Determination of leaf fresh mass after storage between moist paper towels: constraints and reliability of the method

To ensure comparability among leaf fresh mass measurements it is important to handle the leaves in a standardized manner. In the present work constraints of a commonly used method to achieve full turgor, storage between damp paper towels, were investigated. After overnight rehydration in a saturated atmosphere, the fresh mass of leaves of 14 species was measured, and the leaves were stored between paper towels (two treatments: moist and wet) at 4 degrees C. Their mass was measured after 24, 48, and 72 h. Leaf fresh mass increased during the first 24 h of storage between moist paper towels by an average of 1.8%, between wet towels by 3.3%. Among the species, the increase of leaf fresh mass between moist towels correlated with the species' desiccation propensity, indicating that it was rehydration from water loss during initial handling. On the other hand, between wet towels the fresh mass increase was associated with the species' leaf tissue structure, and it continued to increase beyond 24 h, indicating that the increase was a result of water penetration into the leaf air spaces. It is concluded that storage between moist paper towels results in reliable values of leaf fresh mass, and that desiccated leaves rehydrate well between moist towels. However, care has to be taken to avoid too wet conditions as they may lead to erroneously high fresh mass values, especially in species with large air spaces. Furthermore, exposure to unsaturated atmospheric conditions during handling has to be minimized.     

Biofilm functional responses to the rehydration of a dry intermittent stream

Intermittent water flow regimes characterize streams in many world regions, especially those with arid and semiarid climates. During cease to flow conditions, biofilms on streambed sediments may be exposed to desiccation. Environmental conditions and resource availability change with desiccation and may influence biofilm functioning and whole stream ecosystem processes. Rainfall events during the nonflow phase can rehydrate streambed sediments, but the effect of these pulses on biofilm functioning is unclear. This study aimed to analyze the effects of a rehydration event on biofilm functional diversity during the nonflow period in a subtropical Australian stream. Biofilms from three different stream pools on the same reach; one permanently water-covered and the other two differing in their desiccation time were studied. Biofilms initially differed owing to the time they were exposed to dry conditions but rehydration events significantly increased biofilm functional diversity, producing a “reset” effect on the desiccation exposure, as after that bacterial functioning decreased again because of the new dry conditions. The observed rapid biofilm responses to rehydration during flow intermittency might be essential in sustaining biofilm functional diversity in intermittent streams.     

Skin resistance to water gain and loss has changed in cane toads (Rhinella marina) during their Australian invasion

The water‐permeable skin of amphibians renders them highly sensitive to climatic conditions, and interspecific correlations between environmental moisture levels and rates of water exchange across the skin suggest that natural selection adapts hydroregulatory mechanisms to local challenges. How quickly can such mechanisms shift when a species encounters novel moisture regimes? Cutaneous resistance to water loss and gain in wild‐caught cane toads (Rhinella marina) from Brazil, USA (Hawai''i) and Australia exhibited strong geographic variation. Cutaneous resistance was low in native‐range (Brazilian) toads and in Hawai''ian populations (where toads were introduced in 1932) but significantly higher in toads from eastern Australia (where toads were introduced in 1935). Toads from recently invaded areas in western Australia exhibited cutaneous resistance to water loss similar to the native‐range populations, possibly because toads are restricted to moist sites within this highly arid landscape. Rates of rehydration exhibited significant but less extreme geographic variation, being higher in the native range than in invaded regions. Thus, in less than a century, cane toads invading areas that impose different climatic challenges have diverged in the capacity for hydroregulation.     

Desiccation and rehydration dynamics in the epiphytic resurrection fern Pleopeltis polypodioides

The epiphytic resurrection—or desiccation-tolerant (DT)—fern Pleopeltis polypodioides can survive extreme desiccation and recover physiological activity within hours of rehydration. Yet, how epiphytic DT ferns coordinate between deterioration and recovery of their hydraulic and photosynthetic systems remains poorly understood. We examined the functional status of the leaf vascular system, chlorophyll fluorescence, and photosynthetic rate during desiccation and rehydration of P. polypodioides. Xylem tracheids in the stipe embolized within 3–4 h during dehydration. When the leaf and rhizome received water, tracheids refilled after ∼24 h, which occurred along with dramatic structural changes in the stele. Photosynthetic rate and chlorophyll fluorescence recovered to predesiccation values within 12 h of rehydration, regardless of whether fronds were connected to their rhizome. Our data show that the epiphytic DT fern P. polypodioides can utilize foliar water uptake to rehydrate the leaf mesophyll and recover photosynthesis despite a broken hydraulic connection to the rhizome.     

Effects of desiccation and rehydration on nitrogen fixation by epiphylls in a tropical rainforest

Nitrogen fixation rates by epiphyllous microorganisms are affected by desiccation. Rates from leaf samples which had been dried for 12 h were 0.66 ng N/10 cm²/h. In contrast, rates from leaves which had been kept continuously wet were 18.69 ng N/10 cm²/h. Leaf samples which had been rehydrated for 2 and 4 h showed intermediate fixation rates (4.26 and 9.76 ng N/10 cm²/h, respectively). Epiphyllous bryophytes maintain moist conditions on the leaf surface and thus create a microenvironment suitable for prolonged fixation by the microorganisms.     

Survival of Cowpea Rhizobia in Soil as Affected by Soil Temperature and Moisture

Successful inoculation of peanuts and cowpeas depends on the survival of rhizobia in soils which fluctuate between wide temperature and moisture extremes. Survival of two cowpea rhizobial strains (TAL309 and 3281) and two peanut rhizobial strains (T-1 and 201) was measured in two soils under three moisture conditions (air-dry, moist (−0.33 bar), and saturated soil) and at two temperatures (25 and 35°C) when soil was not sterilized and at 40°C when soil was sterilized. Populations of rhizobia were measured periodically for 45 days. The results in nonsterilized soil indicated that strain 201 survived relatively well under all environmental conditions. The 35°C temperature in conjunction with the air-dry or saturated soil was the most detrimental to survival. At this temperature, the numbers of strains T-1, TAL309, and 3281 decreased about 2 logs in dry soil and 2.5 logs in saturated soil during 45 days of incubation. In sterilized soil, the populations of all strains in moist soil increased during the first 2 weeks, but decreased rapidly when incubated under dry conditions. The populations did not decline under saturated soil conditions. From these results it appears that rhizobial strains to be used for inoculant production should be screened under simulated field conditions for enhanced survival before their selection for commercial inoculant production.     

Blood volume and total water content of the woodlouse, Oniscus asellus, in conditions of hydration and desiccation

Changes in the blood volume and total water content of Oniscus asellus were measured by means of inulin dilution and weight. The relationship between body weight and blood volume, total water content, and weight loss does not change during desiccation. The process of desiccation is similar in 0% r.h. and 62% r.h., although the rates of reduction of blood volume, total water content and weight are different in these two humidities. During desiccation most of the water lost by the animal comes from the blood. There is no evidence to suggest that O. asellus takes up an excess of water in moist conditions.     

Reactive oxygen species metabolism in desiccation-stressed thalli of the liverwort Dumortiera hirsuta

Rates of extracellular superoxide radical formation were estimated in the liverwort Dumortiera hirsuta (SW) Nees. Initial experiments showed that D. hirsuta produced extracellular superoxide at high rates, even when unstressed, and that production increased considerably during rehydration following mild desiccation stress. Experiments in which desiccation was artificially induced using polyethylene glycol showed that superoxide was produced during rehydration rather than desiccation. Incubation of plants in water induced the release about 23% of the superoxide-producing activity, although most appeared to be tightly bound to the cell surface. Experiments with metabolic inhibitors indicated that superoxide production was insensitive to the flavoprotein inhibitor DPI, but inhibited by cyanide, suggesting that peroxidases may produce the superoxide. Despite producing large amounts of superoxide, D. hirsuta only produced small quantities of hydrogen peroxide, the natural product of superoxide dismutation. However, experiments showed that D. hirsuta can reduce the concentration of exogenously supplied hydrogen peroxide from 50 µ M to zero within 1 h, suggesting that any hydrogen peroxide produced is rapidly metabolized. The physiological significance of superoxide production is discussed, with special reference to its possible role as a defence against pathogenic fungi and bacteria.     

A case study of the influence of local weather on Aedes aegypti (L.) aging and mortality

The survival rate of mosquitoes is an important topic that affects many aspects of decision‐making in mosquito management. This study aims to estimate the variability in the survival rate of Ae. aegypti, and climate factors that are related to such variability. It is generally assumed that the daily probability of mosquito survival is independent of natural environment conditions and age. To test this assumption, a three‐year fieldwork (2005–2007) and experimental study was conducted at Fortaleza‐CE in Brazil with the aim of estimating daily survival rates of the dengue vector Aedes aegypti under natural conditions in an urban city. Survival rates of mosquitoes may be age‐dependent and statistical analysis is a sensitive approach for comparing patterns of mosquito survival. We studied whether weather conditions occurring on a particular day influence the mortality observed on that particular day. We therefore focused on the impact of daily meteorological fluctuations around a given climate average, rather than on the influence of climate itself. With regard to survival time, multivariate analyses using the stepwise logistic regression model, adjusted for daily temperature, relative humidity, and saturated vapor pressure deficit (SVPD), suggest that age, the seasonal factor, and the SVPD were the most dependent mortality factors. Similar results were obtained using the Cox proportional hazard model, which explores the relationships between the survival and explanatory variables.     

Cold tolerance and dehydration in Enchytraeidae from Svalbard

When cooled in contact with moisture, eight species of arctic Enchytraeidae from Svalbard were killed by freezing within minutes or hours at −3 and −5 °C; an exception was Enchytraeus kincaidi which survived for up to 2 days. When the temperature approached 0 °C the enchytraeids apparently tried to escape from the moist soil. The supercooling capacity of the enchytraeids was relatively low, with mean supercooling points of −5 to −8 °C. In contrast, specimens of several species were extracted from soil cores that had been frozen in their intact state at −15 °C for up to 71 days. Compared to freezing in a moist environment, higher survival rates were obtained during cooling at freezing temperatures in dry soil. Survival was recorded in species kept at −3 °C for up to 35 days, and in some species kept at −6 °C for up to 17 days. Slow warming greatly increased survival rates at −6 °C . The results strongly suggest that arctic enchytraeids avoid freezing by dehydration at subzero temperatures. In agreement with this, weight losses of up to ca. 42% of fresh weight were recorded in Mesenchytraeus spp. and of up to 55% in Enchytraeus kincaidi at water vapour pressures above ice at −3 to −6 °C. All specimens survived dehydration under these conditions. Accepted: 12 December 1997     

Responses to Drying of Recalcitrant Seeds of Quercus nigra L.

It has been suggested that rate of desiccation can influencethe expression of recalcitrant behaviour in seeds, thus complicatingthe task of determining which seeds are truly recalcitrant.The objective of this study was to see if variable rates ofdesiccation influenced such behaviour inQuercus nigra L., atree seed known to be recalcitrant. Acorn moisture content, leachate conductivity, and germinationwere determined at various times during desiccation at threerates at 27 and 40°C. Moisture contents and germinationdecreased as the severity of desiccation increased. Leachateconductivity increased slightly but was not a sensitive indicatorof loss of viability. The critical (lethal) moisture contentfor these acorns was 10–15%, although rehydration within48 h of reaching this level prevented death in about 25% ofthe acorns. At 27°C any desiccation treatment that producedlosses of 30–50mg of moisture per g of acorn dry weightper day should be suitable as a test for recalcitrance in thegenus. Apparent physiological or fungal damage at 40°C rulesout the higher temperature for such a test. Quercus nigra L.; desiccation rate; temperature; recalcitrant; leachate conductivity; germination; viability     

Ultrastructural changes in the pyrenoid of the lichenParmelia sulcata stored under controlled conditions

Summary Observations have been made on ultrastructural changes in pyrenoids of the algal cells in the lichenParmelia sulcata subjected to a range of controlled desiccation and rehydration regimes. Weaker staining of parts of the proteinaceous pyrenoid matrix, interpreted as damage, occurred within 48 hours of transfering moist thalli to 53% r.h. in a 12 hours light 12 hours dark (20°C 14°C) regime. No specific damage resulted from direct dehydration at 0% r.h. or on transfer after 48 hours from 53% r.h. to 0% r.h. Material stored dry under different conditions,e.g., 0%, 53% or 53% to 0% r.h., for 72 hours showed recovery of the pyrenoid matrix when rehydrated in water (30 minutes) or by storage at 100% r.h. for 24 hours. After storage at 53% r.h. or 0% r.h. for 3 months, damage was more extreme, being greater in material originally dried at 53% r.h. Recovery in water, to the original appearance, only occurred after rehydration for 24 hours at 100% r.h. After 3 months desiccation, damage, due to the initial 48 hours at 53% r.h. was still apparent. Severe damage involved expansion of the pyrenoid. As rehydration restored pyrenoids to their original dimensions, pyrenoid proteins probably became dispersed rather than degraded during desiccation.     

Cellular expansion at low temperature as a cause of membrane lesions

Rates of solute leakage from excised discs of cucumber (Cucumis sativus L. cv Straight Eight) cotyledons were altered by temperature during plasmolysis in the manner of a simple diffusion phenomenon; the log of the leakage rate increased in proportion to the temperature. During deplasmolysis, however, leakage rates responded to temperature with a very different pattern: chilling conditions (below about 20°C) caused large increases in leakage rates, indicating disruption of membrane integrity in the tissues. The time course of restoration of normal leakage rates after deplasmolysis/chilling damage indicated a rapid repair of the lesions. A similar sensitivity to low temperatures was found during rehydration after leaf desiccation, with low temperatures again causing high leakage rates. It is suggested that low temperatures interfere with membrane expansion, possibly by lowering elasticity and hindering the incorporation of lipid material into the expanding membrane. The expansion of tissues at low temperatures may cause lesions in cellular membranes, contributing to chilling injury.     

Acetylene reduction byDaviesia mimosoides under Eucalyptus

Summary Daviesia mimosoides is a common understorey legume in Eucalyptus forests of the Brindabella Range in southeastern Australia, capable of fixing atmospheric nitrogen. Rates of N fixation were measured by the acetylene-reduction technique over a growing season in the field. Pot trials under controlled conditions were also carried out to elucidate effects of soil moisture, temperature, and light. Average rates in the field varied from about 1–5 μ mol C₂H₄/g/h (wet weight of nodule), but rates up to 14 μ mol C₂H₄/g/h were measured in optimum controlled conditions. Annual N-fixation rates approximate 4.5–7.0 kg/ha. In pot trials, rate of acetylene reduction decreased with soil moisture to about−10 MPa tension, with a marked depression at about−6 MPa, but within the normal field range of soil moisture there was little correlation of moisture with average acetylene reduction rate. Rates were similar in the temperature range of 20–30°C, but were depressed by either low or high temperature (<10 or >30°C). Diurnal fluctuations in acetylene reduction rates were not correlated with solar radiation, but rates were limited by high mid-day temperatures.     

Soil denitrification in three cropping systems characterized by differences in nitrogen and carbon supply

Rates of denitrification measured over a growing season, both within and between rows of plants, in grass and lucerne leys and in barley plots were related to soil moisture and nitrate levels using bivariate, nonlinear regression models. Both within and between rows in all three crops, moisture explained a significant fraction of the variation in denitrification rates when the rates were regressed as increasing exponentially with increasing soil water content under moist conditions. Only soil moisture explained a significant fraction of the variation of the denitrification rates measured in the soil cores taken in (19.5%) and between (46.3%) plant rows at the barley field and in (42.4%) the plant rows in the lucerne ley. In some treatments, the rates appeared to be unrelated to moisture below a critical moisture threshold.Nitrate proved to be a useful variable in predicting denitrification rates in the grass ley and between lucerne rows. Water and nitrate could explained 36.6% of the within-row variation and 24.2% of the between-row variation in the grass ley and 65% of the between-row sum of squares.An attempt to use the regression model based on data from one year for the grass ley to predict the losses during another year for the same crop was not successful.     

Behavioural flexibility allows an invasive vertebrate to survive in a semi-arid environment

Plasticity or evolution in behavioural responses are key attributes of successful animal invasions. In northern Australia, the invasive cane toad (Rhinella marina) recently invaded semi-arid regions. Here, cane toads endure repeated daily bouts of severe desiccation and thermal stress during the long dry season (April–October). We investigated whether cane toads have shifted their ancestral nocturnal rehydration behaviour to one that exploits water resources during the day. Such a shift in hydration behaviour could increase the fitness of individual toads by reducing exposure to desiccation and thermal stress suffered during the day even within terrestrial shelters. We used a novel method (acoustic tags) to monitor the daily hydration behaviour of 20 toads at two artificial reservoirs on Camfield station, Northern Territory. Remarkably, cane toads visited reservoirs to rehydrate during daylight hours, with peaks in activity between 9.00 and 17.00. This diurnal pattern of rehydration activity contrasts with nocturnal rehydration behaviour exhibited by adult toads in their native geographical range and more mesic parts of Australia. Our results demonstrate that cane toads phase shift a key behaviour to survive in a harsh semi-arid landscape. Behavioural phase shifts have rarely been reported in invasive species but could facilitate ongoing invasion success.     

Desiccation provides photosynthetic protection for crust cyanobacteria Microcoleus vaginatus from high temperature

As the dominant cyanobacterial species in biological soil crusts (BSCs), Microcoleus vaginatus often suffer from many stress conditions, such as desiccation and high temperature. In this study, the activities of light‐harvesting complexes (LHCs) and reaction centers of photosystem II (PS II) in crust cyanobacteria M. vaginatus were monitored under high temperature and desiccation conditions using chlorophyll fluorescence technology. The results showed that all the fluorescence signals were significantly inhibited by high temperature or desiccation treatments. Under high temperature conditions, it was further demonstrated that PS II reaction centers were first destructed within the first hour, then the LHCs gradually dissociated and free phycocyanin formed within 1–5 h, and the activities of all the light harvesting and reaction center pigment proteins were fully suppressed after 24 h of high temperature treatment. Furthermore, the high temperature treated M. vaginatus lost its ability to recover photosynthetic activity. On the contrary, although desiccation also led to the loss of photosynthetic activity in M. vaginatus, after rehydration in the light the fluorescence parameters including Fo, Fv and Fv/Fm could be well recovered within 12 h. It was concluded that desiccation could provide crust cyanobacteria M. vaginatus some protection from other stresses, such as high temperature demonstrated in this experiment. The combine of temperature change and precipitation pattern in the field provide a guarantee for the alternate metabolism and inactivity in crust cyanobacteria. That may be a very important strategy for the survival of crust cyanobacteria in high temperature regions.     

Desiccation tolerant plants as model systems to study redox regulation of protein thiols

While the majority of plants and animals succumb to water loss, desiccation tolerant organisms can lose almost all of their intracellular water and revive upon rehydration. Only about 300 ‘resurrection’ angiosperms and very few animals are desiccation tolerant. By contrast, many bryophytes and most lichens are desiccation tolerant and so are the seeds and pollen grains of most flowering plants. The current literature reveals that the extreme fluctuations in water content experienced by desiccation tolerant organisms are accompanied by equally extreme changes in cellular redox state. Strongly oxidizing conditions upon desiccation can cause irreversible oxidation of free cysteine residues of proteins, which can change protein structure and function, and contribute to protein denaturation. It appears likely that reversible formation of disulphide bonds, in particular through protein glutathionylation, contributes to the set of protection mechanisms that confer desiccation tolerance. Upon rehydration, de-glutathionylation can be catalyzed by glutaredoxins (GRXs) and protein disulphide bonds can be reduced through NADPH-dependent thioredoxins (TRXs). Due to their ability to survive severe oxidative stress, desiccation tolerant plants and seeds are excellent models to study protein redox regulation, which may provide tools for enhancing tolerance to drought and more generally, to oxidative stress, in crops.     

The effect of photoperiod length on respiration in leaves of Saxifraga cernua L., an arctic herb

Abstract Rates of oxygen uptake were measured in leaves of Saxifraga cernua which had been exposed to an 18-h photoperiod. These rates were compared to those in plants which had been exposed to continuous light. Rates of total dark respiration and alternative pathway respiration measured at the end of the photoperiod gradually decreased over the initial 3 d of exposure to an 18-h photoperiod. Thereafter, respiratory rates were constant. Rates of total dark respiration and alternative pathway respiration decreased during the 6h dark period. Rates of normal and alternative pathway respiration are equally affected during the dark period. The respiratory rates had reached a new minimum level 3 d after the initiation of a dark period. These results suggest that respiration rates in arctic plants are high because of the long photoperiod in the arctic. The kinetics of photoperiod induced changes in respiration are slow enough to suggest the involvement of the biological clock in setting respiration rates. Indeed, total dark respiration and alternative pathway respiration show a definite circadian rhythm. Free-running experiments show that normal respiration changes much less (has a smaller amplitude of variation) than alternative pathway respiration and that alternative pathway respiration accounts for most of the rhythmicity of respiration.     

Distinct contractile and cytoskeletal protein patterns in the Antarctic midge are elicited by desiccation and rehydration

Desiccation presents a major challenge for the Antarctic midge, Belgica antarctica. In this study, we use proteomic profiling to evaluate protein changes in the larvae elicited by dehydration and rehydration. Larvae were desiccated at 75% relative humidity (RH) for 12 h to achieve a body water loss of 35%, approximately half of the water that can be lost before the larvae succumb to dehydration. To evaluate the rehydration response, larvae were first desiccated, then rehydrated for 6 h at 100% RH and then in water for 6 h. Controls were held continuously at 100% RH. Protein analysis was performed using 2‐DE and nanoscale capillary LC/MS/MS. Twenty‐four identified proteins changed in abundance in response to desiccation: 16 were more abundant and 8 were less abundant; 84% of these proteins were contractile or cytoskeletal proteins. Thirteen rehydration‐regulated proteins were identified: 8 were more abundant and 5 were less abundant, and 69% of these proteins were also contractile or cytoskeletal proteins. Additional proteins responsive to desiccation and rehydration were involved in functions including stress responses, energy metabolism, protein synthesis, glucogenesis and membrane transport. We conclude that the major protein responses elicited by both desiccation and rehydration are linked to body contraction and cytoskeleton rearrangements.