We studied the effects of oxygen (aerated versus O2 depleted ∼0.5 mg 1−1 O2) and nitrate (none versus 10 μmol 1−1) on the ammonium uptake kinetics and adenylate pools in two wetland plants differing in their degree of flood tolerance (Phalaris arundinacea L. and Glyceria maxima (Hartm.) Holmb.). The study was performed as a random block design in a growth chamber. The -uptake kinetics were estimated by using a computerised nonlinear parameter estimation procedure to fit the differential form of a modified Michaelis–Menten model to solution depletion curves. The uptake kinetics for differed between the two species: Vmax was significantly higher for P. arundinacea (24.7 to 29.6 μmol h−1 g−1 root dry weight) than for G. maxima (4.6–10.3 μmol h−1 g−1 root dry weight). The concentration at which uptake ceases (Cmin) was 0.2 to 0.5 μmol 1−1 for P. arundinacea and significant higher (1.1–2.7 μmol 1−1) for G. maxima.Km varied between 3.1 and 6.2 μmol 1−1 for P. arundinacea, and 1.6 and 3.0 μmol 1−1 for G. maxima. The different uptake kinetics of the two species reflect the different structure of their root systems: P. arundinacea has an extensive root system consisting of many thin roots whereas G. maxima has fewer but thicker roots. The uptake kinetics also suggest that P. arundinacea is adapted to growing at lower ambient concentrations than G. maxima. Oxygen had no consistent effect on uptake kinetics. However, the plants that had in the nutrient solution as well as had slightly higher Vmax values and lower Cmin and Km values than those without . Thus, both species were able to sustain their uptake characteristics at low external O2 concentrations, probably because of internal aeration through the air-space tissue of the plants. Nitrate deprivation also lowered the energy charge ratio and adenine nucleotide content in roots. The roots recovered quickly from deprivation once was resupplied. The stresses imposed by partially O2-depleted conditions and lack of nitrate were therefore relatively mild and reversible. It seems that the inherent aerenchyma development under aerated conditions in these species is sufficient to maintain adequate root oxygenation under partially O2-depleted conditions.

The flooding tolerance strategy of three tropical gallery forest species, Sebastiana klotzchyana, Hymenaea courbaril var. stilbocarpa and Chorisia speciosa was studied. Data from the flood-intolerant Schyzolobium parahyba have been included for comparison.

In the case of S. klotzchyana flooding induced an acceleration of glycolysis, with ethanol as the sole end product of anaerobic metabolism. Oxygen diffusion to the root system was not detectable.

The results obtained for H. courbaril and C. speciosa show a compromise between metabolic and morpho-anatomical adaptations to enable them to occupy seasonally flooded areas. In both species, at the onset of the stress, the roots stop producing high amounts of ethanol and lactate. diverting part of the pyruvate produced to malate. The amount of pyruvate converted into malate is determined by the amount of oxygen diffused from the aerial part to the roots. This is used to oxidise pyruvate through Krebs' cycle and the excess is converted to malate. ATP production in the root system is predominantly aerobic and therefore not linked to malate production.

The low level of acceleration of the fermentative pathway, coupled with the lack of oxygen diffusion to the root system, resulted in a significant decrease in the growth rate of S. parahyba.

Seeds of Erythrina caffra Thunb. are able to germinate anaerobically. Cycloheximide, chloramphenicol and malonate depressed germination. N2-incubated seeds metabolised [2-14C] Na-acetate. Synthesis of ATP in embryonic axes of N2-incubated seeds occurred to the same extent as in air-germinated seeds. Cycloheximide but not chloramphenicol depressed ATP and ethanol contents and respiratory capacity of embryonic axes.

Embryonic axis mitochondrial O2 uptake capacity was similar for seeds incubated for 24 h in air and N2. The activities of five mitochondrial enzymes at this stage were slightly lower in N2-incubated than in air-incubated axes. However, at 12 h, i.e. prior to germination, activities in N2-incubated axes were higher for malate dehydrogenase, oxoglutarate dehydrogenase and pyruvate dehydrogenase, similar for succinate dehydrogenase and slightly lower for cytochrome oxidase than corresponding activities in axes of air-germinated seeds. Mitochondrial protein synthesis occurred in axes of N2-incubated seeds but at a slightly lower rate than in air-incubated seeds.

When assayed anaerobically nitrate reductase activity was found associated with purified mitochondria. The nitrate reductase associated with mitochondria utilised NADH, succinate and to a lesser extent malate as electron donors. The activities measured were much lower than those of typical mitochondrial enzymes.

It is concluded that mitochondrial activity occurs anaerobically in E. caffra embryonic seed axes and might possibly play a role in anoxic germination.

Eleocharis sphacelata (Cyperaceae) is a common emergent hydrophyte in wetlands throughout Australasia, and is notable as a particularly large member of its genus, able to grow in water up to 2 m deep (Sainty & Jacobs 1981). It is a perennial species, supporting live emergent shoots throughout the year, and grows only on flooded sediments. In Australia, the water temperature in E. sphacelata wetlands remains mild in winter (c. 6°C) but is high (25–30°C) during summer (Brix et al. 1992; Sorrell & Boon 1992). In this paper we present an analysis of the mechanisms of internal ventilation in this species and their ability to aerate the submerged tissue.

Plants are killed by prolonged ice encasement at slightly subzero temperatures and under these anoxic conditions a number of metabolites are accumulated. Timothy grass plants encased in ice at −2°C can survive these conditions for many weeks and accumulate mainly CO2 and ethanol and lower levels of malate, oxalate, citrate, fumarate and pyruvate. While the accumulation of most of the metabolites increases towards the end of the encasement period the amount of malate decreases, indicating utilisation. Late in the encasement period lactate, butyrate, and traces of malonate, formate and tartarate were detected which might be of bacterial origin. The bacteria most commonly isolated from ice-encased plants is Pseudomonas fluorescens (Trevisan) which in most cases is saprophytic. All of the above-mentioned metabolites were also detected from thaw water in the field after long duration of ice encasement, butyrate attaining the highest concentration.

Plants tolerant of long-term flooding and oxygen deprivation in their perennating organs such as rhizomes and tubers are able to avoid the deleterious effects of anoxia and minimise the dangers of re-entry to air by reactions with antioxidants such as ascorbic acid and glutathione. In processes of detoxification of oxygen radicals, ascorbic acid is oxidised to dehydroascorbic acid and reduced glutathione to oxidised glutathione. Through the action of enzymes such as monodehydroascorbate reductase (MR) and dehydroascorbate reductase (DHAR), glutathione and ascorbic acid may be regenerated to maintain sufficient levels of antioxidants within the tissue in order to quench oxygen radicals.

The actions of endogenous plant hormones are invoked to explain several morphological adaptations to poor aeration. These include changes to the growth and internal morphology of inundated roots, the promotion of extension growth by stems or leaves of aquatic and semi-aquatic species by submergence, and morphological changes in shoots where only the roots and lower shoot are inundated. This article considers ethylene-promoted aerenchyma formation in maize (Zea mays) and compares the promoting action of ethylene, low oxygen partial pressures and carbon dioxide on shoot extension in rice (Oryza sativa), a rice mimic Echinochloa oryzoides and a submersed aquatic monocot Potamogeton pecrinatus. Different kinds of hormonal messages (positive, negative, accumulative and debit) passing between roots and shoots co-ordinate shoot development with the roots and their environment. Recent progress in quantifying the delivery of abscisic acid (ABA) or the ethylene precursor 1-aminocyclopropane-1-car-boxylic acid (ACC) from roots to shoots in the transpiration stream is summarised in relation to control of stomatal closure and leaf epinastic curvature in flooded plants.

It has been argued, whether or not the Pasteur effect occurs in plant tissues as a response to long-term hypoxia. To study this question roots of wheat seedlings (Triticum aestivum L. cv. Alcedo) were analysed following acclimation to oxygen shortage by a prior 6-d-cultivation in a nitrogen-flushed nutrient solution. A Pasteur Quotient of approximately one suggested the absence of a significant Pasteur effect. This conclusion was supported by finding an accumulation of soluble carbohydrates.

A progressive adaptation of hypoxically pretreated wheat roots was indicated by measurements under low oxygen tension of 2 kPa, when half of the produced carbon dioxide was generated by fermentation (Gas exchange Quotient, GQ≈2.1) with no apparent increase in the glycolytic substrate flux. The remaining oxygen uptake was even higher in hypoxically grown roots than in the aerobically grown control specimens. When whole seedlings were placed in oxygen-free conditions for 2 h, roots of seedlings pretreated hypoxically suffered a 50% loss in the concentration of ATP, while 90% of the ATP was lost in roots transferred from an aerated solution directly into an anaerobic environment. This was interpreted as an improvement in hypoxia tolerance by minimising the fermentation rate (low PQ) but in particular the ATP requirements by metabolic arrest strategies.

During the first 2 h of oxygen re-exposure, the GSH level was almost constant, while the GSSG increased about 10-fold. This results in a decline of the GSH/GSSG ratio, which reflects oxidative stress induced by re-aeration following hypoxic pretreatment. Further evidence for this is an increase in lipid peroxidation measured as thiobarbituric acid-reactive material (TBA-rm) and the affected content of sulfydryl-groups in the root tissues.

In spite of the high level of reduced glutathione in the roots under hypoxia-inducing conditions, they contained a retarded glutathione reductase (GR) activity compared with aerobically grown roots. Re-aeration up to 2 h resulted in a further decrease in GR activity. Only at the end of the 16-h period of re-aeration the enzyme activity was able to recover, by overshooting slightly those values of the continuously aerated controls. This was accompanied by a restoring a high GSH/GSSG ratio and an enhanced level of GSH.

In response to long-term oxygen shortage in the rooting medium (>7days), there was an accumulation of soluble carbohydrates in shoots as well as in roots of wheat seedlings (Triticum aestivum L. cv. Alcedo). The content of fructo-oligosaccharides had increased five-fold in the roots and seven-fold in the shoots. This fails to fit the assumption that higher substrate consumption is accompanied by the fermentation under oxygen shortage.

The hypothesis of reed decline being a consequence of eutrophication has been proved. Rhizomes of P. australis from two habitats differing in nutrient availability were collected during one year. In the nutrient-poor habitat, less aboveground biomass and greater starch reserves were formed in comparison with the nutrient-rich one. In the stand with a high nutrient availability, a decrease in starch content was observed in horizontal rhizomes during autumn.

The oxidation products of unsaturated hydrocarbon α-farnesene that accumulate in cuticular lipids of stored apple fruits and are involved in the development of superficial scald showed UV fluorescence. The fluorescence emission band was localised at about 330 nm; the fluorescence excitation spectrum resembles the absorption spectrum of the products, but was more resolved. The corresponding fluorescence was recorded from the surfaces of intact apple fruits. It appears that the fluorescence analysis of farnesene oxidation products is a useful, specific, and sensitive tool to follow their concentrations both in extracts and intact fruits.

The deteriorative processes leading to the death of a population, individual or part of an individual can be genetically programmed or induced by environmental perturbations, physical damage, pests and diseases. Senescence in multicellular plants is typically a phenomenon resulting from cell differentiation and loss of totipotency. Recycling of nutrients released from senescent cells, abscission layer formation, containment of pathogens and dispersal of progeny are crucial aspects of senescence management. Senescence-related autocatalytic changes induced by substances generally thought to regulate senescence may not mirror the sequence of changes occurring naturally through correlative processes in the intact plant. The chloroplast has a key role in reversing senescence-related degradation of other organelles. Conventional symptoms of senescence used in plant sciences have obscured common theories of senescence regulation for all types of organism.

Various lines of evidence suggest that the controlled increase in the production of oxygen radicals is an important factor involved in phytohormone metabolism, selective changes in membrane permeability, degradation of photosynthetic pigments and unsaturated membrane lipids, oxidative modification and subsequent proteolytic degradation, and other events occurring in ageing and senescing leaves. Increased lipid peroxidation, appearance of chlorophyll allomerised products, as well as fluorescent ‘lipofuscin-like’ pigments have been documented in different systems of leaf senescence. On the other hand, the accumulation of peroxidised products may result from the alteration to native structural organisation and energy dissipation reactions of the photosynthetic apparatus. Although there are contradictory and incomplete data on the activity of the systems involved in the metabolism of oxygen radicals, it seems that senescing leaves retain, at least in part, their defence potential against both activated oxygen species and toxic lipid peroxidation products. This provides the plant with the ability to successfully dismantle its photosynthetic apparatus during senescence in a relatively safe manner.

Acetaldehyde (AA) vapours exogenously applied or endogenously synthesised caused inhibition of fruit ripening process. Exposure of whole tomatoes to 6000 ppm AA vapour, for 6–24 h prior to storage, caused inhibition of fruit ripening, as expressed by reduced colour development. The endogenous levels of AA and ethanol found in the juice were positively correlated with the durations AA applied. Application of CO2 or N2 atmosphere resulted in production of endogenous AA and ethanol which also led to inhibition of ripening. Polygalacturonase (PG) activity was inhibited by the various AA treatments, as well as by CO2 or N2 pretreatments.

Exposure of mango fruits to AA vapour (1500–3300 ppm) caused a delay in colour development of mango fruits cv. ‘Haden’. The higher the AA concentration applied, the more effective it was in delaying fruit colour development. In addition to delaying ripening, AA concentration of 2200 ppm was the most effective in reducing the amount of fruit infected with Alternaria alternata, which is a latent fungus in this fruit. In mango cv. ‘Tommy Atkins’ pretreatment with N2 atmosphere prior to storage delayed ripening as expressed by less colour development.

Plants have evolved a multiplicity of defence mechanisms against pathogen attack. Their modes of action may be to (i) kill the pathogen directly, (ii) block the action of enzymes required for infection, or (iii) erect barriers to pathogen growth. Some of these reactions proceed via free radical intermediates and make use of either atmospheric oxygen or reactive oxygen species. This paper reviews the various types of reaction involving oxygen-derived free radicals that are initiated in plant tissue when it is invaded by pathogenic organisms. Both the production of free radicals by plants in defensive processes and the utilisation of free radicals by pathogens in offensive reactions are considered and particular attention is given to the use of electron paramagnetic resonance (EPR) spectroscopy for the direct observation of such free radical reactions.

The effect of a xanthine oxidase (XO) inhibitor, allopurinol, was studied in three host-rust interactions. Application of allopurinol in an incompatible interaction (wheat– Puccinia recondita f. sp. tritici) did not significantly alter symptom expression nor result in an increase in lipid peroxidation (LP) and electrolyte leakage as compared to non-treated control plants. On the other hand, application of allopurinol in three compatible host–rust interactions suppressed, in a concentration-dependent manner, the formation of uredospores which was accompanied by a transient increase in electrolyte leakage and LP during the biotrophic fungal growth (5–6 days after infection). These results suggest that (i) allopurinol can inhibit fungal growth in later developmental stage (s) developing only in compatible hosts and (ii) XO is not the primary source of the generation of reactive oxygen species (ROS) in incompatible rust combinations.