Nitrate reductase in cotyledons of cucumber seedlings as affected by nitrate, phytochrome and calcium

Regulation by the active form of phytochrome (P_FR) and the effect of Ca²⁺ was examined with nitrate reductase (NR) in etiolated cucumber ( Cucumis sativus cv. Beilpuig). Nitrate reductase activity (NRA) was studied in excised cotyledons of cucumber seedlings grown in distilled water and in darkness for seven days at 24 ± 0.5°C. All experiments were performed in the dark and a dim green safelight was used during analyses. In etiolated cucumber cotyledons NRA was induced by nitrate and a brief irradiation (15 min) with red light (R) resulted in 62% increase in NRA. This effect was nullified when R was followed immediately by a brief (5 min) far-red light (FR). NRA also showed a semidian (12 h) rhythmicity. Both P_FR, and nitrate effects were age dependent. Calcium seemed to be involved since the phytochrome effect was only observed when calcium was supplied in the external solution. The effect of R on NRA depended on the period of calcium nitrate incubation. An external supply of calcium ionophore mimicked the effect of R and, if supplied to R-irradiated cotyledons, produced a higher NR level than that caused by R alone. This suggested that intracellular free calcium was involved.     

Balancing photosynthetic light-harvesting and light-utilization capacities in potato leaf tissue during acclimation to different growth temperatures

We investigated the effect of temperature during growth and development on the relationship between light-harvesting capacity, indicated by chlorophyll concentration, and light-utilization potential, indicated by light- and bicarbonate-saturated photosynthetic oxygen evolution, in Solanum tuberosum L. cv. Norland. Conal plantles were transplanted and grown at 20°C for 2 weeks before transfer to 12, 16, 20, 24 and 28°C for 6 weeks. After 4 weeks of the temperature treatments, leaf tissue fresh weights per area were one-third higher in plants grown at 12°C vs those grown at 28°C. Conversely, chlorophyll content per area in tissue grown at 12°C was less than one-half of that of tissue grown at 28°C at 4 weeks. Photosynthetic capacity measured at a common temperature of 20°C and expressed on a chlorophyll basis was inversely proportional to growth temperature. Leaf tissue from plants grown at 12°C for 4 weeks had photosynthetic rates that were 3-fold higher on a chlorophyll basis than comparable tissue from plants grown at 28°C. These results suggest that the relationship between light-harvesting capacity and light-utilization potential varies 3-fold in response to the growth temperatures examined. The role of this response in avoidance of photoinhibition is discussed.     

Seed dormancy in Rumex species in response to environmental factors

Abstract. Many Rumex species show similar seed dormancy characteristics but there is more information concerning R. crispus and R. obtusifolius than other species. These species respond positively to red or white light. Far-red light applied for short periods may promote or inhibit germination depending on the timing of the irradiation in relation to temperature change; but long periods of far-red inhibit germination. Seeds may also be stimulated to germinate in the dark by low-temperature stratification at 15°C or less providing the temperature of the seeds is subsequently raised to a minimum of about 15°C. Seeds can, however, germinate at lower temperatures providing they have received other appropriate stimulatory treatment. Seeds also respond to alternating temperatures. In a diurnal cycle the minimum upper temperature required is about 15°C and the maximum lower temperature is about 25°C. The optimum period spent at the upper temperature is about 8 h when it is 15–25°C but the optimum period decreases as the upper temperature is increased above this range so that at 45°C, for example, it is only about 30 min. The period spent at the lower temperature in a diurnal cycle is not critical. Providing these criteria are met, the percentage germination increases with the number and amplitude of the cycles. The warming part of the cycle is necessary for the response but so far there is no convincing evidence that cooling itself is important. Secondary dormancy is induced at constant temperatures at a rate dependent on temperature, but apparently only in the presence of oxygen. This feature affects the optimum timing of a temperature change or exposure to light. Strong positive interactions are shown between stimulatory temperature treatments and white or red light. Unlike many other weed species the seeds respond only slightly to nitrate ions. The implications of these responses are discussed in relation to field behaviour.     

Triadimenol Increases Nitrate Levels and Nitrate Reductase Activity in Canola Leaves

Nitrate reductase activity in the first true leaves of canola(Brassica napus L.) seedlings grown in one-quarter strengthHoagland's solution from seeds pretreated with triadimenol (0.3or 30 g (a.i.) kg^–1 of seed) was higher than controlsduring the growth period of 15 to 25 d after planting. Triadimenolalso increased chlorophyll levels, the increase being more pronouncedat its lower concentration. The treatment also increased theweight and nitrate content of the leaves. When seedlings weregrown in nutrient solution containing 1 to 20 mM nitrate, theincrease in nitrate reductase activity by triadimenol was higherat lower rather than at higher nitrate concentrations. The nitratelevels and Kjeldahl nitrogen in the triadimenol-treated leaveswas higher than the controls at concentrations of added nitrateabove 2 mM. Addition of nitrate to plants grown in ammonium,increased nitrate reductase activity more in plants grown fromtriadimenol-treated seeds than controls. However, addition of10µM triadimenol for 24 h to ammonium-grown plants hadlittle effect on enzyme activity, both in the absence as wellas the presence of nitrate. This study demonstrates that triadimenolincreases nitrate reductase activity and nitrate accumulationin the leaves and at least part of the increased enzyme activityis independent of nitrate accumulation. Key words: Triazoles, nitrate content, nitrate reductase activity     

Effect of Temperature on the Formation of Microsclerotia of Verticillium dahliae

Microsclerotium formation by six isolates of Verticillium dahliae was studied at different temperatures both in vitro and in Arabidopsis thaliana . In vitro mycelial growth was optimal at 25°C, but microsclerotium formation was greatest at 20°C (two isolates) or 15–20°C (one isolate). Seedlings of A. thaliana were root-dipped in a conidial suspension, planted, and either placed at 5, 10, 15, or 25°C, or left at 20°C until the onset of senescence, after which some of the plants were placed at 5, 10, 15, or 25°C. The amount of microsclerotia per unit of shoot weight was assessed in relation to isolate and temperature. The optimal temperature for production of microsclerotia was 15–25°C. Two isolates each produced about 10 times more microsclerotia than each of the other four isolates. For these isolates, high R ²_adj.-values of 0.77 and 0.66 were obtained, with temperature and its square as highly significant (P   < 0.001) independent variables. R ²_adj.-values for the other isolates varied between 0.28 and 0.39. Moving plants to different temperatures at the onset of senescence led to microsclerotial densities that were intermediate between densities on plants that had grown at constantly 20°C and plants grown at other temperatures. This suggests that vascular colonization rate and rate of microsclerotium formation are similarly affected by temperature. The senescence rate of plants appeared unimportant except for plants grown at 25°C, which showed the highest amounts of microsclerotia per unit of plant weight in the most rapidly senescing plants.     

温度和气体成分对冬枣果实贮藏期间品质的影响

主要探讨冬枣(Ziziphus jujuba Mill.cv Dongzao)在-1℃的动态气调(CA-Ⅰ,70%O2 0%CO2处理7天,然后转入5%O2 0%CO2中)、普通气调(CA-Ⅱ,5%O2 0%CO2;CA-Ⅲ,10%O2 0%CO2)及普通冷藏和常温(25℃)等条件下,果实发病率、色素、可溶性固形物、可滴定酸、乙醇和乙酸乙酯含量等的变化.结果表明:与普通冷藏相比,气调贮藏能减缓果实的腐烂,抑制色素的分解和减少果肉中乙醇、乙酸乙酯的含量.动态高氧处理能有效地保持果实的颜色,抑制色素降解及果皮褐变.CA-Ⅲ(10%O2 0%CO2)能有效地减少果肉乙醇的含量而CA-Ⅱ(5%O2 0%CO2)能有效地减少果肉乙酸乙酯的含量.气调贮藏的果实可滴定酸及可溶性固形物含量与其他处理的果实没有明显的差异.     

The influence of the rate of temperature change on the activation of dormant seeds of Rumex obtusifolius L.

1. One temperature shift from 20 to 30°C in darkness induces 30–40% germination in Rumex obtusifolius seeds. The same germination percentages are found with heat treatment varying between 1 and 6h duration, indicating that the total heat sum of the temperature shift is not important.
2. Germination is greatly enhanced by three consecutive heat shifts of 1h at 30°C separated by 1h periods at 20°C.
3. The seeds are activated to a small extent after a slow warming (+2°Ch^–1) from 20 to 30°C, followed by incubation for 1h at 30°C. Germination is much higher after rapid heating (+10°Ch^–1) to 30°C, followed by 1h incubation at this temperature. Repeated fast heating treatments on four consecutive days enhances germination. Moderately rapid heatings (+3·3°Ch^–1) give intermediate results.
4. The rate of cooling does not influence the germination percentage. Cooling alone cannot induce germination.
5. Heating alone from 15 to 25°C without cooling also activates germination. In this temperature range the seeds are more activated by rapid warming than by slow warming.
6. The ecological relevance of the response to different warming rate is discussed. The insensitivity of seeds to a slow warming might keep deeply buried seeds in a dormant stage.     

Morphological dormancy in seeds of the autumn-germinating shrub Lonicera caerulea var. emphyllocalyx (Caprifoliaceae)

To better understand the germination ecophysiology of the genus Lonicera , the dormancy class, temperature requirements for embryo growth and radicle emergence and phenology of seedling emergence were determined for Lonicera caerulea var. emphyllocalyx . At maturity, seeds have an underdeveloped embryo (approximately 28% of the length of full-grown embryos). Embryos in fresh seeds grew to full length at 15, 20, 20/10 and 25/15°C within 3 weeks, but failed to grow at ≤ 10°C and at 30°C. Radicles emerged from 86–100% of freshly matured seeds in light at 15, 20, 20/10 and 25/15°C within 28 days, but failed to emerge at 10°C. Radicles emerged equally well in a 12 h photoperiod and in continuous darkness at 25/15°C. Rapid embryo growth and germination over a range of conditions indicate that seeds of this taxon have morphological dormancy (MD); this is the first report of MD in a species of Lonicera . Seeds are dispersed in summer, at which time high temperatures promote embryo growth. Embryos grow to the critical length for germination in approximately 1 month; the peak of seedling emergence occurs in early autumn. Radicles emerged within 2 months from 98% of seeds buried at soil depths of 2 cm and 10 cm in the field in August in Sapporo, Japan; thus, seeds have no potential to form a persistent soil seed bank. However, seeds sown too late in autumn for embryos to grow remained viable and germinated the following summer when temperatures were high enough to promote embryo growth.     

The effect of differential root and shoot temperature on the nitrate reductase activity,assayed in vivo and in vitro in roots ofHordeum vulgare (barley)

There was a large increase in nitrate reductase activity (NAR) assayed both in vivo and in vitro in roots of barley plants (cv. Midas_ grown with roots at 10°C and shoots at 20°C, compared with whole plants grown at 20°C. There were diurnal fluctuations in NRA in roots from both treatments, but they were much greater in roots grown at 20°C, where NRA fell to a very low value in the dark period. The diurnal fluctuations in the malate content of the roots were also related to the root growth temperature. Plants with roots grown at the lower temperature had a higher malate content, especially in the dark period where it was 20 times greater than in plants with roots at 20°C. At all times there was a three-fold increase in soluble carbohydrate in cooled roots and diurnal fluctuations were much less pronounced than those of malate. Growth at low temperatures increased the total flux of amino N into the xylem sap and increased the proportion of reduced N in the total N flux. At certain times of day both 10°C- and 20°C-grown roots responded to exogeneous malate by increasing the flux of amino acid into the xylem sap, although this effect was always more pronounced in 20°C-grown roots.     

Photosensitivity of Rumex obtusifolius seeds for stimulation of germination: Influence of light and temperature

A population of Rumex obtusifolius L. seeds imbibed for 24 h at 25°C exhibits a sigmoid logarithmic fluence-response relationship for stimulation of germination by red light (R), 11.0 μmol m⁻² being necessary for 50% of the response. After 24 h imbibition at 35°C the fluence-response relationship for stimulation of germination by R is biphasic. For 50% response the very sensitive phase (very low fluence-response) requires 4.7 − 10⁻²μmol m⁻² whereas the less sensitive phase (low fluence-response) requires 4.0 μmol m². A few seconds of far-red light (FR) satisfies the germination requirement of the sensitive seeds after 24 h at 35°C. However, a longer period of FR (2 h) results in low germination. The fluence-response relationship for induction of these seeds by R is sigmoid, 4.8 μmol m⁻² being necessary for 50% response, demonstrating that 2 h FR desensitizes the sensitive proportion of the seed population induced by 24 h at 35°C. A proportion of the seed population can be further sensitized by 60 min at 35°C following this desensitization.     

A comparative study of the formation of extracellular proteins by Aeromonas salmonicida at two different temperatures

Aeromonas salmonicida was grown in a supplemented 3% (w/v) tryptone soya broth medium at 10°C, a temperature at the lower end of the range over which furunculosis has been observed to occur in the field, and 25°C, the optimum temperature for growth. Similar bacterial densities in the range 2.35 × 0.05 mg dry wt/ml were achieved in the two cultures at the beginning of the stationary phase of the growth cycle, after 125 h at 10°C and 18 h at 25°C. At this point, at the higher temperature 1.5 times more exoprotein was formed, 80 × 2.8 μg/ml compared with 54 × 1.7 μg/ml. Exoprotein contained the same proportion of haemolysin at both temperatures and twice as much protease at the higher temperature. The most marked difference was in an unidentified 100 kD protein which was formed in a 10-fold greater amount at 10°C.     

Contrasting photo- and thermoperiod-induced changes in abscisic acid and lipid contents in leaves of mungbean seedlings

Seedlings of Phaseolus aureus ROXB were grown under 12/12 h light/dark cycles with the light period at 32.5°C and darkness at 10°C (normal conditions N) or with light at 10°C and darkness at 32.5°C (inverse conditions, I). I-conditions affected the level of chlorophyll and carotenoids (very low), monogalactosyldiacylgycerol (low) and phosphatidylinositol (high) in the leaves. Leaves of I-seedlings showed a sharp and durable decline of relative water content during the low temperature phase. For the N-seedlings, loss of water was restricted to the end of this period. The loss of water was accompanied by visible symptoms of wilting at specific times of day. Although the pigment content remained nearly unchanged, ABA content of leaves of both N-and I-seedlings increased during water stress. Upon return to the warm period, ABA level continued to increase after the leaves had regained turgor, this 'after stress'increase being more pronounced in the leaves of I-seedlings. Exogenous application of ABA induced a slight increase in the content of phospholipids in N- and I-leaves and a decrease in free fatty acids, whereas monogalactosyldiacylglycerol content was significantly reduced in N-leaves after application of ABA. Upon transfer of I-plants to 20°C for 12 h during the light period, pigment and chloroplastic lipid content increased rapidly whereas upon a further exposure to 10°C in light, pigments and especially monogalactosyldiacylglycerol were lost. The control of pigment and lipid metabolism and the role of ABA during chilling stress are discussed.     

Soluble sugar content of white spruce (Picea glauca) seeds during and after germination

In white spruce ( Picea glauca [Moench.] Voss.) seeds, the raffinose family oligosaccharides (RFOs) provide carbon reserves for the early stages of germination prior to radicle protrusion. Some seedlots contain seeds that are dormant, failing to complete germination under optimal conditions. Since dormancy may be imposed through a metabolic block in reserve mobilization, the goal of this project was to identify any impediment to RFO mobilization in dormant relative to nondormant seeds. Desiccated seeds contain primarily, and in order of abundance on a molar basis, sucrose and the first 3 members of the RFOs, raffinose, stachyose and verbascose. Upon radicle protrusion at 25°C, the contents of RFOs decreased to low amounts in all seed parts, regardless of prior dormancy status and sucrose was metabolized to glucose and fructose, which increased in seed parts. During moist chilling at 4°C, RFO content initially decreased before stabilizing and then increasing. In seeds that did not complete germination, the synthesis of RFOs at 4°C favored verbascose, so that at the end of 14 (nondormant) or 35 (dormant) weeks, verbascose contents in megagametophytes exceeded the amount initially present in the desiccated seed. This was also true in the embryos of the dormant seedlot. In seed parts from both seedlots after months of moist chilling, stachyose amounts exceeded raffinose amounts. Upon radicle protrusion at 4°C, RFO contents decreased to amounts most similar to those present in seeds that completed germination at 25°C. Hence, the RFOs are utilized as a source of energy, regardless of the temperature at which white spruce seeds complete germination. Based on the similarity of sugar contents in seed parts between dormant and nondormant seeds that did not complete germination, differences in sugar metabolism are probably not the basis of dormancy in white spruce seeds.     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Germination Responses of Taraxacum platycarpum Seeds to Temperature

Abstract Germination responses of Taraxacum platycarpum seeds to temperature were examined under laboratory conditions to investigate the emergence-season choice mechanism of the seeds. Almost all the newly collected seeds were non-dormant. Under constant temperature conditions, maximum percentage germination (approximately 90%) was attained at temperatures 6–16°C, where simple linear relationships were observed between the temperature and the rates of germination, i.e. the reciprocals of the time taken to germinate by seed subpopulations with 10–80% germination. Thermal time required for germination of the subpopulations ranged from 600 Kh (degree Kelvin × hours) to 1500 Kh with a relatively constant base temperature of about 2.5°C. Lower limit temperature for germination was slightly below 6°C. Higher limit temperature for germination has the normal distribution with the mean ±SD of 19±2.5°C. Pre-exposure of imbibed seeds to temperatures higher than the higher limit temperature for germination, 25 and 30°C, had no effect on the germinability and the rate of germination at a circa-optimum temperature. Moist chilling treatment at 4°C caused an increase in the variation of germination rate within the seed population, but no evidence for dormancy-inducing or breaking effects was obtained.     

Dormancy and germination of olive embryos as affected by temperature

Olive seeds do not germinate promptly when placed under favourable conditions, which is a problem in raising young plants for breeding or experimental purposes. In a series of experiments an investigation of the role of temperature in the germination of olive embryos was conducted. Naked, unchilled olive embryos ( Olea europaea L. cv. Chalkidikis), cultured in vitro at 20°C, had a germination capacity of 73%, whereas that of embryos which had previously been chilled at 10°C for 2 or more weeks reached 96%. Intact seeds did not germinate at 20°C unless they had previously been subjected to 10°C for 3 or 4 weeks. Embryos chilled while in the intact seed and excised just before transfer to 20°C, reacted in a similar way to naked embryos, but reached their maximum germination capacity after 4 weeks at 10°C. Under constant temperature conditions the highest germination percentage of embryos was observed at 10 and 15°C and the highest germination rate at 15°C, while a moderate capacity and rate of germination occurred at 20°C, and a very low percentage and rate at 25 and 30°C. Prechilling at 10°C did not affect germination at 15°C, but improved the percentage and the rate of germination at 20, 25 and 30°C. The germination percentages of embryos chilled for 1 or 2 weeks at 10°C and then transferred to 25°C were lower than those of similarly chilled embryos transferred to 20°C. The chilling effect could not be reversed at 25°C when the embryos had been chilled for 3 or more weeks. The results show that olive seeds exhibit a state of dormancy that is caused by factors residing partly in the endosperm and partly within the embryo.     

Freezing and desiccation tolerance of imbibed canola seed

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

Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea mays L.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Two experiments were conducted to evaluate the effects of phenotypicrecurrent selection for high and low post-anthesis leaf-laminain vivo NRA on nitrate uptake, nitrate partitioning and in vitroNRA of seedling roots and leaves. In Experiment 1, intact plantsof cycle 0, 4, and 6 of the high and low NRA strains were grownon NH₄-N for 11 d, then exposed to 1.0 mol m^–3 KNO₃, andcultures sampled at 6 h and 28 h (induction and post-inductionperiods). Nitrate uptake, tissue nitrate concentration and invitro NRA were determined. The pattern of response to selectionin seedling leaf NRA was similar to that observed for in vivoNRA of field grown plants. Leaf NRA increased between 6 h and28 h. Root NRA was not affected by selection or sampling time.Treatments differed in total fresh weight but not in reductionor uptake of nitrate per unit weight, indicating a lack of correspondencebetween NRA and reduction and supporting the idea that concomitantreduction by NR is not obligatorily linked to nitrate influxin the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of thehigh and low NRA strains were cultured without N, detopped onday 6, transferred the following day to 0-75 mol m^–3 KNO₃and sampled at 6 h and 28 h. In contrast to Experiment 1, selectionpopulations differed in nitrate reduction and root NRA, whichby 28 h reached higher average levels than root NRA of intactplants. Translocation and reduction were inversely related amongstrains within each sampling time. The high level of translocationin detopped plants of the low NRA strain was difficult to reconcilewith its low leaf NRA level of Experiment 1. It is suggestedthat nitrate transport in detopped roots is altered relativeto the intact system in a way which permits greater NRA inductionand nitrate reduction. The results indicate that nitrate partitioningby detopped root systems should be interpreted with caution. Key words: Zea, nitrate reductase activity, nitrate uptake, nitrate reduction, nitrate partitioning, selection     

Temperature control of germination and its possible role in the survival of a non-dormant population of Avena fatua

Germination of freshly harvested seeds of a non-dormant (ND) line (Stonehouse 319) of wild oats ( Avena fatua L.) was inhibited by incubation of the seeds at relatively high temperatures of 25 and 30°C. The germination inhibition in these seeds appeared to be a case of thermo-inhibition which was the direct effect of hightemperature treatment (HIT), since it did not persist after transferring the seeds to an optimum germination temperature of 20°C. Even a prolonged HTT of 30°C for over 5 weeks did not prevent germination of about 80% of the seeds transferred to 20°C. However, in a significant proportion of the seeds, thermo-dormancy was induced by 10 days of HTT at 30°C if the seeds were then incubated at sub-optimal temperatures of 5 to 15°C. This thermo-dormancy would appear to be 'restrictive' in form, since its expression was restricted to very specific conditions. Relatively low inclubation temperaturs of 5 and 10°C markedly slowed germination whether HTT was applied or not. The results suggest that thermo-inhibition and thermo-dormancy, induced during seasonal temperature fluctuations, may provide a survival mechanism for seeds of such ND lines as Stonehouse 319.