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.
Synopsis 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.
SynopsisA twig chamber system was developed for the exposure of mature trees to ozone (O3) under field conditions. The fumigation system allowed the exact control of O3 concentrations in the chambers, the measurement of O3 uptake as well as gas exchange measurements under ambient and controlled conditions during and after O3 fumigation. Because of differences in individual twigs the system should provide the exposure of replicates to different O3 treatments. Tests showed that temperature, humidity and O3 concentrations inside the chambers were comparable with diurnal courses observed in the field. Comparative gas exchange measurements indicated that there were no differences in net photosynthesis and conductance of twigs outside the chambers and twigs which remained within the chambers for 23 weeks receiving ambient air.
Seeds of Erythrina caffra Thunb. are able to germinate anaerobically. Cycloheximide, chloramphenicol and malonate depressed germination. N-2-incubated seeds metabolised [2-C-14] Na-acetate. Synthesis of ATP in embryonic axes of N-2-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 O-2 uptake capacity was similar for seeds incubated for 24 h in air and N-2. The activities of five mitochondrial enzymes at this stage were slightly lower in N-2-incubated than in air-incubated axes. However, at 12 h, i.e. prior to germination, activities in N-2-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 N-2-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.
Cyclitols are low molecular weight substances which accumulate in plant cells in response to various environmental stress situations, for example drought (Ford 1984), salinity (Gorham et al . 1984), low temperature (Richter et al . 1990). Apart from their more general role in osmotic adjustment, only in the case of salt stress is their mode of function well understood. Cyclitols (e.g. pinitol) accumulate when plants are exposed to increasing salt concentration (Paul & Cockburn 1989) and act as compatible solutes (Sommer et al . 1990) as defined by Brown & Simpson (1972). The significance of cyclitol accumulation in stress adaptation of plants to drought and cold still remains uncertain. However, it is generally accepted that drought and cold as well as several other stress situations lead to an enhanced generation of oxygen free radicals (Elstner 1990; Smirnoff & Colombe 1988), including the hydroxyl radical as the most harmful one. The report by Smirnoff & Cumbes (1989) that myo-inositol is an effective hydroxyl radical scavenger prompted us to test other naturally-occuring cyclitols like pinitol, quebrachitol, 1-D-1-O-methyl-muco-inositol, ononitol and quercitol for their ability to scavenge hydroxyl radicals.
SynopsisTypical phytopathological defence reactions of tobacco have been studied in response to ozone. After a single standard ozone pulse (5 h, 150 nl 1−1) β-l,3-glucanase was induced up to 40-fold in the sensitive tobacco cv. Bel W3, which developed necrotic lesions following the treatment. The activity was elevated 10-fold in the tolerant Bel B without concomitant injury. A decrease within 24 h occurred only in Bel B. Tyramine hydroxycinnamoyl-CoA transferase activity was induced 6- to 7-fold in both cultivars with a maximum after 5–8 h. Tyramine conjugates as the product of this reaction could not be isolated and were apparently rapidly integrated into the cell wall. Precultivation of Bel B plants in a greenhouse containing up to 70% of the outdoor ozone concentration resulted in a 4-fold induction of β-l,3-glucanase and a further increase after an administered ozone pulse. Pretreatment (30 nl 1−1 ozone, 7 h per day, 23 days) of sensitive Bel W3 caused increased injury and ethylene formation upon a subsequent ozone pulse. β-l,3-glucanase was induced to the same level as in non-pretreated control plants. In a comparison with other major air pollutants (SO2, NO2), only ozone induced the above reactions. However, a combined treatment of ozone and NO2 led to synergistic effects. These data indicate that ambient ozone is able to change the phytopathological disposition of plants. This finding may have practical significance for the usual plant experiments carried out in greenhouses with non-filtered air.
SynopsisAlthough chilling of Arabidopsis thaliana cells to 4°C reduced respiration and superoxide generation, the latter still appears significant. Superoxide may contribute to the observed chilling-induced cellular lipid peroxidation and through processes that may be facilitated by the up-regulation of Cu,Zn-superoxide dismutase activity.
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.
SynopsisThe responses of photosynthesis to oxygen concentration 2, 21 and 100% by etiolated bean leaves upon illumination were determined. After 0.3, 3, 6, 10, 12 and 24 h of greening the following processes were measured. (1) Functioning of PS II as reflected by changes in Fv/Fm ratios and t1/2 values. (2) Content and composition of pigments. (3) Capacity for carbon dioxide assimilation. The results showed that O2 exerts a complex inhibitory action on examined processes. The magnitudes of inhibition increased with increasing of O2 concentration and time of its action. Under applied conditions the lag phase for development of examined processes (with the exception of carotenoids) was in the range 3-10 h. After 24 h of greening in 2, 21 and 100% of O2 the Fy/Fm ratios attained values 0.55, 0.45, 0.13 respectively as compared with typical 0.80 of normal green leaves of bean. After 24 h of greening the inhibition by 100% O2 as compared with zero inhibition by 2% O2 were, in percentages: Fw/Fm, 78; t1/2, 77; chlorophyll a, 59; chlorophyll b, 69; β-carotene, 78; neoxanthin, 60; zeaxanthin, 43; violaxanthin, 35; CO2 fixation, 95.
SynopsisRates of leakage of protein and electrolytes were measured in soybean embryonic axes from soybean seeds stored for 8–42 months. A significant increase in the content of proteins and electrolytes was determined in the external medium where aged embryonic axes were incubated. Lipid peroxidation was evaluated by the measurement of thiobarbituric acid-reactive substances (TBARS) in isolated axes from aged seeds. No significant differences were detected either in the contentin vivonor in the rate of generationin vitro. To assessin vivooxidative condition of the axes, an assay employing 2′,7′-dichlorofluorescein (DCFH-DA) oxidation was used. Embryonic axes from seeds stored for 8 and 42 months showed 1.1±0.1 and 4.1±0.1 fluorescence units per min per axes, respectively, after 24 h of imbibition. This significant increase in fluorescence was measured at all the tested incubation times. Antioxidant defences, evaluated as chemiluminescence induced by t-butyl hydroperoxide and α-tocopherol content, showed a significant increase with storage time. These data suggest that the damage due to enhanced peroxidative events are limited by the increased amount of potent lipid soluble antioxidant.
In recent years, some experiments with low doses of ionising radiation have been performed, with reproducible results, which indicate a positive stimulus rather than a deletirious effect (Decker & Degner 1983; Wolff 1989).The aim of our own studies was to establish whether low doses of ionising radiation can influence metabolism by means of a modification of the concentration of a plant hormone, in this particular case jasmonate. The multiple physiological effects caused by the plant bioregulator jasmonate, including response to stress, suggest their essential involvement in central genetic and metabolic processes (Sembdner & Parthier 1993).
SynopsisThe effect of a xanthine oxidase (XO) inhibitor, allopurinol, was studied in three host-rust interactions. Application of allopurinol in an incompatible interaction (wheat–Puccinia reconditaf. 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.
Changes of the in vitro morphogenetic state may be achieved for some potato genotypes, but others are unresponsive (recalcitrant). Although the biochemical basis for somatic recalcitrance is unknown, evidence suggests that two different aspects of oxidative stress may be involved. Phenolic oxidation is a major problem in manipulating cultures of woody plant species (Thorpe & Harry 1990) and lipid peroxidation has been associated with recalcitrance in monocotyledonous plants (Cutler et al. 1989; Benson et al. 1992). Both oxidative phenomena are believed to be free-radical mediated, but to date there is no reported direct evidence for the formation of free radicals during plant tissue culture callogenesis. The objectives of the present study were twofold; to assess the feasibility of using electron paramagnetic resonance (EPR) spectroscopy to detect free radicals directly in plant tissue cultures and to investigate free radical activity during dedifferentiation of responsive and unresponsive potato genotypes.
SynopsisPlants 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.
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The Galileo satellite during its recent passes close to the Earth recorded a planet with an unusual red-absorbing pigment, a poisonous atmosphere, simultaneously rich in oxygen and in methane, with strong, modulated, narrow-band, radio emissions in the MHz frequencies (Sagan et al. 1993). To an observer visiting the solar system, these features; the photo-oxidisable pigment chlorophyll, abundant atmospheric oxygen, the existence of reducing conditions and intelligent life might well appear self-contradictory. While intelligent life is a recent event, the presence of other forms of life based on photosynthesis and survival under both oxygen-rich atmospheres and reducing conditions go back to the earliest times (Table 1). Life on Earth has evolved over nearly 4 G years under atmospheric environments ranging from anoxia, to hypoxia, to hyperoxia (relative to the present day), and not always in that sequence.
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SynopsisContemporary plant species show a wide range of responses to oxidative attack. Much of this variation may reflect the different environmental selective pressures operating at different geological periods over the course of angiosperm evolution. Evidence is provided to show that the wide range of contemporary responses to oxidative stress may directly reflect the persistence of genes controlling free radical processes under environments of the past.
Although ozone (O-3) occurs in both the troposphere and the stratosphere, mechanisms of O-3 formation and removal in both regions of the atmosphere are very different. Moreover, the presence of O-3 in the two layers has very different implications for the biosphere, especially vegetation. This introductory paper gives an overall appreciation of the two pools of O-3 and their relative influences upon plant life. In the troposphere, natural and unburnt hydrocarbons (HCs) not only participate in the formation of atmospheric O-3 but are also involved in the formation of free radicals from O-3 which go on to injure plant tissues. In the stratosphere, O-3 absorbs significant amounts of ultraviolet radiation (285-315 nm, UV-B) from sunlight which would otherwise harm biological systems. Depletion of stratospheric O-3 by chlorofluorocarbons (CFCs) has prompted urgent studies to assess the consequences of enhanced UV-B radiation on vegetation. Preliminary assessments of possible changes to UK crops are described in this paper.
SynopsisDuring 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.
