Alterations of gene expression in potato (Solanum commersonii) during cold acclimation

Plantlets of Solanum commersonii stem-culture were acclimated at 5°C day/night temperature for 14 days. Cold hardiness increased from – 3.5°C to – 8.6°C. During the course of acclimation, the synthesis of polypeptides was investigated and poly (A⁺) RNA was isolated. Translation products of poly(A⁺) RNA in a rabbit rcticulocyte lysate system were then analyzed. During the 14 days of acclimation, 23 cold-induced polypeptides were identified. Most of them disappeared following 1 day of de-acclimation at a 20/15°C day/night regime. The synthesis of one group of polypeptides is prominent and stable throughout the acclimation period. The other group is transient. The most prominent and stable polypeptides have molecular weights of 21, 22, 31 and 83 kDa.
Acclimation alters translatable mRNA population during the development of cold hardiness. Two mRNAs encoding in vitro translation products at 26 and 27 kDa were identified during the course of acclimation. These proteins may play important roles in the overall programming for the development of cold hardiness in tuber-bearing S. commersonii.     

Induction of freezing tolerance in potato (Solanum commersonü) suspension cultured cells

The induction of freezing tolerance by abscisic acid (ABA) or cold treatment in suspension cultured cells of Solanum commersonii was studied. Both ABA (50–100 μ M ) at 23°C and low temperature (4°C) increased freezing tolerance in cultured Solanum commersonii cells from a LT₅₀ (freezing temperature at which 50% cells were killed) of —5°C (control) to —11.5°C in 2 days. Cold-induced freezing tolerance reached its maximum at 2 days and remained constant throughout the cold acclimation period of 11 days. The freezing tolerance induced by ABA, however, showed a rapid decline 2 to 5 days after initiation of ABA treatments. Addition of ABA (100 μ M ) to the culture medium at the inception of low temperature treatment did not enhance freezing tolerance of the cells beyond the level attainable by either treatment singly. Poly(A⁺)-RNA was isolated from the respective treatments, translated in a rabbit reticulocyte lysate cell free system, and the translation products were resolved by two dimensional polyacrylamide gel electrophoresis (ID-PAGE). Analysis of the in vitro translated products revealed changes in the abundance of approximately 26 products (encoding for polypeptides with M, of 14 to 69 kDa and pl of 4.90 to 6.60) in ABA-treated cells 12 h after treatment, and 20 (encoding for polypeptides with M_r of 12 to 69 kDa, with pl of 4.80 to 6.42) in cells exposed to 4°C for 12 h. There were only 5 novel translation products observed when the ABA-treated cells reached the highest level of freezing tolerance (2 days after the initiation of ABA treatment). Changes in translatable RNA populations during the induction of freezing tolerance in cells treated with either ABA or low temperature are discussed.     

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.     

Potato cold hardiness development and abscisic acid. I. Conjugated abscisic acid is not the source of the increase in free abscisic acid during potato (Solanum commersonii) cold acclimation

Free and conjugated abscisic acid (ABA) levels in stem-cultured plantlets of potato ( Solanum commersonii Dun, PI 458317) during cold acclimation were measured. The levels of free and conjugated ABA were measured by an enzyme immunoassay (EIA) with rabbit anti-ABA-serum. The use of immunoglobulin G fraction purified from rabbit antiserum and the methylated form of ABA resulted in an improved measuring range (0.01 to 10 pmol ABA) and precision (slope of logit-log plot, −1.35) of EIA, compared to the use of antiserum and free ABA. Estimates of the EIA were consistent with those resulting from a commercial EIA. Under a 4/2°C (day/night) temperature regime, the potato plantlets increased cold hardiness from −5°C (warm-grown control) to −10°C by the 7th day. During the same period, there were two transitory increases in free ABA, the first one three-fold from 1.5 to 5.3 nmol (g dry weight)⁻¹ on the 2nd day and the second one five-fold from 1.5 to 7.6 nmol (g dry weight)⁻¹ on the 6th day. Each increase in ABA concentration was followed by an increase in cold hardiness. There was no significant change in conjugated ABA content (4.2±0.6 nmol [g dry weight]⁻¹) throughout the cold acclimation period. The lack of an interrelationship between levels of free and conjugated ABA suggested that the transitory increase in free ABA during cold acclimation was not a result of the conversion of conjugated ABA. The increase in free ABA due to biosynthesis of ABA during potato cold acclimation is discussed.     

Intergenerational acclimation in aphid overwintering

Abstract.  1. When first instar nymphs and adults of the grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphidiae) were maintained in long-term cultures (>6 months) at 20 °C and 10 °C, the LT₅₀ decreased from −8 and −8.8 °C to −16.0 and −13.5 °C, respectively.
2. When aphids from the 20 °C culture were transferred to 10 °C, there was a progressive increase in cold tolerance through three successive generations. Transfer of newly moulted pre-reproductive adults reared at 10 °C for three generations back to 20 °C resulted in a rapid loss of cold hardiness in their nymphal offspring.
3. In all generations reared at 10 °C, first born nymphs were more cold hardy than those born later in the birth sequence. The LT₅₀ of nymphs produced on the first day of reproduction in the first, second and third generations maintained at 10 °C were −14.8, −17.0 and −16.6 °C, respectively. Thereafter, nymphal cold hardiness decreased over the subsequent 14 days of reproduction in each generation at 10 °C with mean LT₅₀ values of −10.3, −12.6 and −14.8 °C, respectively. By contrast, the cold tolerance of first born nymphs of aphids reared continuously at 20 °C did not differ in comparison with later born siblings. The LT₅₀ of adult aphids was also unaffected by ageing.
4. The ecological relevance of these findings is discussed in relation to the overwintering survival of aphids such as S. avenae .     

A proposed role for the anaerobic pathway during low-temperature sweetening in tubers of Solanum tuberosum

Survival and growth of temperate zone woody plants under changing seasonal conditions is dependent on proper timing of cold acclimation and development of vegetative dormancy, shortening photoperiod being an important primary signal to induce these adaptive responses. To elucidate the physiological basis for climatic adaptation in trees, we have characterized photoperiodic responses in the latitudinal ecotypes of silver birch ( Betula pendula Roth) exposed to gradually shortening photoperiod under controlled conditions. In all ecotypes, shortening photoperiod triggered growth cessation, cold acclimation and dormancy development, that was accompanied by increases in endogenous abscisic acid (ABA) and decreases in indole-3-acetic acid (IAA). There were distinct differences between the ecotypes in the rates and degrees of these responses. The critical photoperiod and the photoperiodic sensitivity for growth cessation varied with latitudinal origin of the ecotype. The northern ecotype had a longer critical photoperiod and a greater photoperiodic sensitivity than the southern ecotype. Compared with the southern ecotypes, the northern ecotype was more responsive to shortening photoperiod, resulting in earlier cold acclimation, dormancy development, increase in ABA content and decrease in IAA content. However, at the termination of the experiment, all the ecotypes had reached approximately the same level of cold hardiness (−12 to −14°C), ABA content (2.1–2.3 µg g⁻¹ FW) and IAA content (17.2–20.3 ng g⁻¹ FW). In all ecotypes, increase in ABA levels preceded development of bud dormancy and maximum cold hardiness. IAA levels decreased more or less parallel with increasing cold hardiness and dormancy, suggesting a role of IAA in the photoperiodic control of growth, cold acclimation and dormancy development in birch.     

Cold hardening of raspberry plants in vitro is enhanced by increasing sucrose in the culture medium

The influence of exogenously applied sucrose on cold hardening of raspberry ( Rubus idaeus L.) in vitro was examined. Raspberry plants (cv. Preussen) were cultured on Murashige-Skoog (MS) media with different levels (1, 3, 5 and 7%) of sucrose and subjected to low-temperature acclimation (3/−3°C day/night temperature, 8-h photoperiod) for 14 days. Cold hardiness (LT₅₀ in controlled freezing), shoot moisture content, osmolality and the amounts of sucrose, glucose and fructose were determined. Exogenously applied sucrose was taken up by plants, but the uptake corresponded to less than 10% of total sugar reserves in the culture. Cold hardiness was primarily affected by acclimation treatment, but sucrose increased cold hardiness of nonacclimated plants and significantly enhanced the effect of acclimation treatment, 5% sucrose in the culture medium being optimal for cold hardening. LT₅₀ values ranged between −4.1 and −7.1°C for nonacclimated, and between −14.2 and −20.7°C for cold-acclimated shoots. Shoot moisture content was inversely related to medium sucrose level and declined only slightly during cold acclimation. After cold acclimation, plant osmolality predicted hardiness better than shoot moisture content. Plant osmolality and sugar content were increased by increasing the medium sucrose level and, to a greater extent, by cold acclimation. Sucrose, glucose and fructose accumulated during hardening. Sucrose was the predominant sugar, and the rate of sucrose accumulation during cold acclimation was independent of the medium sucrose level or the initial plant sucrose content. A close correlation between cold hardiness and total sugars, sucrose, glucose and fructose was established. These results suggest that sugars have more than a purely osmotic effect in protecting acclimated raspberry plants from cold.     

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.     

Cold tolerance of the aphid predator Episyrphus balteatus (DeGeer) (Diptera, Syrphidae)

Abstract. Larvae of the hoverfiy Episyrphus balteatus (DeGeer) are important predators of aphids in the U.K. A large proportion of the U.K. population migrates south to warmer climes at the end of summer, but a small number are thought to overwinter in the U.K., with the mated female being the overwintering morph. The cold tolerance of adult flies was investigated to assess the overwintering potential of E. balteatus in the U.K. The high supercooling point (SCP) of -8.3 ± 0.7°C, and lethal temperature (LTemp₃₀) of -9.1°C for acclimated females suggest that E. balteatus has limited cold hardiness. This was confirmed by experiments where, despite a strong acclimation response in both males and females, there was no long-term survival at 5, 0 or - 5°C. At 5°C, 90% of females had died after 10 days. The weak cold hardiness of adult E. balteatus was corroborated by field experiments which demonstrated a 100% mortality after 10 weeks' exposure to U.K. winter conditions. The ecological significance of this limited cold hardiness is discussed in relation to the overwintering abilities of E. balteatus in the U.K.     

Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.     

Abscisic acid and mefluidide in the regulation of cold hardiness development in Solanum tuberosum L. potato

Plants of Solanum tuberosum L. potato do not cold acclimate when exposed to low temperature such as 5°C, day/night. When ABA (45 M) was added to the culture medium, stem-cultured plantlets of S. tuberosum, cv. Red Pontiac, either grown at 20°C/15°C, day/night, or at 5°C, increased in cold hardiness from –2°C (killing temperature) to –4.5°C. The increase in cold hardiness could be inhibited in both temperature regimes if cycloheximide (70 M) was added to the culture medium at the inception of ABA treatment. Cycloheximide did not inhibit cold hardiness development, however, when it was added to the culture medium 3 days after ABA treatment.When pot-grown plants were foliar sprayed with mefluidide (50 M), ABA content increased from 10 nmol to 30 nmol g^–1 dry weight and plants increased in cold hardiness from –2°C to about –3.5°C. The increases in free ABA and cold hardiness occurred only in plants grown at 20°C/15°C; neither ABA nor cold hardiness increased in plants grown at 5°C.The results suggest that an increase in ABA and a subsequent de novo synthesis of proteins are required for the development of cold hardiness in S. tuberosum regardless of temperature regime, and that the inability to synthesize ABA at low temperature, rather than protein synthesis, appears to be the reason why S. tuberosum does not cold acclimate.     

The influence of thermal parameters on the acclimation responses of pinfish Lagodon rhomboides exposed to static and decreasing low temperatures

Pinfish Lagodon rhomboides acclimation rates were determined by modelling changes in critical thermal minimum ( T _{crit min}, ° C) estimates at set intervals following a temperature decrease of 3–4° C. The results showed that pinfish gained a total of 3·7° C of cold tolerance over a range of acclimation temperatures ( T _acc, ° C) from (23–12° C), that cold tolerance increased with exposure time to the reduced temperature at all T _acc, but that the rate of cold tolerance accruement (mean 0·14° C day⁻¹) was independent of T _acc. A highly significant ( P < 0·001) multivariate predictive model was generated that described the acclimation rates and thermal tolerance of pinfish exposed to reduction in water temperature: log₁₀ T _{crit min}= 0·41597 − 0·01704 T _acc+ 0·04320 T _plunge− 0·08376[log₁₀ ( t + 1)], where T _plunge is plunge temperature (° C) and t is the time (days). A comparison of the present data, with acclimation rate data for other species, suggests that factors such as latitude or geographic range may play a more important role than ambient temperature in determining cold acclimation rates in fishes.     

Inositol 1,4,5-trisphosphate formation in leaves of winter oilseed rape plants in response to freezing, tissue water potential aed abscisic acid

Time courses of formation of inositol 1,4,5-trisphosphate (IP₃) were followed in the leaves of non-acclimated and cold (2°C)-acclimated winter oilseed rape ( Brassica napus L. var. oleifera ) plants, subjected to different freezing temperatures or to polyethylene glycol 8000 (PEG) and abscisic acid (ABA) treatments. Changes in water potential (Ψ_w) and in ABA level in the frost- and PEG-treated tissues were also determined. Results obtained indicate that temperatures sligthly higher than LT₅₀ induced a transient and substantial increase in IP₃ level, both in non-acclimated and cold-acclimated tissues. At comparable freezing temperature (–5°C) the response of cold-acclimated leaves was lower than that of non-acclimated ones. The PEG-depedent decrease in Ψ_w to –0.9 MPa or ABA (0.1 m M ) treatment gave rise to a transient increase in IP₃ content in non-acclimated tissues only. Collectively, the data indicate that cold acclimation of plants may lead to lower cell responsiveness to the factors studied in terms of induction of IP₃ formation. Changes in the IP₃ content, observed in the present experiments, support our previous suggestion that non-killing freezing temperatures may induce the phosphoinositide pathway, both in non-acclimated and cold-acclimated tissues. Lowering of tissue water potential to some threshold value or a high exogenous ABA supply may mimic the freezing-dependent reaction in the non-acclimated leaves.     

The role of ABA in freezing tolerance and cold acclimation in barley

The role of ABA in freezing resistance in nonacclimated and cold‐acclimated barley ( Hordeum vulgare L.) was studied. Eleven nonacclimated cultivars differed in their LT₅₀, ranging from −10.8 to −4.8°C. Sugars, free proline, soluble proteins and ABA were analyzed in nonacclimated cultivars and during cold acclimation of one cultivar. There was an inverse correlation between LT₅₀ and both ABA and sucrose contents. Exogenous ABA caused a decrease in the freezing point of leaf tissue in the cultivar with the lowest level of endogenous ABA, but not in the cultivar with the highest level, suggesting that ABA in the latter may be near the optimum endogenous level to induce freezing tolerance. Plants of cv. Aramir treated with ABA or allowed to acclimate to cold temperature increased their soluble sugar content to a similar level. The LT₅₀ of leaves of cold‐acclimated cv. Aramir decreased from −5.8 to −11.4°C, with biphasic kinetics, accumulating proline and soluble sugars with similar kinetics. The biphasic profile observed during cold acclimation could be a direct consequence of cryoprotectant accumulation kinetics. ABA and soluble protein accumulation showed a single step profile, associated mainly with the second phase of the LT₅₀ decrease. Thus, a significant increase in endogenous ABA is part of the response of barley to low temperature and may be required as a signal for the second phase of cold acclimation. Endogenous ABA contents in the nonacclimated state may determine constitutive freezing tolerance.     

Geographical variation in egg cold hardiness: a study on the adaptation strategies of the migratory locust Locusta migratoria L.

Abstract. 1. For many species of insect, cold hardiness is an important trait that enables a population to develop in the next season and to extend its range. To elucidate the role of cold hardiness of the migratory locust Locusta migratoria L. in its outbreak and distribution areas, egg cold hardiness was examined in locusts derived from four locations from latitude 18°23'N to latitude 41°10'N in eastern China.
2. The supercooling points of eggs from different geographic populations did not differ significantly for the first development stage, with an average ± SE of −24.5 ± 0.51 °C, or for the second stage, −22.06 ± 0.68 °C, however there was a significant difference for the embryonic development phase among the four geographical populations. The egg supercooling point increased gradually from neonatal egg to old egg; eggs prior to hatching always had a much higher supercooling point.
3. Comparisons of the cold hardiness of four populations were carried out by validating the close correlation between latitude and the effects of cold on hatching, low lethal temperature (Ltemp₅₀), and low lethal time (Ltime₅₀). There were significant differences among the four populations; the northern population was more cold hardy than the southern population, and the two mid-latitude populations were intermediately cold hardy.
4. The cold hardiness of all populations was enhanced to various degrees by short-term cold acclimation at 0 °C and 5 °C. For most populations, a 2-day acclimation period seemed to be optimal.     

Protein synthesis at low temperatures in two soybean cultivars differing by their cold sensitivity

The effect of low temperatures (14°C/8°C, day/night) on polypeptide synthesis in leaves of two soybean ( Glycine max [L.] Merr.) cvs (Verdon and Maple Arrow) differing in cold sensitivity was investigated. The two cultivars were initially characterized in terms of cold tolerance according to their growth at the young plant stage at 14°C/8°C. Verdon was found to be more tolerant than Maple Arrow. In vivo [³⁵S]-methionine labeled polypeptides were resolved by two-dimensional electrophoresis. Autoradiograms were computer analyzed to evidence and quantify significative changes occurring after 5 days at 14°C/8°C, and to compare the response of the two cultivars. Most of the observed changes were quantitative. The two cultivars essentially exhibited a common modified polypeptide pattern in response to cold temperatures, but the changes were quantitatively more pronounced in the most tolerant cultivar. Computer analysis of two-dimensional electrophoresis gels allowed, for the first time, characterization of cultivar differences in terms of protein pattern under cold conditions.     

Antagonistic effects of abscisic acid and gibberellic acid on the breaking of dormancy of Fagus sylvatica seeds

Study of the factors involved in the dormancy of Fagus sylvatica seeds shows that such dormancy is due partly to the seed coats and partly to endogenous factors. Seed coat removal accelerates both the release from dormancy and the effects of the other treatments that abolish it. The dormancy of these seeds is eliminated by cold treatment at 4°C over a period longer than 8 weeks, and exogenous application of abscisic acid (ABA) reverses the effects of low temperature, the seeds remaining in an ungerminated state. Additionally, ABA reduces protein synthesis but slightly increases RNA synthesis, which suggests its involvement in the synthesis of RNAs related to this process. In vitro translation of the RNAs isolated from these seeds shows that ABA delays the disappearance of at least 2 polypeptides (of ca 22 and 24 kDa), which are abundant in dormant seeds and under conditions that prevent the release from dormancy, but which disappear under treatments that abolish it. Exogenous application of gibberellic acid (GA₃) proved to be efficient in breaking the dormancy of these seeds and in substituting for cold treatment as well as in antagonizing the effects of ABA on the synthesis of both DNA and proteins. GA₃ also accelerates the disappearance of the two polypeptides abundant in dormant seeds and in ABA-treated seeds. These findings suggest that both ABA and GA₃ could be involved in the regulation of nucleic acid and protein metabolism during dormancy, acting antagonistically in these processes and, specifically, in the regulation of the synthesis of the two proteins that appear to play a role in the maintenance of dormancy in these seeds.     

Correlation between the Shortened Period of Cell Pairing during Genomic Exclusion and the Block in Posttransfer Nuclear Development in Tetrahymena thermophia.

The duration of pairing in crosses of the micronuclearly defective strain A^*V and the amicronuclear strain BI3840 of Tetrahymena thermophila is shorter than that in normal conjugation. In controls, pairing takes about 620 min at 30°C and 60 min more at 28°C. In contrast pairing of crosses of the A^*strain took 470 min at 30°C and 490 min at 28°C, and that of crosses of BI3840 strain took 440 min at 30°C. The course of nuclear development in the tester strain crossed with the A^*strain was similar to that in the control until the 3rd prezygotic division. Unilateral transfer of the pronuclei occurred later than on reciprocal transfer in control crosses; posttransfer divisions in hemikarya was completely blocked in over 90% of the cases examined. Defective cell contact in crosses of the A^*V strain and an amicronuclear BI3840 strain may be correlated with block in nuclear division and in macronuclear development, since unfertilized cells from triplet conjugation remain haploid and develop normally.     

Reappraisal of the effect of temperature on the growth kinetics of Aeromonas salmonicida

The effect of temperature (1–34 °C ) on the maximum specific growth rate of Aeromonas salmonicida could not be described by the classical growth models ; for some strains, two optimal temperatures at 23 °C and 30 °C were observed, as well as an unexpected increase in the pseudolag time above 27 °C. This could be explained by the presence of two subsets, notably S-layer⁺ and S-layer⁻ sub-populations. The A⁻ cells had higher growth parameters (T_opt and μ_opt) than the A⁺ cells and were selected by subcultures above 30 °C. Yet the relative proportion of A⁺ cells did not explain all the variation of μ_max versus temperature, and the growth kinetics of an Aer. salmonicida isolate remained unpredictable.     

Genotype-specific differences in chilling tolerance of maize in relation to chilling-induced changes in water status and abscisic acid accumulation

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m⁻² s⁻¹ from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.