Anoxia tolerance in rice seedlings: exogenous glucose improves growth of an anoxia-'intolerant', but not of a 'tolerant' genotype

This study demonstrated that, in rice seedlings, genotypic differencein tolerance to anoxia only occurred when anoxia was imposedat imbibition, but not at 3 d after imbibition. When seeds wereimbibed and grown in anoxia, IR22 (anoxia-‘intolerant’)grew much slower and had lower soluble sugar concentrationsin coleoptiles and seeds than Amaroo (anoxia-‘tolerant’),while Calrose was intermediate. After 3 d in anoxia, the sugarconcentrations in embryos and endosperms of anoxic seedlingswere nearly 4-fold lower in IR22 than in Amaroo. Sugar deficitin the embryo of IR22 is presumably due to the limitation ofsugar mobilization rather than the capacity of transport asshown by similar sugar accumulation ratios of 1.8 between embryoand endosperm in IR22 and Amaroo at 3 d in anoxia. With 20 molm^–3 exogenous glucose, coleoptile extension and freshweight increments in anoxic seedlings of IR22 were much closerto those in the two other genotypes, nevertheless protein concentrationremained lowest on a fresh weight basis in the coleoptiles ofIR22; indicating that protein synthesis has a lower priorityfor energy apportionment during anoxia than processes crucialto coleoptile extension. In contrast to these responses to anoxiaimposed at imbibition, IR22 had nearly the same high toleranceto anoxia as Calrose and Amaroo, when anoxia was imposed onseedlings subsequent to 48 h aeration followed by 16 h hypoxicpretreatment. In fact, coleoptiles of anoxic IR22 had highersugar concentrations and grew faster than Calrose, and exogenousglucose had no effect on the coleoptile extension of IR22. Excisedcoleoptile tips of IR22 and Amaroo with exogenous glucose hadsimilar rates of ethanol production and were equally tolerantto anoxia. In conclusion, much of the anoxia ‘intolerance’of IR22 when germinated in anoxia could be attributed to limitedsubstrate availability to the embryo and coleoptile, presumablydue to slow starch hydrolysis in the endosperm. Key words: Anoxia, coleoptile, embryo, endosperm, ethanol production, germination, growth, Oryza sativa L., solute net uptake or loss, sugar availability.     

Mitochondrial Ultrastructure of Three Hygrophyte Species at Anoxia and in Anoxic Glucose-Supplemented Medium

Mitochondrial ultrastructure of excised roots of Alisma plantago-aqisaticaL., Lycopus europaeus L and Glyceria fluitans L. were electron-microscopicallystudied at anoxia and in anoxic glucose- supplemented mediumin order to find Out if the roots of the three hygrophytes growingon water-logged anaerobic soils have an increased resistanceto anoxia. Irreversible destruction of mitochondrial membranes and othersubcellular structures was shown to occur in the above plants'roots after 24 h at anoxia or in anoxic glucose-supplementedmedium. Only in roots of Glyceria had exogenous glucose a protectiveaction, yet in this case, too, a 48 h anacrobic exposure resultedin a deep-going degradation of cell ultrastructure. It is concluded that though the plants in question grow on soilsdevoid of O₂ their roots avoid anaerobiosis through translocationof O₂ from aerated parts, which appears to explain why theseplants have not developed a biochemical mechanism of adaptationto anaerobiosis in the process of evolution. Key words: Anaerobiosis, hygrophytes, mitochondrial ultrastructure     

Carbohydrate metabolism in germinating caryopses of <Emphasis Type="Italic">Oryza sativa</Emphasis> L. exposed to prolonged anoxia

Anoxia tolerance can be evaluated not only in terms of growth or survival of plant organs during oxygen deprivation, but also in relation to carbohydrate utilization in the context of a well-modulated fermentative metabolism. Rice (Oryza spp.) is unique among cereals, in that it has the distinctive ability to germinate under complete anaerobiosis by using the starchy reserves in its seeds to fuel the anaerobic metabolism. The aim of the present study was to evaluate the ability of germinating rice seedlings to survive a long-term oxygen deficiency [40 days after sowing (DAS)] and the effects on sugar metabolism, focusing on starch degradation as well as soluble sugars transport and storage under anoxia. No significant decline in vitality occurred until 30 DAS though no recovery was detected following longer anoxic treatments. Growth arrest was observed following anoxic treatments longer that 20 DAS, in concomitance with considerably lower ethanol production. Amylolytic activity in embryos and endosperms had similar responses to anoxia, reaching maximum content 30 days after the onset of stress, following which the levels declined for the remainder of the experiment. Under anoxia, average amylolytic activity was twofold higher in embryos than endosperms. Efficient starch degradation was observed in rice under anoxia at the onset of the treatment but it decreased over time and did not lead to a complete depletion. Our analysis of α-amylase activity did not support the hypothesis that starch degradation plays a critical role in explaining differences in vitality and coleoptile growth under prolonged oxygen deprivation.     

The Effects of Oxygen, Carbon Dioxide and Ethylene on Ethylene Biosynthesis in Relation to Shoot Extension in Seedlings of Rice (Oryza sativa) and Barnyard Grass (Echinochloa oryzoides)

Exposing dark-grown seedlings for 3 d to oxygen deficiency (0or 5 kPa) or to additions of carbon dioxide (10 kPa) or ethylene(0·1 Pa) slowed shoot extension in Echinochloa oryzoides,while in rice it was promoted by these treatments, except that5 kPa oxygen was without effect. In E. oryzoides this was dueto reduced growth of the mesocotyl, and in rice to enhancedgrowth of the coleoptile. These responses to carbon dioxideand oxygen deficiency were not consequences of increased ethyleneproduction, since this remained unchanged by carbon dioxideand depressed by oxygen shortage in both species. Furthermore,exogenous ethylene and the ethylene action inhibitor 2,5-norbornadieneeach failed to influence extension in anoxic seedlings, indicatingno regulatory role for ethylene in the absence of oxygen. However,concentrations of the ethylene precursor 1 -aminocyclopropane-1-carboxylic acid (ACC) were increased by carbon dioxide and0 kPa or 5 kPa oxygen, although after 72 h without oxygen totalACC production (i.e. changes in ethylene + ACC + MACC) was suppressedin both species. There was little effect on bound ACC [putativemalonyl-ACC (MACC)] formation. Transferring anaerobic (0 kPa)seedlings to oxygenated conditions (21 kPa) resulted in abnormallyfast rates of ethylene formation, possibly due to the accumulationof ACC under anoxia. This post-anoxic ethylene may have contributedto the faster extension by rice coleoptiles and slower extensionby mesocotyls of E. oryzoides compared with those of seedlingsmaintained continuously in air. Echinochloa oryzoides [Ard.] Fritsch, barnyard grass, Oryza sativa L, rice, oxygen shortage, carbon dioxide, ethylene biosynthesis, shoot extension, 1-aminocyclopropane-1-carboxylic acid (ACC), malonyl-ACC, GC-MS     

The Role of Shikimic Acid in Waterlogged Roots and Rhizomes of Iris pseudacorus L.

Levels of shikimic acid in the roots and rhizomes of Iris pseudacorusgrowing under natural conditions were measured at monthly intervalsfor a period of one year. Seasonal fluctuations in the shikimatecontent suggest that the high levels during winter floodingand lower levels during the summer period of low water tableare related to a particular flood-tolerance metabolism in theroots and rhizomes. The suggested pathway, involving the condensationof phosphoenolpyruvate and crythrose 4-phosphato (both formedduring anaerobic carbohydrate breakdown) to 3-deoxy-D-arabino-heptulosouicacid 7-phosphate and the subsequent synthesis of shikimie acid,appears to function as a physiological adaptation to floodingin rhizomatous species such as I. pseuducorus. Other pathwaysof anaerobic respiration in waterlogged roots are discussedin relation to the proposed scheme in Iris.     

Production and organ distribution of succinate in rice seedlings during anoxia

Anaerobic production of succinate, a common feature in animals able to sustain anoxia, has seldom been reported in plants. By the use of ¹H-nuclear magnetic resonance spectroscopy we show here that succinate is produced by rice seedlings (Oryza sativa L. cv. Arborio) subjected to anoxic conditions. Starting from levels below I μmol (g fresh weight)⁻¹ in air, after 48 h of anoxia the levels of alanine, succinate and lactate had increased to 23.8, 5.2 and 1.0 μmol (g fresh weight) ⁻¹, respectively, in shoot tissues. Succinate was accumulated in shoots, notably in the coleoptiles, but not in roots of the rice seedlings, suggesting its involvement in rice coleoptile elongation under anoxia. Other possible functions of succinate production in rice seedling, an organism highly tolerant to anoxia, are discussed.     

Supply and partitioning of assimilates to roots of Medicago sativa L. and Lotus corniculatus L. under anoxia

Abstract A current explanation of the mechanism of flooding injury to roots suggests that oxygen deficiency depresses the supply of respirable carbohydrates sufficiently to inhibit fermentation. However, even though it has been shown that phloem transport of assimilate is sharply reduced to anaerobic roots, inhibition of assimilate metabolism has also been suggested to be an important factor. This study examines these hypotheses by relating assimilate supply and metabolic activity in anoxic roots of alfalfa (Medicago sativa L.), a flood-intolerant species, and birdsfoot trefoil (Lotus corniculatus L.), a flood-tolerant plant. Roots were made anoxic (severe O₂ deficiency) for 2, 4 or 6 d and shoots were labelled with ¹⁴CO₂. Assimilate transport to the roots and metabolism to structural components were significantly decreased in both species in response to anoxia. Trefoil exhibited significantly greater ¹⁴C incorporation into the residue fraction at 4 d anoxia than did alfalfa, and this was consistent with the greater flooding tolerance of trefoil. When assimilate supply to O₂-deficient roots was decreased by shoot shading, shoot fresh weight was reduced by both anoxia and light treatments. Root-soluble sugars were significantly decreased by shading but were greatly increased in response to anoxia. Root starch concentration also increased under anoxia. Root K⁺ concentration was reduced by anoxia only. The energy status (ATP/ADP) of roots was significantly decreased by shading; however, anoxia reduced the energy status only in unshaded plants. The data indicate that carbohydrate supply to anaerobic roots does not appear to be a limiting factor in the metabolic response of alfalfa roots. Alternatively, metabolism of assimilate in anoxic roots may be an important determinant of survival.     

Differential anoxic expression of sugar-regulated genes reveals diverse interactions between sugar and anaerobic signaling systems in rice

The interaction between the dual roles of sugar as a metabolic fuel and a regulatory molecule was unveiled by examining the changes in sugar signaling upon oxygen deprivation, which causes the drastic alteration in the cellular energy status. In our study, the expression of anaerobically induced genes is commonly responsive to sugar, either under the control of hexokinase or non-hexokinase mediated signaling cascades. Only sugar regulation via the hexokinase pathway was susceptible for O₂ deficiency or energy deficit conditions evoked by uncoupler. Examination of sugar regulation of those genes under anaerobic conditions revealed the presence of multiple paths underlying anaerobic induction of gene expression in rice, subgrouped into three distinct types. The first of these, which was found in type-1 genes, involved neither sugar regulation nor additional anaerobic induction under anoxia, indicating that anoxic induction is a simple result from the release of sugar repression by O₂-deficient conditions. In contrast, type-2 genes also showed no sugar regulation, albeit with enhanced expression under anoxia. Lastly, expression of type-3 genes is highly enhanced with sugar regulation sustained under anoxia. Intriguingly, the inhibition of the mitochondrial ATP synthesis can reproduce expression pattern of a specific set of anaerobically induced genes, implying that rice cells may sense O₂ deprivation, partly via perception of the perturbed cellular energy status. Our study of interaction between sugar signaling and anaerobic conditions has revealed that sugar signaling and the cellular energy status are likely to communicate with each other and influence anaerobic induction of gene expression in rice.     

Anoxia tolerance and α-amylase activity in four rice cultivars

The relationship between anoxia tolerance and reserved carbohydrate catabolism was investigated in four rice (Oryza sativa L.) cultivars subjected to a 48-h anoxic stress. The coleoptile elongation of all cultivars was suppressed by anoxic stress, however, the elongation of cvs Koshihikari and Awa-akamai was much greater than that of cvs Touzoumochi and Asahimochi. The anoxic coleoptiles of cvs Koshihikari and Awa-akamai contained about 2-fold as much ATP as those of cvs Touzoumochi and Asahimochi. Ethanol production in the anoxic coleoptiles of cvs Koshihikari and Awa-akamai was about 2-fold as much as that of cvs Touzoumochi and Asahimochi, which suggests that ethanolic fermentation is probably more active in cvs Koshihikari and Awa-akamai than in cvs Asahimochi and Touzoumochi. Activity of α-amylase, which catabolizes starch to soluble sugars, in endosperms of cvs Koshihikari and Awa-akamai was about 2-fold that of cvs Touzoumochi and Asahimochi, and soluble sugar concentration in the coleoptiles of cvs Koshihikari and Awa-akamai was about 3-fold greater than that of cvs Touzoumochi and Asahimochi. Soluble sugar concentration and ethanol production rate in the coleoptiles of rice seedlings were correlated well with α-amylase activity in their endosperms, which were also correlated well with anoxia tolerance with respect to the coleoptile elongation and ATP concentration in the coleoptiles. These results suggest that the ability to degrade starch to soluble sugar by α-amylase in endosperm may be important for the anoxia tolerance in rice coleoptiles and it may serve to distinguish the anoxia tolerance of rice coleoptiles.     

Tolerance of crop plants to oxygen deficiency stress: fermentative activity and photosynthetic capacity of entire seedlings under hypoxia and anoxia

The study investigates the reactions of rice, wheat and maize to anoxia (plants without access to oxygen) and hypoxia (roots with very limited access to oxygen). We studied the adaptations of these intact crop plants because they are known to differ widely in their tolerance to oxygen deficiency. In hypoxia, there was an accumulation of sugars, especially in wheat and maize, although both flood-sensitive species significantly increased the activities of fermentative and glycolytic enzymes, clearly more than in rice. In rice, avoiding an oxygen limitation due to the effective aeration system (30% of root cross-sectional area) may have accounted for only a minor metabolic reaction to hypoxia. In anoxia, maize and wheat quickly lost viability and nearly all photosynthetic capacity, while most rice leaves stayed turgid and green, losing only 50% of the photosynthetic capacity. A strong metabolic arrest under anoxia was obvious for the sucrolytic, glycolytic and fermentative enzymes in all tested species, but was most pronounced in rice. Of the 14 enzymes studied, rice showed the lowest activity increase in hypoxia for 11 enzymes, and the strongest activity decrease in anoxia for 8 enzymes. However, rice was able even under anoxia to keep a 1/4 of the ATP level of the aerated control, while it was at the detection limit in maize and wheat. It appears that in anoxic rice, the switch to metabolic dormancy and maintenance of basic shoot meristems diminishes the needs for energy and substrate. Additionally, rice already has lower sugar demand under hypoxia, and sugar supply appears to be sustained under anoxia by a functioning anaerobic amylase and by the photosynthetically active shoot.     

Exogenous Nitrate as a Terminal Acceptor of Electrons in Rice (Oryza sativa) Coleoptiles and Wheat (Triticum aestivum) Roots under Strict Anoxia

To elucidate the physiological role of exogenous nitrate under anaerobic conditions, we studied the effect of 10 mM KNO₃ on the mitochondrial ultrastructure in rice (Oryza sativa L.) coleoptiles and in wheat (Triticum aestivum L.) roots, detached from four-day-old seedlings, under strict anoxia. In wheat roots, following 6-h-long anoxia in the absence of exogenous nitrate, the mitochondrial membranes were partially degraded and, after 9 h under anoxia, the mitochondrial membranes and the membranes of other organelles were completely destroyed. In rice coleoptiles, the partial membrane degradation was observed only after 24 h and their complete breakdown after 48 h of anaerobiosis. In the presence of exogenous nitrate, no membrane destruction was noticed even after 9 and 48 h of anaerobiosis in wheat roots and rice coleoptiles, respectively. These results indicate that exogenous nitrate exerts protective action as a terminal electron acceptor, alternative to the molecular oxygen. Our findings are compared with the results of other researchers concerning the adverse or favorable nitrate action on plant growth, metabolism, and energy status under anaerobic stress.     

Sugar utilization and anoxia tolerance in rice roots acclimated by hypoxic pretreatment

Although most cereal roots cannot elongate under anoxic conditions, primary roots of three-day-old rice (Oryza sativa L.) seedlings were able to elongate during a 24-h period of anoxia. Hypoxic pretreatment (H-PT) increased the elongation of their roots. Sucrose synthase (EC 2.4.1.13), glucokinase (EC 2.7.1.2), fructokinase (EC 2.7.1.4), pyruvate decarboxylase (EC 4.1.1.1) and alcohol dehydrogenase (EC 1.1.1.1) activities were increased by anoxia in both H-PT and non-pretreated (N-PT) roots. However, these activities were greater in the H-PT roots than in the N-PT roots. The average rate of production of ethanol for the initial 6h after the onset of anoxia was 3.7 and 1.4 micromolg(-1) fresh weight h(-1) for the H-PT and N-PT roots, respectively, suggesting that ethanolic fermentation may increase more quickly in the H-PT roots than in the N-PT roots. Roots of the seedlings lost ATP and total adenine nucleotides in anoxia, however, the H-PT roots maintained higher levels of ATP and total adenine nucleotides compared to the N-PT roots. These results show that rice roots are able to utilize the set of enzymes involved in the metabolism of soluble sugars under anoxia. The ability to maintain an active fermentative metabolism for production of ATP by fueling the glycolytic pathway with fermentable carbohydrate is probably greater in H-PT than in N-PT roots.     

Mitochondrial ultrastructure in roots of mesophyte and hydrophyte at anoxia and after glucose feeding

Summary In order to investigate the nature of the tolerance of mesophytes and hydrophytes to root anaerobiosis, changes in the mitochondrial ultrastructure of excised roots (with and without added glucose under anoxia) were studied in plants from two ecologically opposite types-pumpkin and rice.A 12-hour exposure to anoxia led to mitochondrial degradation in roots of adult rice and pumpkin plants. The addition of glucose preserved cell ultrastructure for up to 72–96 hours. During this period mitochondrial ultrastructure changed. In rice roots this primarily involved an increased number of cristae and a change in their arrangement into parallel rows. Cells of pumpkin roots displayed long mitochondria (up to 55 m) of different profiles which fused to form a complex mitochondrial network that was in close association with parts of the endoplasmic reticulum carrying a large number of ribosomes. This may be regarded as an adaptive development that facilitates the transport of glycolytic energy along mitochondrial membranes to the sites of protein synthesis.It is concluded that root cells of a hydrophyte are not more tolerant to anoxia than mesophyte. Thus, the ability of hydrophytes to grow on anaerobic soils should be attributed not so much to peculiar features of the roots' metabolism but to the ability of these plants to perform an easy transport of O₂ from leaves to roots. With respect to mesophytes it is stressed that the supply of assimilates is important for the resistance of roots to soil anaerobiosis.     

The Role of Sugars,Hexokinase, and Sucrose Synthase in the Determination of Hypoxically Induced Tolerance to Anoxia in Tomato Roots

Hypoxic pretreatment of tomato (Lycopersicon esculentum M.) roots induced an acclimation to anoxia. Survival in the absence of oxygen was improved from 10 h to more than 36 h if external sucrose was present. The energy charge value of anoxic tissues increased during the course of hypoxic acclimation, indicating an improvement of energy metabolism. In acclimated roots ethanol was produced immediately after transfer to anoxia and little lactic acid accumulated in the tissues. In nonacclimated roots significant ethanol synthesis occurred after a 1-h lag period, during which time large amounts of lactic acid accumulated in the tissues. Several enzyme activities, including that of alcohol dehydrogenase, lactate dehydrogenase, pyruvate decarboxylase, and sucrose synthase, increased during the hypoxic pretreatment. In contrast to maize, hexokinase activities did not increase and phosphorylation of hexoses was strongly inhibited during anoxia in both kinds of tomato roots. Sucrose, but not glucose or fructose, was able to sustain glycolytic flux via the sucrose synthase pathway and allowed anoxic tolerance of acclimated roots. These results are discussed in relation to cytosolic acidosis and the ability of tomato roots to survive anoxia.     

Activities of proteinases in maize and soybean roots in response to anoxic stress

This study characterized the changes in proteinase activities in maize inbred line H60 and soybean cultivar Keller roots in response to anoxia. After 24 h of anoxia, crude protein extracts from both maize and soybean root tips (10 cm) were assayed for proteinase activities at pHs ranging from 4.5 to 10.2. In anoxic roots of both maize and soybean, activities of proteinases with alkaline pH optima increased, and activities of proteinases with acidic pH optima declined. Proteinases with neutral pH increased in anoxic maize roots, but declined in anoxic soybean roots. Whether the differences in proteinase activities in anaerobic maize and soybean roots contribute to the differental susceptibility of the two species requires further study.Journal Article No. 265-89.     

Effect of Localized Anoxia on Phaseolus vulgaris L. Root Growth

Heterogeneity within the root environment results in differentialgrowth within root systems. The response of five Phaseolus vulgarisL. cultivars to non-uniform root aeration was evaluated. Threetreatments were applied to a split root system for a periodof 72 h. Treatments consisted of an aerated control, a non-aeratedcontrol (both halves non-aerated, using N₂). and localized anoxia(one-half the root system aerated and the remaining half subjectedto N₂). Shoot and root growth were reduced in the anoxic controlbut not in the aerated control or localized anoxia treatment. Root growth was greatest in the aerated portion of the localizedanoxia treatment for all genotypes. Contributions of the rootcomponents to the compensatory responses differed dependingon the plant cultivar examined. The growth of branched and lateralroots present before the treatment period increased by 65% inline 31908. A 50% increase in the growth of lateral roots whichemerged during the treatment period occurred in another line(Swan Valley). Other genotypes responded in an intermediatemanner. These observations indicate differences in cultivarresponses to localized soil stress. Key words: Phaseolus vulgaris L., Anoxia, Root growth     

Evidence for down-regulation of ethanolic fermentation and K+ effluxes in the coleoptile of rice seedlings during prolonged anoxia.

Ethanolic fermentation, the predominant catabolic pathway in anoxia-tolerant rice coleoptiles, was manipulated in excised and 'aged' tissues via glucose feeding. Coleoptiles with exogenous glucose survived 60 h of anoxia, as evidenced by vigorous rates of K+ and phosphate net uptake and growth of roots and shoots when re-aerated. In contrast, coleoptiles without exogenous glucose showed net losses of K+ and phosphates starting 12 h after anoxia was imposed and these did not recover fully when re-aerated after 60 h of anoxia. Ethanol production (micromol x g(-1) FW x h(-1)) declined from about 7.5 during the first 12 h of anoxia to 5 or 2.2 after 48-60 h, in coleoptiles with or without exogenous glucose, respectively. Carbohydrate concentrations changed only slightly in anoxic coleoptiles with exogenous glucose due to net glucose uptake at 2.6 micromol x g(-1) FW x h(-1). Ethanolic fermentation, and therefore ATP production, may have been down-regulated after an initial period of acclimation to anoxia in coleoptiles with exogenous glucose. Maintenance requirements for energy were assessed to be 3.4-7.6-fold lower in these anoxic coleoptiles than published estimates for non-growing aerated leaf tissues. A modest part of the required economy in energy consumption would have been derived from diminished ion transport; anoxia reduced K+ and phosphate net uptake by 70-90% in these coleoptiles. K+ efflux was 10-fold lower in anoxic than in aerated coleoptiles with exogenous glucose. Using the unidirectional efflux equation, the membrane permeability to K+ was estimated to be 17-fold lower in anoxic than in aerated coleoptiles, presumably due to predominantly closed K+ channels.     

Phloem flow and sugar transport in Ricinus communis L. is inhibited under anoxic conditions of shoot or roots

Anoxic conditions should hamper the transport of sugar in the phloem, as this is an active process. The canopy is a carbohydrate source and the roots are carbohydrate sinks. By fumigating the shoot with N₂ or flooding the rhizosphere, anoxic conditions in the source or sink, respectively, were induced. Volume flow, velocity, conducting area and stationary water of the phloem were assessed by non‐invasive magnetic resonance imaging (MRI) flowmetry. Carbohydrates and δ¹³C in leaves, roots and phloem saps were determined. Following flooding, volume flow and conducting area of the phloem declined and sugar concentrations in leaves and in phloem saps slightly increased. Oligosaccharides appeared in phloem saps and after 3 d, carbon transport was reduced to 77%. Additionally, the xylem flow declined and showed finally no daily rhythm. Anoxia of the shoot resulted within minutes in a reduction of volume flow, conductive area and sucrose in the phloem sap decreased. Sugar transport dropped to below 40% by the end of the N₂ treatment. However, volume flow and phloem sap sugar tended to recover during the N₂ treatment. Both anoxia treatments hampered sugar transport. The flow velocity remained about constant, although phloem sap sugar concentration changed during treatments. Apparently, stored starch was remobilized under anoxia.     

Amylolytic activity and carbohydrate levels in relation to coleoptile anoxic elongation in Oryza sativa genotypes

Among starchy seeds, rice has the unique capacity to germinate successfully under complete anaerobiosis. In this conditions, starch degradation is supported by a complete set of starch-degrading enzymes that are absent or inactive in cereals except rice. A characterization of carbohydrate metabolism and starch-degrading enzyme activity across twenty-nine genotypes of Oryza sativa L. is presented here. The zymogram of amylolytic activities present in rice embryos and endosperms under anaerobic conditions seven days after sowing (DAS) revealed marked differences among cultivars. Coleoptile elongation was positively correlated with total amylolytic activities and α-amylase activity in embryos, and negatively correlated with α-amylase activity in endosperm. Moreover, carbohydrate content in embryos was found to be positively correlated with total amylolytic activities under anaerobic conditions, while a negative relationship was recorded in the endosperm. Carbohydrate status in rice seedlings has a primary importance in sustaining coleoptile elongation towards the surface. The relationship between carbohydrate level in embryo and anoxic germination, as well as with total amylolytic activities present in rice embryo under anaerobic condition 7 DAS, is consistent with the role of sugar metabolism to support rice germination under oxygen-deprived environment.     

Anaerobiosis in the nematode Caenorhabditis elegans

In Caenorhabditis elegans, mortality rates and changes in concentrations of carbohydrate stores and anaerobic end products were determined in anoxic (test) and normoxic (control) animals at two different temperatures (10 and 20 degrees C). The anoxic tolerance of the free-living nematode proved to be well-developed: at 10 degrees C, about 50% of animals had survived a period of 50 h of anoxia. The carbohydrate stores (approximately 30 mmol glycosyl units kg-1 freshweight (FW)) were reduced by two-thirds within 24 h of anoxia at both temperatures. L-lactate, acetate, succinate, and propionate were identified as the main anaerobic end products. The amounts and proportions of the end products were dependent on temperature. They did not accumulate very much in the tissues, but were mainly excreted. During anoxia, the metabolism of C. elegans was depressed to 3-4% of the aerobic value. The food-source Escherichia coli was found to be at least partly alive in the gut of the animals. To separate between anaerobiosis in animals and bacteria, cleaning procedures were applied, and additional control measurements were made: anaerobic end products produced either by E. coli alone or by bacteria-free (axenic) bred nematodes were quantified at identical incubation conditions.