Internal oxygen transport inRumex species and its significance for respiration under hypoxic conditions

Rumex thyrsiflorus, Rumex crispus andRumex maritimus show a differential flood-tolerance in the river ecosystem in the Netherlands.R. thyrsiflorus occurs at high-elevated habitats and is flood-intolerant, the other two species occur at lower-elevated habitats and are flood-tolerant. We compared their respiratory activity under aerobic and anaerobic conditions in the root environment and quantified the internal gas transport. The results indicate that aerial oxygen can be used for root respiration in both aerobically and anaerobically grown plants. The amount of oxygen used via internal aeration increased with decreasing oxygen concentration in the root environment. Aerobically grown plants ofR. maritimus andR. crispus already showed a high internal aeration, but there was a significant increase in internal oxygen transport in anaerobic plants, where new, aerenchymatous roots had formed. This indicates the functional significance of new root formation for respiration in these species upon hypoxia. After two weeks of anaerobiosis, more than 50% of the total respiration of the roots of young plants ofR. maritumus and 40% of roots of young plants ofR. crispus was due to internal aeration at low oxygen concentrations in the root environment. InR. maritimus both young and old plants performed in this way, inR. crispus only young plants, whileR. thyrsiflorus showed some internal aeration, but this was hardly detectable. These differences can be explained on the basis of a different morphology and concomitant diffusive resistance of both root and shoot system. In experiments with different submergence levels of the shoot, the amount of internal aeration was positively correlated to the total leaf area protruding above the water surface inR. maritimus. This indicates a functional significance of the petiole and leaf elongation response upon total submergence of this species.     

The relative importance of anaerobiosis and high iron levels in the flood tolerance ofRumex species

In both hydroculture experiments and a greenhouse trial the combined effects of flooding and high iron levels on the growth and occurrence of iron toxicity were investigated in threeRumex species having different flood tolerance.In a hydroculture experiment the plants were subjected to different FeCl₂ concentrations and anaerobiosis. At solution iron concentrations exceeding 750 M, the growth rate of the flood-intolerantR. thyrsiflorus was sharply decreased. The root system was most negatively affected. Differences between the investigated species could be most likely explained from differences in root porosity and are thus closely related to a differential internal oxygen supply to the root systems.In a greenhouse experiment soil flooding was combined with the addition of different ferrous iron concentrations to the soil solution. Flooding in combination with the addition of 5 mM ferrous iron did not result in a significant decrease in biomass production of any of the investigatedRumex species, in spite of the fact that several types of shoot iron toxicity were perceived. Especially at high iron levels significant amounts of bronzing spots on the leaves of all species were observed. Petiole iron toxicity symptoms, which result in a sagging of the petioles, was most clearly observed in the flood-intolerantR. thyrsiflorus.Although the hydroculture experiments revealed a severe effect of anaerobiosis and high iron levels on the root development and plant growth rate of especially the flood-intolerantR. thyrsiflorus, no such adverse effects were registered in the greenhouse experiments in neither of the species. This is most probably due to the fact that under greenhouse conditions theRumex species are able to locally immobilize iron by oxidation, thereby avoiding the actual iron stress. Since biomass production was hardly affected under greenhouse conditions, it is concluded that high iron levels in the soil solution are of minor importance in the different flood tolerance of theRumex species. It also indicates that great care has to be taken in the interpretation of hydroculture experiments to the actual effect of the suggested stress conditions under greenhouse or natural conditions.     

Growth and Survival Responses of Rumex Species to Flooded and Submerged Conditions: The Importance of Shoot Elongation, Underwater Photosynthesis and Reserve Carbohydrates

Plants of Rumex thyrsiflorus Fingerh., R. crispus L. and R.maritimus L., which are zoned along a gradient of elevationin a river foreland ecosystem, and differ in their flood-tolerance,were subjected to different flooding levels. Under conditionsof soil flooding, the growth rates of the flood-tolerant R.crispus and R. maritimus were as high as under drained conditions,but that of the flood-intolerant R. thyrsiflorus was halved.Upon submergence, the low elevation species R. maritimus showedrapid shoot elongation; when elongation resulted in a protrusionof leaves above the water surface, the plants survived. Alternatively,underwater photosynthesis also led to a 100% survival of submergedR. maritimus plants, provided that enough inorganic carbon wasmade available in the water. This could be attributed in partto the use of photosynthetically-derived oxygen for root respiration;in a hydroculture experiment, with 5.0 mM CO₂ in the water inthe shoot environment, photosynthetically-derived oxygen contributedmore than 50% to root oxygen consumption at low oxygen concentrationsin the root environment. The intermediately elevated species R. crispus appeared to bemuch more tolerant towards conditions of prolonged total submergence:older plants survived eight weeks submergence in the dark. Thisresponse was explicable in terms of a dormancy-strategy, whichis characterized by a slow consumption of carbohydrates storedin the tap-root. The differential responses of R. maritimusand R. crispus to total submergence reveal the limitations offlood-tolerance and reflect the different life-histories ofthe species. Key words: Photosynthesis, Rumex, submergence, carbohydrates, growth rate, shoot elongation     

Elongation by primary lateral roots and adventitious roots during conditions of hypoxia and high ethylene concentrations

Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots contained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsiflorus Fingerh., which has little root porosity, was much more affected. Ethylene had an even stronger effect on root elongation than hypoxia, since very small concentrations (0.1 cm³ m^?3) reduced root extension in the two species, and higher concentrations inhibited elongation more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growth. An exception may be the highly aerenchymatous, adventitious roots of R. palustris. Aerenchyma in these roots provides a low-resistance diffusion pathway for both endogenously produced ethylene and shoot-derived oxygen. This paper shows that extension by roots of R. palustris in flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.     

Regulatory role of auxin in adventitious root formation in two species of Rumex,differing in their sensitivity to waterlogging

Adventitious rooting in Rumex plants, in which the root systems were in hypoxic conditions, differed considerably between two species. R. palustris, a species from frequently flooded river forelands, developed a large number of adventitious roots during hypoxia, whereas adventitious root formation was poor in R. thyrsiflorus, a species from seldom flooded dykes and river dunes. Adventitious rooting could also be evoked in aerated plants of both species by application of auxin (1-naphthaleneacetic acid or indoleacetic acid) to the leaves. The response to auxin was dose-dependent, but even high auxin doses could not stimulate R. thyrsiflorus to produce as many adventitious roots as R. palustris. Consequently, the difference between the species in the amount of adventitious root formation was probably genetically determined, and not a result of a different response to auxin. A prerequisite for hypoxia-induced adventitious root formation is the basipetal transport of auxin within the shoot, as specific inhibition of this transport by N-1-naphthylphthalamic acid severely decreased the number of roots in hypoxia-treated plants. It is suggested that hypoxia of the root system causes stagnation of auxin transport in the root system. This can lead to an accumulation of auxin at the base of the shoot rosette, resulting in adventitious root formation.     

Ethylene accumulation in waterlogged Rumex plants promotes formation of adventitious roots

Accumulation of the gaseous plant hormone ethylene is very importantfor the induction of several responses of plants to flooding.However, little is known about the role of this gas in the formationof flooding-induced adventitious roots. Formation of adventitiousroots in Rumex species is an adaptation of these plants to floodedsoil conditions. The large air-spaces in these roots enablesdiffusion of gases between shoot and roots. Application of ethylene to non-flooded Rumex plants resultedin the formation of adventitious roots. In R. palustris Sm.shoot elongation and epinasty were also observed. The numberof roots in R. thyrsiflorus Fingerh. was much lower than inR. palustris, which corresponds with the inherent differencein root forming capacity between these two species. Ethyleneconcentrations of 1.5–2µI I_{– 1} induced a maximumnumber of roots in both species. Quantification of ethylene escaping from root systems of Rumexplants that were de-submerged after a 24 h submergence periodshowed that average ethylene concentrations in submerged rootsreached 1.8 and 9.1 µl I_–1 in R. palustris and R.thyrsiflorus, respectively. Inhibition of ethylene productionin R. palustris by L--(2-aminoethoxyvinyl)-glycine (AVG) or-aminobutyric acid (AIB) decreased the number of adventitiousroots induced by flooding, indicating that high ethylene concentrationsmay be a prerequisite for the flooding-induced formation ofadventitious roots in Rumex species. Key words: Adventitious roots, epinasty, ethylene, flooding, Rumex, shoot elongation     

The Significance of Oxygen Transport and of Metabolic Adaptation in Flood-Tolerance of Senecio Species

Experiments were designed to provide information about the physiological basis of flood-tolerance in Senecio species. The oxygen concentration in roots of S. jacobaea L., S. viscosus L. and S. vulgaris L. became almost zero after transplantation to a solution of low oxygen concentration, and it was concluded that the flood-sensitivity of these Senecio species could be due to insufficient oxygen transport from the shoots to the roots. The oxygen concentration in the roots of the flood-tolerant S. congestus (R.Br.) DC., growing in a solution of low oxygen tension, was almost sufficient to maintain oxygen utilization at the rate observed in roots of plants, grown in an air-saturated solution. Oxygen utilization by roots of the flood-tolerant S. aquaticus Hill, growing in a solution of low oxygen tension, was inhibited 50%. However, the oxygen concentration in the roots of this species remained high enough to maintain cytochrome oxidase activity and oxidative phosphorylation at the rate observed in roots from an air-saturated environment. The activity of a second (“alternative”) oxidase must have been drastically reduced. Alternative NADH-oxidizing enzymes, like nitrate reductase which was induced by anaerobiosis in roots of S. aquaticus, might replace the regulatory function of the alternative oxidase. — Thus, in S. aquaticus root porosity and root length contributed to the maintenance of an oxygen concentration which was sufficient for uninhibited cytochrome oxidase activity and oxidative phosphorylation rate in roots growing in a solution of low oxygen tension.     

The comparative germination ecology of nine Rumex species

The germination requirements, dormancy cycle and longevity of nine Rumexspecies were studied in field conditions and laboratory experiments to show theadaptations of the related species to their specific habitat. Within one genus,rather striking differences were observed in germination ecology. However, theclosely related species, R. acetosa and R.scutatus, are very similar: they fruit in early summer; theirseeds can germinate immediately after dispersal, and they are nondormant andshort-lived. R. acetosella also has fruits insummer, but the seeds do not germinate the first season after dispersal. Theyare long-lived, but buried seeds do not show a dormancy cycle; they mightgerminate in different seasons after exposure to light. Seeds of four species (R. conglomeratus,R. maritimus, R. sanguineus andR. crispus) are long-lived and undergo aseasonal dormancy cycle, with a low level of dormancy in winter and early springand a deep dormancy in summer as was already known for R.obtusifolius. These seeds are shed in the autumn, and they germinatemainly in the spring in consecutive years. R. maritimusalso germinates in summer and autumn on drying muddy soils. The seeds of R. hydrolapathum only germinate onwaterlogged soils, which explains its growth at the edge of streams and ponds.Its seeds are rather short-lived. The seeds of the species on very wetplaces require a higher temperature for germination.     

Metabolic response of Medicago sativa L. and Lotus corniculatus L roots to anoxia

Abstract Differential rates of fermentation and energy production have been implicated in the response of plant species to extended root anoxia. This study describes the metabolic response to anaerobiosis of waterlogging-tolerant birdsfoot trefoil (Lotus corniculatus L.) and waterlogging-sensitive alfalfa (Medicago sativa L.). Studies were carried out on glasshouse-grown plants subjected to root anaerobiosis in nutrient solution. Rate of fermentation, as estimated by CO₂ evolution, declined significantly upon anaerobiosis in both species but was proportionally less, relative to the aerobic control, in trefoil. Another indicator of carbon flux through glycolysis, the concentration of glucose-6-phosphate, was also significantly lower in trefoil roots relative to aerobic controls. Both species showed significantly increased root exudation of K⁺, sugars and andno-N, especially during the first 2 d of root anaerobiosis, indicating changes in membrane selective permeability. The energy status of roots subjected to anaerobiosis declined sharply in both species but trefoil roots maintained higher ATP/ADP ratios for up to 4 d of anaerobiosis. The results are consistent with the hypothesis that increased fermentation activity maintains a more favourable root energy status. This higher energy status may facilitate survival by maintaining crucial root activities, such as maintenance of membrane stability.     

Root responses of flood-tolerant and flood-sensitive tree species to soil redox conditions

Summary Under controlled rhizotron conditions, roots of Taxodium distichum L., Quercus lyrata Walt, and Q. falcata var. pagodaefolia Ell. were subjected to low soil redox potentials. Root elongation was inhibited at low soil redox potentials. In T. distichum, redox potentials below +200 mV resulted in a significant inhibition of root elongation. In Q. falcata var. pagodaefolia and Q. lyrata, redox potentials below +350 mV resulted in complete cessation of root growth. Studies on root anatomy indicated that low soil redox potenials resulted in a changed cellular structure in the cortex of T. distichum. However, little change was noted in stress roots of oak species. Alcohol dehydrogenase activity in T. distichum roots was approximately doubled compared to control plants, indicating stimulated alcoholic fermentation. In T. distichum, alcoholic fermentation and anatomical changes contribute to flood tolerance but oak species lack these characteristics.     

Hormone sensitivity and plant adaptations to flooding

Plant hormones play a key role as mediators between environmental signals and adaptive plant responses. Auxin, ethylene and gibberellins are involved in the initiation of adaptive plant responses such as the development of adventitious roots and stimulated shoot elongation upon flooded conditions. These adaptive plastic responses in plants are frequently linked to changes in the concentrations of the hormones involved, but only rarely to shifts in sensitivity. Examples from ecophysiological research performed with species from the genusRumex demonstrate the importance of the hormone sensitivity concept in plant adaptations to flooding: (a)Rumex species can be grouped into three response categories according to the ethylene sensitivity of the youngest petioles: positive, negative and indifferent; (b) Sub-ambient oxygen concentrations sensitize petioles of wetlandRumex species to ethylene; (c) Enhanced ethylene levels sensitize petioles of wetlandRumex species to gibberellin; (d) Auxin is the primary plant hormone responsible for the initiation of adventitious roots in wetlandRumex species. However, a factor related to waterlogging, possibly ethylene, is required to sensitize the root-shoot junction to endogenous auxin.     

Effects of aluminium on growth and cation uptake in seedlings of Eucalyptus mannifera and Pinus radiata

This experiment was designed to examine the effects of aluminium (Al) on the growth of Pinus radiata (D. Don) and Eucalyptus mannifera subsp. mannifera (Mudie) seedlings in culture solutions in a glasshouse to help explain the failure of radiata pine trees on some acid, low fertility soils in Australia on which the native eucalypts flourish. Aluminium (Al) in culture solution increased the growth of roots and shoots of seedlings of both species but while growth of the eucalypt continued to increase with increases in Al to 2.222 μM, growth of the pine was largest at 370 μM Al. In addition to total root length, specific root length (length per unit dry weight), a measure of fineness of the root, increased in the eucalypt seedlings as the substrate Al increased. Growth of the shoots and roots of the pine in the absence of any added Al was extremely poor suggesting that Al, in low concentrations, may be an essential element or ameliorate some other factors in solution culture at low pH. Root and shoot concentrations of K increased with increasing Al, whilst Ca and Mg Concentrations decreased and Mn concentrations were unaffected in both species. Tissue Ca and Mg concentrations were 2 to 3 times higher in the eucalypt seedlings than the pine at all levels of added Al due to greater uptake of these elements by the eucalypt. In contrast, at the highest concentration of Al in the medium, shoot Al concentrations were lower in the cucalypt than in the pine due to a greater proportion of Al being retained in the eucalypt roots. These differences between the seedlings in terms of root growth and tissue cation concentrations may help explain the ability of eucalypt species to maintain vigorous growth on acid soils high in Al and low in Ca and P, where growth of the pines failed.     

An amalgamation between hormone physiology and plant ecology: A review on flooding resistance and ethylene

Both distribution of terrestrial plants and species composition in flood plain communities are strongly influenced by flooding (waterlogging, partial submergence, or submergence). The interaction between a plant's flooding resistance and the seasonal timing, duration, depth, or frequency of flooding often determines plant distribution in flood plains. Flooding may be accompanied by marked physical changes in light, carbon availability, diffusion rate of gases, and density of the environment. Various physiological processes may be affected by these flooding-induced physical changes, including aerobic respiration, photosynthesis, and processes in which light acts as a source of information (e.g., phytochrome photoequilibrium). Certain plant species acclimatize and adapt to these physical changes to relieve the constraints imposed by the flooded environment. Underwater photosynthesis, enhanced shoot elongation, adventitious roots, and aerenchyma formation are typical adaptive responses which are believed to improve the oxygen status of submerged plants. Ethylene and other plant hormones play a central role in the initiation and regulation of most of these adaptive responses, which permit escape from anaerobiosis. Mechanisms of direct tolerance of anaerobic conditions, such as a vigorous fermentative respiratory pathway, are of particular importance when the plant is very deeply submerged, or during the night and when the water is sufficiently turbid to exclude light.Studies on the cosmopolitan genus Rumex, distributed in a flooding gradient on river flood plains, have integrated plant hormone physiology with plant ecology. Rumex species showed a high degree of interspecific variation in ethylene production rates, endogenous ethylene concentrations, ethylene sensitivity, and ethylene-mediated growth responses. The field distribution of Rumex species in flooding gradients is explained in terms of a balance between endogenous ethylene concentrations and sensitivity towards this growth regulator (ethylene economy). Much data has been gathered using a recently developed laser-driven photoacoustic detection technique capable of detecting six parts of ethylene in 10¹² parts air flowing continuously over the plant.     

Host-specificity of a root borer,Bembecia chrysidiformis [Lep.: Sesiidae], a potential control agent forRumex spp. [Polygonaceae] in Australia

Bembecia chrysidiformis (Esper) [Lep.: Sesiidae] was examined for its natural history and specificity toRumex spp. (Polygonaceae) which are weeds in Australia. Adults of this southern European insect appear in late spring to summer. Eggs are laid on the dried, seed bearing stems of perennialRumex plants. The larvae tunnel inside the root during summer through to the next spring. In nature, the larvae are round inRumex species of the subgeneraRumex andAcetosa. In host-specificity tests with 1st instar larvae, the roots of a number of genera within thePolygonaceae were attacked. Larvae died on a range of plants from other families except inPersea americana Miller(Lauraceae), Helianthemum nummularium (L.) Miller (Cistaceae) andQuercus ilex L. (Fagaceae) where larvae fed on the stems. The insect was judged safe for release in Australia by assessing aspects of its biology, its known host plants, and the lack of reported attack on other plants.      

Comparison of two serpentine species with different nickel tolerance strategies

The effects of nickel were studied in two serpentine species with different metal tolerance strategies:Silene italica L., which limits nickel uptake and translocation, andAlyssum bertolonii Desv., a serpentine endemic, which accumulates nickel mostly in the leaves. InS. italica, nickel 7.5 μM inhibited root growth and depressed mitotic activity in root tips. Peroxidase activity and phenol concentration both in roots and shoots were increased; under the same conditions nickel did not produce any relevant effect onA. bertolonii. InS. italica an adequate calcium concentration (25 mM) was able to reverse the effects of nickel on root growth and metabolism. InA. bertolonii the same calcium concentration reduced root growth, confirming this species adaptation also to low calcium concentrations, typical of serpentines.     

The effect of a foliar disease (rust) on the development of Gastrophysa viridula (Coleoptera: Chrysomelidae)

Abstract.

• 1 Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae) and the pathogenic rust fungus Uromyces rumicis (Schum.) Wint. both occur on leaves of Rumex crispus L. and R.obtusifolius L. Individual stages of beetle development, and egg laying, were compared on healthy and infected leaves of each plant species in the laboratory. Oviposition choice was investigated in the field and laboratory.
• 2 Beetles reared on infected leaves of each species had greater larval mortality and slower development than those reared on healthy leaves. Although larvae feeding on infected leaves consumed up to 2.5 times more dry weight than those reared on healthy leaves, they had a lower relative growth rate and pupated at a lower weight. These changes were consistent with the reduced nutritive quality of rust-infected Rumex leaves.
• 3 Fecundity of beetles reared on infected leaves of both species was considerably reduced. Eggs laid by beetles feeding on infected R.crispus leaves also had a reduced viability.
• 4 The beetle developed consistently poorer on healthy R.crispus than on healthy R.obtusifolius throughout its life-cycle. Differences in larval performance were greater between host species than between infected and healthy leaves.
• 5 Oviposition was similar on infected and healthy R.crispus in both the laboratory and field. However, adults consumed less, and laid fewer eggs on infected than on healthy R.obtusifolius. The pattern of egg laying on different aged leaves was affected by rust infection: a greater proportion of eggs was laid on the older, infected leaves, than on the equivalent aged leaves on the healthy plants. Few larvae survived from eggs laid on rusted leaves in the field.

     

Changes in growth,porosity, and radial oxygen loss from adventitious roots of selected mono‐ and dicotyledonous wetland species with contrasting types of aerenchyma

Growth in stagnant, oxygen‐deficient nutrient solution increased porosity in adventitious roots of two monocotyledonous (Carex acuta and Juncus effusus) and three dicotyledonous species (Caltha palustris, Ranunculus sceleratus and Rumex palustris) wetland species from 10 to 30% under aerated conditions to 20–45%. The spatial patterns of radial oxygen loss (ROL), determined with root‐sleeving oxygen electrodes, indicated a strong constitutive ‘barrier’ to ROL in the basal root zones of the two monocotyledonous species. In contrast, roots of the dicotyledonous species showed no significant ‘barrier’ to ROL when grown in aerated solution, and only a partial ‘barrier’ when grown in stagnant conditions. This partial ‘barrier’ was strongest in C. palustris, so that ROL from basal zones of roots of R. sceleratus and R. palustris was substantial when compared to the monocotyledonous species. ROL from the basal zones would decrease longitudinal diffusion of oxygen to the root apex, and therefore limit the maximum penetration depth of these roots into anaerobic soil. Further studies of a larger number of dicotyledonous wetland species from a range of substrates are required to elucidate the ecophysiological consequences of developing a partial, rather than a strong, ‘barrier’ to ROL.     

How does glutamine synthetase activity determine plant tolerance to ammonium?

The wide range of plant responses to ammonium nutrition can be used to study the way ammonium interferes with plant metabolism and to assess some characteristics related with ammonium tolerance by plants. In this work we investigated the hypothesis of plant tolerance to ammonium being related with the plants’ capacity to maintain high levels of inorganic nitrogen assimilation in the roots. Plants of several species (Spinacia oleracea L., Lycopersicon esculentum L., Lactuca sativa L., Pisum sativum L. and Lupinus albus L.) were grown in the presence of distinct concentrations (0.5, 1.5, 3 and 6 mM) of nitrate and ammonium. The relative contributions of the activity of the key enzymes glutamine synthetase (GS; under light and dark conditions) and glutamate dehydrogenase (GDH) were determined. The main plant organs of nitrogen assimilation (root or shoot) to plant tolerance to ammonium were assessed. The results show that only plants that are able to maintain high levels of GS activity in the dark (either in leaves or in roots) and high root GDH activities accumulate equal amounts of biomass independently of the nitrogen source available to the root medium and thus are ammonium tolerant. Plant species with high GS activities in the dark coincide with those displaying a high capacity for nitrogen metabolism in the roots. Therefore, the main location of nitrogen metabolism (shoots or roots) and the levels of GS activity in the dark are an important strategy for plant ammonium tolerance. The relative contribution of each of these parameters to species tolerance to ammonium is assessed. The efficient sequestration of ammonium in roots, presumably in the vacuoles, is considered as an additional mechanism contributing to plant tolerance to ammonium nutrition.     

Effect of phosphorus supply to the surface roots of wheat on root extension and rhizosphere chemistry in an acidic subsoil

Seedlings of two cultivars of wheat (Triticum aestivum L.) differing in tolerance to aluminium (Al) were grown using a split-root sand/soil culture technique. Each culture tube was divided horizontally into a surface (0–150 mm) compartment and a subsurface (150–250 mm) compartment separated by a root-permeable paraffin wax barrier. Thus phosphorus (P) supplied to surface roots could not percolate or diffuse into the soil in the subsurface compartment. The soil in the subsurface compartment was divided into ‘rhizosphere’ and ‘non-rhizosphere’ zones using a porous (5 μm) membrane. Root growth of both cultivars into the subsurface zone was enhanced by increased P supply to surface roots, but did not conform to known relationships between root growth and soil pH, extractable-Al, or pH, Al or P concentrations in soil solution. Concentrations of Al in soil solution in the rhizosphere were greater than those in solution in the bulk soil. Concentrations of Al reactive with pyrocatechol violet (30s-RRAI) in the rhizosphere soil solution were generally greater than those in non-rhizosphere soil. With the Al-sensitive cultivar, root dry weight and length increased as concentrations of RRAl in the rhizosphere soil solution increased. Increased concentrations of Al in rhizosphere soil solutions were not related to the presence of organic ligands in solution. The effect of P in promoting root penetration into the acidic subsurface stratum was not related to differential attainment of maturity by the plant shoots, but appeared to be related to the effect of P in enhancing the rate of root growth. Thus, suboptimal supply of P to the surface roots of a plant, even at levels sufficient to preclude development of nutritional (P) stress symptoms, may seriously reduce tolerance to Al, and hence diminish the ability of roots to penetrate into acidic subsoils.     

Influence of root zone oxygen stress on potassium and ammonium absorption by Myrobalan plum rootstock

Summary Growth chamber experiments were conducted with French prune (Prunus domestica L.) scions grafted on Myrobalan 29C (P. cerasifera Ehrh.) rootstocks grown in nutrient solution to characterize K and NH₄ uptake before, during, and after anaerobiosis. Conditions of oxygen stress were imposed by removing the source of aeration and bubbling solutions with nitrogen gas.At solution oxygen concentrations less than 1%, K leaked from plant roots. After 18 h of anaerobic conditions, aeration was restored and K depletion from solution occurred within 2 h. Uptake of K the following day was similar to that before oxygen stress was imposed.Under similar conditions with solution oxygen concentrations less than 1%, both K and NH₄ uptake were inhibited. Potassium leakage from roots was significantly greater than that of NH₄. The presence of NH₄ had no significant effect on K leakage from roots. Signs of wilting during oxygen stress appeared first on those trees that received NH₄. Potassium uptake by rootstocks in the presence of NH₄ was inhibited prior to and following anaerobiosis. However, the extent of NH₄-induced inhibition of K uptake before anaerobiosis was similar to the K uptake inhibition after anaerobiosis.