Dehydration of earthworm cocoons exposed to cold: a novel cold hardiness mechanism

Mechanisms involved in cold hardiness of cocoons of the lumbricid earthworm Dendrobaena octaedra were elucidated by osmometric and calorimetric studies of water relations in cocoons exposed to subzero temperatures. Fully hydrated cocoons contained ca. 3 g water · g dry weight^-1; about 15% of this water (0.5 g·g dry weight^-1) was osmotically inactive or bound. The melting point of the cocoon fluids in fully hydrated cocoons was-0.20°C. Exposure to frozen surroundings initially resulted in supercooling of the cocoons dehydrated (as a result of the vapour pressure difference at a given temperature between supercooled water and ice) to an extent where the vapour pressure of water in the body fluids was in equilibrium with the surrounding ice. This resulted in a profound dehydration of the cocoons, even at mild freezing exposures, and a concomitant slight reduction in the amount of osmotically inactive water. At temperatures around-8°C, which cocoons readily survive, almost all (>97%) osmotically active water had been withdrawn from the cocoons. It is suggested that cold injuries in D. octaedra cocoons observed at still lower temperatures may be related to the degree of dehydration, and possibly to the loss of all osmotically active water. The study indicates that ice formation in the tissues is prevented by equilibrating the body fluid melting point with the exposure temperature. This winter survival mechanism does not conform with the freeze tolerance/freeze avoidance classification generally applied to cold-hardy poikilotherms. Implications of this cold hardiness mechanism for other semi-terrestrial invertebrates are discussed.Abbreviations DSC differential scanning calorimetry - dw dry weight - MP melting point(s) - II water potential - R universal gas constant - T absolute temperature - V specific volume of water     

Physiology of cold hardiness in cocoons of five earthworm taxa (Lumbricidae: Oligochaeta)

Earthworm cocoons are mostly found in the uppermost soil layers and are therefore often exposed to low temperatures during winter. In the present study, cocoons of five taxa of earthworms were investigated for their tolerance to freezing, melting points of cocoon fluids and dehydration of cocoons when exposed to a frozen environment. Embryos of the taxa investigated were freeze intolerant. The melting points of fully hydrated cocoon fluids were high (above –0.3°C) and thermal hysteresis factors were absent. Exposure to a frozen environment caused the cocoons to dehydrate drastically and dehydrated cocoons showed significantly lower super-cooling points than fully hydrated cocoons, reducing the risk of freezing for dehydrated cocoons. It is proposed therefore that the cold-hardiness strategy of the earthworm cocoons is based on dehydration upon exposure to subzero temperatures in the frozen environment. Cocoons of three surface-dwelling taxa, Dendrobaena octaedra, Dendrodrilus rubidus tenuis and Dendrodrilus rubidus norvegicus had lower supercooling points and survived frost exposure better than cocoons of two deeper-dwelling taxa, Aporrectodea caliginosa and Allolobophora chlorotica. One of the investigated taxa, D. r. norvegicus, was collected from a cold alpine habitat. However, it was not more cold hardy than the closely related D. r. tenuis collected from a lowland temperate habitat. D. octaedra was the most cold hardy taxon, its cocoons being able to withstand –8°C for 3 months and –13.5°C for 2 weeks in frozen soil.Abbreviations dw dry weight - fw fresh weight - SCP supercooling point     

Low Temperature Tolerance of Pacu, Piaractus mesopotamicus

Pacu, Piaractus mesopotamicus (=Colossoma mitrei), is a South American warm water fish species found in the temperature range of 15–35°C. The culture of a warm water species in temperate regions demands knowledge on its temperature requirements. Pacu introduction into the Israeli fish culture system is being considered. Temperature range in the region is 8–33°C, thus the minimum winter water temperatures might be a limiting factor. To determine what is the minimum temperature pacu would tolerate, and hence which overwintering operations in warm-temperate regions are required for this warm water species, low temperature tolerance tests in the laboratory and observations in the field were carried out. Laboratory experiments reducing temperature by 1–3°C per day were carried out with fish of 150–200g, about the size pacu reach after one culture season. The field observations compared survival of two-year-old pacu of 1.3kg mean weight overwintered in outdoors and in greenhouse ponds. For one-year-old fish 7.5°C was found to be the lower temperature tolerance limit. Two-year-old fish withstand short exposures to this temperature rather well and their lower tolerance limit might be lower. This indicates that in warm-temperate regions pacu should survive in outdoors ponds. In this case some loss of weight should be expected, and suspension of feeding when temperature drops below 16–18°C is recommended to avoid wasting feed that the fish will not consume anyway. To be in the safe side, inflow of the warmest available water into the ponds is recommended if maximum water temperature drops to 10°C or below. Overwintering in greenhouses or other heated facilities would be recommended if an exceptionally cold winter is expected and for regions with lower winter minimum temperatures.     

Drought survival in Dactylis glomerata and Festuca arundinacea under similar rooting conditions in tubes

Drought survival in perennial forage plants involves different adaptative responses such as delay of dehydration through water uptake, limitation of water loss and tolerance of tissues to dessication. To compare the importance of these responses in contrasting cultivars of forage grasses at the whole plant level, we carried out two experiments under glasshouse conditions. Plants of cocksfoot (Dactylis glomerata L.) cultivars, cvs. Currie, Medly (both of Mediterranean origin) and Lutetia (of continental origin), and of tall fescue (Festuca arundinacea L.) cv. Centurion (Mediterranean) were grown in 60 cm-deep cylinders to eliminate the effect of differences of root depth on water availability whilst allowing severe drought to be imposed at a realistic rate. In both experiments, the cvs. were ranked similarly for plant survival, with high mortality for Centurion, low for the Mediterranean cocksfoots Currie and Medly, and intermediate for Lutetia. These differences could not be ascribed to water use during most of the drought period since water uptake and decrease in leaf extension were not significantly different between species and cultivars. However, resistant cvs. of cocksfoot were able to extract water for a longer period and at a lower soil water potential (s) than other cvs. The critical s at plant death was –3.8 and –3.6 MPa for Medly and Currie and –3.0-,–2.6 MPa for Lutetia and Centurion. Moreover, at a low soil water reserve (15–2%), membrane stability and water content were maintained for longer in enclosed immature leaf bases of cocksfoots cultivars, whereas the fescue Centurion exhibited accelerated lamina senescence and steady increase of membrane damage in surviving tissues. Therefore, it is proposed that the drought resistance of tall fescue in the field can mainly be ascribed to its ability to develop a deep root system. In cocksfoot, dehydration tolerance in surviving tissues and the ability of roots to extract water at low soil water potentials may, in addition to root depth, contribute significantly to plant survival under severe drought.     

Response to simultaneous dehydration and thermal stress in three species of Puerto Rican frogs

Summary The response to simultaneous temperature and dehydration stress was examined in three species of Puerto Rican frogs.Eleutherodactylus antillensis is found primarily in hot, arid grasslands at low altitudes.Eleutherodactylus portoricensis is restricted to cool, forested montane habitats above 300 m.Eleutherodactylus coqui occurs in both the lowlands and the highlands. The physiological tolerance of the frogs to temperature was measured at 20, 25, and 30°C at various levels of dehydration using an index derived from jumping performance. Although jump distance of fully hydratedE. antillensis was unaffected by temperature, this species tolerated significantly more dehydration at high temperatures than low.Eleutherodactylus portoricensis died at 30°C, but in this species both distance jumped and dehydration tolerance were unaffected by lower temperatures. Distance jumped byE. coqui increased with increasing temperature, but tolerance of dehydration remained unchanged.Eleutherodactylus coqui was less tolerant of high temperatures thanE. antillensis but more tolerant thanE. portoricensis.Abbreviations DT dehydration tolerance - SVL snout vent length     

干旱-复水对两种石斛属植物叶水势的影响

石斛属植物多附着在其他植物体或岩石上,水分获取困难,其特殊的水分利用策略是其生存和发展的重要保证.为弄清石斛属植物对干旱胁迫的适应能力和机制,该文选用3年生金钗石斛和铁皮石斛,通过盆栽控水进行干旱胁迫和复水处理,探讨在不同干旱历时和干旱后复水条件下两种石斛的叶水势变化情况.结果表明:随着干旱时间的延长,两种石斛叶水势均...     

Combined effects of drought and high temperature on water relations of wheat and sorghum

Drought and high temperature are major factors limiting crop production. The two stresses occur together in many regions, but they usually are investigated separately. This study tested the hypothesis that high temperature interacts with drought to affect water relations, and the effect is greater in heat-sensitive wheat (Triticum aestivum L.) than in sorghum (Sorghum bicolor L. Moench). Wheat and sorghum were grown in soil that was well watered or not watered in controlled chambers at 15/10, 25/20, 35/30 and 40/35 °C day/night. Soil water content (SWC), leaf relative water content (RWC), leaf water potential (), leaf osmotic potential (), leaf turgor potential (P) and osmotic adjustment (OA) were determined at 2-d intervals. All values held nearly constant at all temperatures when soil was well watered but were affected strongly by high temperature when water was withheld. The combined stresses reduced SWC, RWC, Psi and , and unevenly raised P over time, particularly in sorghum. Sorghum also exhibited marked OA at high temperature, which was usually lethal to wheat. High temperature appeared to interact with drought to affect water relations by altering SWC and not by influencing OA. The results demonstrated that crops maintain nearly stable water relations regardless of temperature when moisture is ample, but high temperature strongly affects water relations when water is limiting. Increasing the thermotolerance of wheat might improve its potential to acclimate to both high temperature and drought.     

The respiratory metabolism of selected lumbricidae

Summary Open-system infra red gas analysis was used to measure the CO₂ output throughout a year of four species of earthworm. The respiratory quotients (R.Q.s) of the four species were determined by means of a Warburg apparatus and it was found that they varied with season. In some instances R.Q.s did not fall within the expected range of 0.7 to 1.0 and the low values were attributed to calciferous gland activity and the fixation of metabolic CO₂.The results from CO₂ output measurements at 10°C and R.Q.s were used to calculate oxygen uptake, this varied seasonally but the mean annual values at 10°C for adult, large immature and small immature A. rosea were 64.17, 72.66 and 78.56 l O₂ g^-1 fresh wt h^-1 respectively. Mixed size groups of L. castaneus had a mean annual oxygen consumption at 10°C of 155.83 l O₂ g^-1 fresh wt h^-1 and equivalent values for D. rubida and O. cyaneum were 112.02 and 69.35 l O₂ g^-1 fresh wt h^-1. The apparent relationship between a high respiratory rate per unit weight and a litter dwelling habit (e.g. L. castaneus and D. rubida) disappeared when allowance was made for the weight of gut contents. Mean annual values for oxygen uptake in l O₂ g^-1 gut free fresh wt h^-1 at 10°C were L. castaneus (194.79), D. rubida (142.22), A. rosea (95.70) and O. cyaneum (139.28). No size specific metabolism could be demonstrated either within or between species, this is believed to be correlated with the different levels of activity shown by different species and their life stages.Rates of oxygen consumption per unit weight for A. rosea were shown to be proportional to ambient temperature. Q ₁₀ slopes of this relation, between 6 and 15°C, were higher for large immature A. rosea (2.42) and small immatures (1.96) than for adult clitellate worms (1.42). The mean Q ₁₀ relationship for all size classes of A. rosea was 1.93 over the same temperature range and the equivalent value for cocoons was 1.63. The relationship between the oxygen consumption rate of all size classes of A. rosea and ambient temperature was not significantly affected by acclimatisation at 5 and 10° C prior to measurements being made at 6, 10 and 15° C.     

Effects of High Temperature and Water Deficit on Photosystem II in Hordeum vulgare Leaves of Various Ages

Structural and functional characteristics of photosystem II (PSII) were examined in leaves of 4-, 7-, and 11-day-old barley seedlings exposed to high temperature (40°C, 3 h) and water deficit (replacement of nutrient medium with 3% PEG 6000 solution, 45 h). In young seedlings, the effective quantum yield of PSII photochemical reactions decreased upon heat treatment but did not change after dehydration. Both stressful factors diminished the Q _B-reducing capacity of PSII in 4- and 7-day-old plants. This was caused by the increase in the reduction level of plastoquinone and by the impairment of the Q _B-binding site of the D1 protein. The increase in the content of plastoquinol after the heat shock was due to the impaired oxidizing capacity of cytochrome f (Cyt f). The dehydration did not alter the content of functionally active Cyt f but elevated the microviscosity of the lipid bilayer in thylakoid membranes, which presumably impeded the lateral diffusion of plastoquinones and reduced their reoxidation rate. The heating and dehydration of old leaves reduced the amount of -type PSII reaction centers, thereby inhibiting the linear electron transport. It is concluded that PSII responses to heat treatment and water deficit are subject to variations depending on leaf age. Measurements of the redox potentials for plastoquinones, Cyt f, and cytochrome b ₅₅₉ upon senescence, hyperthermia, and water deficit allowed us to propose that heat and water stresses activate cyclic electron transport around PSII.     

Effects of dehydration on organ metabolism in the frogPseudacris crucifer: hyperglycemic responses to dehydration mimic freezing-induced cryoprotectant production

The metabolic effects of evaporative water loss at 5 °C were assessed for both fall- and spring-collected spring peepersPsuedacris crucifer. Frogs readily endured the loss of 50% of total body water. During dehydration organ water content was defended with no change in water content in skeletal muscle, gut, and kidney of 50% dehydrated frogs and reduced water content in liver, brain and heart. Dehydration stimulated a rapid and massive increase in liver glucose production. In fall-collected frogs liver glucose rose by 120-fold to 2690±400 nmol · mg protein^-1 or 220 mol · g ww^-1 in 50% dehydrated frogs and glucose in other organs increased by 2.6- to 60-fold. Spring-collected frogs showed the same qualitative response to dehydration although absolute glucose levels were lower, rising maximally by 8.4-fold in liver. Glucose synthesis was supported by glycogenolysis in liver and changes in the levels of glycolytic intermediates in liver indicated that an inhibitory block at the phosphofructokinase locus during desiccation helped to divert hexose phosphates into the production of glucose. Liver energy status (ATP, total adenylates, energy charge) was maintained even after the loss of 35% of total body water but at 50% dehydration all parameters showed a sharp decline; for example, energy charge fell from about 0.85 to 0.42. Severe dehydration also led to an accumulation of lactate in four organs, probably hypoxia-induced the to impaired circulation. The hyperglycemic response ofP. crucifer to dehydration mimics the cryoprotectant synthesis response seen during freezing of this freeze-tolerant frog, suggesting that these share a common regultory mechanism and that the cryoprotectant response may have arisen out of pre-existing volume regulatory responses of amphibians. The hyperglycemic response to dehydration might also be utilized during winter hibernation to help retard body water loss by raising the osmolality of the body fluids in situations where hibernaculum conditions become dry.Abbreviations bin body mass - bw body water - CrP creatine phosphate - dw dry weight - F6P fructose-6-phosphate - FBP fructose-1,6-bisphosphate - G6P glucose-6-phosphate - PEP phosphoenolpyruvate - PFK phosphofructokinase - PYR pyruvate - ww wet weight     

Cold hardiness and geographic distribution of earthworms (Oligochaeta,Lumbricidae, Moniligastridae)

Cold hardiness of 12 species and 2 subspecies of earthworms from Northern Eurasia was studied. Supercooling temperatures, the water content and the thresholds of tolerated temperatures of worms and their cocoons were determined. The threshold values varied within ?1…?35°C for worms and within ?1…?196°C for cocoons. Earthworms of 4 species and 2 subspecies survived freezing. Cocoons of all species except Eisenia fetida possessed a protective dehydration mechanism which prevented their freezing. During wintering at subzero temperatures, earthworms lost up to 20% of water, cocoons up to 37%. Species of the same life form can overwinter at different phases and have different cold hardiness values. On the whole, epigeic and epi-endogeic species (except for Eisenia fetida) were more resistant to cold than endogeic ones. The following preliminary classification of earthworms according to their tolerance to negative temperatures is proposed: (1) both onthogenetic phases are tolerant; (2) only cocoons are tolerant; (3) both onthogenetic phases are intolerant. The geographic distribution of all the studied species (except for Eisenia nordenskioldi nordenskioldi) is partially or completely limited by insufficient resistance of the worms to negative temperatures. A significant cold hardiness of cocoons of most species is nonadaptive, since the worms hatched from the eggs in spring die without having enough time to reach maturity and to lay cocoons before the onset of subzero temperatures. Only 3 species (Eisenia nordenskioldi nordenskioldi, Eisenia atlavinyteae, and Dendrobaena octaedra) can live in permafrost regions; this is the main reason for a drastically reduced diversity of earthworm assemblages in eastern Siberia except for its southern, mountain parts. In general, the reasons for the impoverishment lie in the modern climatic conditions correlated with the ecophysiological capacities of earthworms.     

Electrotransformation of intact and osmotically sensitive cells of Corynebacterium glutamicum

Summary Intact and osmotically sensitive cells of Corynebacterium glutamicum can be efficiently transformed by electroporation. This was shown by using the plasmid vector pUL-330 (5.2 kb), containing the kanamycin resistance gene of transposon Tn5. The following electric parameters yielded efficient transformation. For intact cells: one exponentially decaying field pulse with time constants and with initial field intensities of E ₀=35–40 kV cm^-1; prepulse temperature 20°C. Cell regeneration (survival) was 100%–80%. Transformation efficiency can be increased by an additional freeze and thaw cycle of the cells, prior to electroporation. Lysozyme treated cells (osmotically sensitive) were transformed with three successive pulses of E ₀=25–30 kV cm^-1. Cell regeneration under these conditions was found to be 20–30%. The optimum yield of transformants/g plasmid-DNA was 3×10³ for intact cells, 2×10⁴ for intact cells which were frozen and thawed twice and 7×10⁴ for osmotically sensitive cells if the cell suspension was pulsed at a cell density of 1–3×10⁸/ml and at a DNA concentration of 0.2 g/ml up to 2 g/ml. The data obtained for osmotically sensitive cells suggest that the temperature increase accompanying the electric field pulse enhances colony formation and transformation efficiency if the initial prepulse temperature is 20°C, although regeneration of electroporated C. glutamicum cells starts to decrease at temperatures20°C.     

Effects of Ration and Temperature on Reproduction of a Semelparous and an Iteroparous Triclad

Summary

This paper compares aspects of the reproduction of a semelparous triclad, Dendrocoelum lacteum, with those of an iteroparous triclad, Dugesia polychroa. Cocoon output was less sensitive to ration and possibly temperature for the former as compared with the latter species. Cocoons of D. lacteum were on average bigger and contained more hatchlings than those of D. polychroa. The sizes of hatchlings did not differ between species, but for D. lacteum only, the hatchlings emerging from late cocoons were significantly larger, and less numerous, than those emerging from earlier cocoons. Possible reasons for these differences are considered.     

Physiological ecology of Juniperus virginiana in oldfields

Juniperus virginiana plants grow faster than other associated tree species in abandoned fields. During the summer the needles of the species do not light saturate even at 1,750 E m^-2 s^-1, reach optimum photosynthesis at 20°C, and maintain maximum photosynthesis at-8 to-12 bar twig water potential. In the field, the plants experience pronounced daily changes in water potential. The magnitude of the changes becomes more pronounced later in the summer. Leaves of the mature plants have highest rate of photosynthesis, young trees intermediate, and seedlings lowest. In winter there is a slight shift in optimum temperature for photosynthesis and the plants photosynthesize at 0°C. The rates of photosynthesis are lower in winter than in summer. On sunny days with calm winds, mature individuals and seedlings maintain significantly higher temperatures than air temperature while intermediate plants do not. The latter exhibit a lower photosynthetic rate than both mature plants and seedlings. The trends of photosynthesis, in the 3 size classes, both in winter and summer, correspond to the chlorophyll content of their leaves. It is concluded that J. virginiana grows well in open field habitats because it is a sun-adapted, drought resistant species with a long growing season which includes winter. The species is excluded from mature forests because it is shade-intolerant.     

Gas exchange characteristics and temperature relations of two desert annuals: A comparison of a winter-active and a summer-active species

Summary The gas exchange characteristics of two C₃ desert annuals with contrasting phenologies, Geraea canescens T. & G. (winter-active) and Dicoria canescens T. & G. (summer-active), both Asteraceae, were determined for plants grown under a moderate (25°/15° C, day/night temperature) and a high (40°/27° C) growth temperature regime. Both species had high photosynthetic capacities; maximum net photosynthetic rates were 38 and 48 mol CO₂ m^-2 s^-1 for Geraea and Dicoria, respectively, and were not influenced by growth temperature regime. However, the temperature optima of net photosynthesis shifted from 26° C for Geraea and from 28° C for Dicoria when grown under the moderate temperature regime to 31° C for both species when grown under the high temperature regime. Although the shifts in temperature optima were smaller than those observed for many desert perennials, both species showed substantial increases in photosynthetic rates at high temperatures when grown at 40°/27° C. In general, the gas exchange characteristics of Geraea and Dicoria were very similar to each other and to those reported for other C₃ desert annuals. Geraea and Dicoria experienced different seasonal patterns of change in several environmental variables. For Geraea, maximum daily air temperature (T _a) increased from 24° to 41° C over its growing season while Dicoria experienced maximum T _a at midseason (45° C). At points during their respective growing seasons when midday T _a ranged between 35° and 40° C, leaf temperatures (T ₁) of both species were below T _a and, therefore, were closer to the photosynthetic temperature optima measured in the laboratory. Leaf conductances to water vapor (g ₁) and water potentials () were high at these times, but later in their growing seasons Dicoria maintained high g ₁ and while Geraea showed large decreases in these quantities. The ability of Dicoria to successfully growth through the hot, dry summers of the California deserts may be related to its ability to acquire the available water in locally mesic habitats.     

The functional morphology of juvenile plants tolerant of strong summer drought in shaded forest understories in southern Spain

It has been hypothesized that plants cannot tolerate combined shade and drought, as a result of morphological trade-offs. However, numerous plant species are reportedly widespread in shaded forest understories that face drought, whether seasonal or occasional. We studied juveniles of six plant species that cope with strong summer drought in the understoreys of mixed Quercus forests in southern Spain: the tall-shrubs Phillyrea latifolia and Viburnum tinus, the perennial herb Rubia peregrina, the small shrub Ruscus aculeatus, and climbers Hedera helix and Smilax aspera. All of these species persist in evergreen shade (c. 3% daylight). Two other species were studied as comparators, Ruscus hypoglossum, less tolerant of drought, and Ceratonia siliqua, less tolerant of shade. Morphological and chemical variables relevant to shade and drought tolerance were measured for juveniles in a range of sizes, and also for the leaves of mature plants. The species converge in features that confer tolerance of shade plus drought by reducing demand for resources. Demand for water is reduced through a moderate to high below-ground mass fraction and low to moderate specific leaf area (respectively 0.22–0.52 and 112–172 cm² g^–1 at 1.00 g total dry mass). Demand for both irradiance and water is reduced through a low to moderate foliar nitrogen concentration and long-lived, physically protected leaves (2 yr). The species also converge in features that confer tolerance of either low irradiance or drought through specialized capture of resource, without precluding the other tolerance. These features include deep roots relative to shoot size, moderately higher specific leaf area in shade (1.2–2.0 × that in sun) and higher chlorophyll:nitrogen ratio in shade. Foliar chlorophyll per unit mass was higher in shade, but chlorophyll was not necessarily synthesized in greater amounts; rather, it was higher apparently due to shade effects on structural features linked with specific leaf area. In contrast, N per unit mass was higher in sun leaves independently of specific leaf area. Despite these convergences, the species diverge considerably in their root mass allocation and architecture, leaf saturated water content, density of stomata and guard cell size. No single narrowly defined functional type is needed for tolerance of shade plus drought.     

Cold tolerance of Littorinidae from southern Africa: intertidal snails are not constrained to freeze tolerance

All intertidal gastropods for which cold tolerance strategies have been assessed have been shown to be freeze tolerant. Thus, freeze tolerance is considered an adaptation to the intertidal environment. We investigated the cold tolerance strategies of three species of subtropical and temperate snails (Gastropoda: Littorinidae) to determine whether this group is phylogenetically constrained to freeze tolerance. We exposed dry acclimated and wet rehydrated snails to low temperatures to determine temperature of crystallisation (Tc), lower lethal temperature and LT₅₀ and to examine the relationship between ice formation and mortality. Tc was lowest in dry Afrolittorina knysnaensis (–13.6±0.4 °C), followed by dry Echinolittorina natalensis (–10.9±0.2 °C) and wet A. knysnaensis (–10.2±0.2 °C). The Tc of both A. knysnaensis and E. natalensis increased with rehydration, whereas Tc of dry and wet Afrolittorina africana did not differ (–9.6±0.2 and –9.0±0.2 °C respectively). Wet snails of all species exhibited no or low survival of inoculative freezing, whereas dry individuals of A. knysnaensis could survive subzero temperatures above –8 °C when freezing was inoculated with ice. In the absence of external ice, Afrolittorina knysnaensis employs a freeze-avoidance strategy of cold tolerance, the first time this has been reported for an intertidal snail, indicating that there is no family-level phylogenetic constraint to freeze tolerance. Echinolittorina natalensis and A. africana both showed pre-freeze mortality and survival of some internal ice formation, but were not cold hardy in any strict sense.     

Subtle relationships: freshwater fishes and water chemistry in southern South America

We investigated the relationships between water chemistry and the occurrence, distribution, physiology, and morphology of fish faunas. We examined 34 species (ca. 10% of the Argentinean freshwater fish fauna) from 120 localities (5 areas) situated between 26°15 S (Trancas, Tucumán) and 38°30 S (Sierra de la Ventana, Buenos Aires). Fourteen chemical features are described by: conductivity, total dissolved solids, temperature, pH, CO ₃ ^2– , CO₃H^–, Cl^–, SO ₄ ^2– Ca²⁺, K⁺, Mg²⁺, Na⁺, Mg/Ca, Mg+Ca/Na+K. Three Basic Data Matrices considering the mean, maximum and minimum values of each variable for each fish species were used in a Cluster and Principal Component Analysis. Groups of species clustered in similar ways to particular water chemistries. Similarity was the common occurrence of species in a defined area and preference for a common range of the factors considered. Groups of species so defined showed patterns of distribution related to climate, environment, trophic state and hydrographic complexity. Each cluster included some eurytopic species which appeared together at extreme chemical and geographic characteristics. Twenty four species had ranges of tolerance for the 14 variables and evidence of a grouping according to these ranges. Eighteen species which occurred at maximum or minimum absolute values for more than one factor were ordered along an eurytopy — stenotopy axis. We support the statement that species with a larger tolerance range for most factors have a higher probability of being widely distributed. Astyanax fasciatus and A. bimaculatus tolerated the highest number of maximum and minimum values, followed by Jenynsia l. lineata, A. eigenmanniorum and Trichomycterus corduvensis. Groups of species based on chemical factors showed differences in the relative number of basic morphological types.This paper was submitted at the symposium Fish Ecology in Latin America during the 1993 meeting of the ASIH at Austin.Consejo Nacional de Investigaciones CientíficasInstituto de Limnología de La PlataFacultad de Medicina, Universidad Nacional de La Plata     

Dry matter production and photosynthetic capacity in Gossypium hirsutum L. under conditions of slightly suboptimum leaf temperatures and high levels of irradiance

Summary Gossypium hirsutum L. var. Delta Pine 61 was cultivated in controlled-environment chambers at 1000–1100 mol photosynthetically active photons m^-2 s^-1 (medium photon flux density) and at 1800–2000 mol photons m^-2 s^-1 (high photon flux density), respectively. Air temperatures ranged from 20° to 34°C during 12-h light periods, whereas during dark periods temperature was 25° C in all experiments. As the leaf temperature decreased from about 33° to 27° C, marked reductions in dry matter production, leaf chlorophyll content and photosynthetic capacity occurred in plants growing under high light conditions, to values far below those in plants growing at 27° C and medium photon flux densities. The results show that slightly suboptimum temperatures, well above the so-called chilling range (0–12° C), greatly reduce dry matter production in cotton when combined with high photon flux densities equivalent to full sunlight.Abbreviations DW dry weight - F _v variable fluorescence yield - F _M maximum fluorescence yield - PFD photon flux density (400–700 nm)     

Dehydration-responsive features of <Emphasis Type="Italic">Atrichum undulatum</Emphasis>

Drought is an increasingly important limitation on plant productivity worldwide. Understanding the mechanisms of drought tolerance in plants can lead to new strategies for developing drought-tolerant crops. Many moss species are able to survive desiccation—a more severe state of dehydration than drought. Research into the mechanisms and evolution of desiccation tolerance in basal land plants is of particular significance to both biology and agriculture. In this study, we conducted morphological, cytological, and physiological analyses of gametophytes of the highly desiccation-tolerant bryophyte Atrichum undulatum (Hedw.) P. Beauv during dehydration and rehydration. Our results suggested that the mechanisms underlying the dehydration–recovery cycle in A. undulatum gametophytes include maintenance of membrane stability, cellular structure protection, prevention of reactive oxygen species (ROS) generation, elimination of ROS, protection against ROS-induced damage, and repair of ROS-induced damage. Our data also indicate that this dehydration–recovery cycle consists not only of the physical removal and addition of water, but also involves a highly organized series of cytological, physiological, and biochemical changes. These attributes are similar to those reported for other drought- and desiccation-tolerant plant species. Our findings provide major insights into the mechanisms of dehydration-tolerance in the moss A. undulatum.