Rates Of Desiccation Research Articles

Growth Process and Characteristics of Drying Cracks on the Soil Surface of Yangguanzhai Site Cemetery During Archaeological Excavations

ABSTRACT The primary challenge for the earthen sites of great historic and scientific value in China is attributed to drying cracks. We investigated 105 exposed partial-cave tombs in the Yangguanzhai Site Cemetery to understand drying cracks’ growth process and characteristics. Real-time monitoring and observations were further conducted toward one specific tomb during archaeological excavation for finer-grain records. Results show that these cracks grew rapidly on the soil surface during the early stage of the excavation and some then transformed to more severe deterioration, such as gullies and collapses. Intriguingly, most drying cracks found on the side surface grew initially from top areas and extended downward vertically and independently. By contrast, their counterparts on the bottom surface were curved and twisted with one another. The changes of the soil properties and water content in soil at different depths suggest that larger porosity and smaller-size soil particles facilitate faster water absorption and evaporation of superficial soil, whereas low rates of water evaporation and soil desiccation on the flat bottom surface slows down the formation of drying cracks. Further investigations and field experiments are the next steps in building complete growth models of drying cracks and fully understanding the underlying mechanism.

Effects of desiccation rate and inhibition of protein synthesis and the violaxanthin cycle on the rewetting recovery of Porella platyphylla

Following 5 different rates of 1-week desiccation to which Porella platyphylla (a desiccation-tolerant leafy liverwort) was subjected, physiological recovery upon rehydration (after 1 h, 24 h, 48 h) in light was monitored by cytological, photosynthetic (e-transport rate, photoprotective mechanisms), and other metabolic parameters using nuclear- and chloroplast-encoded protein synthesis inhibitors and a violaxanthin cycle inhibitor (DTT: dithiothreitol). Desiccation tolerance is mainly constitutive in P. platyphylla, allowing survival of rapid drying, and employs an active rehydration-induced repair and recovery mechanism. Following 24 hours of rehydration after the one-week dehydration in laboratory air (∼35% relative humidity /RH/), the chloroplasts had an irregular round shape. The recovery within 1 h of the thylakoid-function-related photosynthetic processes was extremely fast and independent of protein synthesis, while the overall recovery (except plants dried up at 76% RH, which depended on protein synthesis only to a limited extent) required de novo protein synthesis during rewetting. During the course of rehydration protein synthesis affects rehydrin expression, osmolite and general carbohydrate metabolism, the main repair and antioxidant mechanisms. Recovery was the best in samples dried up at 76% RH. Membrane permeability increased upon rehydration following 1-week dehydration. Normal membrane function recovered after 24 h (max. 48 h), except in plants that suffered drastic dehydration. Total protein content of plants dried under different conditions and of samples taken during recovery was generally lower than in the control plants at full turgor. The xanthophyll cycle has a great importance during the recovery in light; we confirmed the existence of a greater zeaxanthin-dependent and a smaller zeaxanthin-insensitive NPQ (nonphotochemical quenching). Plants dried under more favorable conditions had better light protection. The highest values of the osmotic potential belonged to the plants subjected to intense dehydration. Total soluble carbohydrate content was well-balanced and seemed to be unchanged following the different treatments. There was a significant increase in proline, while total fructan showed lower values at different degrees of 1-week desiccation. The cytoplasmic protein synthesis is likely to be involved in the change of the fructan content during rehydration.

Pathogenic Dynamics During Colony Ontogeny Reinforce Potential Drivers of Termite Eusociality: Mate Assistance and Biparental Care

As an ecologically dominant taxon, termites appear to be resilient to environmental stressors. However, swarming alates (winged-individuals) encounter a myriad of environmental pressures that drastically reduce the probability of colony foundation. Dispersing alates face high rates of predation, desiccation, nitrogen limitation, and risks of infection, among others. We propose that alates benefit from mate assistance and biparental care to overcome some of these challenges. We assessed whether the bacteria, Serratia marcescens (an ecologically relevant, gram-negative, facultative termite pathogen), negatively affected the growth of newly founded termite colonies. Additionally, we revealed the significance of the king’s presence in improving successful establishment of incipient colonies. Virgin queens of the dampwood termite, Zootermopsis angusticollis, were subjected to one of four treatments: naive (untreated), or injections with either sterile saline, heat-killed S. marcescens, or a sublethal does of live S. marcescens. Each queen was then paired with a naive, virgin king. The incipient colonies underwent censuses every four days for 80 days. We estimated survival rates and compared the onset of oviposition and hatching, overall egg production and larval hatching success, all as a function of queen treatment and the presence of a mate. We identified factors that, under pathogenic stress, influenced these fitness-related milestones. Queen infection significantly reduced the number of successfully established colonies. Moreover, both the presence of a king and his mass significantly influenced the queen’s survival, her onset of oviposition, overall egg production, and hatching success. We conclude that termite colonies incur significant fitness costs after a queen suffers an acute infection and that the presence of a king (and his stored resources) may help mitigate the negative effects of a queen’s infection. Pathogenic pressures, combined with the significant role of kings in colony success, appear to reinforce two-parent colony foundation, mate assistance, biparental care, and ultimately the overlap of generations, all of which have been considered preadaptations for eusociality. By studying the fitness consequences of pathogenic stress during the ontogeny of a termite colony, we can infer some of the conditions and pressures under which termite sociality likely emerged.

Moisture requirements for the migration of Haemonchus contortus third stage larvae out of faeces

The abomasal nematode Haemonchus contortus causes severe disease and production loss in small ruminants in warmer regions and is also an emerging threat in many temperate climates. Specific knowledge of the effects of climate on the epidemiology of H. contortus is needed to effectively apply sustainable control strategies, which rely on prediction of infection risk. Although the effects of temperature and rainfall on larval development in this species have been characterised, much less is known about migration out of faeces and onto herbage. This is an important deficit in our understanding of the epidemiology of haemonchosis in regions with relatively low and particularly erratic rainfall. Methods were developed to assess the migration of third stage larvae (L3) out of faeces under simulated rainfall in the laboratory. These were applied in a series of experiments, which showed that rainfall is required for migration. However, a single rainfall event was not sufficient for migration from faeces of which the crust has hardened after having been kept in dry conditions. Light and regular rainfall resulted in rapid emergence from moist faeces kept in humid conditions, but much slower emergence from dry faeces in dry conditions. Ambient relative humidity therefore appears to act through faecal moisture content to modify the effect of rainfall on larval migration. Larvae survived well in dry faeces for a number of days, but did not migrate in the absence of rainfall, so sheep faeces could potentially act as a larval reservoir in dry conditions, with peaks of infection following rainfall. Rates of faecal desiccation and rehydration on pasture could therefore be highly relevant to temporal patterns of larval availability.

Ovicidal activity of three insect growth regulators against Aedes and Culex mosquitoes

Interspecific variations in the susceptibility of freshly and embryonated eggs of Aedes albopictus, Ae. aegypti, Ae. atropalpus and Culex pipiens were tested against three classes of insect growth regulators (IGRs) including ecdysone agonist (azadirachtin), chitin synthesis inhibitor (diflubenzuron) and juvenile hormone analog (pyriproxyfen) at 0.001, 0.01, 0.1 and 1.0ppm concentrations. Egg hatching inhibition was dose dependent, the highest being at 1.0ppm concentration for freshly laid eggs of Ae. albopictus (pyriproxyfen: 80.6%, azadirachtin: 42.9% and diflubenzuron: 35.8%). Aedes aegypti showed lower egg hatching inhibition when exposed to pyriproxyfen (47.3%), azadirachtin (15.7%) and diflubenzuron (25.5%). Freshly laid eggs of Cx. pipiens were most susceptible to diflubenzuron. Aedes atropalpus eggs were tolerant to all three classes of IGRs. Embryonated eggs of Ae. albopictus, Ae. aegypti, Ae. atropalpus and Cx. pipiens were resistant to pyriproxyfen, azadirachtin and diflubenzuron than freshly laid eggs. The median desiccation time (DT50) of Ae. atropalpus eggs was maximum (5.1h) as compared to Ae. aegypti (4.9h), Ae. albopictus (3.9h) or Cx. pipiens (1.7h) eggs. Insignificant relationship between the rates of desiccation and egg hatching inhibition suggests other factors than physical providing eggs the ability to tolerate exposures to various IGRs. Egg hatching inhibition was due to the alteration in embryonic development caused by IGRs. Changes in the egg shell morphology and abnormal egg hatching from the side of the egg wall instead of operculum, was observed at higher concentrations of diflubenzuron. Morphological and physiological variations in eggs may be the key factor to influence the ovicidal efficacy of IGRs. The present data provide a base line for the improvement of the ovicidal efficacy of the insecticide and its formulation.

A comparative study of desiccation responses of seeds of three drought-resistant Mediterranean oaks

The recalcitrance of many Quercus species has received considerable attention. However, considerably less is known about the desiccation sensitivity of drought resistant Mediterranean oaks. The objective of this study was to estimate the seed likelihood sensitivity to desiccation of the three Mediterranean oaks (Quercus coccifera, Quercus pubescens, Quercus pedunculiflora), to see how the severity of desiccation affects acorn moisture content and germination behavior, and to explore any relation of initial acorn characteristics to acorn rate of water loss. Acorns were subjected to variable rates of desiccation after air-drying in room temperature. Results showed that there were significant differences in drying rates between species. Acorn moisture content and germination decreased as the severity of desiccation increased, while the germination capacity of Q. pedunculiflora was unaffected from the applied desiccation period, and remained high after 15days of drying. In Q. coccifera acorns, water loss rate was correlated to the most of acorn morphological characteristics, while for the other two species this correlation was low. The critical moisture content below which Q. coccifera and Q. pubescens seeds start losing their viability is approximately 26%, which is achieved after a drying period of 5 and 7days, respectively, while for the species Q. pedunculiflora a longer desiccation period should be investigated to find out the minimum critical moisture content.

Desiccation as a Long-Term Survival Mechanism for the Archaeon Methanosarcina barkeri

Viable methanogens have been detected in dry, aerobic environments such as dry reservoir sediment, dry rice paddies and aerobic desert soils, which suggests that methanogens have mechanisms for long-term survival in a desiccated state. In this study, we quantified the survival rates of the methanogenic archaeon Methanosarcina barkeri after desiccation under conditions equivalent to the driest environments on Earth and subsequent exposure to different stress factors. There was no significant loss of viability after desiccation for 28 days for cells grown with either hydrogen or the methylotrophic substrates, but recovery was affected by growth phase, with cells desiccated during the stationary phase of growth having a higher rate of recovery after desiccation. Synthesis of methanosarcinal extracellular polysaccharide (EPS) significantly increased the viability of desiccated cells under both anaerobic and aerobic conditions compared with that of non-EPS-synthesizing cells. Desiccated M. barkeri exposed to air at room temperature did not lose significant viability after 28 days, and exposure of M. barkeri to air after desiccation appeared to improve the recovery of viable cells compared with that of desiccated cells that were never exposed to air. Desiccated M. barkeri was more resistant to higher temperatures, and although resistance to oxidative conditions such as ozone and ionizing radiation was not as robust as in other desiccation-resistant microorganisms, the protection mechanisms are likely adequate to maintain cell viability during periodic exposure events. The results of this study demonstrate that after desiccation M. barkeri has the innate capability to survive extended periods of exposure to air and lethal temperatures.

The role of the persistent fruit wall in seed water regulation in Raphanus raphanistrum (Brassicaceae)

Dry fruits remain around the seeds at dispersal in a number of species, especially the Brassicaceae. Explanations for this vary, but usually involve mechanisms of innate dormancy. We speculate that, instead, a persistent fruit may give additional protection through control of dehydration, to species growing in arid or Mediterranean environments where water is sporadic. X-rays and weight measurements were used to determine the extent to which Raphanus raphanistrum seeds within mature fruits imbibe water, and germination tests determined the roles of the fruit and seed coat in seed dormancy. Rates of water uptake and desiccation, and seedling emergence were compared with and without the fruit. Finally, germinability of seeds extracted from fruits was determined after various periods of moist conditions followed by a range of dry conditions. Most seeds rapidly take up water within the fruit, but they do not fully imbibe when compared with naked seeds. The seed coat is more important than the dry fruit wall in maintaining seed dormancy. The presence of a dry fruit slows emergence from the soil by up to 6-8 weeks. The fruit slows the rate of desiccation of the seed to a limited extent. The presence of the fruit for a few days during imbibition somehow primes more seeds to germinate than if the fruit is absent; longer moist periods within the pod appear to induce dormancy. The fruit certainly modifies the seed environment as external conditions change between wet and dry, but not to a great extent. The major role seems to be: (a) the physical restriction of imbibition and germination; and (b) the release and then re-imposition of dormancy within the seed. The ecological significance of the results requires more research under field conditions.

Three centuries of change in the Peace–Athabasca Delta, Canada

The Peace–Athabasca Delta in northern Alberta, Canada, is a dynamic wetland ecosystem. Climatic, hydrologic, biological, and historical data are synthesized to elucidate how the ecosystem has changed over the past 300 years. Annual temperature is now higher than it has been in the past 300 years. For much of the 1700s, the Delta was colder in winter and had a lower flood frequency than that of the last 30 years. The 1800s were characterized by long and cold winters, 4–12 year-long episodes of high or low water, and repeated human epidemics. The early twentieth century was relatively moist and cool. Since mid-twentieth century the Delta has experienced periods of both intense warmth and cold, desiccation and recharge. Since the mid-1960s, local and regional mean annual temperatures have increased 0.30°C to 0.48°C per decade while winter temperatures have increased 0.68°C to 0.92°C per decade; annual snowfall has decreased 12 to 41 cm per decade while winter snowfall has decreased 12 to 34 cm per decade. Major events in the past 45 years include climatic changes favoring a warmer, drier ecosystem; cultural and socioeconomic changes; building of the Bennett Dam; prevention of the Athabasca River mainstem avulsion in 1972; the Cree Creek avulsion of 1982; large fluctuations in water, vegetation, and wildlife; and the development of the Alberta Tar Sands. Increased rates of basin desiccation and wildfire activity and upstream land disturbances may combine to alter the Delta’s biotic composition. There appears to be no relevant historical analogue of the present Delta.

The environmental physiology of Antarctic terrestrial nematodes: a review.

The environmental physiology of terrestrial Antarctic nematodes is reviewed with an emphasis on their cold-tolerance strategies. These nematodes are living in one of the most extreme environments on Earth and face a variety of stresses, including low temperatures and desiccation. Their diversity is low and declines with latitude. They show resistance adaptation, surviving freezing and desiccation in a dormant state but reproducing when conditions are favourable. At high freezing rates in the surrounding medium the Antarctic nematode Panagrolaimus davidi freezes by inoculative freezing but can survive intracellular freezing. At slow freezing rates this nematode does not freeze but undergoes cryoprotective dehydration. Cold tolerance may be aided by rapid freezing, the production of trehalose and by an ice-active protein that inhibits recrystallisation. P. davidi relies on slow rates of water loss from its habitat, and can survive in a state of anhydrobiosis, perhaps aided by the ability to synthesise trehalose. Teratocephalus tilbrooki and Ditylenchus parcevivens are fast-dehydration strategists. Little is known of the osmoregulatory mechanisms of Antarctic nematodes. Freezing rates are likely to vary with water content in Antarctic soils. Saturated soils may produce slow freezing rates and favour cryoprotective dehydration. As the soil dries freezing rates may become faster, favouring freezing tolerance. When the soil dries completely the nematodes survive anhydrobiotically. Terrestrial Antarctic nematodes thus have a variety of strategies that ensure their survival in a harsh and variable environment. We need to more fully understand the conditions to which they are exposed in Antarctic soils and to apply more natural rates of freezing and desiccation to our studies.

Functional Changes of Photosystem II in the Moss Rhizomnium punctatum (Hedw.) Induced by Different Rates of Dark Desiccation

Summary An effect of different rates of dark desiccation on Photosystem II (PSII) in the drought medium-tolerant moss Rhizomnium punctatum (Hedw.) was studied by exposing the detached leaves for 24 h to air of relative humidities of 85 %, 70 % and 30 %. A similar decrease in O2 evolution after 24 h of desiccation, but diverse changes of the O-J-I-P chlorophyll fluorescence transient during desiccation, were observed. Whereas the Fp value remained fairly constant, the Fo value increased with increasing time and rate of desiccation, leading to a decrease in the Fv/Fp value for 70 % and 30 % RH. Concerning the relative height of the J step, two different trends in the time courses of the rFj parameter were found. At 85 % RH, only a slight rFj increase was observed while at 30 % RH an rFj decrease dominated. The samples exposed to 70 % RH exhibited both phases: the increasing one was followed by the decreasing one. The Western-blot method indicated that no D1 degradation occurred during desiccation. Similarly, no changes in the E475/E436 band ratio of the Chl fluorescence excitation spectrum at 77K were found for 30 % RH. It is suggested that damage to the acceptor side of PSII dominating at 85 % RH is followed by a functional disconnection of P680 from the antennae systems, which was apparent at higher rates of desiccation. The notion of different reactions to fast and slow desiccation was further supported by the experiment with the chemically closed PSII.

Histochemical, Physiological, and Ultrastructural Changes in the Maize Embryo during Artificial Drying

When drying high moisture(450–500 g H2O kg−1 fw) Zea mays L. seeds, a preconditioning period of drying at low temperatures(PC) may prevent injury due to subsequent high temperature drying. We preconditioned maize seeds under four different environments(35°C / 35% RH, 35°C / 90% RH, 20°C / 35% RH, and 20°C / 90% RH) and determined the level of seed deterioration in terms of reserve stability, enzyme activity, respiration, and ultrastructural changes in the radicle meristem. During PC, respiration rates were significantly higher for the 35°C than the 20°C treatments; however, overall respiration during PC was considerably lower than during germination. After 48 h of PC, amylase activity was present in all of the PC treatments with the highest levels and additional isozymes in the 35°C / 90% RH treatment. This treatment was also the only one in which we found degradation of starch grains in the histochemical analysis. During PC, transmission electron micrographs revealed migration of lipid bodies toward the cell wall and formation of protein bodies within vacuoles. Both of these processes were more likely to appear in those treatments that allowed higher rates of desiccation or in which the seeds were more mature (Harvest 2). Mitochondrial cristae were well differentiated with no evident impairment of their integrity in any of the PC treatments. Better seed quality was obtained when seed were preconditioned at moderate (35°C) temperature and low (35%) relative humidity. We conclude from these results that the positive effects of preconditioning may occur as a balance between metabolic and morphological processes resulting in high temperature desiccation tolerance, while minimizing the negative impact of the catabolic events such as degradation of starch grains and higher respiration rates.

Water relations in Eucalyptus regnans nursery plants following root exposure after lifting

Water relations of Eucalyptus regnans seedlings during root exposure and associated desiccation were studied in three trials. In the first, changes in water potential and relative water content (RWC) of leaves and hydraulic resistance in the lower stem were recorded during exposure. In the two subsequent trials, different exposure conditions resulted in different rates of desiccation. After various exposure times, roots were soaked for 24 h in aerated distilled water under continuous light before measuring root and whole plant hydraulic resistance. Exposure for 5–8 h and accompanying short term leaf water stress to below −2 MPa resulted in significant increases in hydraulic resistance of stem, roots and whole plant. Although leaf water potentials at the longest exposure times in one trial fell below the probable turgor loss point, there was no apparent permanent leaf damage. In the two trials in which roots were soaked before hydraulic resistances were measured, there was a marked difference in the response of leaf water potential to soaking. Although hydraulic resistance increased with longer exposure in both trials, the results from one trial suggested a further increase during soaking. Differences between the two trials suggested differences between plants before treatment or the soaking treatment itself may also have influenced water relations after exposure. Results are discussed in terms of the practical implications, particularly effects of nursery management on the condition of trees at lifting and subsequent water relations of lifted seedlings during and after transplanting. Some aspects of regulation of water stress by E. regnans seedlings are also discussed.

Responses to Drying of Recalcitrant Seeds of Quercus nigraL.

It has been suggested that rate of desiccation can influence the expression of recalcitrant behaviour in seeds, thus complicating the task of determining which seeds are truly recalcitrant. The objective of this study was to see if variable rates of desiccation influenced such behaviour in Quercus nigraL., a tree seed known to be recalcitrant. Acorn moisture content, leachate conductivity, and germination were determined at various times during desiccation at three rates at 27 and 40°C. Moisture contents and germination decreased as the severity of desiccation increased. Leachate conductivity increased slightly but was not a sensitive indicator of loss of viability. The critical (lethal) moisture content for these acorns was 10–15%, although rehydration within 48 h of reaching this level prevented death in about 25% of the acorns. At 27°C any desiccation treatment that produced losses of 30–50mg of moisture per g of acorn dry weight per day should be suitable as a test for recalcitrance in the genus. Apparent physiological or fungal damage at 40°C rules out the higher temperature for such a test.

Moisture relationships of the high arctic collembolan Onychiurus arcticus

Abstract. Rates of desiccation, rehydration and survival, under conditions of 0 and 100% relative humidity, were investigated in the high arctic collembolan Onychiurus arcticus (Tullberg) over a range of temperatures from ‐3 to 10d̀C. Desiccation, measured over 4h in a dry environment, was rapid and highly significantly correlated with saturated vapour pressure deficit (SVPD). At 10d̀C animals lost over 60% of body moisture in under 1h. Under moist conditions body weight remained constant. Survival, measured over 4h under dry conditions, showed a highly significant negative correlation with SVPD across the range of temperatures. Survival in moist controls was 100%. Partially desiccated animals were able to rehydrate using free water or moisture from a saturated atmosphere, but uptake of the former was faster. Rates of water uptake were around 30 times slower than rates of loss through desiccation. Animals given free water regained initial weights in 24‐144h, dependent on temperature. Uptake rates of free water, measured over the time necessary fully to rehydrate, were highly significantly correlated with temperature, whereas those for atmospheric moisture were not. Over the time scales used in the experiments, three linear regression equations accurately predict rates of desiccation, rehydration and survival from SVPD or temperature. Ecological adaptation by O.arcticus to the highly contrasting climatic environments of the arctic summer and winter seasons is discussed in the context of climate change.

Community Organization of Erigeron Glaucus Folivores: Effects of Competition, Predation, and Host Plant

Experiments were conducted to determine the relative importance of interspecific competition, predation, and host plant quality on the success of the common herbivores of Erigeron glaucus, the seaside daisy. Leaves of E. glaucus were attacked by spittlebugs (Philaenus spumarius), plume moth caterpillars (Platyptilia williamsii), and thrips (Apterothrips secticornis). Each species of herbivore was affected strongly by only one of the biotic factors examined. For spittlebugs, damage caused by caterpillars increased rates of desiccation and mortality of nymphs. This competitive effect probably caused the negative association between late—instar spittlebug nymphs and caterpillar damage wherever both species were encountered along the California coast. Plume moth caterpillars were most strongly affected by predator exclusion, at least during the late spring. Chicken—wire cages that excluded birds resulted in greater numbers of caterpillars on single rosettes and on whole clonal mats. Passerculus sandwichensis, Savannah Sparrows, were likely responsible for this effect which was only found during the season when adult birds were feeding insects to nestlings. On San Miguel Island, Savannah Sparrows were absent, densities of caterpillars were greater than on the mainland, and caging had no effect on caterpillar survival; these observations were all consistent with the hypothesis that Savannah Sparrows were important predators on the mainland. The factor that was most important for each herbivore species did not interact with other biotic factors. The relative ranking of the factors for each species was similar among the 3 yr of the experiments, with only one exception. Observations suggested that experiments conducted at Bodega Bay were representative of this community at other locations in California where all of these species were present. A scheme is proposed as a first step in predicting the importance of various biotic factors in herbivore community organization. If effects of predation, parasitism, or physical disturbance are very strong, host plant related competition and host quality are likely to be unimportant. If predation and disturbance are relatively less important, factors that reduce the choices available to an individual herbivore make host quality and interspecific competition more likely to play a significant role.

Chemical desiccation of potato vines

Potato (Solanum tuberosum L., ‘Russet Burbank’) vine desiccation was compared with sulfuric acid (89 kg ha−1 sulfur), dinoseb [2-(1-methylpropyl)-4, 6-dinitrophenol] (2.4 kg ha−1), diquat (6, 7-dihydrodipyrido [1,2-α:2′,1′-c] pyrazinediium ion) (0.28 kg ha−1) and endothal (7-oxabicyclo[2.2.1] heptane-2,3-dicarboxylic acid) (1.1 kg ha−1). All treatments were field-applied in southern Idaho during August and September. Vine (stem and leaf) desiccation was most rapid and consistent with sulfuric acid which averaged 42% desiccation at day one after treatment. Dinoseb and diquat at rates used were similar with desiccation of 19% and 17%, respectively, at day one. Endothal followed with 9% desiccation at day one. As diquat rates increased from 0.28 kg ha−1 to 0.55 kg ha−1, desiccation increased from 16% to 21% at day one. As diquat rate was further increased to 1.65 kg ha−1 vine desiccation was 25%. Major differences in desiccation rate occurred the first day. After the first day, rates of desiccation were mostly similar. Immature vines were desiccated more slowly than mature vines that were beginning to senesce. This difference between mature and immature vine desiccation was least with sulfuric acid. Several crop oils and surfactants evaluated with dinoseb and diquat were similar, except Mor-act that slightly increased dinoseb activity. Changes in tuber yield and quality were not associated with chemicals except as expected from desiccation.

The tardigrade cuticle II. Evidence for a dehydration-dependent permeability barrier in the intracuticle

Tardigrades weighed during desiccation in high or low humidities show a short period of rapid transpiration followed by an abrupt decline in transpiration which virtually arrests water loss. The amount of water retained following this ‘permeability slump’ is greater at low rates of desiccation but the slump is not a metabolic phenomenon, being reproducible in dead or narcotised animals. Tardigrades rinsed in hot chloroform (62 °C) for 5 hr still show the characteristic permeability decline when desiccated in 80% RH. However, 25hr rinsing in hot chloroform apparently obliterates the slump. Estimates of the bound water content of tardigrades by DSC show that this can account for the dehydrated masses of these chloroform-rinsed animals and that all free water is probably transpired. Lipid analysis of the 25 hr chloroform extracts by GC-MS reveals several lipid classes, predominantly free fatty acids (C 12–C 18); these are not detectable in the 5 hr extracts. Control rinsing in hot water has no apparent effect on the permeability slump. TEM tracer studies with lanthanum show the lipid-rich intracuticle to serve as a transpiration barrier in dehydrated animals but not in fully hydrated specimens. There is thus strong support for the role of intracuticular lipids in effecting the permeability slump. A model to explain this phenomenon on the basis of lipid phase changes is postulated.

Temperature and Relative Humidity Effects on Aerial Exposure Tolerance in the Freshwater BivalveCorbicula fluminea

The exposure tolerance, aerial respiratory behaviors, and the rates of water loss of the Asian freshwater clam, Corbicula fluminea, were assessed under three temperature conditions (15°, 25° and 35°C) and five relative humidity (RH) treatments (5%, 33%, 53%, 75% and 95%).C. fluminea displayed low tolerance to aerial exposure (range of median tolerance times: 23.8-24.9 h at 35°C, 71.4-78.2 h at 25°C, and 248.5-341.6 at 15°C). Relative humidity had no effect on median tolerance time except at 15°C. Body size was reciprocally related to water loss rate at all temperatures, and on longevity at 25° and 35°C. Cumulative rates of water loss at 95% and 75% RH were lower than the other humidities at 15°C, but no differences were found at 25° or 35°C. Mantle edge exposure behavior was inhibited by low humidity and high temperature. Exposing mantle tissues did not increase rate of water loss except at humidities below which the behavior was very rare. The occurrence or extent of the behavior did not affect individual clam longevity. The results suggest that C. fluminea can detect rates of desiccation and make behavioral adjustments.

Photosynthetic recovery of the resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring: effects of prior desiccation rate and mechanisms of desiccation damage.

The effects of desiccation rate on photosynthetic recovery of the resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring were examined. Gas exchange over a 24 h rehydration period, ribulose bisphosphate carboxylase conservation in desiccated fronds, de novo protein synthesis rate during rehydration, and frond leakage rate during rehydration were measured following desiccation at various rates. Seven-day hydrated, fully recovered plants were dried at four rates. The initial decline in tissue fresh weight/dry weight ratio was essentially linear to a ratio value of 1.35, the point at which frond curling occurred, but slowed thereafter. Frond curling required 5.5, 52, 94, and 175 h of desiccation, respectively, for the four treatments.The rate of net photosynthesis after 24 h of rehydration was greatest for the two intermediate drying speeds; both very rapid and very slow drying were associated with significantly reduced rates. Electrolyte leakage was greatest following very rapid drying and de novo protein synthesis was impaired following very slow drying. Ribulose bisphosphate carboxylase conservation was not significantly affected by drying speed. These results support the hypothesis that desiccation injury is multicausal and that specific components of the photosynthetic recovery process respond differently to desiccation rate. However, in spite of these significant desiccation rate effects, overall photosynthetic recovery in S. lepidophylla appears to be relatively tolerant of widely varying rates of desiccation.