, leaves have been subjected to two different preliminary treatments to improve permeability of cellular membranes, viz. chloroform and liquid nitrogen treatments, the latter being chosen since it was quicker. Sap has been obtained by espressing at 272 Atm., then filtered and analyzed. Diffusion pressure deficit has been determined by equilibration with vapour. A series of samples has been placed in moist chambers with known water potential until equilibrium has been attained. DPD corresponds to the water potential of the chamber where the sample neither gains nor loses water.