Influence of simulated snow cover on the cold tolerance and freezing injury of yellow-cedar seedlings

It has been hypothesized that yellow‐cedar [Chamaecyparis nootkatensis (D. Don) Spach] decline may result from root freezing injury following climate change‐induced reductions in protective snow cover. To test this hypothesis, we measured the freezing tolerance and injury expression of yellow‐cedar seedlings in three treatments that differed in the insulative protection they provided to soils during winter and spring: (1) full exposure to ambient temperatures (exposed treatment), (2) continuous protection from ambient temperatures via addition of perlite over pots (full protection), and (3) perlite protection only during winter and exposure to ambient temperatures during spring (partial protection). Foliage from all treatments was cold tolerant enough to prevent foliar freezing injury throughout the study period. However, on all sample dates, roots of seedlings from all treatments were only tolerant to about ?5 °C – a level considerably warmer than the reported maximum cold tolerance for the species and well above the soil temperature recorded in the exposed treatment. As a result of this limited root cold tolerance, visibly uninjured roots of seedlings from the exposed treatment had significantly higher relative electrolyte leakage (REL) throughout the winter and early spring than seedlings in soil protection treatments. Seedlings from the exposed treatment also had significantly higher foliar REL values and greater visual foliar injury than seedlings from the other treatments starting in early spring. For both roots and foliage, REL measurements consistently detected tissue damage before visual injury was evident. Patterns of injury from both REL and visual injury assessments showed the same pattern: damage began with freezing injury to roots and subsequently became evident as foliar browning after spring temperatures increased. All seedlings in the exposed treatment eventually had 100% fine root damage and died. This progression of initial root damage followed by foliar browning and mortality after the onset of warming conditions is consistent with reports of yellow‐cedar decline symptom development in the field.     

The induction of freezing tolerance in jack pine seedlings: The role of root plasma membrane H+- ATPase and redox activities

The acquired freezing tolerance of jack pine seedlings (Pinus banksiana Lamb.) conditioned at low nonfreezing temperatures and short photoperiods was determined by comparison of seedling survival to that of nonconditioned (control) seedlings following exposure to ?5 and ?10°C. Compared to that of controls, survival of conditioned seedlings was markedly increased following exposure to freezing temperatures. A 1-week conditioning treatment significantly increased the survival of the seedlings after exposure to ?5°C, but was less effective on seedlings exposed to ?10°C. Conditioning periods of 2 and 4 weeks resulted in higher survival of seedlings exposed to both ?5 and ?10°C. The changes of two root-plasma-membrane-associated enzyme activities, H⁺-ATPase and NADH-dependent ferricyanide reductase, were studied in enriched plasma membrane fractions during conditioning and after freezing. Post-freezing activities of both enzymes were enhanced by conditioning at low temperatures and short photoperiods. These changes may be related to the increased frost hardiness also induced by conditioning.     

Influence of soil temperature on niacin and thiamine in honey mesquite

Summary The influence of soil temperature was examined on niacin and thiamine concentration in honey mesquite (Prosopis glandulosa var.glandulosa) seedlings. The seedlings were grown in soil temperature regimes of 21, 27, and 32°C in a controlled environment growth room. Nodulation randomly occurred on the roots of the seedlings, necessitating separate analysis according to the occurrence of nodulation. Roots of nodulated seedlings from the 21°C soil temperature regime contained greater quantities of niacin and thiamine compared to root samples from seedlings grown in either 27 or 32°C regimes. Niacin concentration of non-nodulated seedlings was highest in samples from seedlings grown in the 27°C soil temperature regime and lowest in samples from seedlings grown in the 21°C regime. Thiamine concentration was the greatest from non-nodulated seedlings grown in the 27°C soil temperature regime, while the thiamine concentration of non-nodulated samples from the 32°C regime was the least. Optimal soil temperature for honey mesquite root growth appears to be about 27°C. At sub-optimal soil temperatures niacin might have limited ‘growth’ while at supra-optimal soil temperatures, thiamine might be a limiting factor. College of Agricultural Sciences Contribution No. T-9-164.     

Elevated growth temperatures reduce the carbon gain of black spruce [Picea mariana (Mill.) B.S.P.]

We explored the effect of high‐growth temperatures on a dominant North American boreal tree, black spruce [Picea mariana (Mill.) B.S.P.]. In 2004 and 2005, we grew black spruce at either 22 °C/16 °C day/night temperatures [low temperature (LT)] or 30°/24 °C [high temperature (HT)] and determined how temperature affected growth, leaf morphology, photosynthesis, respiration and thermotolerance. HT spruce were 20% shorter, 58% lighter, and had a 58% lower root : shoot ratio than LT trees. Mortality was negligible in the LT treatment, but up to 14% of HT seedlings died by the end of the growing season. HT seedlings had a higher photosynthetic temperature optimum, but net photosynthesis at growth temperatures was 19–35% lower in HT than LT trees. HT seedlings had both a lower apparent maximum ribulose‐1,5‐bisphosphate carboxylation capacity (V_cmax) and a lower apparent maximum electron transport rate (J_max) than LT trees, indicating reduced allocation to photosynthetic components. Consistently, HT needles had 26% lower leaf nitrogen content than LT needles. At each measurement temperature, HT seedlings had 20–25% lower respiration rates than LT trees; however, this did not compensate for reduced photosynthetic rates at growth temperature, leading to a greater ratio of dark respiration to net carbon dioxide assimilation rate in HT trees. HT needles had 16% lower concentrations of soluble sugars than LT needles, but similar starch content. Growth at high temperatures increased the thermotolerance of black spruce. HT trees showed less PSII inhibition than LT seedlings and no increase in electrolyte leakage when briefly exposed to 40–57 °C. While trees that develop at high temperatures have enhanced tolerance for brief, extreme heat events, the reduction in root allocation indicates that seedlings will be more susceptible to episodic soil drying and less competitive for belowground resources in future climates of the boreal region.     

Freezing tolerance of ectomycorrhizal fungi in pure culture

The ability to survive freezing and thawing is a key factor for the existence of life forms in large parts of the world. However, little is known about the freezing tolerance of mycorrhizal fungi and their role in the freezing tolerance of mycorrhizas. Threshold temperatures for the survival of these fungi have not been assessed experimentally. We grew isolates of Suillus luteus, Suillus variegatus, Laccaria laccata, and Hebeloma sp. in liquid culture at room temperature. Subsequently, we exposed samples to a series of temperatures between +5°C and −48°C. Relative electrolyte leakage (REL) and re-growth measurements were used to assess the damage. The REL test indicated that the lethal temperature for 50% of samples (LT₅₀) was between −8.3°C and −13.5°C. However, in the re-growth experiment, all isolates resumed growth after exposure to −8°C and higher temperatures. As many as 64% of L. laccata samples but only 11% in S. variegatus survived −48°C. There was no growth of Hebeloma and S. luteus after exposure to −48°C, but part of their samples survived −30°C. The fungi tolerated lower temperatures than was expected on the basis of earlier studies on fine roots of ectomycorrhizal trees. The most likely freezing tolerance mechanism here is tolerance to apoplastic freezing and the concomitant intracellular dehydration with consequent concentrating of cryoprotectant substances in cells. Studying the properties of fungi in isolation promotes the understanding of the role of the different partners of the mycorrhizal symbiosis in the freezing tolerance.     

Cold tolerance in sporobolus virginicus (L.) Kunth robust form, tissue cultures and whole plants

Abstract Sporobolus virginicus (L.) Kunth robust form, is a coastal C4 grass species in tropical and subtropical regions. An artificial freeze test was used to determine the response of tissue exposed to low temperature. The response was monitored by three methods: measuring respiration rates as carbon dioxide flux before and after freezing, conducting a triphenyl tetrazolium chloride (TTC) viability assay, and observing growth of tissue cultures and the regrowth of shoots and roots from rhizome buds. The TTC assay overpredicted the survival temperature in rhizomes, when based on a 50% of control as lethal value, but was a good indicator of survival in callus and suspension cultures. Respiration rates of callus tissue declined with low temperature exposure and paralleled the TTC results. Rhizome tissue however had a more complex respiratory response. High, post-freeze, respiration rates during thawing of rhizomes at + 5.0°C were correlated with injury, detected both by a TTC assay and by the measurement of carbon dioxide flux. High respiration rates, measured after the thawing period, due to disabled rhizomes that were ultimately inviable, provide an explanation for the overprediction of the TTC assay. S. virginicus was found to be freeze-sensitive, LT₅₀= - 2.5°C, with no hardening ability and it was concluded that cellular resistance defines the limits of freezing tolerance. However, avoidance of freezing strain through an underground perenniating organ probably allows S. virginicus to survive on the polar end of its range where episodic frosts may decrease temperatures to the lower limits of the cellular tolerance of a species.     

Changes in Sugar Content during Cold Acclimation and Deacclimation of Cabbage Seedlings

The relationship between freezing tolerance and sugar contentin cabbage seedlings was investigated. Seedlings exposed tonon-freezing low temperature (5 °C) acquired freezing tolerancedown to -6 °C. The degree of freezing tolerance increasedwith duration of exposure to low temperature (up to 10 d). Sucrose,glucose, fructose and myo -inositol were detected as solublesugars in cabbage leaves, and all soluble sugars, except formyo -inositol, and starch increased gradually during cold acclimationsuch that their levels were positively correlated with the degreeof freezing tolerance. The induced freezing tolerance was attributednot to ontogenetic changes but to cold acclimation. However,the induced freezing tolerance was lost after only 1 d of deacclimationat control temperatures, and this change was associated witha large reduction in sugar content. These results reveal that the sugar content of cabbage leavesis positively correlated with freezing tolerance. Brassica oleracea L.; cabbage; cold acclimation; deacclimation; freezing tolerance; sugars     

The effect of soil freezing on the survial of winter-sown white lupins (Lupinus albus L.)

Specially constructed soil-freezing growth boxes were used to study the effects of the intensity and duration of soil freezing on root injury and the survival of white lupin seedlings of different ages under controlled conditions. The extent of root damage depended on both the intensity of soil freezing and the stage of seedling development (measured as the extent of lignification of the central stele of the primary root). Seedlings whose secondary root development was well advanced, and in which the endodermis was completely lignified, survived intense soil freezing intact. Young seedlings with weakly lignified roots were damaged by moderate soil freezing (> 5 days at ?1°C) and killed by more intense freezing (5 days at ?2°C). The extent of root development and ligmfkation was correlated with the number of leaf primordia produced at the shoot apex so that the susceptibility to soil freezing damage could be accurately predicted by a simple physiological/leaf production model.     

Exposure to subfreezing temperature and a freeze-thaw cycle affect freezing tolerance of winter wheat in saturated soil

Winter wheat is sown in the autumn and harvested the following summer, necessitating the ability to survive subfreezing temperatures for several months. Autumn months in wheat-growing regions typically experience significant rainfall and several days or weeks of mild subfreezing temperatures at night, followed by above-freezing temperatures in the day. Hence, the wheat plants usually are first exposed to potentially damaging subfreezing temperatures when they have high moisture content, are growing in very wet soil, and have been exposed to freeze-thaw cycles for a period of time. These conditions are conducive to freezing stresses and plant responses that are different from those that occur under lower moisture conditions without freeze-thaw cycles. This study was conducted to investigate the impact of mild subfreezing temperature and a freeze-thaw cycle on the ability of 22 winter wheat cultivars to tolerate freezing in saturated soil. Seedlings that had been acclimated at +4°C for 5 weeks in saturated soil were frozen to potentially damaging temperatures under three treatment conditions: (1) without any subzero pre-freezing treatment; (2) with a 16-h period at ?3°C prior to freezing to potentially damaging temperatures; and (3) with a freeze-thaw cycle of ?3°C for 24 h followed by +4°C for 24 h, followed by a 16-h period at ?3°C prior to freezing to potentially damaging temperatures. In general, plants that had been exposed to the freeze-thaw cycle survived significantly more frequently than plants frozen under the other two treatments. Plants that had been exposed to 16 h at ?3° (without the freeze-thaw cycle) before freezing to potentially damaging temperatures survived significantly more frequently than plants that were frozen to potentially damaging temperatures without a subzero pre-freezing treatment. These results indicated that cold-acclimated wheat plants actively acclimate to freezing stress while exposed to mild subfreezing temperatures, and further acclimate when allowed to thaw at +4°C for 24 h. The cultivar Norstar had the lowest LT₅₀ (temperature predicted to be lethal to 50% of the plants) of the 22 cultivars when frozen with either of the subzero pre-freezing treatments, but several cultivars had lower LT₅₀ scores than Norstar when frozen without a subzero pre-freezing treatment. We conclude it may be possible to improve winterhardiness of wheat grown in saturated soil by combining the ability to effectively respond to mild subzero pre-freezing temperatures with a greater ability to withstand freezing to damaging temperatures without a subzero pre-freezing exposure.     

Regulation of proline and ethylene levels in rape seedlings for freezing tolerance

This study was aimed to investigate the possibility of regulating free proline content and ethylene production in the resistant to abiotic stress cv. ‘Hornet H’ and the tolerant to stress cv. ‘Sunday’ of winter rapeseed seedlings by pretreatment with exogenous L-proline and L-glutamine in non-acclimated and cold-acclimated seedlings in relation to freezing tolerance. The ratio of proline content in acclimated (at 4°C) versus non-acclimated (18°C) ‘Hornet H’ seedlings increased 2.12-fold and in ‘Sunday’ seedlings 1.95-fold. Exogenously applied, proline and glutamine produced a positive effect on free proline content in both cold-acclimated and non-acclimated seedlings. At a temperature of -1°C the proline content significantly increased in non-acclimated and especially in cold-acclimated seedlings. At an intensified freezing temperature (?3°C, ?5°C, ?7°C), the proline content decreased in comparison with that at ?1°C, but glutamine, especially proline, in cold-acclimated seedlings takes part in free proline level increase and in seedlings’ resistance to freezing. Ethylene production increased in cold-acclimated conditions and under the effect of exogenous proline and glutamine. In freezing conditions, ethylene production decreased, but in cold-acclimated seedlings and under pretreatment of proline and glutamine the ethylene synthesis was intensive. Thus, free proline content and ethylene production increase in cold-acclimated winter rapeseed seedlings and under pretreatment with glutamine and especially with proline. Free proline is involved in the response to cold stress, and its level may be an indicator of cold-hardening and freezing tolerance, but the role of ethylene in the regulation of cold tolerance remains not quite clear.     

Root–shoot communication of the seedlings of Japanese larch and a hybrid species grown in different soil-temperature regimes

We studied the effects of soil temperature (7, 15, and 25°C) on the growth and photosynthesis of seedlings of the Japanese larch (Larix kaempferi) and its hybrid larch (L. gmelinii × L. kaempferi) to simulate early stages of regeneration after disturbance. At a soil temperature of 7°C, the root length per unit root biomass, chlorophyll concentration, and photosynthetic nitrogen-use efficiency (PNUE) were markedly lower in the Japanese larch than in the hybrid larch, which may indicate that the hybrid larch is better at acquiring water and nutrients. At ambient temperatures of 17–25°C, the light-saturated photosynthesis rate (P _sat) of both seedlings grown at a soil temperature of 7°C was lower than at 15 or 25°C. By the 16th week, the needle area, root area, and biomass in seedlings of both types were lower at a soil temperature of 7°C than at soil temperatures of 15 or 25°C. At a soil temperature of 25°C, P _sat and nitrogen uptake were lower in both larch species than at 15°C. The growth of the Japanese larch declined sharply from 15 to 25°C; however, the growth of the hybrid larch decreased only slightly from 15 to 25°C. We conclude that an increased soil temperature may retard larch growth in cold regions, especially in the case of the Japanese larch.     

Effects of constant and fluctuating soil temperature on growth,development and nutrient uptake of maize seedlings

Summary The effect of constant and fluctuating soil temperature and two soil moisture regimes on the growth, development, transpiration and nutrient uptake by maize seedlings was studied in a greenhouse investigation. The constant root temperatures were maintained at 30, 34, 35, 36, 37, and 38°C for both 250 and 750 cm of soil moisture suctions. The fluctuating root temperature, for 250 cm of soil moisture suction only, of 30–35, 30–39, 30–40, 30–45 and 30–48°C were maintained to simulate the soil temperature regime under field conditions. The constant root temperature of 35°C and fluctuating temperature between 30–40°C significantly decreased the shoot and root growth and transpiration rate. On the average, there was 1.3 and 0.7 g decrease in fresh shoot weight and 0.36 and 0.30 g in fresh root weight per degree increase in root temperature for 250 and 750 soil moisture suction, respectively. In general, the effect of high soil moisture suction on maize seedlings was more severe when at high root temperature. The shoot and root concentration of N, P, and K decreased while that of B increased with increase in root temperature. The root concentration of Zn also decreased with increase in root temperature.     

Root growth of subalpine and montane Eucalyptus seedlings at low soil temperatures

 This study examines the effect of different soil temperatures on root growth in seedlings of Eucalyptus pauciflora Sieber ex Sprengel subsp. pauciflora and Eucalyptus nitens (Deane & Maiden) Maiden. Seedlings were grown in a glasshouse in pots containing soil. Pots were held in water baths maintained at 3, 7 or 13°C, whilst shoots were exposed to ambient glasshouse temperatures. The experiments were designed to separate direct effects of soil temperature from effects due to differences in seedling size. In the first experiment, seedlings were grown to constant height (25 cm for both species), in the second to constant time (100 days for E. pauciflora and 64 days for E. nitens) and in the third experiment seedlings were transferred between soil temperatures. The rate of growth of both species increased with increasing soil temperature. E. nitens grew faster than E. pauciflora at 7 and 13°C, but E. pauciflora grew faster than E. nitens at 3°C. The rate of browning of roots increased with decreasing soil temperature and at a faster rate in E. nitens than E. pauciflora. Root length was highly correlated to root mass within diameter and colour classes (r² > 0.7). However, brown roots were heavier than white roots. Consequently, changes in root mass did not reflect changes in root length when the proportion of brown to white root also changed. For example, at a constant height of 25 cm at 3°C, E. nitens had greater root mass but lesser root length than E. pauciflora. E. pauciflora at 3°C grew faster, and had more root length and less brown roots than E. nitens. This supports the argument that E. pauciflora is better adapted than E. nitens to survive and grow at lower soil temperatures. Received: 16 December 1996 / Accepted: 2 April 1997     

Partition of photosynthates between shoot and root in spring wheat (Triticum aestivum L.) as a function of soil water potential and root temperature

Low soil water potential and low or high root temperatures are important stresses affecting carbon allocation in plants. This study examines the effects of these stresses on carbon allocation from the perspective of whole plant mass balance. Sixteen-day old spring wheat seedlings were placed in a growth room under precisely controlled root temperatures and soil water potentials. Five soil water potential treatments, from −0.03 MPa to −0.25 MPa, and six root temperature treatments, from 12 to 32°C were used. A mathematical model based on mass balance considerations was used, in combination with experimental measurements of rate of net photosynthesis, leaf area, and shoot/root dry masses to determine photosynthate allocation between shoot and root. Partitioning of photosynthates to roots was the lowest at 22–27°C root temperature regardless soil water potential, and increased at both lower and higher root temperatures. Partitioning of photosynthates to the roots increased with decreasing soil water potential. Under the most favourable conditions, i.e. at −0.03 MPa soil water potential and 27°C root temperature, the largest fraction, 57%, of photosynthates was allocated to the shoots. Under the most stressed conditions, i.e. at −0.25 MPa soil water potential and 32°C root temperature, the largest fraction, more than 80%, of photosynthates was allocated to roots.     

Arabidopsis: Sensitivity of growth to high temperature

Arabidopsis thaliana seedlings as measured by an electrolyte leakage assay, have been found to be extremely sensitive to high temperature stress as compared to a high temperature tolerant variety (Tracy) of soybean. Over 50% ion leakage occurred in Arabidopsis leaves during a 15-minute exposure to 50°C, indicating a heat killing time of less than 15 minutes. In contrast, the heat killing time for soybean at 50°C was over five times longer. When soybean or Arabidopsis seedlings in culture plates were exposed to 37°C for 2 hours and then returned to 23°C, they suffered no apparent short-term or long-term damage. Soybean seedlings given a 42°C, treatment for 2 hours also showed no damage. Arabidopsis seedlings after a 42°C treatment for 2 hours showed no apparent immediate damage, but 48 hours after return to 23°C severe damage symptoms were visible and after 96 hours all the seedlings were dead. Both soybean and Arabidopsis seedlings synthesize heat shock proteins (hsps) when exposed to 42°C for 2 hours. The hsps synthesized are of similar molecular weights, although the relative abundances of the different size classes are very different in the two plants. Even though hsps are produced in Arabidopsis seedlings after a 2 hour exposure to 42°C their presence is not sufficient for the seedlings to recover from the effects of rhe heat shock when returned to 23°C. Our results show that Arabidopsis has a heat sensitive genotype. This along with its other characteristics should make it a good model system in which to assay in transgenic plants, the functions of homologous and heterologous genes that might be candidates for determining heat tolerance in plants.     

Root respiration in citrus acclimates to temperature and slows during drought

Citrus seedlings were grown in soil columns in which the root system was hydraulically separated into two equal layers; this enabled us to maintain roots in the upper layer without water for 110 d. The columns were placed into waterbaths modified so that soil temperatures in the top layer could be maintained at 25°C or at 35°C, while temperature in the bottom layer was maintained at 25°C. We hypothesized that, if citrus plants were grown in dry soil for an extended period, root mortality would increase if the cost of maintaining the roots was increased by elevating the soil temperature. However, during the drought period we did not observe any root mortality, even at the higher soil temperature. Moreover, we did not find that root respiration was increased by prolonged exposure to drought and higher soil temperature. We did find that root respiration rates slowed in dry soil. Furthermore, when the soil columns were switched from one temperature treatment to another, root respiration rates in wet soil rapidly increased when moved to a higher temperature or rapidly decreased when moved to a lower temperature. But after only 4 d, respiration rates returned to their original level; root respiration in dry soil was not affected by either short-or long-term shifts in soil temperature. Root respiration in citrus appears to acclimate rapidly to changes in soil temperature.     

Water stress preconditioning and cotton root pressure-flux relationships

Summary A model was developed to describe interactive effects of exposure time and treatment on thermostability of excisedIllicium parviflorum Michx. root cell membranes using electrolyte leakage (L_c) procedures. Roots were moved from 25°C to treatment temperatures between 35°C and 60°C for 30 to 300 min. A sigmoidal response described L_c increases with increasing temperature at selected time exposures and the lethal exposure time decreased exponentially as temperature increased. The lethal temperature (52.0±1.1°C) for a 15 min exposure using this technique was comparable to the critical temperature (52.2±1.2°C) when roots were exposed to gradually increasing temperatures (4°C per h). Total protein content of roots began to decrease as temperatures increased from 35 to 40°C and the temperature corresponding to 50% reduction in total proteins was 49.1±2.2°C.     

Sensitivity of the common bean (Phaseolus vulgaris L.) symbiosis to high soil temperature

Summary Experiments were done to test whether N fixation is more sensitive to high soil temperatures in common bean than in cowpea or soybean. Greenhouse experiments compared nodulation, nitrogenase activity, growth and nitrogen accumulation of several host/strain combinations of common bean with the other grain legumes and with N-fertilization, at various root temperatures. Field experiments compared relative N-accumulation (in symbiotic relative to N-fertilized plants) of common bean with cowpea under different soil thermal regimes. N-fertilized beans were unaffected by the higher temperatures, but nitrogen accumulation by symbiotic beans was always more sensitive to high root temperatures (33°C, 33/28°C, 34/28°C compared with 28°C) than were cowpea and soybean symbiosis. Healthy bean nodules that had developed at low temperatures functioned normally in acetylene reduction tests done at 35°C. High temperatures caused little or no suppression of nodule number. However, bean nodules produced at high temperatures were small and had low specific activity. ForP. vulgaris some tolerance to high temperature was observed among rhizobium strains (e.g., CIAT 899 was tolerant) but not among host cultivars. Heat tolerance ofP. acutifolius andP. lunatus symbioses was similar to that of cowpea and soybean. In the field, high surface soil temperatures did not reduce N accumulation in symbiotic beans more than in cowpea, probably because of compensatory nodulation in the deeper and cooler parts of the soil.     

Differential responses of Arabidopsis ecotypes to cold, chilling and freezing temperatures

Arabidopsis plants show an increase in freezing tolerance in response to exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. In the present study, we evaluated the physiological and morphological responses of various Arabidopsis ecotypes to continuous growth under chilling (14°C) and cold (6°C) temperatures and evaluated their basal freezing tolerance levels. Seedlings of Arabidopsis plants were extremely sensitive to low growth temperatures: the hypocotyls and petioles were much longer and the angles of the second pair of true leaves were much greater in plants grown at 14°C than in those grown at 22°C, whereas just intermediate responses were observed under the cold temperature of 6°C. Flowering time was also markedly delayed at low growth temperatures and, interestingly, lower growth temperatures were accompanied by longer inflorescences. Other marked responses to low temperatures were changes in pigmentation, which appeared to be both ecotype specific and temperature dependent and resulted in various visual phenotypes such as chlorosis, necrosis or enhanced accumulation of anthocyanins. The observed decreases in chlorophyll contents and accumulation of anthocyanins were much more prominent in plants grown at 6°C than in those grown at 14°C. Among the various ecotypes tested, Mt‐0 plants markedly accumulated the highest levels of anthocyanins upon growth at 6°C. Freezing tolerance examination revealed that among 10 ecotypes tested, only C24 plants were significantly more sensitive to subzero temperatures. In conclusion, Arabidopsis ecotypes responded differentially to cold (6°C), chilling (14°C) and freezing temperatures, with specific ecotypes being more sensitive in particular traits to each low temperature.     

Induced Heat Sensitivity of Wheat Roots and Protecting Effect of Ethanol and Kinetin

Wheat seedlings were subjected to heat shock for 2 min at 45°C. The seedlings were then incubated at 25°C or higher temperatures (usually 35°C). At 25°C the root tips survived the heat shock, but not at temperatures above 34°C, unless they had been pretreated with ethanol or kinetin, After 1 h in ethanol and after more than 15 h in kinetin the root meristem survived a high incubation temperature after the heat shock. Immediately after heat treatment the glyceride content in treated root tips was higher than in untreated roots. The same was observed after heat treatment of root tips pretreated in ethanol and kinetin. The content of ether extractable lipids was not changed by the heat shock.