Calorimetric studies of the state of water in seed tissues

To understand the physical state of water in hydrating biological tissues, thermodynamic properties of water in cotyledons of pea and soybean with moisture contents ranging from 0.01 g H₂O/g dw to 1.0 g H₂O/g dw were studied using differential scanning calorimetry. The heat capacity of the tissues increased abruptly at moisture contents above 0.08 and 0.12 g H₂O/g dw for soybean and pea cotyledons, respectively. Melting transitions of water were observed at moisture contents >0.23 and 0.26 g H₂O/g dw for soybean and pea. However, freezing of water was not observed unless moisture contents exceeded 0.33-0.35 g H₂O/g dw. In both seed tissues, the temperatures of the freezing and melting varied with moisture content and showed hysterisis. The energy of the transition also varied with moisture content and was similar to the heats of fusion and crystallization of pure water only at moisture contents >0.54 and 0.58 gH₂O/g dw for soybean and pea seeds, respectively. The thermal properties of water change distinctly as seed moisture content changes: at least five states or water can be identified.     

Water and lipid relations in beech (Fagus sylvatica L.) seeds and its effect on storage behaviour

Beech (Fagus sylvatica L.) seeds indicate intermediate storage behaviour. Properties of water in seed tissues were studied to understand their requirements during storage conditions. Water sorption isotherms showed that at the same relative humidity (RH) the water content is significantly higher in embryo axes than cotyledons. This tendency maintains also after recalculating the water content for zero amount of lipids in tissues. Differential thermal analysis (DTA) indicated water crystallization exotherms in the embryo axes at moisture content (MC) higher than 29% and 16% in the cotyledons. In order to examine the occurrence of glassy state in the cytoplasm of beech embryos as a function of water content, isolated embryo axes were examined using electron spin resonance (ESR) of nitroxide TEMPO probe located inside axes cells. TEMPO molecules undergo fast reorientations with correlation time varied from 2 x 10(-9) s at 180 K to 2 x 10(-11) s at 315 K. Although the TEMPO molecules label mainly the lipid bilayers of cell membranes, they are sensitive to the dynamics and phase transformation of the cytoplasmic cell interior. The label motion is clearly affected by a transition between liquid and glassy state of the cytoplasm. The glass transition temperature (T(g)) raises from 253 to 293 K when water content decreases from 18% to 8%. Far from T(g) the motion is described by Arrhenius equation with very small activation energy E(a) in the liquid state and is relatively small in the glassy state where E(a)=1.5 kJ/mol for 28% H(2)O and E(a)=4.7 kJ/mol for 8% H(2)O or less. The optimal storage conditions of beech seeds are proposed in the range from 255 K for 15% H(2)O to 280 K for 9% H(2)O.     

Axes and cotyledons of recalcitrant seeds of Castanea sativa Mill. exhibit contrasting responses of respiration to drying in relation to desiccation sensitivity

Oxidative damage originating from uncontrolled metabolism is thought to be responsible for the sensitivity to drying in recalcitrant seeds. This study compares the responses of respiration to drying and the loss of membrane integrity in isolated axes and cotyledons of the recalcitrant seeds of Castanea sativa Mill. Electron spin resonance spectroscopy of two nitroxide spin probes introduced into the seed tissues was used to assess the cytoplasmic viscosity and the membrane permeability during fast and slow drying. Drying rates had not effect on the rise in viscosity in axes and cotyledons. In both tissues, the cytoplasmic viscosity during drying remained constant at 0.2 Poise until 1.6 g water/g DW (g/g), thereafter it increased exponentially. Axes were found to be more tolerant to drying than cotyledons: membranes showed minor changes in their permeability during drying and 50% viability was retained in dried axes containing 0.12 g/g. In contrast, plasma membranes in cotyledons lost their integrity below 0.6 g/g, regardless of the drying rate. Drying axes and cotyledons exhibited contrasting responses of their metabolism to drying. At the onset of drying, the rates of O2 uptake declined rapidly in drying axes. However, respiration in drying cotyledons sequentially increased to c. 1.4-fold at 1.2 g/g then decreased concomitantly with the loss of membrane integrity. The respiratory quotients (CO2 output/O2 input) remained constant around 0.9 until the loss of membrane integrity, then rose to 2.8. As a symptom of mitochondrial injury, the levels of reduction of cytochromes were assessed in situ in fresh and dried cotyledons using light spectroscopy. The levels of reduced cytochrome c and aa3 were lower in dried C. sativa cotyledons than in dried orthodox cotyledons of cowpea, indicating that a disruption in the electron transport chains may have occurred during drying. Desiccation sensitivity in recalcitrant seeds may be due to the inability to actively depress their metabolism during drying, thereby increasing the chances of initiating peroxidative damage during drying.Key words: Castanea satia, membrane permeability, recalcitrant seed, respiration, viscosity.      

Embryo Water Status and Survival in the Recalcitrant Species Quercus robur L: Evidence for a Critical Moisture Content

The responses of Q. robur L. fruits, seeds and embryonic axesto desiccation are characterized and discussed in relation tocurrent knowledge of recalcitrant seed behaviour. A relationshipbetween viability and seed moisture content is described. Thisrelationship was unaffected by rate of drying, year of harvestor presence of the pericarp. Desiccation sensitivity did notincrease with storage. Excised embryonic axes survived to lower moisture contents thanintact seeds. However, in the intact seed, loss of viabilityappeared to be determined by a critical moisture content inthe cotyledons. Consequently, the level of desiccation tolerancewithin the axis attached to cotyledons was not determined byaxis drying rate. A link is drawn between the difference in the desiccation toleranceof embryonic axes and of cotyledons, and estimates of theirdifferent levels of matrix-bound water. The results presentedare consistent with a critical moisture content for survivalwhich is determined by the loss of all free cellular water.This hypothesis takes account of the differential desiccationsensitivity of seed tissues and differences in desiccation tolerancebetween species.     

Changes in extreme high-temperature tolerance and activities of antioxidant enzymes of sacred lotus seeds

Sacred lotus (Nelumbo nucifera Gaertn. ‘Tielian’) seed is long-lived and extremely tolerant of high temperature. Water content of lotus and maize seeds was 0.103 and 0.129 g H2O [g DW] ?1, respectively. Water content, germination percentage and fresh weight of seedlings produced by surviving seeds gradually decreased with increasing treatment time at 100℃. Germination percentage of maize (Zea mays L. ‘Huangbaogu’) seeds was zero after they were treated at 100℃ for 15 min and that of lotus seeds was 13.5% following the treatment at 100℃ for 24 h. The time in which 50% of lotus and maize seeds were killed by 100℃ was about 14.5 h and 6 min, respectively. With increasing treatment time at 100℃, relative electrolyte leakage of lotus axes increased significantly, and total chlorophyll content of lotus axes markedly decreased. When treatment time at 100℃ was less than 12 h, subcellular structure of lotus hypocotyls remained fully intact. When treatment time at 100℃ was more than 12 h, plasmoly-sis gradually occurred, endoplasmic reticulum became unclear, nuclei and nucleoli broke down, most of mitochondria swelled, lipid granules accumulated at the cell periphery, and organelles and plas-molemma collapsed. Malondialdehyde (MDA) content of lotus axes and cotyledons decreased during 0-12 h of the treatment at 100℃ and then increased. By contrast, the MDA content of maize embryos and endosperms increased during 5-10 min of the treatment at 100℃ and then decreased slightly. For lotus seeds: (1) activities of superoxide dismutase (SOD) and glutathione reductase (GR) of axes and cotyledons and of catalase (CAT) of axes increased during the early phase of treatment at 100℃ and then decreased; and (2) activities of ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) of axes and cotyledons and of CAT of cotyledons gradually decreased with increasing treat-ment time at 100℃. For maize seeds: (1) activities of SOD and DHAR of embryos and endosperms and of GR of embryos increased during the early phase of the treatment at 100℃ and then decreased; and (2) activities of APX and CAT of embryos and endosperms and of GR of endosperms rapidly decreased with increasing treatment time at 100℃. With decrease in seed germination, activities of SOD, APX, CAT, GR and DHAR of axes and cotyledons of lotus seeds decreased slowly, and those of embryos and endosperms of maize seeds decreased rapidly.     

Plant vitrification solution 2 lowers water content and alters freezing behavior in shoot tips during cryoprotection

Plant shoot tips do not survive exposure to liquid nitrogen temperatures without cryoprotective treatments. Some cryoprotectant solutions, such as plant vitrification solution 2 (PVS2), dehydrate cells and decrease lethal ice formation, but the extent of dehydration and the effect on water freezing properties are not known. We examined the effect of a PVS2 cryoprotection protocol on the water content and phase behavior of mint and garlic shoot tips using differential scanning calorimetry. The temperature and enthalpy of water melting transitions in unprotected and recovering shoot tips were comparable to dilute aqueous solutions. Exposure to PVS2 changed the behavior of water in shoot tips: enthalpy of melting transitions decreased to about 40 J g H2O(-1) (compared to 333 J g H2O(-1) for pure H2O), amount of unfrozen water increased to approximately 0.7 g H2O g dry mass(-1) (compared to approximately 0.4 g H2Og dry mass(-1) for unprotected shoot tips), and a glass transition (T(g)) at -115 degrees C was apparent. Evaporative drying at room temperature was slower in PVS2-treated shoot tips compared to shoot tips receiving no cryoprotection treatments. We quantified the extent that ethylene glycol and dimethyl sulfoxide components permeate into shoot tips and replace some of the water. Since T(g) in PVS2-treated shoot tips occurs at -115 degrees C, mechanisms other than glass formation prevent freezing at temperatures between 0 and -115 degrees C. Protection is likely a result of controlled dehydration or altered thermal properties of intracellular water. A comparison of thermodynamic measurements for cryoprotection solutions in diverse plant systems will identify efficacy among cryopreservation protocols.     

Role of relative humidity, temperature, and water status in dormancy alleviation of sunflower seeds during dry after-ripening

The effect of various combinations of temperature and relative humidity on dormancy alleviation of sunflower seeds during dry after-ripening was investigated. The rate of dormancy alleviation depended on both temperature and embryo moisture content (MC). Below an embryo MC of 0.1 g H(2)O g(-1) dw, dormancy release was faster at 15 °C than at higher temperatures. This suggests that dormancy release at low MC was associated with negative activation energy, supported by Arrhenius plots, and low Q(10) values. At higher MC, the rate of dormancy alleviation increased with temperature, correlating well with the temperature dependence of biochemical processes. These findings suggests the involvement of two distinct cellular mechanisms in dormancy release; non-enzymatic below 0.1 g H(2)O g(-1) dw and associated with active metabolism above this value. The effects of temperature on seed dormancy release above the threshold MC were analysed using a population-based thermal time approach and a model predicting the rate of dormancy alleviation is provided. Sunflower embryo dormancy release was effective at temperatures above 8 °C (the base temperature for after-ripening, Tb(AR), was 8.17 °C), and the higher the after-ripening temperature above this threshold value, the higher was the rate of dormancy loss. Thermodynamic analyses of water sorption isotherms revealed that dormancy release was associated with less bound water and increased molecular mobility within the embryonic axes but not the cotyledons. It is proposed that the changes in water binding properties result from oxidative processes and can, in turn, allow metabolic activities.     

Ascorbic acid metabolism in ageing recalcitrant sal (Shorea robusta Gaertn. f.) seeds

Changes in ascorbate content and its enzymatic utilization pattern were studied in embryonic axes and cotyledons of sal seeds undergoing rapid loss of viability, at ambient conditions. Ascorbate levels were significantly higher initially in the embryonic axes (0.32 mg/g fresh weight) and cotyledons (0.21 mg/g fresh weight) of freshly mature, relatively hydrated (42.2% moisture content) and 100% viable sal seeds. It declined sharply as the tissues; embryonic axes and cotyledons, desiccated with absolutely no detectable amount in non-viable seeds (21% moisture content). Significantly strong correlation was obtained between desiccation of embryonic axes (r = 0.96) and cotyledon (r = 0.97) with loss of ascorbate levels and loss of germinability. Higher rates of ascorbic acid utilization (AAU) recorded in the embryonic axes of 100% viable seed declined sharply as the seed viability reduced due to desiccation below 36.8% moisture content. AAU was not detected in the cotyledons.     

Physiological aspects of Taxus brevifolia seeds in relation to seed storage characteristics

Water relations, desiccation tolerance and longevity of Taxus brevifolia (Nutt.) seeds were studied to determine the optimal stage of development and storage conditions for seeds of this species. Seeds equilibrated to a range of relative humidities (RHs) had unusually low water contents which can be accounted for by the high lipid content of gametophyte tissues (71% of the dry mass). Water relations of embryonic tissue were more typical of those reported for other seed species. The water content below which freezing transitions were not observable in the embryo was ca 0.24 g H₂O (g dry weight)⁻¹ (g g⁻¹) for all maturity classes studied. Embryos did not achieve significant levels of desiccation tolerance (survival to water contents less than 0.5 g g⁻¹) until the latter stages of development when dry matter was maximal. Mature embryos could be dried to 0.025 g g⁻¹ (seed water content of 0.010 g g⁻¹) with no loss of viability. Thus, at the latter stages of development, embryo water content could be optimized to avoid both desiccation and freezing damage. Survival of mature seeds declined over a 2-year period when seeds were stored at temperatures between 5 and 35°C and RHs between 14 and 75%, corresponding to seed water contents between 0.015 and 0.07 g g⁻¹. The deterioration rate was slowest for seeds stored at the lowest RH and temperature. Our data indicate that seeds of Taxus brevifolia show orthodox rather than recalcitrant storage characteristics, but that the optimum water content for storage was extremely low. The results suggest that even if stored at optimal water contents and low temperatures, T. brevifolia seeds will be relatively short lived. The high quantity of lipids or reducing sugars may be contributing factors in the poor storage characteristics.     

Production and scavenging of reactive oxygen species in Fagus sylvatica seeds during storage at varied temperature and humidity

The accumulation of reactive oxygen species (ROS) in seed tissues plays an important role in the loss of seed viability during storage. In the present study, we examined whether the loss of germination capacity and viability of beech (Fagus sylvatica L.) seeds during storage under different temperatures (4, 20 and 30 degrees C) and relative humidity levels (45% and 75% RH) is associated with: (1) an increase in the level of ROS, such as superoxide radical (O2*-), oxygen peroxide (H2O2); and, (2) changes in low molecular antioxidants (ascorbate and glutathione) and enzymatic scavengers such as ascorbate peroxidase dehydroascorbate reductase, glutathione reductase, catalase, superoxide dismutase and guaiacol peroxidase. Beech seeds progressively lost their ability to germinate during 9 weeks of storage under the above conditions. The deleterious effects of temperature treatments increased with growing seed moisture content at higher humidity. The loss of seed viability was correlated with the generation of ROS during storage, which was more intensive at higher temperatures and humidity levels. The ascorbate content significantly increased in seeds stored in all temperature and humidity variants, when the seeds lost the ability to germinate to a large degree. At the same time, glutathione content dramatically decreased, but it was possible to observe a defensive reaction in seeds stored at 20 degrees C. Activities of all scavenging enzymes, measured after slow imbibition of seeds, significantly increased in comparison to the non-treated control (8-9% MC, -10 degrees C). This increase was higher in embryo axes than in cotyledons. Our results suggest that the loss of viability of beech seeds during storage at different temperatures, above zero, and at different humidity levels is closely related to ROS production, and that the antioxidative system is not sufficient to protect them.     

Changes in extreme high-temperature tolerance and activities of antioxidant enzymes of sacred lotus seeds

Sacred lotus (Nelumbo nucifera Gaertn. ‘Tielian’) seed is long-lived and extremely tolerant of high temperature. Water content of lotus and maize seeds was 0.103 and 0.129 g H₂O [g DW] ⁻¹, respectively. Water content, germination percentage and fresh weight of seedlings produced by surviving seeds gradually decreased with increasing treatment time at 100°C. Germination percentage of maize (Zea mays L. ‘Huangbaogu’) seeds was zero after they were treated at 100°Cfor 15 min and that of lotus seeds was 13.5% following the treatment at 100°C for 24 h. The time in which 50% of lotus and maize seeds were killed by 100°C was about 14.5 h and 6 min, respectively. With increasing treatment time at 100°C, relative electrolyte leakage of lotus axes increased significantly, and total chlorophyll content of lotus axes markedly decreased. When treatment time at 100°C was less than 12 h, subcellular structure of lotus hypocotyls remained fully intact. When treatment time at 100°C was more than 12 h, plasmolysis gradually occurred, endoplasmic reticulum became unclear, nuclei and nucleoli broke down, most of mitochondria swelled, lipid granules accumulated at the cell periphery, and organelles and plasmolemma collapsed. Malondialdehyde (MDA) content of lotus axes and cotyledons decreased during 0 −12 h of the treatment at 100°C and then increased. By contrast, the MDA content of maize embryos and endosperms increased during 5–10 min of the treatment at 100°C and then decreased slightly. For lotus seeds: (1) activities of superoxide dismutase (SOD) and glutathione reductase (GR) of axes and cotyledons and of catalase (CAT) of axes increased during the early phase of treatment at 100°C and then decreased; and (2) activities of ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) of axes and cotyledons and of CAT of cotyledons gradually decreased with increasing treatment time at 100°C. For maize seeds: (1) activities of SOD and DHAR of embryos and endosperms and of GR of embryos increased during the early phase of the treatment at 100°C and then decreased; and (2) activities of APX and CAT of embryos and endosperms and of GR of endosperms rapidly decreased with increasing treatment time at 100°C. With decrease in seed germination, activities of SOD, APX, CAT, GR and DHAR of axes and cotyledons of lotus seeds decreased slowly, and those of embryos and endosperms of maize seeds decreased rapidly.     

Lipid participation in intracellular freezing avoidance mechanisms of lettuce seed

Freeze-fracture electron microscopy was used to study water content related freezing resistance in Grand Rapids lettuce seeds. Consistent and recognizable conformational changes occurred in lipid-water phases of lettuce seeds at different moisture contents. In air-dry lettuce seed cotyledons, the lipids lying in spherical lipid bodies near the cell wall appeared amorphous, while the structure was crystalline above 20% water content. The lipid bodies interassociated into membrane bilayers in seeds containing 20 to 25% water. Such lyotropic phase transitions in membrane lipids during lettuce seed hydration are believed to contribute to the biphasic freezing behavior observed in lettuce seeds at different moisture contents and to provide a natural freezing tolerance mechanism for highly desiccated plant tissues such as seeds.     

Desiccation tolerance of recalcitrant Theobroma cacao embryonic axes: the optimal drying rate and its physiological basis

Recalcitrant seed axes were reported to survive to lower water contents under fast-drying conditions. The present study was to examine the hypothesis that drying rate and dehydration duration could interact to determine desiccation tolerance through different physico-chemical mechanisms. The effect of drying rate on desiccation tolerance of Theobroma cacao seed axes at 16 degrees C was examined. Rapid-drying at low relative humidity (RH) and slow-drying at high RH were more harmful to cocoa axes, because electrolyte leakage began to increase and axis viability began to decrease at high water contents. Maximum desiccation tolerance was observed with intermediate drying rates at RH between 88% and 91%, indicating the existence of an optimal drying rate or optimal desiccation duration. This maximum level of desiccation tolerance for cocoa axes (corresponding to a critical water potential of -9 MPa) was also detected using the equilibration method, in which axes were dehydrated over a series of salt solutions or glycerol solutions until the equilibrium. These data confirmed that the physiological basis of the optimal drying rate is related to both mechanical stress during desiccation and the length of desiccation duration during which deleterious reactions may occur. The optimal drying rate represents a situation where combined damages from mechanical and metabolic stresses become minimal.     

Desiccation tolerance of protoplasts isolated from pea embryos

To facilitate studies of desiccation tolerance at the cellular level, a technique to isolate protoplasts from desiccation-tolerant pea (Pisum sativum L. cv. Alaska) embryos has been developed. Using FDA (fluorescein diacetate) as a probe, viability of the protoplasts was investigated before and after drying to determine whether the protoplasts could survive desiccation in a manner similar to the tissue from which they were isolated. Protoplasts were isolated from 12 h imbibed pea axes, suspended in several different sugar solutions, then dried to water contents less than 0.2 g H(2)O g(-1) DW. Protoplasts only survived drying if the rate was rapid (<2 h), while slow drying (24 h) was lethal. Maximal survival (75%) was obtained after drying protoplasts with a mixture of sucrose and raffinose, while pure sucrose and trehalose were somewhat less effective protectants. Low survival was obtained after drying protoplasts with monosaccharides and pure raffinose. Protoplasts isolated from germinated seedlings did not survive dehydration below 0.2 g H(2)O g(-1) DW. Transmission electron microscopy revealed that dried desiccation-tolerant protoplasts appeared shrunken, with folded membranes, while dried protoplasts from sensitive tissue had disrupted membranes. While isolated protoplasts maintained some of the desiccation tolerance of orthodox seeds, their inability to survive complete drying and their sensitivity to drying rate is similar to the behaviour of recalcitrant embryos.     

Imbibition temperature sensitivity of lima bean seeds controlled by initial seed moisture

Excised embryonic axes and whole seeds of Phaseolus lunatus L. were previously shown to be injured if exposed to low (5°-15°) temperature during the initial stages of imbibition. Present data show that this chilling injury during imbibition of liquid water can be prevented if axes are first allowed to absorb water vapor. The increase of initial water content to 20% increases growth even of unchilled axes, and reduces leaching of 264 mμ absorbing compounds. Protection resulting from increased water content is at first independent of the temperature at which water vapor was absorbed. However, longer exposure of high moisture axes to low temperature results in typical chilling injury. The response to initial seed moisture is repeatedly reversible with changes in water content. Because the same response occurs in intact seeds, it may be possible both to protect them against low temperature injury and to increase vigor by increasing seed water-content prior to planting.     

A comparative study of water distribution, free radical production and activation of antioxidative metabolism in germinating pea seeds

The aim of this study was to investigate whether there is a relationship between hydration of the embryo axes and cotyledons and the resumption of the oxidative metabolism in both organs of germinating seeds of pea (Pisum sativum L. cv. Piast). Nuclear magnetic resonance (¹H-NMR) spectroscopy and imaging were used to study temporal and spatial water uptake and distribution in pea seeds. The observations revealed that water penetrates into the seed through the hilum, micropyle and embryo axes, and cotyledons hydrate to different extents. Thus, inhomogeneous water distribution may influence the resumption of oxidative metabolism. Electron paramagnetic resonance (EPR) measurements showed that seed germination was accompanied by the generation of free radicals with g₁ and g₂ values of 2.0032 and 2.0052, respectively. The values of spectroscopic splitting coefficients suggest that they are quinone radicals. The highest content of free radicals was observed in embryo axes immediately after emergence of the radicle. Glutathione content decreased during the entire germination period in both embryo axes and cotyledons. A different profile was observed for ascorbate, with significant increases in embryo axes, coinciding with radicle protrusion. Electrophoretic analysis showed that superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) were present in dry seeds and were activated later during germination, especially in embryo axes. The presence of all antioxidative enzymes as well as low molecular antioxidants in dry seeds allowed the antioxidative machinery to be active as soon as the enzymes were reactivated by seed imbibition. The observed changes in free radical levels, antioxidant contents and enzymatic activities in embryo axes and cotyledons appear to be more closely related to metabolic and developmental processes associated with preparation for germination, and do not correspond directly to the hydration of the tissues.     

Genetic engineering of glycinebetaine synthesis in tomato protects seeds, plants, and flowers from chilling damage

Tomato (Lycopersicon esculentum Mill.) plants, which normally do not accumulate glycinebetaine (GB), are susceptible to chilling stress. Exposure to temperatures below 10 degrees C causes various injuries and greatly decreases fruit set in most cultivars. We have transformed tomato (cv. Moneymaker) with a chloroplast-targeted codA gene of Arthrobacter globiformis, which encodes choline oxidase to catalyze the conversion of choline to GB. These transgenic plants express codA and synthesize choline oxidase, while accumulating GB in their leaves and reproductive organs up to 0.3 and 1.2 micromol g(-1) fresh weight (FW), respectively. Their chloroplasts contain up to 86% of total leaf GB. Over various developmental phases, from seed germination to fruit production, these GB-accumulating plants are more tolerant of chilling stress than their wild-type counterparts. During reproduction, they yield, on average, 10-30% more fruit following chilling stress. Endogenous GB contents as low as 0.1 micromol g(-1) FW are apparently sufficient to confer high levels of tolerance in tomato plants, as achieved via transformation with the codA gene. Exogenous application of either GB or H2O2 improves both chilling and oxidative tolerance concomitant with enhanced catalase activity. These moderately increased levels of H2O2 in codA transgenic plants, as a byproduct of choline oxidase-catalyzed GB synthesis, might activate the H2O2-inducible protective mechanism, resulting in improved chilling and oxidative tolerances in GB-accumulating codA transgenic plants. Thus, introducing the biosynthetic pathway of GB into tomato through metabolic engineering is an effective strategy for improving chilling tolerance.     

Trichiliadregeana胚轴的脱水敏感性与抗坏血酸的抗氧化作用

以顽拗性TrichiliadregeanaSond.种子为材料,研究其胚轴的脱水敏感性与抗坏血酸的抗氧化作用。T.dregeana胚轴的脱水耐性随着脱水进程逐渐下降,50%的胚轴被脱水致死的含水量(W50)大约为0.16gH2O/gDW。在脱水过程中,胚轴的电解质渗漏速率逐渐增加,超氧物歧化酶(SOD)、抗坏血酸过氧化物酶(APX)、过氧化氢酶(CAT)、谷胱苷肽还原酶(GR)和脱氢抗坏血酸还原酶(DHAR)的活性下降,硫代巴比妥酸(TBA)-活性产物的含量增加。2.5~10.0mmol/L抗坏血酸处理能有效地增加胚轴的脱水耐性和SOD、APX、CAT和GR的活性,降低电解质渗漏速率和TBA活性产物的含量。结果表明,T.dregeana胚轴的脱水耐性与抗氧化酶的活性增加和脂质过氧化作用的降低密切相关。     

Effects of cooling rate on seeds exposed to liquid nitrogen temperatures

The effect of cooling rate on seeds was studied by hydrating pea (Pisum sativum), soybean (Glycine max), and sunflower (Helianthus annuus) seeds to different levels and then cooling them to − 190°C at rates ranging from 1°C/minute to 700°C/minute. When seeds were moist enough to have freezable water (> 0.25 gram H₂O/gram dry weight), rapid cooling rates were optimal for maintaining seed vigor. If the seeds were cooled while at intermediate moisture levels (0.12 to 0.20 gram H₂O per gram dry weight), there appeared to be no effect of cooling rate on seedling vigor. When seeds were very dry (< 0.08 gram H₂O per gram dry weight), cooling rate had no effect on pea, but rapid cooling rates had a marked detrimental effect on soybean and sunflower germination. Glass transitions, detected by differential scanning calorimetry, were observed at all moisture contents in sunflower and soybean cotyledons that were cooled rapidly. In pea, glasses were detectable when cotyledons with high moisture levels were cooled rapidly. The nature of the glasses changed with moisture content. It is suggested that, at high moisture contents, glasses were formed in the aqueous phase, as well as the lipid phase if tissues had high oil contents, and this had beneficial effects on the survival of seeds at low temperatures. At low moisture contents, glasses were observed to form in the lipid phase, and this was associated with detrimental effects on seed viability.