Growth and survival of Azolla filiculoides in Britain. II. Sexual reproduction

Sporulation in the floating fern Azolla filiculoides Lam. is both frequent and widespread in Britain and might therefore play a greater part in the population dynamics of the species than has been suggested by earlier reports. In laboratory experiments, increasing plant density and/or phosphorus supply resulted in increased sporulation. It was estimated that a thick mat of 8 kg m² fresh biomass can produce 380000 microsporocarps and 85000 megasporocarps per m².
Light and temperatures >10°C were necessary for sporocarp germination. Sporocarps could survive exposure to both low temperatures (5°C for at least 3 months) and sub-zero temperatures (−10°C for at least 18 d). Sporocarps were found to survive storage in water for 3 yr and to germinate from mud samples collected in the field. In laboratory culture, sporeling growth and survival were optimal at 15°C.
There is some evidence that A. filiculoides might have adapted to the British climate since its introduction.     

Shoot temperatures and thermal tolerances for succulent species of Haworthia and Lithops

Abstract. Temperatures of small succulent plants, such as species in the genera Haworthia and Lithops , are highly influenced by temperatures of the surrounding soil. Indeed, the minimum and the maximum temperatures of the upper leaf epidermis of Haworthia retusa. H. turgida. Lithops leslei , and L. turbiniformis were generally within 1°C of the accompanying soil surface temperatures. An energybudget model closely predicted such soil-to-plant temperature differences as well as the effect of the greater convective exchange for the protruding Haworthia species compared with the Lithops species, which were flush with the soil surface. Although a lower shortwave absorptance would reduce maximum shoot temperatures, the shortwave absorptances of all four species were similar to those of the soil in their respective native habitats in South Africa. Tolerances of the four species to low and to high temperatures at three different day/night air temperatures (15°C/5°C, 30°C/20°C, and 45°C/35°C) were analysed using cellular accumulation of a vital stain, neutral red. Chlorenchyma cells were slightly more tolerant of extreme temperatures than were cells of the water-storage parenchyma. In this regard, H. retusa survived low and high temperatures that killed the water-storage parenchyma but not the chlorenchyma. Acclimation to low temperatures and to high temperatures, which was exhibited by all four species, led to estimated tolerances to 1 h at −16°C and 68°C. Although the low temperature tolerance is not particularly noteworthy, very few vascular plants are reportedly able to tolerate such high temperatures.     

THE COMBINED EFFECTS OF LOW TEMPERATURE AND SO2+NO2 POLLUTION ON THE NEW SEASON'S GROWTH AND WATER RELATIONS OF PICEA SITCHENSIS

Picea sitchensis (Bong.) Carr. seedlings were exposed to SO₂, NO₂ and SO₂+ NO₂ during dormancy in controlled environments, and were taken to night temperatures of 4, 0, −5, −10 and −15 °C in a freezer. Conditions in the freezer were carefully monitored during the low–temperature treatments. In two experiments, different photoenvironments and temperature regimes were imposed prior to the cold treatments, and different effects were observed. In the first, only limited frost hardiness was achieved and night temperatures of −15 °C were lethal. Temperatures of −5 and − 10 °C led to poor survival of lateral buds, particularly in plants exposed to 45 ppb SO₂. The poor bud break in plants exposed to SO₂ and to − 5 °C resulted in a loss of the effectiveness of this temperature as a chill requirement. Pressure-volume analysis showed that the shoots of plants exposed to NO₂ had greater elasticity (lower elastic moduli, e), so that loss of turgor occurred at lower relative water contents. In contrast, a hardening period (2 weeks in night/day temperatures of 3/10 °C and 8 h days at 50 μmol m⁻² s⁻¹ PAR) gave decreased elasticity and lower solute potentials of spruce shoots. In the second experiment, exposure to 30 ppb SO₂ and SO₂+ NO₂ led to slight, but consistent, increases in frost injury to the needles of plants frozen to − 5 and − 10 °C. The results suggest that the main interaction of low temperatures and winter pollutants may be on bud survival rather than on needle damage, but that effects are subtle, only occurring with certain combinations of pollutant dose and cold treatment.     

SEASONALITY AND THERMAL ACCLIMATION OF REACTIVE OXYGEN METABOLISM IN FUCUS VESICULOSUS (PHAEOPHYCEAE)

The intertidal brown macroalga Fucus vesiculosus L. acclimates its defense against reactive oxygen in response to both (1) growth at different temperatures in laboratory culture and (2) seasonal changes in environmental conditions. Fucus vesiculosus was grown in seawater at 0° C, 20° C, and at 0° C with a 3-h daily emersion at −10° C. Algae grown at low temperature, both with and without freezing, produced less reactive oxygen after severe freezing stress than those grown at 20° C. These differences were correlated with growth temperature-induced changes in activities of superoxide dismutase (SOD), glutathione reductase, and ascorbate peroxidase. The contents of tocopherols increased with increased cultivation temperature, whereas the activity of catalase and the content of glutathione and ascorbate did not change. Growth at 0° C increased the resistance of photosynthesis to freezing and reduced photoinhibition in high light at 5° C; the latter effect was further increased in algae subject to daily freezing. These data suggest that elevated activity of reactive oxygen scavenging enzymes, especially SOD, increases the resistance to photoinhibition, at least at low temperature, as well as being important for freezing tolerance. Seasonal changes in reactive oxygen metabolism showed a similar pattern to those elicited by temperature in laboratory culture. Summer samples had lower activities of most reactive oxygen scavenging enzymes than algae collected in autumn and winter when water temperatures were lower. In contrast to the laboratory experiments, ascorbate content did change and was lower during the winter than summer, whereas the content of glutathione was not influenced by season. Overall, the data not only indicate that temperature plays an important role in the regulation of stress tolerance and reactive oxygen metabolism but also suggest that other factors are also involved.     

The effects of selection for cold tolerance on cross-tolerance to other environmental stressors in Drosophila melanogaster

Abstract  Cross tolerance, whereby tolerance to one environmental stress is correlated with tolerance to other stressors, is thought to be widespread in insects. We used lines of Drosophila melanogaster Meigen (Diptera: Drosophilidae) selected for survival at a 1-h exposure to −5°C to examine the extent to which this selection results in increased tolerance to other stresses, including high and low temperatures, desiccation and starvation. While selection improved tolerance to acute cold exposure and survival at −5°C, there was little effect of selection regime on tolerance to other stressors. There was no correlation between tolerances to any of the stressors, suggesting different mechanisms of tolerance. This supports arguments that correlations between stress tolerances during selection experiments with D. melanogaster may be coincidental. The magnitude of heat-hardening was apparently constrained by basal tolerance among lines, but the magnitude of the rapid cold-hardening response was not correlated with basal cold tolerance, implying that the relationship between inducible and basal tolerances differs at high and low temperatures.     

Effect of temperature on development and survival in Delias nigrina (Fabricius) (Lepidoptera: Pieridae)

Abstract The effect of temperature on rate of development and survival of the immature stages of a subtropical population of the black jezebel, Delias nigrina , was studied under laboratory conditions at a range of constant temperatures. Mean developmental times from first-instar larva to adult varied from 29 days at 27°C to 52 days at 19°C; the development threshold temperature and thermal constant were estimated to be 9°C and 494 degree-days, respectively. Larval developmental rates reached physiological maximum at the higher temperatures tested (25−27°C). Pupal development, by contrast, was not affected in the same way as larvae by higher temperature. Survival of the immature stages varied inversely with temperature: survival was highest at 19°C and significantly reduced at 27°C. Mortality at the higher temperature was attributable mainly to final-instar larvae and pupae. These findings indicate that, compared with other tropical pierids that have been studied, D. nigrina has: (i) a comparatively low temperature threshold; (ii) a slow rate of development; and (iii) a poor tolerance to moderately high temperatures. Physiologically, these features are more characteristic of a temperate butterfly than a tropical one. This physiological response appears to be reflected by the temperate nature of the genus as a whole, which may be related to its period of origin and evolution during past climatic events.     

The effect of temperature on the life cycle, growth and fecundity of Branchiura sowerbyi (Oligochaeta: Tubificidae)

Branchiura sowerbyi Beddard was probably introduced to Britain with exotic plants consigned to botanic gardens. The British populations occur mainly in artificially warmed habitats such as lily ponds in botanic gardens and power station effluents but a few are found at natural temperatures.
Branchiura completed its life cycle in about a year both in the river Avon, Wiltshire, at natural temperatures and in the warm effluent from a power station discharging into the river Thames. In a worm culture maintained at natural temperatures the rate of cocoon production reached a peak in summer and was closely related to temperature.
In the laboratory it was found that the optimum temperature for cocoon laying in mature worms, also growth in sexually immature worms, was near 25°C. The optimum temperature for growth in sexually mature worms, however, was lower (10°C in the Avon population and 15°C in the Thames population). It followed that at 20°–25°C the growth of sexually mature worms was probably depressed by the high rate of cocoon production.     

Rapid cold-hardening in a Karoo beetle, Afrinus sp.

Abstract.  In the insect rapid cold-hardening response, survival at subzero temperatures is greatly improved by a brief pre-exposure at a milder temperature. It is predicted that insects with minimal cold tolerance capabilities living in variable environments should use rapid cold-hardening to survive sudden cold snaps. This is tested in Afrinus sp., a beetle that lives in an exposed habitat on rock outcrops in the Karoo Desert, South Africa, where microclimate temperatures drop infrequently to below freezing. Afrinus sp. shows a significant rapid cold-hardening response: survival of a 2-h exposure to −6.5 °C is much improved after pre-exposure to −2 °C, to 0 °C with a 2-h return to the rearing temperature, and to 40 °C, but not after pre-exposure to 0 °C. Little is known about the mechanism of the rapid cold-hardening response, although the data suggest that rapid cold-hardening may be mediated via several different mechanisms.     

Freezing and desiccation tolerance of imbibed canola seed

Depending on the environmental conditions, imbibed seeds survive subzero temperatures either by supercooling or by tolerating freezing-induced desiccation. We investigated what the predominant survival mechanism is in freezing canola ( Brassica napus cv. Quest) and concluded that it depends on the cooling rate. Seeds cooled at 3°C h⁻¹ or faster supercooled, whereas seeds cooled over a 4-day period to −12°C and then cooled at 3°C h⁻¹ to−40°C did not display low temperature exotherms. Both differential thermal analysis and nuclear magnetic resonance (NMR) spectroscopy confirmed that imbibed canola seeds undergo freezing-induced desiccation at slow cooling rates. The freezing tolerance of imbibed canola seed (LT₅₀) was determined by slowly cooling to −12°C for 48 h, followed with cooling at 3°C h⁻¹ to −40°C, or by holding at a constant −6°C (LD₅₀). For both tests, the loss in freezing tolerance of imbibed seeds was a function of time and temperature of imbibition. Freezing tolerance was rapidly lost after radicle emergence. Seeds imbibed in 100 μ M abscisic acid (ABA), particularly at 2°C, lost freezing tolerance at a slower rate compared with water-imbibed seeds. Seeds imbibed in water either at 23°C for 16 h, or 8°C for 6 days, or 2°C for 6 days were not germinable after storage at −6°C for 10 days. Seeds imbibed in ABA at 23°C for 24 h, or 8°C for 8 days, or 2°C for 15 days were highly germinable after 40 days at a constant −6°C. Desiccation injury induced at a high temperature (60°C), as with injury induced by freezing, was found to be a function of imbibition temperature and time.     

Survival at low temperature of larvae of the pine processionary moth Thaumetopoea pityocampa from an area of range expansion

1 Larvae of Thaumetopoea pityocampa (Lepidoptera: Notodontidae) develop throughout the winter, although their feeding activity and survival can be impaired by adverse climatic factors. The present study investigated the survival at low temperature of larvae originating from a population with range expansion in an alpine valley in Northern Italy.
2 The supercooling point of individually analysed larvae averaged at −7 °C. This value insufficiently described the cold hardiness of the larvae; 39% of the tested larvae were alive when returned to room temperature immediately after freezing. When larval colonies inside their nest were exposed to −17 °C for 1 h after gradual temperature decrease, survival was 70.4%.
3 Rearing of larvae in the laboratory at different day/night temperatures indicated an effect of cumulative chill injury on larvae. A logistic regression explained the relationship between negative thermal sum (h°C below 0 °C) received in the laboratory experiment and larval survival. A similar relationship was demonstrated between negative thermal sum and survival of larval colonies in the field.
4 In the laboratory experiment, some tested larvae were able to survive for up to 8 weeks without feeding depending on rearing temperature. As expected, feeding occurred only when larvae were reared at temperatures of 9 °C day/0 °C night.
5 We classify the larvae of T. pityocampa as being moderate freezing tolerant. The winter behaviour allows this species to track climate warming by a rapid expansion into those areas that become compatible with the insect's development.     

Contact with the host plant enhances aphid survival at low temperatures

1. The hypothesis, suggested by previous studies, that host plant contact reduces the cold tolerance of anholocyclic aphids was tested under laboratory conditions. Adult and first-instar Rhopalosiphum padi (L.) were exposed to temperatures of 0, –5 and –10 °C on intact plants, excised leaves and in the absence of contact with plant material.
2. Median lethal time (LT₅₀) values at all three temperatures indicate that aphids exposed in association with plant material survive longer than aphids that have no contact with their host plant. The difference in survival was most pronounced at –10 °C. Therefore, the above hypothesis is rejected for aphids on cereals because host plant contact apparently enhances cold tolerance.
3. Exposure on excised leaves also enhanced aphid survival at low temperature but was less effective than the intact plant. This suggests that plant quality as well as the presence or absence of plants is important in the cold tolerance of aphids on cereals.     

Ultrastructure-function correlative studies for cardiac cryopreservation. II. Hearts frozen to various temperatures without a cryoprotectant

Isolated rat hearts were perfused with balanced salt solution (BSS) for 20 min, sealed in a metal cannister, and cooled in a −20 °C acetone bath at a rate of 1 °C/min to one of four subzero core temperatures (−10, −12, −17, or −20 °C). Upon attainment of the desired temperature the hearts were rapidly thawed (40–50 ° C/min) and reperfused with BSS for an additional 20 min. Approximately half of the hearts cooled to −10 or −12 °C resumed spontaneous contractile activity after thawing. One of 16 hearts survived cooling to −17 °C, while no heart survived cooling to −20 °C. Nonfrozen controls gave a positive inotropic response to a standard test dose of ouabain; none of the thawed survivors did.     

Ultrastructural changes arising from freezing of leaf blade cells of rye (Secale cereale): an investigation using freeze-substitution

The survival at sub-zero temperatures of leaf blade cells of rye ( Secale cereale L. cv. Voima), which had not been cold acclimated, was determined by measuring the efflux of ninhydrin-positive substances: 50% of the cells were dead at −4°C (LT₅₀) and none survived at −12°C or below. Examination of ultrastructural changes during cold hardening and freezing injury requires frozen tissues prepared for transmission electron microscopy without thawing. Specimens were prepared from leaf blade segments at room temperature, −4°C or −12°C by plunge freezing at 3 m s⁻¹ into a cooling medium at −170°C followed by freeze-substitution in acetone with OsO₄ fixation. Comparisons of room temperature specimens were made with those prepared by chemical fixation using glutaraldehyde/paraformaldehyde/tannic acid. On freezing to −12°C, the cells were severely dehydrated and distorted, the vacuoles severely shrunken and the cytoplasm and mitochondria disorganized whereas the chloroplasts were little affected. On freezing to −4°C, some cells were as disorganized as those at −12°C, others were relatively intact, and some showed evidence of intracellular ice crystal formation.     

High and low temperature tolerances and their relationships to distribution of agaves

Abstract. The potential influence of tissue tolerances to extreme temperatures on distributional limits was investigated for 15 taxa (14 species) of leaf-succulent agaves from the south-western United States and northern Mexico. As a group, the agaves exhibited a moderate low temperature tolerance of – 11°C (based on a 50% inhibition in the number of mesophyll cells taking up a stain, neutral red). However, nearly all of the species were able to tolerate extremely high tissue temperatures of over 60°C. Nocturnal acid accumulation by these crassulacean acid metabolism plants was about 6°C more sensitive to temperature extremes than was cellular membrane integrity.
High and low temperature acclimation in response to changing day/night air temperatures was observed in all 15 taxa, with high temperature acclimation averaging two-fold greater than low temperature acclimation (3.8°C versus 2.0°C per 10°C change in ambient temperature). Species occupying the coldest habitats exhibited the greatest low temperature tolerances and acclimation; several such species, such as Agave utahensis and A. schottii , had small rosette sizes which resulted in higher minimum leaf temperatures. Species from the hottest habitats had among the greatest high temperature tolerances and acclimation; the two species from open desert scrub habitats, A. deserti and A. lecheguilla , had the lowest leaf shortwave absorptances observed, which would result in lower maximum leaf temperatures. Thus morphology and tissue tolerances to stressful temperatures reflect the temperature extremes of a plant's native habitat, although low temperature tolerance appears to limit the distribution of agaves more than high temperature tolerance.     

Potato cold hardiness development and abscisic acid. II. De novo synthesis of proteins is required for the increase in free abscisic acid during potato (Solanum commersonii) cold acclimation

During cold acclimation of potato plantlets ( Solanum commersonii Dun, PI 458317), there are two transitory increases in free ABA content corresponding to a three-fold increase on the 2nd day and a five-fold increase on the 6th day (Ryu and Li 1993). During this period, plantlets increased in cold hardiness from −5°C (killing temperature, control grown at 22/18°C, day/night) to −10°C by the 7th day of exposure to 4/2°C (day/night). This increase in free ABA was not found when cycloheximide (CHI), an inhibitor of cytoplasmic protein synthesis, was added to the culture medium 6 h before exposure to low temperatures. Plantlets treated with CHI did not acclimate to cold, maintaining a hardiness level (−5°C) similar to that of the 22/18°C-grown plantlets. When the CHI-treated plantlets were exposed to low temperatures for 3 days, transferred to CHI-free culture medium and grown at low temperatures, the plantlets showed a transitory increase in free ABA 2 days later. This increase was followed by the development of cold hardiness (−8°C). Application of CHI to the culture medium after 3 days of cold acclimation, when the first ABA peak and a partial development of cold hardiness (−8°C) had occurred, blocked the second transitory increase in free ABA and resulted in no further development of cold hardiness. These results suggest that de novo synthesis of proteins is required for these transitory increases in free ABA during cold acclimation of potato plantlets.     

Stress tolerance and burrowing behaviour in the southern African millipede Alloporus uncinates

Laboratory experiments were carried out to assess the tolerance and behaviour of the southern African spirostreptid millipede Alloporus uncinates (Attems) to moisture and temperature stress. Rates of water loss in dry air were 0–026 mg H₂0 cm^-2 hour^-1 for females and 0021 for males and remained relatively constant with an increase in temperature from 20 to 30°C but at higher temperatures there was a rapid increase in water loss, especially for females. In dry laboratory conditions at 30°C all individuals died within 30 days, whilst at 20°C at least 90% of individuals survived this period. Aggregation appeared to have no significant effect on survivorship but small body size conferred a survivorship advantage. Females burrowed earlier and to a greater depth than males, and initial moisture content of soil had a significant effect on mean burrow depth. Alloporus uncinatus , like other spirostreptid millipedes, appears to have considerable tolerance to dry conditions but the wide geographic and habitat range of this species may mean that the intensity of selection for tolerance to moisture stress may vary.     

Primary and secondary induction requirements for flowering of Festuca rubra

Root and shoot temperatures were varied independently to determine the importance of root temperature during cold acclimation. Spinach ( Spinacia oleracea L. cvs Harbin and Bloomsdale) plants were subjected to 20/20°C. 20/5°C, 5/20°C, and 5/5°C (shoot/root) temperature treatments. Leaf freezing tolerance, water potential, stomatal resistance, osmotic potential, and water content were measured at 0.25. 1.25, 3.25, and 7.25 days of treatment. There was no change in freezing tolerance or the water relations of the 20/20°C treated plants during the course of the experiment. Freezing tolerance was increased by the 5°C shoot temperature treatments, but was not enhanced by water stress induced by the low root temperature. Leaf water potential and water content decreased and stomatal resistance increased within 6 h in the 20/5°C plants. By day 3, osmotic potential began decreasing in the 20/5°C plants. Leaf water content, osmotic potential, and water potential decreased more gradually in plants grown with 5°C shoot temperature, irrespective of root temperature. Decreased water content and osmotic potential were not correlated with increased freezing tolerance as reported for other herbaceous crop plants.     

Field ecology of freeze tolerance: interannual variation in cooling rates, freeze-thaw and thermal stress in the microhabitat of the alpine cockroach Celatoblatta quinquemaculata

Microclimate recordings were made over four years on the Rock and Pillar Range, New Zealand. These are used in conjunction with mortality and freezing data derived in the laboratory to make inferences about the winter thermal ecology of the freeze-tolerant alpine cockroach Celatoblatta quinquemaculata (Dictyoptera: Blattidae). Threshold temperatures are identified through laboratory experiments at ecologically relevant cooling rates: 0°C; −3.1°C (FP₅: the temperature at which 5% of cockroaches are expected to be frozen); and −4.5°C (FP₅₀). The maximum cooling rate in the microhabitat across any of the thresholds was 0.06°C min⁻¹, considerably slower than the 1°C min⁻¹ normally used in laboratory studies. Freeze-thaw events occurred regularly in the field, and temperature minima occasionally fell to temperatures lethal to C. quinquemaculata . Variability in snow cover contributed to interannual variation in microclimate temperatures. Decreased snow cover is predicted with climate change scenarios, and this will probably result in more freeze-thaw cycles and more extreme minimum temperatures in this environment. It is concluded that the limited environmental tolerances of the animals living in this habitat make the few degrees of interannual variation ecologically relevant.     

Effects of temperature on the establishment potential in the U.K. of the non-native glasshouse biocontrol agent Macrolophus caliginosus

Abstract This study investigated the effect of temperature on the development and winter survival of the predatory mirid Macrolophus caliginosus Wagner, recently introduced into the U.K. as a biocontrol agent for glasshouse whitefly Trialeurodes vaporariorum . The developmental threshold for M. caliginosus calculated by three methods was between 7.3 and 8.4 °C, with a day-degree requirement per generation varying between 472 and 524 day-degrees. It was estimated that under outdoor conditions M. caliginosus could complete two generations per year in the U.K. All life stages of M. caliginosus had supercooling points around −20 °C, with some pre-freeze mortality evident in both acute and chronic low temperature exposures. Acclimation increased survival of nymphal M. caliginosus from approximately 24–52 days when exposed to a constant 0 °C. Provision of prey extended survival of nymphs in the laboratory at a constant 5 °C from 39 to 64 days and in the field by c . 150 days. The results are discussed in the context of the occurrence and establishment of M. caliginosus in the U.K. and the need to develop a reliable risk assessment system for non-native species used in glasshouse biocontrol.     

Structural changes and impairment of function associated with freezing and thawing in muscle, nerve, and leucocytes

In some tissues, especially of skeletal muscle, leucocytes, and peripheral nerves, the effects of freezing to low temperatures, e.g., −78 or −150 °C, and thawing have been a total loss of function and a severe cellular structural disturbance. In the case of muscle, the fibers are converted during thawing into a highly condensed form, called thaw-rigor, in which the individual sarcomeres are shortened beyond a supercontracted state. This effect must be avoided if muscle is to be frozen and stored successfully. Leucocytes and especially the neutrophils are seriously affected by exposure to low temperature. The result is a general swelling of the cell and of its nucleus which is suggestive of damage to the membranes of the cell. Rat cutaneous nerves frozen and exposed to temperature below −15 °C showed after thawing a general disturbance of the myelin involving rupture and separaration of the lamellae. In the axoplasm the neurofilaments and neurotuabules are converted into small aggregates and the mitochondria appear to be swollen. Nerves frozen at −5 and some at −10 °C showed little change in structure, the myelin and axoplasm exhibiting a normal appearance. A check on viability by excitation from an electric stimulus indicated survival after exposure to −5 and in a few cases to −10 °C.