Resurrection grasses in India

Summary A search for desiccation tolerant grasses in India revealed nine grass species (in the genera Eragrostiella, Oropetium and Tripogon) whose ability to recover from airdryness had not previously been reported. Dry leaves of these species recover uninjured within 24 h of plants receiving water. Desiccation tolerance limits were 0%–11% RH (at 28°C). Some species were relatively tall and vigorous and may be useful for grazing purposes in arid areas.     

Changes in soluble sugars in relation to desiccation tolerance and effects of dehydration on freezing characteristics of Acer platanoides and Acer pseudoplatanus seeds

The role of soluble sugars in desiccation tolerance was investigated in seeds of two species from the genus Acer: Norway maple (Acer platanoides L.) — tolerant and sycamore (Acer pseudoplatanus L.) — intolerant to dehydration. During two years of observations it was found that seeds of Norway maple acquire desiccation tolerance at the end of August i.e. about 125 days after flowering (DAF). During seed development, the transition from intolerant to tolerant state in Norway maple seeds was accompanied by the accumulation in seed tissues of raffinose, stachyose and sucrose. The sucrose/raffinose ratio in Norway maple seeds was lower than in sycamore. In mature Norway maple seeds sucrose and raffinose contents were higher than in sycamore. It was concluded, that soluble sugars such as sucrose, raffinose and stachyose may play an important role in desiccation tolerance and/or intolerance of Norway maple and sycamore seeds. Differential thermal analysis (DTA) was used to study the relationship between desiccation sensitivity and the state of water in seed tissues. The level of non-freezable water was the same in both analysed seed species, but the temperature of water crystallization during desiccation was lower in sycamore seeds.     

Desiccation increases sucrose levels in Ramonda and Haberlea, two genera of resurrection plants in the Gesneriaceae

A few genera of angiosperms are known as 'resurrection plants' since their leaves withstand complete desiccation. In many organisms, including some resurrection plants, desiccation tolerance is associated with the accumulation of special carbohydrates. We examined whether this is also true for the two European angiosperm genera of resurrection plants, Ramonda and Haberlea in the Gesneriaceae. Using gas chromatography, non-structural carbohydrates were determined as a percentage of the dry weight in leaves of Ramonda nathaliae subjected to various desiccation regimes. Sucrose was the predominant soluble carbohydrate in all samples, and its level steadily increased from 2 to 10% during desiccation. Starch amounted to ca 2% in control leaves and disappeared completely within 8 days of desiccation. Considerable amounts (1–2.5%) of raffinose and smaller amounts of its precursor galactinol (1-a-galactosyl- myo -inositol) were present in control leaves; these carbohydrates showed only minor changes upon desiccation. Similar results were obtained when excised leaves of Ramonda nathaliae, Ramonda myconi and Haberlea rhodopensis were subjected to desiccation. These data indicate that sucrose accumulation is connected to desiccation tolerance in Gesneriaceae; the presence of raffinose may be a pre-adaptation since this sugar prevents crystallization of sucrose during drying.     

The effect of dehydration and rehydration on the nitrogen content of various fractions from resurrection plants

Nitrogen contents were determined in 20 species of “resurrection plants”,i.e. plants with leaves which are able to revive from an air-dry state (viz. Boea hygroscopica, Borya nitida, Cheilanthes sieberi, Coleochloa pallidior, C. setifera, Craterostigma plantagineum, Myrothamnus flabellifolia, Oropetium capense, Pellaea calomelanos, P. falcata, P, viridis, Polypodium polypodioides, Ramondia pyrenaica, Selaginella lepidophylla, Sporobolus stapfianus, Talbotia elegans,Tripogon loliiformis, Xerophyta retinervis, X. villosa, X. viscosa), and in three desiccation sensitive species (Eragrostis tenuifolia, Selaginella kraussiana andSporobolus pyramidalis). In a preponderance of resurrection plants insoluble nitrogen content fell during dehydration of intact plants and soluble non-protein N rose. Both changes were particularly marked in species which lose chlorophyll and thylakoid structure during drying. These trends were usually only partially reversed after 24 h rehydration. Recovery of¹⁴C-leucine incorporation in rehydrating leaves was slow. Leaves of desiccation sensitive vascular plants tended on the average to lose soluble protein rather than insoluble N during drying, and tended to have higher soluble non-protein N contents than tolerant plants. However, similarity in the changes in N-contents inXerophyta villosa leaves killed by airdrying compared to leaves surviving air-drying, opposes the view that death was due to excessive loss of protein.     

Water Content, Raffinose, and Dehydrins in the Induction of Desiccation Tolerance in Immature Wheat Embryos

Desiccation tolerance is initiated in wheat (Triticum aestivum L.) embryos in planta at 22 to 24 d after anthesis, at the time that the embryo water content has decreased from about 73% fresh weight (2.7 g water/g dry weight) to about 65% fresh weight (1.8 g water/g dry weight). To determine if desiccation tolerance is fully induced by the loss of a relatively small amount of water, detached wheat grains were treated to reduce the embryo water content by just a small amount to approximately 69% (2.2 g water/g dry weight). After 24 h of such incipient water loss, subsequently excised embryos were able to withstand severe desiccation, whereas those embryos that had not previously lost water could not. Therefore, a relatively small decrease in water content for only 24 h acts as the signal for the development of desiccation tolerance. Embryos that were induced into tolerance by a 24-h water loss had no detectable raffinose. The oligosaccharide accumulated at later times even in embryos of detached grains that had not become desiccation tolerant, although tolerant embryos (i.e. those that previously had lost some water) contained larger amounts of the carbohydrate. It is concluded that desiccation tolerance and the occurrence of raffinose are not correlated. Immunodetected dehydrins accumulated in embryos in planta as desiccation tolerance developed. Detachment of grains induced the appearance of dehydrins at an earlier age, even in embryos that had not been made desiccation tolerant by incipient drying. It is concluded that a small reduction in water content induces desiccation tolerance by initiating changes in which dehydrins might participate but not by their interaction with raffinose.     

Sugar effects on membrane damage during desiccation of pea embryo protoplasts

Desiccation tolerance of protoplasts isolated from germinating pea (Pisum sativum L. cv. 'Alaska') embryonic axes depends, in part, on the osmotic strength and composition of the suspending medium. To determine the reason for this dependence and whether treatment with different solutions results in different types of damage, protoplast recovery and survival were assessed after dehydration to a range of water contents. Protoplasts were derived from germinating axes that had intermediate desiccation tolerance. Protoplasts were isolated and resuspended in buffers containing sucrose/raffinose (85:15, w/w) or sorbitol, which were isotonic or hypertonic to the cells of the embryonic axis, then were flash-dried to a range of water contents. Protoplasts were rehydrated and stained with fluorescein diacetate (FDA) to assess survival and to estimate two types of membrane injury: lysis and the loss of semipermeability. In all treatments, protoplast survival dropped sharply during the initial phase of dehydration due to lysis. Protoplast survival was greater in hypertonic sucrose/raffinose buffer than in isotonic sucrose/raffinose buffer, or in the latter made hypertonic by the addition of sorbitol. When sorbitol was substituted for sucrose/raffinose in either the isolation or desiccation buffer, or both, protoplast survival at intermediate and low hydrations decreased due to a loss of membrane semipermeability. The results indicate that additional sucrose/raffinose is beneficial for the desiccation tolerance of protoplasts, the benefit is not due to a simple osmotic effect, and the benefit is greatest at water contents less than 0.5 g g(-1) DW, where the presence of the sugars appears to protect membrane semipermeability.     

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.     

Cold-induced accumulation of raffinose family oligosaccharides in somatic embryos of Norway spruce (<Emphasis Type="Italic">Picea abies</Emphasis>)

Summary Exposure of mature cotyledonary somatic embryos of Picea abies to low temperature (4°C) resulted in the accumulation of raffinose family oligosaccharides (RFOs)—raffinose and stachyose. The RFO content represented approximately 20% of the total soluble saccharides with the RFO: sucrose ratio being almost 1∶3 (molar basis) after 3 wk of cold exposure. This treatment, like desiccation, brings the endogenous saccharide spectrum nearer to that of mature zygotic embryos of the same species (zygotic embryos, RFO: sucrose ratio 1∶1.5 on a molar basis). Based on indications that RFOs are at least partly responsible for the positive effects of desiccation, we propose cold treatment as an alternative to slow desiccation for conifer somatic embryogenesis protocols.     

Effects of chilling stress on the accumulation of soluble sugars and their key enzymes in <Emphasis Type="Italic">Jatropha curcas</Emphasis> seedlings

As osmolytes and signaling molecules, soluble sugars participate in the response and adaptation of plants to environmental stresses. In the present study, we measured the effect of chilling (12 °C) stress on the contents of eight soluble sugars in the leaves, cotyledons, stems, and roots of Jatropha curcas seedlings, as well as on the activities of eight rate-limiting enzymes that are critical to the metabolism of those soluble sugars. Chilling stress promoted both starch hydrolysis and soluble sugar accumulation. The soluble sugar contents of the leaves and cotyledons were affected more than that of the stems and roots. Meanwhile, the activities of the corresponding metabolic enzymes (e.g., β-amylase, uridine diphosphate glucose phosphorylase, and sucrose phosphate synthase) also increased in some organs. The gradual increase of soluble neutral alkaline invertase activity in the four studied organs suggested that sucrose catabolic production, such as glucose and fructose, was especially important in determining resistance to chilling stress and hexose signal transduction pathway. In addition, the substantial accumulation of raffinose family oligosaccharides and increase in corresponding metabolic enzyme activity suggested that galactinol and raffinose play an important role in determining the chilling resistance of J. curcas. Together, these findings establish a foundation for determining the relationship between the chilling resistance and soluble sugar accumulation of J. curcas and for investigating the mechanisms underlying sugar signaling transduction and stress responses.     

Induction of tolerance to fast desiccation in black spruce (Picea mariana) somatic embryos: relationship between partial water loss,sugars, and dehydrins

Events associated with the induction of tolerance to fast desiccation in black spruce ( Picea mariana ) somatic embryos were investigated. An experimental approach using an initial period of partial water loss was developed to induce either no, partial, or complete tolerance to fast desiccation. Tolerance to subsequent fast desiccation was not promoted by decreasing embryo water content from 1.5 to 1.1 g H₂O g⁻¹ DW (g g⁻¹) throughout the first 24 h of slow desiccation. However, tolerance increased from 10 to 95% germination during the second 24-h period of slow desiccation after partial water loss from 1 to 0.55 g g⁻¹. Emphasis was also placed on the relationship between observed tolerance, and sugar and dehydrin contents. Compared to controls, sucrose content in embryos doubled after 24 h of slow desiccation and more than tripled after 48 h. Conversely, starch content was decreased by one half after 24 h and by three quarters after 48 h. Sucrose abundance and raffinose occurrence after 48 h of slow desiccation were congruent with complete tolerance to fast desiccation. The period of slow desiccation between 24 and 48 h also increased the content of a 24-kDa dehydrin and the appearance of a 42-kDa dehydrin. The relationship between partial water loss, sugars and dehydrins is discussed with respect to tolerance to fast desiccation in black spruce somatic embryos.     

Influence of freezable/non-freezable water and sucrose on the viability of <Emphasis Type="Italic">Theobroma cacao</Emphasis> somatic embryos following desiccation and freezing

Encapsulated cocoa (Theobroma cacao L.) somatic embryos subjected to 0.08–1.25 M sucrose treatments were analyzed for embryo soluble sugar content, non-freezable water content, moisture level after desiccation and viability after desiccation and freezing. Results indicated that the higher the sucrose concentration in the treatment medium, the greater was the extent of sucrose accumulation in the embryos. Sucrose treatment greatly assisted embryo post-desiccation recovery since only 40% of the control embryos survived desiccation, whereas a survival rate of 60–95% was recorded for embryos exposed to 0.5–1.25 M sucrose. The non-freezable water content of the embryos was estimated at between 0.26 and 0.61 g H₂O g⁻¹dw depending on the sucrose treatment, and no obvious relationship could be found between the endogenous sucrose level and the amount of non-freezable water in the embryos. Cocoa somatic embryos could withstand the loss of a fraction of their non-freezable water without losing viability following desiccation. Nevertheless, the complete removal of potentially freezable water was not sufficient for most embryos to survive freezing.     

Carbohydrate metabolism in the developing and maturing wheat embryo in relation to its desiccation tolerance

Embryos of wheat (Triticum aestivum L. cv. Sappo) were studiedthroughout their development and maturation to investigate therelationships among starch, sucrose and raffinose and the onsetof desiccation tolerance. Starch accumulated in axes and scutellafrom about 20 d post anthesis (dpa) to reach a maximum at approximately35 d. The starch content then declined to a very low value inlate maturation. Extractable -amylase activity increased inembryos throughout the period of starch deposition and showeda substantial rise coincident with starch breakdown. In earlymaturation (approximately 26 dpa) sucrose and raffinose appeared,and continued to increase. The rise in the amount of sucroseparalleled the accumulation of starch, but the major increasein raffinose approximated to the fall in starch content. Embryoswere desiccation intolerant prior to the age when free sucroseand raffinose accumulated: the development of desiccation tolerancewas associated with increasing raffinose: sucrose ratios. Possiblemetabolic and physiological relationships among starch, raffinose,sucrose and the onset of desiccation tolerance are discussed. Key words: Wheat embryos, development, maturation, starch, raffinose, sucrose, desiccation tolerance     

Desiccation tolerance in the vegetative tissues of the fern Mohria caffrorum is seasonally regulated

As there is limited information on the mechanisms of vegetative desiccation tolerance in pteridophytes, we undertook a comprehensive anatomical, ultrastructural, physiological and biochemical study on the fern Mohria caffrorum . Our data show that this species is desiccation-tolerant during the dry season, and desiccation-sensitive in the rainy season. This system allows the verification of protection mechanisms by comparison of tolerant and sensitive tissues of the same species at the same developmental age. Tolerant fronds acquire protection mechanisms during drying that are mostly similar to those reported for angiosperms. These include: (i) chlorophyll masking by abaxial scales and frond curling; (ii) increased antioxidant capacity that is maintained in dry tissues; (iii) mechanical stabilization of vacuoles in the dry state; (iv) de novo production of heat stable proteins (at least one identified as a putative chaperonin); (v) accummulation of protective carbohydrates (sucrose, raffinose family oligosaccharides and cyclitols). This study has implications for the biotechnological production of drought-tolerant crops, and allows speculation on the evolution of vegetative desiccation tolerance.     

Expression of a GALACTINOL SYNTHASE gene is positively associated with desiccation tolerance of Brassica napus seeds during development

Desiccation tolerance of seeds is positively correlated with raffinose family oligosaccharides (RFOs). However, RFOs’ role in desiccation tolerance is still a matter of controversy. The aim of this work was to monitor the accumulation of RFO during acquisition of desiccation tolerance in rapeseed (Brassica napus L.). Rapeseeds become desiccation tolerant at 21-24 d after flowering (DAF), and the time was coincident with an accumulation of raffinose and stachyose. A gene encoding galactinol synthase (GolS; EC2.4.1.123), involved in RFO biosynthesis, was cloned and functionally characterized. Enzymatic properties of recombinant galactinol synthase were also determined. Accumulation of BnGOLS-1 mRNA in developing rapeseeds was concomitant with dry weight deposition and the acquisition of desiccation tolerance, and was concurrent with the formation of raffinose and stachyose. The physiological implications of BnGOLS-1 expression patterns in developing seeds are discussed in light of the hypothesized role of RFOs in seed desiccation tolerance.     

The relationship between carbohydrate composition of some tree seeds and their longevity

Fresh mature tree seeds of 16 species plus soybean were usedfor the analysis of soluble neutral sugar content, as well asfor the determination of longevity in terms of the time requiredfor the seeds to decrease to 50% of the original germinationunder 5C and 80–91% relative humidity. Oligosaccharideswere sometimes found in species of recalcitrant; although themass ratio of oligosaccharide/total sugar or mole ratio of stachyoseand raffinose/sucrose was less than 0.05 or 0.044, respectively.The time required for seeds to decrease to 50% germination variedfrom a few days to 8.3 months for the seeds of desiccation-sensitivespecies. The low ratio of oligosaccharide to sucrose is, however,most unlikely to be a cause of short life-span in recalcitrantseeds. It is suggested that 0.05 mole of oligosaccharide needs to beassociated with one mole sucrose to confer the seeds with desiccationtolerance. Orthodox seeds which have a high ratio of oligosaccharide/totalsugars happened to have a low ratio of disaccharide/total sugarindicating active biosynthetic activity of oligosaccharide.There was a positive correlation between the longevity and themass ratio of oligosaccharide/total sugar or oligo-/disaccharidefor those desiccation-tolerant seeds tested. These results supportthe conclusion that the ratio of oligo-/disaccharide plays arole in the desiccation tolerance and, consequently, the longevityof orthodox seeds. Key words: Tree seed, storage behaviour, recalcitrant, soluble carbohydrate, oligosaccharide, seed longevity     

摘除雌花对甜瓜成熟叶片中糖及相关酶活性的影响

甜瓜有果株的成熟叶片中蔗糖、葡萄糖、果糖含量与无果株的无显著差异,水苏糖与棉子糖含量略低于无果株,肌醇半乳糖苷(合成水苏糖的前体)含量显著低于无果株,蔗糖磷酸合成酶(SPS)和肌醇半乳糖苷合成酶活性与无果株的无显著差异,水苏糖合成酶活性显著高于无果株。     

Different ratios of sucrose/raffinose-induced membrane depolarizations in the mesophyll of species with symplasmic (Catharanthus roseus, Ocimum basilicum) or apoplasmic (Impatiens walleriana, Vicia faba) minor-vein configurations

In mesophyll cells of species with a symplasmic (Ocimum basilicum, Catharanthus roseus, Magnolia denudata) or an apoplasmic (Vicia faba, Impatiens walleriana, Bellis perennis) minor-vein configuration, membrane depolarizations in response to 20 or 200 mol·m^–3 raffinose and sucrose were measured. Ageing period and resting potential marginally affected the degree of depolarization. The symplasmic species showed similar depolarization responses to 20 and 200 mol·m^–3 sucrose or raffinose. In the apoplasmic species, depolarization increased statistically significantly from 20 to 200 mol·m^–3 sucrose, whereas the depolarization response to raffinose was equal at both concentrations. In the apoplasmic species, moreover, the depolarization response to raffinose was significantly weaker than to sucrose at all concentrations. A major difference between symplasmic and apoplasmic species seems to lie in the scantiness of raffinose carriers in the mesophyll plasma membrane of species with the apoplasmic mode of phloem loading.Abbreviations 20R(200R) 20(200) mol·m^–3 raffinose - 20S(200S) 20(200) mol·m^–3 sucrose