Relationship between water content and afterripening in red rice

Reactions regulating seed dormancy can proceed at water contents which are probably too low to permit metabolic activity. The loss of dormancy via afterripening of red rice. ( Oryza sativa L.) seeds was examined as a representative case. Equilibration of seeds to various moisture contents showed that afterripening was most rapid at 6–14% moisture content (dry weight basis). Afterripening did not occur at > 18% moisture content and was severely inhibited at < 5% moisture content. Seed viability was greater than 95% for all treatments. Utilization of moisture isotherms to calculate water-binding enthalpy values identified the optimal afterripening range as approximately the boundary between water-binding region 1 and region 2. From these findings, it is suggested that afterripening may involve some oxidative reactions which are inhibited at lower water contents by the rising free-energy and at the higher side by metabolic reactions.     

Water Content and the Conversion of Phytochrome Regulation of Lettuce Dormancy

In an effort to determine which biological reactions can occur in relation to the water content of seeds, the regulation of lettuce seed dormancy by red and far red light was determined at various hydration levels. Far red light had an inhibiting effect on germination for seeds at all moisture contents from 4 to 32% water. Germination was progressively stimulated by red light as seed hydration increased from 8 to 15%, and reached a maximum at moisture contents above 18%. Red light was ineffective at moisture contents below 8%. Seeds that had been stimulated by red light and subsequently dried lost the enhanced germinability if stored at moisture contents above 8%. The contrast between the presumed photoconversion of phytochrome far red-absorbing (Pfr) to (Pr) occurring at any moisture content and the reverse reaction occurring only if the seed moisture content is greater than 8% may be explained on the basis of the existence of unstable intermediates in the Pr to Pfr conversion. Our results suggest that the initial photoreaction involved in phytochrome conversion is relatively independent of water content, while the subsequent partial reactions become increasingly facilitated as water content increases from 8 to 18%.     

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.     

Possible mechanisms of afterripening in Xanthium seeds

Breaking dormancy in some seeds requires a period of dry storage. In the seeds of Xanthium pennsylvanicum Wallr., the process of afterripening proceeds optimally at water contents between 7 and 14%: this range of dehydration can be identified with water binding region 2, in which water is bound with low enthalpy. At water contents below 7%. Seeds remained primarily dormant over 3 years. Attempts to alter the afterripening with atmospheres of elevated nitrogen showed no effect. and with oxygen there was no consistent effect. There were no changes is osmotic value of the seed sap, or in its sugar or amino acid contents. We speculate that afterripening in Xanthium may involve some nonenxymatic reactions which remove substances which inhibit germination. Candidates for these reactions include the Amadori and Maillard reactions.     

Theoretical basis of protocols for seed storage

The protocols presently established for optimum seed storage do not account for the chemical composition of different seed species, the physiological status of the seed, and the physical status of water within the seed. The physiological status of seeds from five species with varying chemical compositions was determined by measurements of rates of oxygen uptake and seed deterioration. The physical status of water was determined by water sorption characteristics. For each species studied, there was a specific moisture content for the onset of respiration, chemical reactions, and accelerated aging rates. The moisture contents at which these physiological levels were observed varied among the species and correlated with the lipid content of the seed. However, the changes in physiological activities and the physical status of water occurred at specific relative humidities: 91% for the onset of respiration, 27% for the increased rates of thermal-chemical reactions, and 19% for optimum longevity. Based on these observations, we propose that equilibrating seeds between 19 and 27% relative humidity provides the optimum moisture level for maintaining seed longevity during longterm storage.     

The glassy state in corn embryos

The possibility is examined whether seeds may survive the desiccated state in part by vitrification, or the formation of a glassy state. Embryos excised from viable corn (Zea mays L.) seeds at low moisture contents show a series of low temperature first- and second-order phase transitions in the differential scanning calorimeter. These embryos produce normal seedlings if moistened. The thermal events can be duplicated almost entirely in both extracted lipids and purified commercial corn oil. They are therefore associated primarily with these bulk lipids, since membrane phospholipids are present in too small an amount to produce a detectable signal. When the bulk lipids have been extracted, a glass transition appears in the remaining material. At low water contents, it occurs above +40°C and systematically falls to below −60°C as the water content of the embryo rises to 20%. These data are consistent with our hypothesis that the desiccated state in seeds is a glassy state, and that imbibition of water reduces the glass transition temperature below ambient, allowing biochemical activity to resume.     

Chlorophyll fluorescence characteristics associated with hydration level in pea cotyledons

In order to study the effects of desiccation on a photosynthetic system, light harvesting and light-induced electron transport processes were examined in pea cotyledons at various moisture levels, using in vivo fluorescence excitation spectra and fluorescence induction kinetics. Water sorption isotherms yielded thermodynamic data that suggested very strong water binding between 4 to 11% water, intermediate sorption between water contents of 13 to 22%, and very weak binding at moisture contents between 24 to 32%. The fluorescence properties of the tissue changed with the moisture contents, and these changes correlated generally with the three regions of water binding. Peak fluorescence and fluorescence yield remained at low levels when water content was limited to the tightly bound regions, below 12%. Several new peaks appeared in the chlorophyll a excitation spectrum and both peak fluorescence and fluorescence yield increased at intermediate water-binding levels (12-22%). At moisture contents where water is weakly bound (>24%), peak fluorescence and fluorescence yield were maximum and the fluorescence excitation spectrum was unchanging with further increases in water content.

The state of water is an important component in the energy transfer and electron transport system. At hydration levels where water is most tightly bound, energy transfer from pigments is limited and electron transport is blocked. At intermediate water binding levels, energy transfer and electron transport increase and, in the region of weak water binding, energy transfer and electron transport are maximized.

     

沙芥种子发育过程中的脱水耐性

以发育过程中经脱水和未脱水处理的沙芥种子为试验材料,测定了其含水量、萌发率和抗氧化酶系统,探讨了沙芥种子脱水耐性与抗氧化系统之间的关系。结果表明：在20～60DAF,沙芥种子含水量逐渐下降,干重逐渐增加;60DAF种子具有萌发能力,萌发率为24％;且脱水可促进沙芥种子的萌发,人工脱水至含水量为12％和5％时,萌发率分别为56％和44％,自然脱水至含水量为12％和5％时,萌发率分别为52％和60％;发育过程中沙芥种子SOD活性逐渐降低,而CAT、POD、LOX活性以及MDA含量均呈上升趋势;在脱水过程中,随着种子含水量的下降SOD活性逐渐降低,CAT和LOX活性逐渐升高,而POD活性呈先降低后升高的变化趋势。脱水后,种子中MDA含量均高于CK。60DAF的沙芥种子已获得脱水耐性。     

Influence of soil moisture on supercooling capacity and associated physiological parameters of overwintering larvae of rice stem borer

Influence of soil moisture on the supercooling capacity and associated physiological parameters of overwintering larvae of the rice stem borer Chilo suppressalis was examined by exposing larvae to soil moistures of 25, 50, 75 and 100% of saturated soil water content (SSWC) at ambient temperature for 30 d from December 2007 to January 2008 in Beijing, China. At the end of the exposure, supercooling points (SCPs) varied significantly among the treatments, the lowest being in the larvae exposed to soil moisture of 25% SSWC. Fresh weight was significantly higher in the larvae exposed to soil moisture of 100% SSWC than in those kept at 25 and 50% SSWC. Dry weight and body water content (% fresh weight) were not different among the treatments. Glucose and trehalose contents were markedly lower, and glycerol content was significantly higher in the larvae confined to soil moisture of 25% SSWC than in those exposed to the other soil moisture treatments. It is suggested that variation in body water content (% fresh weight) contributes to the differences in SCPs of the overwintering C. suppressalis larvae in all treatments, but the influence of soil moisture treatments on supercooling capacity are caused through changes in glycerol content.     

Viability of Lettuce Seeds: II. SURVIVAL AND OXYGEN UPTAKE IN OSMOTICALLY CONTROLLED STORAGE

Lettuce seeds were stored under aerobic conditions at varioustemperatures from 20 ?C to 35 ?C, under anaerobic conditionsat 35 ?C, and at various moisture contents ranging from 17%to 44%, controlled osmotically by polyethyleneglycol. Survivalof these seeds was compared with others maintained over a similarrange of temperatures and moisture contents but in which themoisture contents were controlled by hermetic storage. Irrespectiveof the method of controlling moisture content, under anaerobicconditions the trend (shown previously in drier seeds) of decreasein longevity with increase in hydration continued up to about27% moisture content, but above this value there was no furtherchange in longevity. In contrast, under aerobic conditions longevityincreased exponentially up to 44% moisture content, this increasestarting at about 15% moisture content at 20 ?C and at about24% at 35 ?C. Under all circumstances longevity was less athigher temperatures. On a water substrate, thermodormant seeds showed a rapid increasein rate of oxygen uptake, followed by a rapid decline, and thena slower decline to a steady state. Similar phases were shownin seeds of various moisture contents controlled osmoticallydown to 17%, although the corresponding rates were lower withdecrease in moisture content and temperature, and the phaseswere extended at lower temperatures. The implications of these results are discussed in relationto the conditions needed to sustain sub-cellular repair andturnover as postulated by Villiers. Key words: Lettuce, Lactuca sativa L, Seed viability, Seed storage, Respiration     

A study of water relations in neem (Azadirachta indica) seed that is characterized by complex storage behaviour

Neem (Azadirachta indica) seed is reputed to have limited tolerance to desiccation, to be sensitive to chilling and imbibitional stress, and to display intermediate storage behaviour. To understand this behaviour the properties of water in seed tissues were studied. Water sorption isotherms showed that at similar relative humidity (RH), the water content was consistently higher in axes than in cotyledons, mainly due to the elevated lipid content (51%) in the cotyledons. Using differential scanning calorimetry, melting transitions of water were observed at water contents higher than 0.14 g H2O g-1 DW in the cotyledons and 0.23 g H2O g-1 DW in the axes. Beside melting transitions of lipid, as verified by infrared spectroscopy, changes in heat capacity were observed which shifted with water content, indicative of glass-to-liquid transitions. State diagrams are given on the basis of the water content of seed tissues, and also on the basis of the RH at 20 degrees C. Longevity was considerably improved, and the sensitivity to chilling/subzero temperatures was reduced when axis and cotyledons were dehydrated to moisture contents < or = of approximately 0.05 g H2O g-1 DW. However, longevity during storage at very low water contents was limited. A possible mechanism for the loss of sensitivity to chilling/subzero temperatures at low water contents is discussed. The results suggest that dry neem seeds in the glassy state have great potential for extended storability, also at subzero temperatures.     

土壤水分变化对玉米苗期吸收积累镉的影响

采用土壤盆栽试验研究不同土壤水分含量对玉米苗期吸收积累 Cd的影响。试验结果表明 ,玉米生物量及其吸收 Cd量在玉米不同的生长时期差异较大。2 2 d收获时 ,玉米地上部和地下部生物量均随着田间持水量 (35 %～ 85 % )的增加而提高 ;而 16 d收获时 ,玉米生物量在田间持水量为 35 %和 85 %时比在其它水分时低许多。 16 d和 2 2 d收获时 ,玉米地上部 Cd含量在田间持水量 5 5 %时分别达到最大值 ,5 5 .4 1mg/ kg和 39.33mg/ kg;而在田间持水量 85 %时分别达最小值 ,2 7.97mg/ kg和 2 3.5 2 m g/kg。在玉米根系的影响下 ,土壤溶液 Cd含量基本上随着玉米的不断生长而降低。田间持水量为 6 5 %时的土壤溶液 Cd含量比田间持水量为 75 %和 85 %时大。玉米总吸 Cd量与水分蒸腾量之间呈极显著的线性正相关关系     

Changes in seed water status as characterized by NMR in developing soybean seed grown under moisture stress conditions

Changes in water status of developing seeds of Soybean (Glycine max L. Merrill.) grown under different moisture stress conditions were characterized by proton nuclear magnetic resonance (NMR)- spin–spin relaxation time (T₂). A comparison of the seed development characteristics, composition and physical properties indicated that, characteristics like seed weight, seed number/ear, rate of seed filling increased with development stages but decreased with moisture stress conditions. The NMR- spin–spin relaxation (T₂) component like bound water increased with seed maturation (40–50%) but decreased with moisture stress conditions (30–40%). The changes in seed water status to increasing levels of moisture stress and seed maturity indicates that moisture stress resulted in more proportion of water to bound state and intermediate state and less proportion of water in free-state. These changes are further corroborated by significant changes in protein and starch contents in seeds under high moisture stress treatments. Thus seed water status during its development is not only affected by development processes but also by moisture stress conditions. This study strongly indicated a clear moisture stress and development stage dependence of seed tissue water status in developing soybean seeds.     

Supercooling capacity of seeds and seedlings in Arabidopsis thaliana.

The influence of the water content of seeds and seedlings of Arabidopsis thaliana (Ecotype Columbia:2) on their supercooling capacity was investigated. Equilibration of the seeds to various air relative humidities resulted in final moisture contents ranging from 8 to 82% (dry weight basis). No supercooling point could be detected when the water content remained below 32.5%, and in seeds at just above this moisture level ice crystals started to form at -26 degrees C. However, cooling partly affected the germination of seeds down to a water content of 26.5%. Upon imbibition, the supercooling point of the seeds remained around -21.6 degrees C and rose sharply to -14.7 degrees C when visible germination started. It remained around -13 degrees C during the following 96 h while the water content of the seedlings increased from 155 to 870%. Hydrated seeds (above 32.5% water content), germinated seeds, and seedlings of Arabidopsis cannot survive being frozen.     

Viability of Lettuce Seeds: I. SURVIVAL IN HERMETIC STORAGE

Seeds which show orthodox storage characteristics conform toa common pattern of survival over a wide range of storage conditionswhich can be described by a single equation. Two aspects ofthis are that in a constant environment the life-spans of individualseeds in a population are normally distributed, and that thereis negative linear relationship between log moisture contentand log life-span. However, when orthodox seeds are fully hydratedthey survive much longer than would be predicted by extrapolatingfrom lower moisture contents; but the moisture content at whichthe change occurs has not previously been investigated. In this paper the viability of lettuce seeds was examined aftervarious periods of hermetic storage at different moisture contents,temperatures and initial partial pressures of oxygen. At moisturecontents below 15% the pattern of survival is typical of otherorthodox seeds but above this value the responses change infour ways: instead of being deleterious, oxygen becomes beneficialto survival; instead of life-spans being normally distributed,they become skewed; the relative effect of temperature on decreasinglongevity is slightly diminished; and the decrease in eurvivalperiod with increase in moisture content begins to become lessmarked, so that ultimately, above 20% to 30% moisture content,there is no further decrease in longevity. These results, which indicate substantial physiological changesat about 15% moisture content, are discussed in relation tothe hypothesis, postulated by Villiers, that repair and turnovermechanisms are absent from dry seeds but are activated on hydration. Key words: Lettuce, Lactuca saliva L., Seed viability, Seed storage     

Mechanisms of seed ageing under different storage conditions for Vigna radiata (L.) Wilczek: lipid peroxidation,sugar hydrolysis,Maillard reactions and their relationship to glass state transition

Two primary biochemical reactions in seed ageing (lipid peroxidation and non-enzymatic protein glycosylation with reducing sugars) have been studied under different seed water contents and storage temperatures, and the role of the glassy state in retarding biochemical deterioration examined. The viability loss of Vigna radiata seeds during storage is associated with Maillard reactions; however, the contribution of primary biochemical reactions varies under different storage conditions. Biochemical deterioration and viability loss are greatly retarded in seeds stored below a high critical temperature (approximately 40 degrees C above glass transition temperature). This high critical temperature corresponds to the cross-over temperature (T(c)) of glass transition where molecular dynamics changes from a solid-like system to a normal liquid system. The data show that seed ageing slows down significantly, even before seed tissue enters into the glassy state.     

Wheat Viability at High Moisture Content under Hermetic and Aerobic Storage Conditions

Wheat seeds (Triticum durum) were stored under both hermeticand aerobic conditions at 25 °C with moisture contents from15 to 33 per cent. Under hermetic storage, seeds lost viabilitymore rapidly the higher the moisture content, whereas in aerobicstorage, seed longevity was enhanced as the moisture contentwas increased from 24 to 31 per cent, and over this range ofmoisture content the seeds survived longer under aerobic thanhermetic storage. On the contrary, an apparent reversal of thistrend occurred when moisture content was increased above 31per cent. The possibility that the changes in longevity occurring at highermoisture contents might be due to the activation of seed metabolismwas supported by the enhanced incorporation of [³H]leucine intoTCA insoluble material (indicating increased protein synthesis)and the reduced leakage of glucose (indicating increased membranerestitution) when seeds were allowed to achieve higher moisturecontents during the prehydration period. The highest level ofseed activation was found in seeds preconditioned to about 31per cent moisture content. Moreover, these seeds, when subsequentlystored under aerobic conditions, maintained a higher rate ofprotein synthesis and lower membrane permeability during thestorage period than seeds at lower water contents. It is suggested that seeds stored at a sufficient hydrationlevel in the presence of oxygen can sustain an effective metabolismduring extended storage, thus permitting the repair of cellulardamage. However, it might be possible that at about 33 per centmoisture content seed could suffer from an excessive advancementof metabolism. Triticum durum, seed storage, effects of high moisture content and oxygen     

果树对水分胁迫反应研究进展(综述)

综述近十多年果树对水分胁迫反应的研究进展。对果树在水分胁迫下叶片、根系的形态和显微结构的变化以及气孔反应、光合作用、水势变化、碳水化合物代谢、矿质营养、活性氧代谢、多胺代谢、内源激素、脯氨酸和甜菜碱等生理生化指标的反应作了全面的阐述。     

Storage of ethyl methanesulphonate-treated barley seeds with low moisture contents

Barley seeds were treated with ethyl methanesulphonate (EMS) for 3 h at 25° C, washed with tap water for 24 h at 25° C, redried at 40° C to different moisture contents below 15% and stored at 25° C in desiccators or in sealed plastic bags. The criteria used for expressing the effect of storage were the M₁ seedling height and the frequency of chromosomal aberrations. With 14·9% seed moisture a strong increase of biological injury occurred in the course of a 2-week storage, while storage of seeds having an initial moisture content of 11·7% led to a significant increase of injury only after 6 weeks. Superdry EMS-treated seeds with 5% or less moisture can be stored at 25° C without any changes in the biological effects. A method is recommended to avoid the EMS-storage effects.     

Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival

In most species, arrest of growth and a decrease in water content occur in seeds and pollen before they are dispersed. However, in a few cases, pollen and seeds may continue to develop (germinate). Examples are cleistogamy and vivipary. In all other cases, seeds and pollen are dispersed with a variable water content (2-70%), and consequently they respond differently to environmental relative humidity that affects dispersal and maintenance of viability in time. Seeds with low moisture content shed by the parent plant after maturation drying can generally desiccate further to moisture contents in the range of 1-5% without damage and have been termed 'orthodox'. Pollen that can withstand dehydration also was recently termed orthodox. Seeds and pollen that do not undergo maturation drying and are shed at relatively high moisture contents (30-70%) are termed 'recalcitrant'. Since recalcitrant seeds and pollen are highly susceptible to desiccation damage, they cannot be stored under conditions suitable for orthodox seeds and pollen. Hence, there are four types of plants with regard to tolerance of pollen and seeds to desiccation. Orthodoxy allows for dispersal over greater distances, longer survival, and greater resistance to low relative humidity. The advantage of recalcitrance is fast germination. Orthodoxy and recalcitrance are often related to environment rather than to systematics. It has been postulated that certain types of genes are involved during presentation and dispersal of pollen and seeds, since molecules (sucrose, polyalcohols, late embryogenic abundant proteins, antioxidants, etc.) that protect different cell compartments during biologically programmed drying have been detected in both.