The effect of incubation media on the water exchange of snapping turtle (Chelydra serpentina) eggs and hatchlings

Summary The effect of two different incubation media, sand and vermiculite, on the water exchange of eggs and the mass of hatchlings of snapping turtles (Chelydra serpentina) was assessed. The eggs were incubated fully buried in either sand or vermiculite at 30 °C and egg mass was measured periodically throughout incubation. The wet and dry masses of each hatchling and its residual yolk were measured at the end of incubation. The media had similar water potentials () but their thermal conductivities differed 2.8-fold. The eggs experienced a net water gain during incubation. The rates of water uptake between treatments were not statistically different throught the first 36 days of incubation but were statistically different thereafter, with eggs incubating in sand taking up water at about twice the rate of eggs incubating in vermiculite. Hatchling masses were similar to both media but hatchling water contents were significantly different. Hatchlings incubated in sand had lower water contents than hatchlings incubated in vermiculite even though the eggs in sand took up more water. Hatchling mass was correlated with egg water exchange for eggs incubated in vermiculite but not for eggs incubated in sand. The difference in egg water exchange in the two media appears to be attributable to differences in the thermal conductivity of the media. The presence of such a thermal effect supports the hypothesis that the eggs exchanged water with the media as water vapor. Egg water exchange was limited by the shell and shell membranes and not by the media. The shell and shell membranes appear to present an effective barrier to water uptake.Abbreviations M _{H 2 O} water flux (cm³·day^-1) - L _p hydraulic conductivity (cm·day^-1·kPa^-1) - A shell area (cm²) - A _p pore area (cm²) - l shell thickness (cm) - r pore radius (cm) - viscosity (kPa·day) - P _{EH 2 O} egg water potential (kPa) - P _{AH 2 O} medium water potential (kPa) - G _{H 2 O} water vapor conductance (cm³·day^-1·kPa^-1) - D _{H 2 O} diffusion coefficient (cm²·day^-1) - R gas constant (cm³·kPa·K^-1·cm^-3) - T temperature (K) - P _{EH 2 O} egg water vapor pressure (kPa) - P _{AH 2 O} medium water vapor pressure (kPa) - d egg diameter - K soil hydraulic conductivity (cm²·day^-1·kPa^-1) - DHM hatchling dry mass - WHM hatchiling wet mass - WU water uptake - IM initial egg mass     

The effect of water exchange on bacterioplankton depletion and inorganic nutrient dynamics in coral reef cavities

We studied the effect of water exchange on the depletion (or accumulation) of bacterioplankton, dissolved organic matter and inorganic nutrients in small open framework cavities (50–70 l) at 15 m depth on the coral reef along Curaçao, Netherlands Antilles. The bacterioplankton removal rate in cavities increased with increasing water exchange rates up to a threshold of 0.0045 s⁻¹, reaching values of 50–100 mg C m⁻² total interior cavity surface area (CSA) per day. Beyond the threshold, bacterioplankton removal dropped. The cryptic community is apparently adapted to the average water exchange in these cavities (0.0041 s⁻¹). Dissolved inorganic nitrogen (DIN), nitrate + nitrite (NO _x ) in particular, accumulated in cavity water and the accumulation decreased with increasing water exchange. Net NO _x effluxes exceeded net DIN effluxes from cavities (average efflux rate of 1.9 mmol NO _x vs. 0.8 mmol DIN m⁻² interior CSA per day). The difference is ascribed to net ammonium losses (NH₄) in cavities at reef concentrations >0.025 μM NH₄, possibly due to enhanced nitrification. Dissolved inorganic phosphate accumulated in cavities, but was not related to water exchange. The cryptic biota in cavities depend on water exchange for optimization of consumption of bacterioplankton and removal of inorganic nitrogen. Coral cavities are an evident sink of bacterioplankton and a source of NO _x and PO ₄ ³⁻ .     

Roles of Aquaporin-3 Water Channels in Volume-Regulatory Water Flow in a Human Epithelial Cell Line

Membrane water transport is an essential event not only in the osmotic cell volume change but also in the subsequent cell volume regulation. Here we investigated the route of water transport involved in the regulatory volume decrease (RVD) that occurs after osmotic swelling in human epithelial Intestine 407 cells. The diffusion water permeability coefficient (Pd) measured by NMR under isotonic conditions was much smaller than the osmotic water permeability coefficient (Pf) measured under an osmotic gradient. Temperature dependence of Pf showed the Arrhenius activation energy (Ea) of a low value (1.6 kcal/mol). These results indicate an involvement of a facilitated diffusion mechanism in osmotic water transport. A mercurial water channel blocker (HgCl₂) diminished the Pf value. A non-mercurial sulfhydryl reagent (MMTS) was also effective. These blockers of water channels suppressed the RVD. RT-PCR and immunocytochemistry demonstrated predominant expression of AQP3 water channel in this cell line. Downregulation of AQP3 expression induced by treatment with antisense oligodeoxynucleotides was found to suppress the RVD response. Thus, it is concluded that AQP3 water channels serve as an essential pathway for volume-regulatory water transport in, human epithelial cells.     

Gill morphology during hypercapnia in brown bullhead (Ictalurus nebulosus): role of chloride cells and pavement cells in acid-base regulation

The role of gill chloride cells (CCs) and pavement cells (PVCs) in acid-base regulation was evaluated in brown bullhead catfish (Ictalurus nebulosus) subjected to acute hypercapnia (water Pco₂=15 torr). Chronic (10 day) cortisol treatment was used as a tool to cause CC proliferation to permit a comparison of the regulatory capacities in groups of fish with widely different gill CC populations. Cortisol (4mg kg^?1 day^?1) caused a pronounced increase (170%) in the surface area of CCs exposed to the water based on scanning and transmission electron microscope analysis. The density of PVC apical membrane microvilli was significantly increased (20%) by cortisol treatment. Exposure of either group of fish to hypercapnia caused similar changes in gill epithelial morphology including: (i) a marked reduction in the surface area of exposed CCs (52 and 78% reduction in the control and cortisol-treated fish, respectively); and (ii) pronounced increases in PVC apical membrane microvilli density (21 and 27% in the control and cortisol-treated fish, respectively). The rates of Cl^? uptake (J_incl^?) and Na⁺ uptake (J_inNa⁺) were elevated (150 and 262%, respectively) in the cortisol-treated fish. Regardless of treatment, J_incl^? was markedly reduced to approximately the same levels after 6 h of hypercapnia, J_inNa⁺ was stimulated in the control group and reduced in the cortisol-treated group and thus, after 6 h of hypercapnia, J_inNa⁺ was equal in each group. The similar morphological responses in fish possessing different initial populations suggests that the predominant mechanism of acid-base regulation during hypercapnia, reduction of C1^?/HCO₃^? exchange, is accomplished by removal of the CC-associated C1-/HCO₃^? exchange sites from the water. The increase in PVC microvilli density during hypercapnia suggests a role for the PVC in acid-base regulation.     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science     

Structural and functional processes during water vapour uptake and desiccation in selected lichens with green algal photobionts

Structural alterations of the photobiont and mycobiont cells of lichens have been related to CO₂-gas exchange during experiments involving water vapour uptake and desiccation of liquid-water-saturated thalli. Increasing water vapour uptake of air dry lichens led to a gradual unfolding of the photobiont cells in Lobaria pulmonaria, Pseudevernia furfuracea, Ramalina maciformis and Teloschistes lacunosus as studied by low-temperature scanning electron microscopy. The data indicated that globular, probably turgid, cells and also slightly infolded or even heavily collapsed cells contributed to positive net photosynthesis, which was reached after water vapour uptake by the four species studied. During desiccation of fully water-saturated thalli of L. pulmonaria, extrathalline water films gradually evaporated before maximum values of CO₂-gas exchange were measured and before photobiont cells started to shrivel. In contrast, in P. furfuracea the CO₂-gas exchange maximum was reached when a considerable percentage of photobiont cells had already collapsed and while other parts of the thalli were still covered with liquid water. Further desiccation led to cavitation of the cortical cells in both species, this occurring at water contents at which net photosynthesis was still positive.Abbreviations EF exoplasmic fracture face - LTSEM low-temperature scanning electron microscopy - NP net photosynthesis - PAR photosynthetic active radiation (400–700 nm) - PF plasmic fracture face We thank D. Pichier, P. Hatvani, H. Müller, Birmensdorf, and J.B. Winkler, Kiel, for technical assistance, and J. Innes, Birmensdorf, for correcting the English text. Stimulating discussion with R. Honegger (Institut für Pflanzenbiologie, Universität Zürich, Switzerland), L. Kappen (Botanisches Institut, Universität Kiel, Germany), T.G.A. Green (Department of Biological Sciences, Hamilton, New Zealand), and O.L. Lange (Julius-von-Sachs-Institut für Biowissenschaften, Universität Würzburg, Germany) are gratefully acknowledged.     

Herbivore perception decreases photosynthetic carbon assimilation and reduces stomatal conductance by engaging 12‐oxo‐phytodienoic acid,mitogen‐activated protein kinase 4 and cytokinin perception

Herbivory‐induced changes in photosynthesis have been documented in many plant species; however, the complexity of photosynthetic regulation and analysis has thwarted progress in understanding the mechanism involved, particularly those elicited by herbivore‐specific elicitors. Here, we analysed the early photosynthetic gas exchange responses in Nicotiana attenuata plants after wounding and elicitation with Manduca sexta oral secretions and the pathways regulating these responses. Elicitation with M. sexta oral secretions rapidly decreased photosynthetic carbon assimilation (A_C) in treated and systemic (untreated, vascularly connected) leaves, which were associated with changes in stomatal conductance, rather than with changes in Rubisco activity and 1‐5 ribulose‐1,5‐bisphosphate turnover. Phytohormone profiling and gas exchange analysis of oral secretion‐elicited transgenic plants altered in phytohormone regulation, biosynthesis and perception, combined with micrografting techniques, revealed that the local photosynthetic responses were mediated by 12‐oxo‐phytodienoic acid, while the systemic responses involved interactions among jasmonates, cytokinins and abscisic acid signalling mediated by mitogen‐activated protein kinase 4. The analysis also revealed a role for cytokinins interacting with mitogen‐activated protein kinase 4 in CO₂‐mediated stomatal regulation. Hence, oral secretions, while eliciting jasmonic acid‐mediated defence responses, also elicit 12‐oxo‐phytodienoic acid‐mediated changes in stomatal conductance and A_C, an observation illustrating the complexity and economy of the signalling that regulates defence and carbon assimilation pathways in response to herbivore attack.     

Water exchange of flexible-shelled eggs and its effect on hatchling traits in the Chinese skink,<Emphasis Type="Italic"> Eumeces chinensis</Emphasis>

Flexible-shelled eggs of the Chinese skink (Eumeces chinensis) were used to determine the routes of water exchange with the environment and the effects of water exchange on hatchling traits. Eggs were incubated under three different hydric conditions at 28 °C. One group (ground eggs) was half-buried in incubation substrate. The second group (upper eggs) rested on the ground eggs but did not contact the substrate directly, and the third group was placed on a plastic platform above the substrate. Ground and upper eggs increased in mass and hatched successfully, while all eggs resting on the plastic platform lost mass and died within a week. Hence, E. chinensis eggs cannot sustain water balance through exchange of water vapor in air alone, but can absorb water from the substrate and other eggs and develop well. Water gained by upper eggs was only 20% of that absorbed by ground eggs, but hatchlings from the two treatments did not differ in morphology or locomotor performance. Therefore, the degree of water exchange of E. chinesis eggs has little effect on embryonic development and hatchling traits. The insensitivity of the eggs to variations in water exchange may be an adaptation against extreme hydric conditions in a species that deposits its eggs close to the soil surface.Communicated by I.D. Hume     

Effect of dDAVP on basolateral cell surface water permeability in the outer medullary collecting duct

We report a novel approach for assessing the volume of living cells which allows quantitative, high-resolution characterization of dynamic changes in cell volume while retaining the cell functionality. The aim of this study was to evaluate the short-term effect of vasopressin on basolateral cell surface water permeability in the outer medullary collecting duct (OMCD). The permeability of the basolateral cell membrane was determined in the tubules where the apical membrane was blocked with oil injected into the lumen. The apparent coefficient of water permeability (P _f) was evaluated by measuring the cell swelling after the step from hypertonic to isotonic medium (600 mosm to 300 mosm). Desmopressin (dDAVP) induced an increase of the basolateral P _f from 113.7±8.5 μm/s in control cells to 186.6±11.4 μm/s in micro-dissected fragments of the OMCD incubated in vitro (10⁻⁷ M dDAVP, 30 min at 37 °C) (P<0.05). Mercury caused pronounced inhibition of basolateral water permeability (26.0±6.9 μm/s; P<0.05). The effect of mercury (1.0 mM HgCl₂) was reversible: after washing the fragments with PBS for 20 min, P _f values were restored to the control levels (125.0±9.5 μm/s). The results of the study indicate the existence of a mechanism controlling the osmotic water permeability of the basolateral cell membrane in the OMCD epithelium.     

Water vapor uptake and photosynthesis of lichens: performance differences in species with green and blue-green algae as phycobionts

Summary Dry lichen thalli were enclosed in gas exchange chambers and treated with an air stream of high relative humidity (96.5 to near 100%) until water potential equilibrium was reached with the surrounding air (i.e., no further increase of weight through water vapor uptake). They were then sprayed with liquid water. The treatment took place in the dark and was interrupted by short periods of light. CO₂ exchange during light and dark respiration was monitored continuously. With no exception water uptake in all of the lichen species with green algae as phycobionts lead to reactivation of the photosynthetic metabolism. Further-more, high rates of CO₂ assimilation were attained without the application of liquid water. To date 73 species with different types of Chlorophyceae phycobionts have been tested in this and other studies. In contrast, hydration through high air humidity alone failed to stimulate positive net photosynthesis in any of the lichens with blue-green algae (Cyanobacteria). These required liquid water for CO₂ assimilation. So far 33 species have been investigated, and all have behaved similarly. These have included gelatinous as well as heteromerous species, most with Nostoc phycobionts but in addition some with three other Cyanophyceae phycobionts. The same phycobiont performance differences existed even within the same genus (e.g. Lobaria, Peltigera) between species pairs containing green or blue-green phycobionts respectively. Free living algae also seem to behave in a similar manner. Carbon isotope ratios of the lichen thalli suggest that a definite ecological difference exists in water status-dependent photosynthesis of species with green and blue-green phycobionts. The underlying biochemical or biophysical mechanisms are not yet understood. Apparently, a fundamental difference in the structure of the two groups of algae is involved.     

Environmental effects on CO2 efflux from riparian tundra in the northern foothills of the Brooks Range,Alaska, USA

Summary Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO₂ m^–2 day^–1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO₂ efflux in this habitat. In parallel with the field measurements of CO₂ efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO₂ efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO₂ efflux and carbon balance.     

Fluorescent measurements of intracellular free calcium in isolated toad urinary bladder epithelial cells

Summary Sodium-calcium exchange has been suggested to play a pivotal role in the regulation of cytosolic free calcium (Ca _f ) by epithelial cells. Using isolated epithelial cells from the toad urinary bladder, Ca _f has been measured using the intracellular Casensitive fluorescent dyes Fura 2 and Quin. 2. Dye loading did not alter cell viability as assessed by measurements of ATP and ADP content or cell oxygen consumption. When basal Ca _f was examined over a wide range of cell dye content (from 0.04 to 180 nmol dye/mg protein) an inverse relationship was observed. At low dye content, Ca _f was 300–380 nM and, as dye content was increased, Ca _f progressively fell to 60 nM. Using low dye content cells, in which minimal alteration in Ca steady state would be expected, the role for plasma membrane Na–Ca exchange was examined using either medium sodium substitution or ouabain. While medium sodium substitution increased Ca _f , prolonged treatment with ouabain had no effect on Ca _f despite a clear increase in cell sodium content. The lack of effect of ouabain suggests that Na–Ca exchange-mediated Ca efflux plays a minimal role in the regulation of basal Ca _f . However, exchange-mediated Ca efflux may play a role in Ca _f regulation when cytosolic calcium is elevated.     

Effect of water restriction on energy and water balance and osmoregulation of the fruit bat Rousettus aegyptiacus

The energy budget, water balance and osmoregulation of the fruit bat, Rousettus aegyptiacus, were studied during normal hydration and during water restriction (oven-dried apple diet). The water input and output were balanced during both normal hydration and water restriction. The kidney of the fruit bat is well adapted to handle the water load from its fruit diet by excreting large volumes (14% of the body mass per day) of dilute urine (113±25 mosmol·kg H₂P^-1) as well as reducing urine volume (-95%) and increasing urine osmotic concentration (555±280 mosmol·kg H₂O^-1) during water restriction. The haematocrit, plasma haemoglobin and total protein concentrations did not increase during water restriction and heat exposure, suggesting the conservation of plasma volume. Gross energy intake was not alfected by water restriction. However, digested energy intake and digestibility were significantly reduced. The effective regulation of energy and water budgets during water restriction suggests that the fruit bat can cope with seasonal climatic changes and with variable fruit supply during various seasons.Abbreviations BM body mass - DEI digested energy intake - EWL evaporative water loss - GEL gross energy intake - NH normal hydration - T _a ambient temperature - WR water restriction     

Growth and gas exchange response to water shortage of a maize crop on different soil types

The effect of water shortage on growth and gas exchange of maize grown on sandy soil (SS) and clay soil was studied. The lower soil water content in the SS during vegetative growth stages did not affect plant height, above-ground biomass, and leaf area index (LAI). LAI reduction was observed on the SS during the reproductive stage due to early leaf senescence. Canopy and leaf gas exchanges, measured by eddy correlation technique and by a portable photosynthetic system, respectively, were affected by water stress and a greater reduction in net photosynthetic rate (A _N) and stomatal conductance (g _s) was observed on SS. Chlorophyll and carotenoids content was not affected by water shortage in either condition. Results support two main conclusions: (1) leaf photosynthetic capacity was unaffected by water stress, and (2) maize effectively endured water shortage during the vegetative growth stage.     

Effects of water stress on gas exchange of field grown <Emphasis Type="Italic">Zea mays</Emphasis> L. in Southern Italy: an analysis at canopy and leaf level

Zea mays is cultivated in the Mediterranean regions where summer drought may lead to photoinhibition when irrigation is not available. In this work the response of maize to water stress was evaluated by gas exchange measurements at the canopy and leaf level. Leaf gas exchange was assessed before, during and after water stress, while canopy turbulent fluxes of mass and energy were performed on a continuous basis. In the early growth period, a linear increment of net ecosystem photosynthetic rate (P _NE) to incoming of photosynthetic photon flux density (PPFD) was found and net leaf photosynthetic rate (P _NL) showed the tendency to saturate under high irradiance. During water stress, the relationship between P _NE and PPFD became curvilinear and both P _NE and P _NL saturated in a range between 1,000 and 1,500 μmol (photons) m⁻² s⁻¹. Leaf water potential (ψ_l) dropped from −1.50 to −1.88 MPa during water stress, indicating that leaf and canopy gas exchanges were limited by stomatal conductance. With the restoration of irrigation, P _NE, P _NL and ψ_l showed a recovery, and P _NE and P _NL reached the highest values of whole study period. Leaf area index (LAI) reached a value of 3.0 m² m⁻². The relationship between P _NE and PPFD remained curvilinear and P _NE values were lower than those of a typical well-irrigated maize crop. The recovery in P _NE and P _NL after stress, and ψ_l values during stress indicate that the photosynthetic apparatus was not damaged while soil moisture stress after-effects resulted in a sub-optimal LAI values, which in turn depressed P _NE.     

Water exchange in manganese-based water-oxidizing catalysts in photosynthetic systems: From the water-oxidizing complex in photosystem II to nano-sized manganese oxides

The water-oxidizing complex (WOC), also known as the oxygen-evolving complex (OEC), of photosystem II in oxygenic photosynthetic organisms efficiently catalyzes water oxidation. It is, therefore, responsible for the presence of oxygen in the Earth's atmosphere. The WOC is a manganese–calcium (Mn₄CaO₅(H₂O)₄) cluster housed in a protein complex. In this review, we focus on water exchange chemistry of metal hydrates and discuss the mechanisms and factors affecting this chemical process. Further, water exchange rates for both the biological cofactor and synthetic manganese water splitting are discussed. The importance of fully unveiling the water exchange mechanism to understand the chemistry of water oxidation is also emphasized here. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

The correlation between crassulacean acid metabolism and water uptake in Senecio medley-woodii

The combination of a chamber for CO₂ gas exchange with a potometric measuring arrangement allowed concomitant investigations into CO₂ gas exchange, transpiration and water uptake by the roots of whole plants of Senecio medley-woodii, a species which exhibits Crassulacean acid metabolism. The water-uptake rate showed the same daily pattern as malate concentration and osmotic potential. The accumulation of organic acids resulting from nocturnal CO₂ fixation enhanced the water-uptake rate from dusk to dawn. During the day the water-uptake rates decreased with decreasing organic-acid concentration. With gradually increasing water stress, CO₂ dark fixation of S. medley-woodii was increased as long as water could be taken up by the roots. It was also shown that a reestablished water supply after drought caused a similar increase which in both cases ameliorated the water uptake in order to conserve a positive water balance for as long as possible. This water-uptake pattern shows that Crassulacean acid metabolism is not only a water-saving adaptation but also enhances water uptake and is directly correlated with the amelioration of the plant water status.Abbreviation CAM Crassulacean acid metabolism     

Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T _1/2=1 to 27 s were found. The volumetric clastic modulus () of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10^-6 cm s^-1 bar^-1. For cortical cells P=5 to 11 bar, T _1/2=0.3 to 1 s, Lp=0.4 to 9·10^-5 cm s^-1 bar^-1 and =6 to 215 bar. The T _1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T _1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T _1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not , but the reason for the change in cell permeability to water is not known.In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T _1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423–429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h^-1 and 5% h^-1, respectively. Thus, although the T _1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.Abbreviations P turgor pressure - T _1/2 half-time of water exchange of individual cell - Lp hydraulic conductivity - volumetric elastic modulus - t _1/2 average half-time of water exchange of tissue     

Responses in metabolic rate to changes in temperature in diapausing Colorado potato beetle Leptinotarsa decemlineata from three European populations

Many insects survive adverse periods in seasonal environments by entering diapause, a deep resting stage, during which energy consumption is typically low and gas exchange is in the form of a discontinuous gas exchange cycle (DGC). Because insects in high‐latitude environments are severely time constrained during summer, an effective diapause termination with careful regulation of metabolic rate is important. The present study examines whether diapausing Colorado potato beetles Leptinotarsa decemlineata Say originating from three latitudinally different regions in Europe differ in their quantitative or qualitative gas exchange patterns in response to an increasing temperature. Overall production of gaseous CO₂, as well as qualitative patterns relating to the DGC, are measured at a late stage of diapause at four different temperatures in increasing order from 13, 18, 23 to 28 °C. Overall CO₂ production is found to be lower in the two northern populations (61°49′N and 55°75′N) compared with the southernmost population (45°48′N) but increases as a function of temperature in all populations in a similar way. However, in the northern populations, raising the temperature increases the amount of CO₂ discharged during single DGC peaks, whereas the DGC frequency remains relatively unchanged. By contrast, in the southernmost population, the amount of CO₂ discharged during individual DGC peaks remains relatively unchanged, whereas the DGC frequency increases as a function of temperature. The observed differences may relate to water retention benefits or, alternatively, energetic benefits relating to heightened gas exchange efficiency in hypoxic or hypercapnic environments. Overall, the results suggest that, although populations of L. decemlineata may have similar thermal sensitivities, they have different qualitative strategies to regulate metabolic re‐activation at diapause termination.     

Response of photosynthesis and respiration of resurrection plants to desiccation and rehydration

Using non-invasive techniques (CO₂ gas exchange, light scattering, light absorption, chlorophyll fluorescence, chlorophyll luminescence), we have analysed the response of respiration and photosynthesis to dehydration and rehydration of leaves of the resurrection plants Craterostigma plantagineum Hochst., Ramonda mykoni Reichb. and Ceterach officinarum Lam. et DC. and of the drought-sensitive mesophyte spinach (Spinacia oleracea L.). The following observations were made: (i) The rate of water loss during wilting of detached leaves of drought-tolerant resurrection plants was similar to that for leaves of the sensitive mesophyte, spinach. Leaves of Mediterranean xerophytes lost water much more slowly. (ii) Below a residual water content of about 20%, leaves of spinach did not recover turgor on rewatering, whereas leaves of the resurrection plants did. (iii) Respiration was less sensitive to the loss of water during wilting in the resurrection plants than in spinach. (iv) The sensitivity of photosynthesis to dehydration was similar in spinach and the resurrection plants. Up to a water loss of 50% from the leaves, photosynthesis was limited by stomatal closure, not by inhibition of reactions of the photosynthetic apparatus. Photosynthesis was inhibited and stomates reopened when loss of water became excessive. (v) After the leaves had lost 80% of their water or more, the light-dependent reactions of photosynthetic membranes were further inhibited by rewatering in spinach; they recovered in the resurrection plants. (vi) In desiccated leaves of the resurrection plants, slow rehydration reactivated mitochondrial gas exchange faster than photosynthetic membrane reactions. Photosynthetic carbon assimilation recovered only slowly.