In the present work we compare responses of the carbohydrate content to hypoxia of related plant species, which naturally grow on sites prone to flooding (Senecio aquaticus HILL-flooding tolerant), with plants from habitats with only a low risk of oxygen shortage (Senecio jacobaea L.-flooding sensitive). The sum of water soluble carbohydrates in shoots of Senecio aquaticus was nearly tripled following one day of hypoxic treatment. While the amounts of glucose and fructose, changed only slightly, those of sucrose and fructans were two-to fourfold. Following oxygen shortage in the root environment the fructans became the main pool of water soluble carbohydrates in S. aquaticus. The shoots of the flooding tolerant species S. aquaticus formed more fructans when grown under oxygen deficiency, than their flooding intolerant relative S. jacobaea. The substantial increase in sucrose and fructan content occurred in spite of diminished photosynthetic rates under restricted oxygen supply. Fructans were found to accumulate as a response to oxygen deficiency in both flooding tolerant and intolerant species but with higher absolute values and ratios between fructan to starch in the flooding tolerant species. Adding 50 mM sucrose to the nitrogen-flushed nutrient solution did not lead to a further increase of fructan in shoots of S. aquaticus but it occurred in S. jacobaea, but the CO2 fixation decreased to nearly 50% in both species.

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.

SynopsisIn 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.

This paper shows the effects of re-aeration on the glutathione pool following a prolonged period of root hypoxia. An increased content of total glutathione has been measured in roots of wheat seedlings (Triticum aestivum L. cv. Alcedo), grown in a nitrogen-flushed nutrient solution (HI) with their shoots in air compared with roots of aerobically grown plants (C). Re-aeration of hypoxically pretreated roots causes oxidative injury indicated by the oxidation of reduced glutathione (GSH), decrease of total thiol groups and increased formation of TBA reactive material (lipid peroxidation).Re-admission of oxygen results in a 50% rise in oxygen uptake over the whole 16 h re-aeration period compared with the control. During this time the overall glutathione pool of HI treatment increases to almost double that of the control, essentially reflected in the amount of oxidized glutathione (GSSG). Hypoxically pretreated roots showed lower glutathione reductase activity (GR) than the control. Immediately following re-aeration the activity was further decreased to a limiting value which seems to prevent full reduction of the newly formed glutathione. Therefore, the capacity to reduce the GSSG pool is below the capacity for net glutathione synthesis. This results in a decline of the GSH/GSSG ratio which reflects oxidative stress. The enzyme activity recovers slowly after re-aeration exceeding the values of aerobically grown roots only after 16 h correlating with a high reduction state of the glutathione pool.Copper, known to induce the formation of reactive oxygen species, strengthened the effect of re-aeration and enhanced the post-anoxic injury irreversibly.The importance of the glutathione system in roots to cope with varying oxygen tension is discussed.

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.

The extent to which isolated root systems of 14-d-old seedlings of wheat (Triticum aestivum) and maize (Zea mays) were depleted of oxygen by respiration was measured after immersing them in outgassed olive oil to exclude oxygen entry from the air. At intervals over 45 min, gas from the roots was removed under partial vacuum and oxygen partial pressures measured by gas chromatography. Contrary to earlier findings (Erdmann and Wiedenroth, 1988), roots were able to utilize almost all of their oxygen within 20 min at 25°C, including that dissolved in the thin water covering interposed between roots and oil. The rate of aerobic respiration could be estimated readily from the time course of oxygen depletion.

The current-response method was used to characterize effect of oxygen deficiency on functional state of membranes along the roots of wheat seedlings. The results show apical parts of older roots being most affected by hypoxia, while the youngest roots behaved as effectively adapted.

We studied fructan accumulation in wheat seedlings (Triticum aestivum L. cv. Alcedo) caused by oxygen shortage around the root. Plants were cultivated in either nitrogen-flushed or aerated nutrient solution. In response to the nitrogen treatment there was an accumulation of soluble carbohydrates in shoots as well as in roots. The main contribution was due to fructans. The content of fructo-oligosaccharides had been increased five-fold in the roots and seven-fold in the shoots. This is incompatible with the assumption that higher substrate consumption follows enhanced fermentation under oxygen shortage. Re-aeration of the nitrogen-flushed nutrient solution resulted in enhanced consumption of stored carbohydrates, mainly of fructans, accompanied by high growth rates of the roots. The possibility of utilizing fructans quickly if oxygen is available is a possible advantage for plants adapted to hypoxia. Particularly however, it is suggested that the low energy requirements for sucrose:sucrose fructosyltransferase, allowing fructan synthesis even under oxygen shortage, may improve the fitness of plants to survive temporary hypoxia in the rhizosphere.

The effect of oxygen deficiency in the rhizosphere on structural and physiological parameters of the root, and correlative reactions of the shoot, were studied with special regard to the reversibility of stress-induced growth inhibitions by re-aeration. Experiments with wheat seedlings, including investigations of their morphological and anatomical adaptations, gas exchange, accumulation of respiratory substrates, ATP content, as well as 86Rb-uptake, enabled conclusions to be drawn as to how plants cope with temporary hypoxic injury. Every adaptive response needs additional energy. Attention is focussed on the achievement of suitable energy balances between root and shoot under adverse environmental conditions.

Details of the morphology and anatomy of the coleoptile of wheat plants are given; these have not been described adequately previously. The investigations focussed on the hexaploid summer wheat Triticum aestivum L. cv. Hatri, and three taxa with different ploidy levels. In darkness the longitudinal growth of the coleoptile was delayed by nearly 24 h but the final length, reached after 120 h, was double that of coleoptiles of plants cultivated under continuous light. As soon as the coleoptile has grown, the primary leaf pushes through a pore pre-formed during the meristematic stage and located 1–1·5 mm behind the apex. The pore is stabilized mechanically by anastomosing of the originally free ends of the vascular bundles, as well as by increased lignification in this region. The species investigated differ in length, f. wt and d. wt, size of epidermal cells, and especially in the size of guard cells of the coleoptiles. The number of parenchyma layers, however, shows no specificity.

Effet de conditions anoxiques, en presence ou en absence d'azote dans la solution nutritive, sur le rendement photosynthetique net, le point de compensation en CO 2 , l'intensite respiratoire, l'accumulation de substances minerales dans la matiere seche des parties aeriennes, et sur la masse seche des racines

Changes in pH, total alkalinity and O2 concentration were followed in an aquatic medium with excised wheat roots (Tritkum aestivum L.). Concentrations of total inorganic carbon and free CO2 were calculated from total alkalinity and pH according to carbonate equilibria. The total inorganic carbon was estimated by flow-injection infra-red gas analysis. Total alkalinity increased in the root medium during incubation. Respiratory CO2 production was estimated best from the increase in total inorganic carbon measured with an infra-red gas analyser.

The oxygen status in roots of wheat seedlings (Triticum aestivum) was determined by a volumetric micro-absorption method. Plants grew in nutrient solution (aerated or nitrogen-flushed) or on flooded sand up to the 10th day. The roots were then exposed to aerated or hypoxic conditions for several hours before gas was extracted by reducing the pressure within a concentrated salt solution or by physical crushing. The oxygen content of the extracted gas bubbles was measured with pyrogallol. Comparative experiments with the helophytes Phalaris arundinacea and Carex acutiformis yielded similar oxygen concentrations to those already described in literature. The concentrations of oxygen (13–16%) in young wheat roots were surprisingly high when exposed to nutrient solution flushed with nitrogen gas. Removal of the shoots decreased the oxygen concentration in the roots, indicating some internal oxygen transport from shoots to roots. Detached, submerged roots of wheat still contained 6% oxygen following 20 h of submergence in nitrogen-flushed solution. A linear relationship was found between the oxygen concentration in roots of Triticum aestivum, Zea mays and the two helophytes and the volume of extractable gas per volume of root. This ratio corresponded to the extent of aerenchyma formation. Hence, a certain amount of oxygen may have been adsorbed onto the inner surfaces of the lacunae of the roots. However, the large amount of oxygen in the roots of intact wheat plants suggest that some parts of the root system are unlikely to suffer from the oxygen shortage imposed by oxygen-deficient external conditions such as flooded soil.

Anatomical alterations in the root-shoot junction following hypoxic conditions were studied in young wheat plants (Triticum aestivum L. cv. Hatri) grown in nutrient solution flushed with air or nitrogen gas. The root-shoot junction was characterized by densely packed tissues with only small intercellular spaces. Seven days of hypoxia did not alter the anatomy of this region, suggesting that it does not constitute an important pathway for oxygen diffusion from aerial shoot to the aerenchymatous roots. A likely alternative path for oxygen movement is the gas-filled interspace between coleoptile and shoot base. Roots emerging from more apical parts of the stem elongated more quickly in hypoxic conditions than those from more basal parts. This was related to the path length from the main point of entry of atmospheric oxygen into the plant. Additionally, oxygen shortage in the ambient root medium decreased the number of mitoses per root tip, as determined by the Feulgen method. This effect was most severe in the basally inserted roots, that are presumed to be the most oxygen deficient.

Morphological and anatomical parameters which are variable under root anaerobiosis in Triticum aestivum were checked on five taxa of primitive and modern wheats (and the related genus Aegilops). The plants were grown in nutrient solution which was either aerated or flushed with nitrogen. When the plants were flushed with nitrogen a general retardation in longitudinal root growth occurred in all of them, but only Triticum aestivum showed a clear promotion of growth of later appearing roots enabling it to maintain the same root/shoot ratio even under stress conditions. There was an increase in the volume of intercellular space in the root cortex of nearly all the plants investigated. The diameter and the lignin content of the roots and the form of their cortical cells also varied. All these changes were expressed in the primitive wheats to a lesser extent than in the advanced Triticum aestivum indicating that there is a clear increase in the adaptive response in the latter.

Anatomical changes in roots of wheat seedlings (Triticum aestivum L. cv. Hatri) following oxygen deficiency in the rooting medium were investigated. The response of the plant to stress was tested at a very early developmental stage when the first adventitious roots had just emerged. In order to analyze the adaptation of different roots, respiration rates of the roots 1–3 and 4–n were compared with the respiration rates of the total root system. Oxygen deficiency was induced either by flushing nutrient solution with nitrogen or flooding of sand. In contrast to plants grown in well aerated media, both stress variants led to a significant increase of the intercellular space of the root cortex in seminal and first adventitious roots. Radial cell enlargement of cortical cells near the root tip, cell wall thickenings in flooded sand cultures and an increase in phloroglucinol-stainable substances were found to be further indicators of low oxygen supply. The roots 4–n which were promoted in growth under hypoxia showed higher respiration rates; hence the total root respiration was not restricted.

Morphological changes of roots and shoots following oxygen deficiency in the root medium and after partial pruning of the root system were analyzed to obtain easily measurable parameters of the adaptive capacity of the root system against stress. Wheat seedlings (Triticum aestivum L. cv. Hatri) were cultivated on nutrient solution which was either aerated or flushed with nitrogen, or were cultivated on flooded sand. On the third day after grain swelling in two pruning variants, roots 1–3 or 4–8 were excised. Root anaerobiosis retarded longitudinal growth and biomass accumulation of the shoot and the seminal roots, and stimulated the development of adventitious roots. Partial removal caused a general compensative growth of the remaining roots under aerobic conditions. Root pruning plus anaerobiosis exceeded the compensatory capacity of the seedlings and thus caused a strong delay of elongation and biomass accumulation of both roots and shoots, including decrease of the root/shoot ratio. Roots became independent of endosperm reserves on the seventh day under aerobic conditions though caryopses were not completely exhausted at this time. Additionally, oxygen deficiency delayed the reserve exhaustion process.

Oxygen deficiency in the root medium of Triticum aestivum seedlings leads to changes in root morphology. The gas exchange of the shoots is influenced only to a small extent, whereas in the roots respiration is gradually decreased and substituted by fermentation. These consequences are partially compensated by exudation and degradation of the intermediates in toxic concentrations and an increased substrate decomposition to ensure the ATP level. Compensatory mechanisms are discussed in a general concept of regulatory mechanisms valid for normal development as well as under resistance physiological load, ensuring the energy state of the organism. Under normal conditions cells will work following the overflow concept. Under stress conditions gradually compensation mechanisms become effective, starting with short term effects (by changing redox state and/or phosphorylation activity) over processes which need longer times (by changes of pool sizes, substrate availability etc. ) to limited changes in the realization of the genetic program (from de novo synthesis of participating enzymes to variations in organelle numbers).

Apparent photosynthesis (Pn) and root respiration (Rn) of 8 to 10 d old intact bean plants were studied separately at different temperatures and oxygen concentrations acting on the roots using IRGA technique. Pn is reversibly decreasing during 3 h root cooling (+ 5 °C) in accord with bending down of the primary leaves, and closing of the stomata. Plants adapted to low root temperatures show no effect of increasing the latter on Pn. Even 3 h oxygen deficit in the root medium has no influence on Pn but is increasing Rr in consequence of alteration in ’ metabolism from respiration to fermentation. One must distinguish between short time reaction and long time adaptation of the plant on root stress.