Growth,sugar accumulation,and dark respiration of suspension cell cultures derived from contrasting alfalfa cultivars

Fall dormancy results in decumbent, slow shoot growth of alfalfa (Medicago sativa L.) in autumn and reduced shoot regrowth rates after herbage removal in summer. Although fall dormancy is used to predict alfalfa adaptation, we possess a poor understanding of the biological mechanisms underlying fall dormancy. Our objective was to examine growth and carbohydrate metabolism of suspension cell cultures derived from contrasting alfalfa cultivars that genetically differed in fall dormancy. Suspension cells were grown in B5h media containing 2% sucrose. Cells derived from fall non-dormant plants accumulated sugars more rapidly after transfer to fresh media and to higher concentrations than did cells derived from fall dormant alfalfa cultivars. Dark respiration rates of cells derived from non-dormant plants were similar to those derived from fall dormant plants when growth was limited at low cell sugar concentrations. However, both cell growth and dark respiration rates increased in cells derived from non-dormant cultivars in response to greater cell sugar concentrations. High growth rates of cells derived from rapid growing, fall non-dormant alfalfa cultivars were associated with rapid sugar uptake and higher cell respiration rates when compared to cells derived from dormant alfalfa cultivars.     

Cryopreservation of alfalfa (Medicago sativa L.) cells by encapsulation-dehydration

Our objective was to establish a cryopreservation protocol for alfalfa (Medicago sativa L.) cells and study the physiological changes occurring in cells during cryopreservation treatment. Cell cultures of Pioneer cvs. 5262 (fall-dormant, winter-hardy) and 5929 (non-dormant, non-hardy) plants initiated regrowth after cryopreservation by encapsulation-dehydration (ED). Pre-treatment of the encapsulated cells for 4 days in B5 medium containing 0.75 M sucrose and dehydration for 4 h in a laminar flow hood were necessary to achieve maximum cell viability after ED and cryopreservation in liquid N₂ (EDN). Viability (measured as triphenyl tetrazolium chloride reduction) of the cv. 5262 cells after cryopreservation was two- to three-fold greater than that of the cv. 5929 cells. Cold acclimation of the cells (10 days at 2°C) improved viability after cryopreservation. The addition of 7.6 µM ABA to the B5 medium enhanced viability in ED but did not improve cell cryopreservability. Cold-acclimated cells had higher protein concentrations, but neither ABA nor cold acclimation influenced protein composition of cold-acclimated cells determined using SDS-PAGE. Encapsulated cells pre-treated for 4 days in B5 medium containing 0.75 M sucrose showed an increased concentration of cell protein and an altered protein composition. Suspension cultures were re-initiated from both ED and EDN treatments by transferring beads sequentially to B5 media containing 0.75, 0.5, 0.25 M sucrose and then to fresh B5 medium. The ED cells resumed rapid growth after two subcultures, whereas EDN cells needed four or five subcultures to resume rapid growth.     

Alfalfa water status and cold hardiness as influenced by cold acclimation and water stress

Abstract Water stress at a nonacclimating temperature (18–20°C) increased the cold hardiness of Medicagosativa L. (alfalfa) plants. This increased cold hardiness was retained when the previously water-stressed plants were cold acclimated (2–9°C) in the absence of water stress. Water stress during cold acclimation also increased cold hardiness. Alfalfa was demonstrated to suffer injury, measured as decreased growth following freezing, at sub-lethal temperatures. During cold acclimation the turgor potential (ψ) of watered plants increased, whereas the solute potential and the water content per unit dry weight decreased. The large positive psgrdap of acclimated plants indicates that the decreased water content per unit dry weight is related to an increased proportion of tissue dry matter rather than to tissue dehydration.     

In vitro screening for cold hardiness of raspberry cultivars

Raspberry (Rubus idaeus L.) cultivars ‘Festival’, ‘Titan’ and ‘Willamette’ were cultured in vitro on three different media: (A) MS medium supplemented with 1.0 mg l^-1 BAP and 0.1 mg l^-1 IBA, (B) MS medium without growth regulators, and (C) MS medium with reduced sucrose (10 g l^-1), and exposed to different low temperature acclimation treatments: (1) control, no acclimation, (2) 1 week at +15 °C, 1 week at +2 °C, 24 h at -2 °C and 3 days at +2 °C, and (3) 2 weeks at +15 °C, 2 weeks at +2 °C, 24 h at −2 °C and 3 days at +2 °C. After acclimation, shoot moisture content was measured, and cold hardiness (LT₅₀) was determined by controlled freezing. Shoot moisture content was generally lower on culture medium B compared to the other media, but not affected by acclimation treatment. In non-acclimated plants, medium composition had no effect on cold hardiness and no cultivar differences in hardiness were observed. After acclimation, plants on culture medium B were on average more cold hardy than on the other media. Acclimation treatment 3 on media A and B allowed the best discrimination between the hardy cultivar ‘Festival’ and less cold hardy ‘Titan’ and ‘Willamette’. When acclimation treatments were tested further using 11 raspberry cultivars with different levels of cold hardiness, discrimination between cultivars was satisfactory only after acclimation treatment 3 on culture medium B. This revised version was published online in June 2006 with corrections to the Cover Date.     

Low temperature effects on growth and actin cytoskeleton organisation in suspension cells of winter oilseed rape

Rhodamine-phalloidin staining of winter oilseed rape suspension cells revealed that the structure of actin cytoskeleton changes with the phase of cell growth. In small, 4-day-old cells, entering the exponential phase of growth, a dense and uniformly distributed cortical microfilament networks was seen. In six-day-old vacuolated cells, which reached the stationary phase of growth, the actin cytoskeleton was composed of thicker microfilament cables in irregular arrangements. In cells acclimated in cold for 7 days a dense, uniformly distributed and cortical microfilament network was still seen. The fine microfilament network was sensitive to extracellular freezing since the structures underwent depolymerization at −3 °C (in the presence of extracellular ice), both in non-acclimated and cold-acclimated cells. The thicker transvacuolar cables in cells of the stationary growth phase resisted freezing to −7 °C. Acclimation of suspensions at 2 °C resulted in slowing down growth of cells and in the increased freezing tolerance of cells as indicated by a decrease of LT₅₀ from −11 °C to −17.5^o or to −25 °C when determined 7 or 20 days after the beginning of the cold treatment, respectively. Freezing tolerance of non-acclimated cells decreased from −11 °C to −8 °C during subculture, showing a transient increase to −17 °C on the day 6. Results indicate that the arrangement of actin microfilaments and their sensitivity to freezing-induced depolymerization depends on the phase of cell growth rather than on cell acclimation status. Possible mechanisms involved in the freezing-induced depolymerization of actin microfilaments are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Temperature on Cold Acclimation and Deacclimation in Flower Buds of Evergreen Azaleas

The effects of various storage temperature/duration combinations(5, 10 and 17°/4, 8, 12 and 16 weeks) on cold acclimationand deacclimation of flower buds were studied in four speciesof evergreen azaleas having different natural distribution andcold hardiness. The freezing process and the exotherm temperaturedistribution of florets in excised whole buds determined bydifferential thermal analysis were used as the diagnostics todetermine the degree of bud acclimation and deacclimation. Theacclimation in buds lasted for as long as 12 to 16 weeks at5°C storage, and from 8 to 12 weeks at 10°C, and itappeared to be maintained after the chilling requirement forbreaking bud dormancy had been satisfied. Therefore, bud acclimationseems to be maintained independently from bud dormancy. Thedehardening effect on acclimated buds occurred as a result ofshort exposures to higher temperatures or long exposures tolower temperatures, and there was no relation between the rateof deacclimation and the degree of hardiness in each species.Among three storage temperatures examined, 5°C was the mosteffective for the maintenance of cold acclimation in flowerbuds and the small difference of floret water contents at 5and 10°C storage is not significant. (Received August 28, 1982; Accepted February 4, 1983)     

Chill-responsive dehydrins in blueberry: Are they associated with cold hardiness or dormancy transitions?

To survive winters, woody perennials of temperate zones must enter into endodormancy. Resumption of spring growth requires sufficient exposure to low temperature (chill units, CUs) in winter (chilling requirement), which also plays a role in the development of cold hardiness (cold acclimation). Physiological studies on dormancy breaking have focused on identifying markers, such as appearance or disappearance of proteins in response to varying degrees of chill unit accumulation. However, whether these changes are associated with dormancy transitions or cold acclimation is not clear. In the present study, greenhouse-grown blueberry (Vaccinium section Cyanococcus) plants were used to address this question. Three blueberry cultivars, Bluecrop, Tifblue, and Gulfcoast having chilling requirement of approximately 1 200, 900 and 600 CUs, respectively, were first exposed to 4°C for long enough to provide chill units equivalent to one-half of their respective chilling requirement. This treatment was expected to result in cold acclimation. A fraction of plants was then subjected to a 15/12°C (light/dark) regime for 2 weeks, a treatment expected to be “dormancy-neutral” but cause deacclimation. Before and after each treatment, cold hardiness and dormancy status of floral buds were determined; proteins were extracted from the buds collected on the same sampling date, and separated by one-dimensional SDS-PAGE. Dehydrin-like proteins were identified by immunoblotting, using anti-dehydrin antiserum. Results indicate that the chilling treatment resulted in cold acclimation as indicated by increased bud hardiness in all three cultivars. Data also indicate a distinct accumulation of three dehydrin-like proteins of 65, 60, and 14 kDa during cold acclimation. The cold hardiness and levels of dehydrin proteins decreased during the exposure to 15/12°C for 2 weeks. Results also confirmed that this treatment had no negative effect on chill unit accumulation. Densitometric scans of protein gels indicated a close association between the abundance of dehydrins and degree of cold hardiness in these cultivars. In addition, levels of the dehydrin proteins and cold hardiness remained about the same between 100% and >100% satisfaction of chilling requirement. These results suggest that changes in dehydrin expression are more closely associated with cold hardiness than with dormancy transitions.     

Mechanisms underlying the effects of fall dormancy on the cold acclimation and winter hard- iness of Medicago sativa

As a global planting forage legume, alfalfa (Medicago sativa) is a valuable material to study the evolutionary and ecological mechanisms on plant adaptation to freezing due to their contrasting winter hardiness induced by fall dormancy (FD). This paper reveals that FD is an important growth characteristic that is adaptative to short-day and reducing temperature in late autumn, followed by a higher overwintering rate. Alfalfa cultivars are grouped into 11 FD ratings (numbered from 1 to 11), and this phenomenon is related to the extensive spread and cultivation for thousands of years in the globe. Alfalfa cultivars are under different climate habitats for a long time, leading to FD adaptive evolution, which provides rich genetic resources for human. In general, adaptative process associated with cold acclimation and winter hardiness in alfalfa is affected by FD, thus differences in winter hardiness exist among alfalfa cultivars. So far, regulation of FD by light and temperature and effects of FD on physiological and ecological processes involved in cold acclimation have been reported largely. However, signal transduction and the regulatory network associated with gene expression, especially the molecular mechanisms by which antifreeze proteins function in cold adaptation, are still poorly understood. Several scientific problems that need to be addressed in the future studies are highlighted in this review.     

紫花苜蓿秋眠性对低温驯化过程与越冬耐寒适应的作用机理

紫花苜蓿(Medicago sativa)是全球性的栽培牧草, 因其秋眠性强弱而导致的抗寒性差异为深刻解析植物耐寒适应的进化生态学机制提供了一个很好的研究模式。作为多年生豆科植物, 秋眠性是紫花苜蓿适应晚秋日照缩短、温度降低, 以及提高越冬存活率的一种生长特性。秋眠性的形成与几千年来紫花苜蓿在全球的传播扩展和栽培利用有关, 长期处于不同的气候生境, 导致秋眠性的适应进化, 这为人类利用提供了丰富的遗传资源。根据秋眠性的强弱, 学术界目前将之分为11个等级。一般而言, 秋眠性的强弱影响了紫花苜蓿的低温驯化与越冬耐寒适应等过程, 导致不同品种间的抗寒性存在差异。迄今, 关于秋眠性的光温调控, 以及秋眠性影响低温驯化的生理生态过程研究较多, 而对相关的细胞信号转导与基因表达调控途径, 尤其是对秋眠性如何影响越冬紫花苜蓿抗冻蛋白作用等耐寒适应的分子机制, 尚知之甚少。针对目前研究中存在的问题, 该文提出了未来需要重点关注的科学问题。     

Survival of poplar callus at super-low temperatures after cold acclimation

Callus derived from the cambial area of poplar twigs survivedfreezing down to –120?C or even the temperature of liquidnitrogen (–196?C), after cold acclimation. Exposure tocool preconditioning temperatures for a length of time beforebeing acclimated at 0?C is essential for enhancing fully thecallus hardiness. ¹ Contribution No. 1248 from the Institute of Low TemperatureScience. (Received May 30, 1973; )     

Effect of frost nights and day and night temperature during dormancy induction on frost hardiness, tolerance to cold storage and bud burst in seedlings of Norway spruce

For trees, the ability to obtain and maintain sufficient levels of frost hardiness in late autumn, winter and spring is crucial. We report that temperatures during dormancy induction influence bud set, frost hardiness, tolerance to cold storage, timing of bud burst and spring frost hardiness in seedlings of Norway spruce (Picea abies (L.) Karst.). Bud set occurred later in 12°C than in 21°C, and later in cool nights (7°C) than in constant temperature. One weekly frost night (−2.5°C) improved frost hardiness. Cool nights reduced frost hardiness early, but improved hardiness later during cold acclimation. Buds and stems were slightly hardier in 21°C than in 12°C, while needles were clearly hardier in 12°C. Cold daytime temperature, cool nights and one weekly frost night improved cold storability (0.7°C). Seedlings receiving high daytime temperatures burst buds later, and were less injured by light frost some days after bud burst.     

Extended alternating-temperature cold acclimation and culture duration improve pear shoot cryopreservation

Meristems of many pear genotypes can be successfully cryopreserved following 1 week of cold acclimation, but an equal number do not survive the process or have very little regrowth. This study compared commonly used cold acclimation protocols to determine whether the cold acclimation technique used affected the cold hardiness of shoots or the regrowth of cryopreserved meristems. In vitro-grown pear (Pyrus L.) shoots were cold acclimated for up to 16 weeks, then either the shoot tips were tested for cold hardiness or the meristems were cryopreserved by controlled freezing. Cold acclimation consisted of alternating temperatures (22 degrees C with light/-1 degrees C darkness with various photo- and thermoperiods) or a constant temperature (4 degrees C with an 8-h photoperiod or darkness). Compared with nonacclimated controls, both alternating- and constant-temperature acclimation significantly improved postcryopreservation regrowth of P. cordata Desv. and P. pashia Buch. -Ham. ex D. Don meristems. Alternating-temperature acclimation combined with either an 8-h photoperiod or darkness was significantly better than constant-temperature acclimation. Alternating-temperature shoot acclimation for 2 to 5 weeks significantly increased postcryopreservation meristem regrowth, and recovery remained high for up to 15 weeks acclimation. Postcryopreservation meristem regrowth increased with 1 to 5 weeks of constant-temperature acclimation and then declined with longer acclimation. Shoot cold hardiness varied with the acclimation procedure. The LT(50) of shoots acclimated for 10 weeks with alternating temperatures was -25 degrees C; that with constant temperature was -14.7 degrees C; and that of the nonacclimated control was -10 degrees C. Less frequent transfer of cultures also improved acclimation of shoots. Shoots grown without transfer to fresh medium for 6-12 weeks had higher postcryopreservation recovery with shorter periods of acclimation than shoots with a 3-week transfer cycle.     

Cold hardiness in summer and winter diapause and post-diapause pupae of the cabbage armyworm, Mamestra brassicae L. under temperature acclimation

Cold hardiness and biochemical changes were investigated in winter and summer pupae of the cabbage armyworm Mamestra brassicae at the diapause and post-diapause stages under temperature acclimation. Diapause pupae were successively acclimated to 25, 20 and then 10 degrees C (warm-acclimated group). Pupae at the diapause and post-diapause stages were successively acclimated to 5, 0, -5 and then -10 degrees C (cold-acclimated groups). Supercooling point values in winter and summer pupae remained constant regardless of the diapause stages and acclimated temperatures. Warm-acclimated pupae at the diapause stage did not survive the subzero temperature exposure, whereas, cold-acclimated pupae achieved cold hardiness to various degrees. Winter pupae were more cold hardy than summer pupae, and pupae at the post-diapause stage were more cold hardy than those at the diapause stage. Trehalose contents in winter pupae rose under cold acclimation. Summer pupae accumulated far lower trehalose contents than winter pupae, with the maximal level occurring in winter pupae at the post-diapause stage. Glycogen content remained at a high level in diapause pupae after warm acclimation, whereas it decreased after cold acclimation. Alanine, the main free amino acid in haemolymph after cold acclimation, increased at lower temperatures in both diapause and post-diapause pupae, but the increase was greater in the diapause pupae. These results suggest that cold hardiness is more fully developed in winter pupae than in summer pupae, and cold acclimation provides higher cold hardiness in winter pupae at the post-diapause stage than at the diapause stage.     

Thermal physiology and energetics in male desert hamsters (<Emphasis Type="Italic">Phodopus roborovskii</Emphasis>) during cold acclimation

The adjustments in thermal physiology and energetics were investigated in male desert hamsters (Phodopus roborovskii) which were acclimated to 5°C for 4 weeks. Mean core body temperature in cold acclimated animals decreased by 0.21°C compared with controls. Further analysis revealed that the decrease mainly occurred in the scotophase, while in the photophase core body temperature remained constant during the whole cold acclimation. Thermogenic capacity, represented by resting metabolic rate and nonshivering thermogenesis increased in cold acclimated hamsters from initial values of 1.38 ± 0.05 and 5.32 ± 0.30 to 1.77 ± 0.08 and 8.79 ± 0.31 mlO₂ g⁻¹ h⁻¹, respectively. After cold acclimation, desert hamsters maintained a relative stable body mass of 21.7 ± 0.1 g very similar to the controls kept at 23°C (21.8 ± 0.1 g). The mean values of food intake and digestible energy (metabolisable energy) in cold acclimated hamsters were 5.3 ± 0.1 g day⁻¹ and 76.3 ± 0.9 kJ day⁻¹ (74.8 ± 0.9), respectively, which were significantly elevated by 76.7 and 80.4% compared to that in control group. The apparent digestibility was 81.0 ± 0.3% in cold acclimated animals which was also higher than the 79.7 ± 0.2% observed in controls. This increase corresponded with adaptive adjustments in morphology of digestive tracts with 20.2 and 36.8% increases in total length and wet mass, respectively. Body fat mass and serum leptin levels in cold acclimated hamsters decreased by 40.7 and 67.1%, respectively. The wheel running turns and the onset of wheel running remained unchanged. Our study indicated that desert hamsters remained very active during cold acclimation and displayed adaptive changes in thermal physiology and energy metabolism, such as enhanced thermogenic and energy processing capacities.     

Dehydration and cold hardiness in the Arctic Collembolan Onychiurus arcticus Tullberg 1876

Specimens of the Arctic Collembolon Onychiurus arcticus were exposed to desiccation at several subzero temperatures over ice and at 0.5 °C over NaCl solutions. The effects of desiccation on water content (WC), body fluid melting point (MP), supercooling point (SCP) and survival were studied at several acclimation temperatures and relative humidities. Exposure to temperatures down to −19.5 °C caused a substantial and increasing dehydration. At the lowest exposure temperature unfrozen individuals lost 91.6% of the WC at full hydration but more than 80% of the individuals survived when rehydrated. Exposure at 0.5 °C to decreasing relative humidities (RH) from 100% to 91.3% caused increasing dehydration and increasing mortality. Survival of equally dehydrated individuals was higher at subzero temperatures than at 0.5 °C. Concurrent with the decline in WC a lowering of the MP was observed. Animals exposed to −3 °C and −6 °C over ice for 31 days had a MP of −3.8 and < −7.5 °C, respectively. Specimens from a laboratory culture had a mean SCP of −6.1 °C, and acclimation at 0 or −3 °C had little effect on SCPs. Exposure at −8.2 °C over ice for 8 days, however, caused the mean SCP to decline to −21.8 °C due to the severe dehydration of these individuals. Dehydration at 0.5 °C in 95.1 and 93.3% RH also caused a decline in SCPs to about −18 °C. Individuals that had been acclimated over ice at −12.4 °C or at lower temperatures apparently did not freeze at all when cooled to −30 °C, probably because all freezeable water had been lost. These results show that O. arcticus will inevitably undergo dehydration when exposed to subzero temperatures in its natural frozen habitat. Consequently, the MP and SCP of the Collembola are substantially lowered and in this way freezing is avoided. The increased cold hardiness by dehydration is similar to the protective dehydration mechanism described in earthworm cocoons and Arctic enchytraeids. Accepted: 5 January 1998     

Effects of ambient temperature and of temperature acclimation on nitrogen excretion and differential catabolism of protein and nonprotein resources in the intertidal snails,Littorina saxatilis (Olivi) and L. obtusata (L.)

Nitrogenous excretion in two snails, Littorina saxatilis (high intertidal) and L. obtusata (low intertidal) was studied in relation to temperature acclimation (at 4° and 21°C), including total N excretion rates, the fraction of urea in N excretion, corresponding O:N ratios and the partitioning of deaminated protein between catabolic and anabolic processes at 4°, 11° and 21°C. Aggregate N excretion rates in both species showed no significant compensatory adjustments following acclimation. Total weight specific N excretion rates at 21°C were higher in standard 3 mg L. saxatilis (739 ng N mg⁻¹ h⁻¹) than standard 5 mg L. obtusata (257 ng N mg⁻¹ h⁻¹) for snails acclimated to 21°C. Comparisons of Q₁₀ values of total weight specific N excretion to Q₁₀ values for weight specific oxygen consumption ({xxV}O₂) between 4° to 11 °C and 11° to 21°C indicated that, while total rates of catabolic metabolism ({xxV}O₂) and protein deamination in L. obtusata were essentially parallel, the relationship between N excretion and {xxV}O₂ in L. saxatilis revealed the partitioning of a larger share of deaminated protein carbon into anabolism at 4° and 21°C than at 11°C. Urea N accounted for a larger share of aggregate N excreted in L. saxatilis than in L. obtusata, but in both species urea N is a greater proportion of total N excreted when acclimated at 4°C (urea N: ammonia N ratio range: 1 to 2.15) than in snails acclimated to 21°C (urea N: ammonia N ratio range: 0.46 to 1.39). Molar O:N ratios indicate that the proportion of metabolism supported by protein catabolism is greater in L. saxatilis (O:N range: 2.5–8.4) than in L. obtusata (O:N range: 7.3–13.0). In both species, regardless of acclimation temperature, the O:N ratios are generally lowest (high protein catabolism) at 4°C and highest at 21°C.     

The Antarctic notothenioid fish <Emphasis Type="Italic">Pagothenia borchgrevinki</Emphasis> is thermally flexible: acclimation changes oxygen consumption

Antarctic marine organisms are considered to have extremely limited ability to respond to environmental temperature change. However, here we show that the Antarctic notothenioid fish Pagothenia borchgrevinki is an exception to this theory. P. borchgrevinki was able to acclimate its resting metabolic rate and resting ventilation frequency after a 5°C rise in temperature. Acute exposure to 4°C resulted in an elevation in metabolic rate (57.8 ± 4.79 mg O₂ kg⁻¹ h⁻¹) and resting ventilation rate (40.38 ± 1.61 breaths min⁻¹) compared with fish at −1°C (metabolic rate 34.45 ± 3.12 mg O₂ kg⁻¹ h⁻¹; ventilation rate 29.88 ± 3.72 breaths min⁻¹). However, after a 1-month acclimation period, there was no significant difference in the metabolic rate (cold fish 29.52 ± 3.01; warm fish 31.13 ± 2.30 mg O₂ kg⁻¹ h⁻¹), or the resting ventilation rate (cold fish 28.75 ± 0.98; warm fish 34.25 ± 2.28 breaths min⁻¹) of cold and warm acclimated fish. Acclimation changes to the rate of oxygen consumption following exhaustive exercise were complex. The pattern of oxygen consumption during recovery from exhaustive exercise was not significantly different in either cold or warm acclimated fish.     

Freezing Tolerance and Alteration of Translatable mRNAs in Alfalfa (Medicago sativa L.) Hardened at Subzero Temperatures

We analyzed changes in populations of translatable mRNAs occurringin crowns of the cold-tolerant alfalfa (Medicago sativa L.)cv. Apica (CT) and the cold-sensitive cv. CUF-101 (CS) aftertheir acclimation at low nonfreezing temperatures and at subzerotemperatures. Both cultivars showed very similar translationprofiles under all treatments. Low temperatures induced significantchanges in the populations of translatable mRNAs. We observeda relationship between the accumulation of cold-regulated (COR)translation products and freezing tolerance within cultivars.Moreover, at least three COR translation products were specificto the CT and might be related to hardiness potential in alfalfa.Whereas extension of the cold acclimation period at 2C reducedcold tolerance, incubation at subzero temperatures increasedor maintained freezing tolerance. This increased hardiness wasassociated with enhanced translation of COR polypeptides andalso with the appearance of new translatable mRNAs. This is,to our knowledge, the first report of altered gene expressionin plants incubated at subzero temperatures. Marked changesin populations of translatable mRNAs at temperatures below freezingmight be related to previous reports that alfalfa achieves maximumhardiness under snow cover when the soil has frozen. Translationin the presence of [³H]glycine showed that a large proportionof the COR genes encode for glycine-rich proteins (GRPs) andthat some of the GRPs are specific to the CT. (Received May 29, 1992; Accepted October 13, 1992)     

Differential cold hardiness in adults and nymphs of the peach-potato aphid Myzus persicae

The LT₅₀ (lethal temperature) of first instar and adult stages of the peach-potato aphid Myzus persicae was lowered following long term acclimation at low temperatures.
First instars consistently showed greater cold hardiness than adult stages at each acclimation temperature, with the differential increasing as the temperature was lowered. When maintained at 5°C (the lowest acclimation regime) nymphs and adults had dLT₅₀8.3°C and 4.7°C respectively lower than those for non-acclimated individuals.
When 10°C acclimated adults were returned to 20°C, the acclimation effect was retained in full for 6 days but complete deacclimation occurred by day 10. In contrast the LT₅₀ of their progeny increased gradually from the first day of adult deacclimation towards the level of the unacclimated control over a period of 10 days.
A change in cold hardiness was observed in first instars according to their position in the birth sequence. The LT₅₀ of first-born nymphs (day 1 of reproduction) from 20°C parents was - 15.9°C rising to - 8.3°C by day 4 and remaining at this level until the end of the reproductive period.
The differential mortality between nymphs and adults observed in the laboratory was supported by the results of a field experiment. Adult aphids kept in clip-cages on a crop of oilseed rape showed greater mortality compared with those introduced as nymphs when the minimum temperature fell below -4°C for the first time in winter. At - 10°C mortality of aphids introduced as adults approached 100% whereas more than 50% of those introduced as nymphs were still alive at this temperature.     

Thermal maxima for juvenile shortnose sturgeon acclimated to different temperatures

Many populations of shortnose sturgeon, Acipenser brevirostrum, in the southeastern United States continue to suffer from poor juvenile recruitment. High summer water temperatures, which may be exacerbated by anthropogenic activities, are thought to affect recruitment by limiting available summer habitat. However, information regarding temperature thresholds of shortnose sturgeon is limited. In this study, the thermal maximum method and a heating rate of 0.1°C min⁻¹ was used to determine critical and lethal thermal maxima for young-of-the-year (YOY) shortnose sturgeon acclimated to temperatures of 19.5 and 24.1°C. Fish used in the experiment were 0.6 to 35.0 g in weight and 64 to 140 days post hatch (dph) in age. Critical thermal maxima were 33.7°C (±0.3) and 35.1°C (±0.2) for fish acclimated to 19.5 and 24.1°C, respectively. Critical thermal maxima significantly increased with an increase in acclimation temperature (p < 0.0001). Lethal thermal maxima were 34.8°C (±0.1) and 36.1°C (±0.1) for fish acclimated to 19.5 and 24.1°C, respectively. Lethal thermal maxima were significantly affected by acclimation temperature, the log₁₀ (fish weight), and the interaction between log₁₀(fish weight) and acclimation temperature (p < 0.0001). Thermal maxima were used to estimate upper limits of safe temperature, thermal preferences, and optimal growth temperatures of YOY shortnose sturgeon. Upper limits of safe temperature were similar to previous temperature tolerance information and indicate that summer temperatures in southeastern rivers may be lethal to YOY shortnose sturgeon if suitable thermal refuge cannot be found.