Influence of stem lacunar structure on gas transport: relation to the oxygen transport potential of submersed vascular plants

The influence of stem lacunar structure on the potential of diffusion and mass flow to meet estimated root O₂ demands was evaluated and compared in four submersed aquatic plant species. Internodal lacunae formed large continuous gas canals which were constricted at the nodes by thin, perforated diaphragms. Gas transport studies showed that nodes had little effect on diffusion, but significantly reduced mass flow. Measured diffusive resistances approximated those predicted by Fick's first law, ranged from 203 to 5107 × 10⁸ s m⁻⁴ and increased as lacunar area decreased in Potamogeton praelongus, two Myriophyllum species and Elodea canadensis. Our analysis suggested that diffusion could satisfy estimated root O₂ demands given the development of relatively steep O₂ gradients (0.15–0.35 mol O₂ mor⁻¹ per 0.5 m stem) between shoots and roots. Plants with high resistances (e.g. > 750 × 10⁸ s m⁻⁴) and long lacunar pathlengths may be unable, even during active photosynthesis, to support the O₂ demands of a large root system by diffusion alone. Measured nodal resistances to mass flow approximated those predicted by Hagen-Poiseuille law and ranged from 46 to 2029 × 10⁸ Pa s m⁻³. Our analysis suggested that these resistances were quite low and that relatively small pressure differentials (< 150 Pa per 0.5 m stem) could drive mass flow at rates which would support root O₂ demands. Possible mechanisms whereby plant architecture may serve to maintain these pressure differentials are proposed.     

Oxygen diffusion and dark respiration in aquatic macrophytes

Abstract. The supply of oxygen to respiring shoot tissue was investigated for three submerged macrophytes (Potamogeton crispus L., Egeria densa Planch, and Myriophyllum triphyllum Orchard). For all species, the response of oxygen uptake rates to the external O₂ concentration was a rectangular hyperbola over the range 0–5.0 × 10^?3m³ m^?3. However, the response pattern for material with water-infiltrated lacunar airspaces was non-hyperbolic over this range. The change in response was interpreted as an increased substrate (O₂) limitation, resulting from lower radial diffusion rates within the infiltrated material. Neither the uninfiltrated nor the infiltrated responses obeyed the linear and logarithmic formulae of the type observed for submerged macrophytes by earlier authors. These results suggest that the responses observed are affected by factors such as water velocity, internal restrictions to diffusion and the range of oxygen tensions investigated. Therefore, it is unlikely that one response formula can adequately account for the effects of oxygen concentration on submerged macrophyte oxygen uptake. The lacunar airspaces also represent a possible oxygen source for dark respiration. The consumption of oxygen from the airspaces was investigated by displacing the gas from the lacunae and measuring the subsequent increase in the rate of oxygen assimilation from the external liquid. Approximately 30% of the oxygen consumed by E. densa and P. crispus, and more than 40% of that consumed by M. triphyllum, was derived from the lacunar system. This O₂ supply is a consequence of the higher oxygen concentration in the lacunae than in the external medium, due to the low solubility of oxygen in water. Storage of photosynthetically-produced oxygen in the lacunae could not be identified during a light/dark transient, due to rate changes caused by the effects of light on the respiratory metabolism. However, O₂ partial pressure gradients artificially set up between the lacunae and water equilibrated within an hour, suggesting that excess oxygen would be lost to the water within this time.     

Stem hypertrophic lenticels and secondary aerenchyma enable oxygen transport to roots of soybean in flooded soil

Background and Aims

Aerenchyma provides a low-resistance O₂ transport pathway that enhances plant survival during soil flooding. When in flooded soil, soybean produces aerenchyma and hypertrophic stem lenticels. The aims of this study were to investigate O₂ dynamics in stem aerenchyma and evaluate O₂ supply via stem lenticels to the roots of soybean during soil flooding.

Methods

Oxygen dynamics in aerenchymatous stems were investigated using Clark-type O₂ microelectrodes, and O₂ transport to roots was evaluated using stable-isotope ¹⁸O₂ as a tracer, for plants with shoots in air and roots in flooded sand or soil. Short-term experiments also assessed venting of CO₂ via the stem lenticels.

Key Results

The radial distribution of the O₂ partial pressure (pO₂) was stable at 17 kPa in the stem aerenchyma 15 mm below the water level, but rapidly declined to 8 kPa at 200–300 µm inside the stele. Complete submergence of the hypertrophic lenticels at the stem base, with the remainder of the shoot still in air, resulted in gradual declines in pO₂ in stem aerenchyma from 17·5 to 7·6 kPa at 13 mm below the water level, and from 14·7 to 6·1 kPa at 51 mm below the water level. Subsequently, re-exposure of the lenticels to air caused pO₂ to increase again to 14–17 kPa at both positions within 10 min. After introducing ¹⁸O₂ gas via the stem lenticels, significant ¹⁸O₂ enrichment in water extracted from roots after 3 h was confirmed, suggesting that transported O₂ sustained root respiration. In contrast, slight ¹⁸O₂ enrichment was detected 3 h after treatment of stems that lacked aerenchyma and lenticels. Moreover, aerenchyma accelerated venting of CO₂ from submerged tissues to the atmosphere.

Conclusions

Hypertrophic lenticels on the stem of soybean, just above the water surface, are entry points for O₂, and these connect to aerenchyma and enable O₂ transport into roots in flooded soil. Stems that develop aerenchyma thus serve as a ‘snorkel’ that enables O₂ movement from air to the submerged roots.     

Oxygen loss from Spartina alterniflora and its relationship to salt marsh oxygen balance

Spartina alterniflora has been reported to lose significant amounts of oxygen to its rhizosphere with potentially important effects on salt-marsh biogeochemical cycling and plant productivity. The potential significance of this oxidative pathway was evaluated using laboratory split-chamber experiments to quantify oxygen loss from intact root systems under a wide variety of pre-treatment and incubation conditions including antibiotics to inhibit microbial respiration. The aerenchyma system of S. alterniflora was found to transport O₂, N₂, Ar, and CH₄ from above-ground sources to its below-ground roots and rhizomes. While non-respiratory gases were observed to move from the lacunae to water bathing the root systems, net O₂ loss did not occur; instead oxygen present outside of the roots/rhizomes was consumed. Net oxygen loss was found when resistance to gas transport was reduced in the lacunae-rhizosphere pathway by placing the root systems in a gas phase and when plant respiration was significantly reduced. Root system respiration appeared to be the major variable in the plant oxygen balance. When root and rhizome respiration was inhibited using poisons or lowered by cooling, the oxygen deficit was greatly reduced and oxygen loss was indicated. The effect of seasonal temperature changes on root system oxygen deficit presents a possible explanation as to why Spartina produces root systems with respiration rates that cannot be supported by gas transport. Overall, while oxygen loss from individual plant roots is likely, integrating measured root system oxygen loss with geochemical data indicates that the mass amount of oxygen lost from S. alterniflora root systems is small compared to the total oxygen balance of vegetated salt marsh sediments.     

Oxygen consumption by purified plasmalemma vesicles from wheat roots: Stimulation by NADH and salicylhydroxamic acid (SHAM)

Plasmalemma vesicles from wheat (Triticum aestivum L.) roots consumed O₂ and the addition of 1 mM NADH increased the rate ~ 3-fold (to 15-30 nmol O₂·mg⁻¹·min⁻¹). The NADH-dependent O₂ uptake was abolished by catalase. In the presence of salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase pathway in plant mitochondria, NADH-dependent O₂ consumption was stimulated 10–20-fold (to 200–400 nmol·mg^1&#x0304;·min⁻¹). Catalase also abolished this stimulation, which was KCN-sensitive but antimycin A-insensitive, and the production of H₂O₂ during SHAM-stimulated NADH-dependent O₂ uptake was demonstrated. Irrespective of the mechanism, SHAM-stimulated respiration by root plasmalemma makes it difficult to interpret results on root respiration obtained using KCN and SHAM.     

Oxygen fixation into hydroxyproline in etiolated maize seedlings : verification by tandem mass spectrometry

Etiolated maize (Zea mays L.) seedlings were grown in the dark for 5 days in an atmosphere enriched with 10.0 atom% ¹⁸O₂. Hydroxyproline was isolated from root and shoot tissues, purified, and methylated. It was not possible to determine ¹⁸O incorporation into hydroxyproline by conventional mass spectrometry because the final product was not sufficiently pure. The final product was analyzed successfully by tandem mass spectrometry. The ¹⁸O content of the hydroxyl oxygen atom was 10 ± 0.7 atom%. This result demonstrates that the hydroxyl oxygen atom in hydroxyproline was derived exclusively from molecular oxygen.     

Radial Oxygen Losses from Intact Rice Roots as Affected by Distance from the Apex, Respiration and Waterlogging

Radial oxygen losses (ROL) from the roots of intact rice plants were assayed by the cylindrical Pt electrode technique. At 23°C losses from roots grown in waterlogged soil proved to be about double those from non-waterlogged plants. Cooling which lowers respiratory activity led to increased ROL and it was estimated that at 23°C respiratory activity had been reducing oxygen loss by 8 to 10 10^–8g O₂ cm^–2 root surface min^–1 (c. 50 %) in the non waterlogged, and by 4.5 to 5.5 10^–8g O₂ cm^–2 min^–1 (2C–30 %) in the waterlogged roots. Lacunae formation occurred nearer to the apex and was eventually more extensive in the waterlogged roots while the presence of more intact and presumably functional tissue in the non-waterlogged roots coincides with the greater respiratory effect noted. Estimated flux rates at 23°C (respiration inactive) were respectively 15–17 × 10^–8g O₂ cm^–2 min^–1 (non-waterlogged) and 20–23 × 10^–8g O₂ cm^–2 min^–1 (waterlogged). A major part of this difference can probably be accounted for directly by the differences in root porosity, and Meakiness' superimposed upon lower porosity in the non-waterlogged plants may account for the remainder. ROL was also examined in relation to distance from the apex. With respiratory activity lowered by cooling, two patterns of oxygen loss were detected. Pattern I was a property of younger roots of length between 5–9 cm, while pattern 2 was found in longer roots 11–16 cm bearing numerous emergent laterals. In both, ROL fell rapidly towards the base and at 4–5 cm approached zero in pattern 1 and near zero to about 16% of the maximum in pattern 2. The rapid drop in oxygen loss in both patterns which indicates a concomitant decrease in root wall permeability was associated with the appearance of cortical lacunae at 2–3 cm from the apex. In pattern 2 a rise in ROL began at 5–6 cm from the apex. The presence of lateral root initials in both the pericycle and unbroken segments of cortex was associated with maintained permeability in this pattern as well as with the basal increase in ROL. With a 3-electrode system placed around the apical 3 cm regions of waterlogged roots, it was found that ROL was substantially affected by respiratory activity at 0.5 cm, a little less so at 1.7 cm, but much less or not at all at 3 cm from the apex. The drop in respiratory effect parallelled the formation of cortical lacunae.     

Oxygen distribution and movement,respiration and nutrient loading in banana roots (Musa spp. L.) subjected to aerated and oxygen-depleted environments

Excessive soil wetness is a common feature where bananas (Musa spp.) evolved. Under O₂ deficiency, a property of wet soils, root growth and functions will be influenced by the respiratory demand for O₂ in root tissues, the transport of O₂ from the shoot to root and the supply of O₂ from the medium. In laboratory experiments with nodal roots of banana, we examined how these features influenced the longitudinal and radial distributions of O₂ within roots, radial O₂ loss, solute accumulation in the xylem, root hydraulic conductivity, root elongation and root tip survival. In aerated roots, the stele respired about 6 times faster than the cortex on a volume basis. Respiratory O₂ consumption decreased substantially with distance from the root apex and at 300–500 mm it was 80% lower than at the apex. Respiration of lateral roots constituted a sink for O₂ supplied via aerenchyma, and reduced O₂ flow towards the tip of the supporting root. Stelar anoxia could be induced either by lowering the O₂ partial pressure in the bathing medium from 21 to 4 kPa (excised roots) or, in the case of intact roots, by reducing the O₂ concentration around the shoot. The root hair zone sometimes extended to 1.0 mm from the root surface and contributed up to a 60% drop in O₂ concentration from a free-flowing aerated solution to the root surface. There was a steep decline in O₂ concentration across the epidermal-hypodermal cylinder and some evidence of a decline in the O₂ permeability of the epidermal-hypodermal cylinder with increasing distance from the root apex. The differences in O₂ concentration between cortex and stele were smaller than reported for maize and possibly indicated a substantial transfer rate of dissolved O₂ from cortex to stele in banana, mediated by a convective water flow component. An O₂ partial pressure of 4 kPa in the medium reduced net nutrient transfer into the vascular tissue in the stele within 1 or 2 h. Hypoxia also caused a temporary decrease in radial root hydraulic conductivity by an order of magnitude. In O₂ deficient environments, the stele would be among the first tissues to suffer anoxia and O₂ consumption within the root hair zone might be a major contributor to root anoxia/hypoxia in banana growing in temporarily flooded soils.     

Oxygen and reactive oxygen species-dependent regulation of plant growth and development

Oxygen and reactive oxygen species (ROS) have been co-opted during evolution into the regulation of plant growth, development, and differentiation. ROS and oxidative signals arising from metabolism or phytohormone-mediated processes control almost every aspect of plant development from seed and bud dormancy, liberation of meristematic cells from the quiescent state, root and shoot growth, and architecture, to flowering and seed production. Moreover, the phytochrome and phytohormone-dependent transmissions of ROS waves are central to the systemic whole plant signaling pathways that integrate root and shoot growth. The sensing of oxygen availability through the PROTEOLYSIS 6 (PRT6) N-degron pathway functions alongside ROS production and signaling but how these pathways interact in developing organs remains poorly understood. Considerable progress has been made in our understanding of the nature of hydrogen peroxide sensors and the role of thiol-dependent signaling networks in the transmission of ROS signals. Reduction/oxidation (redox) changes in the glutathione (GSH) pool, glutaredoxins (GRXs), and thioredoxins (TRXs) are important in the control of growth mediated by phytohormone pathways. Although, it is clear that the redox states of proteins involved in plant growth and development are controlled by the NAD(P)H thioredoxin reductase (NTR)/TRX and reduced GSH/GRX systems of the cytosol, chloroplasts, mitochondria, and nucleus, we have only scratched the surface of this multilayered control and how redox-regulated processes interact with other cell signaling systems.

Oxygen and reactive oxygen species regulate plant growth, development, and differentiation through multiple interlinked signaling pathways.

Advances

• Developmentally regulated hypoxia and reactive oxygen species (ROS) production are key features of the stem cell niches, providing information about stem cell position, the environment, and metabolic state.
• Protein cysteine oxidation is central to oxygen and ROS signaling. However, S-nitrosylation, S-glutathionylation, S-sulfhydration, and S-sulfenylation modifications can occur on the same cysteine. The influence of each modification on stability, localization, and function remains unknown.
• Numerous intersecting ROS signaling pathways are probable and likely depend on the site of ROS production and the nature of the oxidized receptor protein. ROS sensors such as the hydrogen peroxide (H₂O₂)-INDUCED Ca²⁺ INCREASES 1 (HPCA1) leucine rich receptor kinase translate redox signals into protein modifications to regulate signaling cascades. H₂O₂ perception/transduction is dependent on thiol-dependent mechanisms policed by the ferredoxin/thioredoxin (TRX), NAD(P)H TRX reductase C (NTRC), reduced glutathione (GSH), and glutaredoxin (GRX) systems.
• ROS waves transmit redox signals from cell to cell in the apoplast, and probably through plasmodesmata. Long-distance transport of H₂O₂ and other ROS, therefore, appears to be unnecessary. Similarly, contact sites between organelles allow ROS transfer.
• Convergence points for oxygen and ROS signaling occur on proteins such as ROH OF PLANT 2 (ROP2) GTPase,RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), and TRX-h to regulate meristematic activity via TARGET OF RAPAMYCIN (TOR) kinase activity.

     

Role of seagrass photosynthesis in root aerobic processes

The role of shoot photosynthesis as a means of supporting aerobic respiration in the roots of the seagrass Zostera marina was examined. O₂ was transported rapidly (10-15 minutes) from the shoots to the root-rhizome tissues upon shoot illumination. The highest rates of transport were in shoots possessing the greatest biomass and leaf area. The rates of O₂ transport do not support a simple gas phase diffusion mechanism. O₂ transport to the root-rhizome system supported aerobic root respiration and in many cases exceeded respiratory requirements leading to O₂ release from the subterranean tissue. Release of O₂ can support aerobic processes in reducing sediments typical of Z. marina habitats. Since the root-rhizome respiration is supported primarily under shoot photosynthetic conditions, then the daily period of photosynthesis determines the diurnal period of root aerobiosis.     

Partitioning of oxygen consumption between “maintenance” and “growth” in developing herring Clupea harengus (L.) embryos

Growth and oxygen consumption was measured in developing herring Clupea harengus (L.) embryos. By considering the variations in oxygen consumption with embryonic size and growth rate, an attempt was made to partition oxygen consumption between growth related and growth unrelated (i.e., “maintenance”) processes. The metabolic cost of growth was estimated as ≈ 150 ng O₂ · μg dry wt tissue formed⁻¹. This estimate compares favourably with the biochemical estimate of the costs of transport and net biosynthesis. The “maintenance” component was proportional to embryonic mass (77 ng O₂ · μg⁻¹· d⁻¹). Over the entire embryonic period, growth processes were responsible for ≈ 25% of the cumulated oxygen consumption.     

Glucose transport across the intestinal wall of the freshwater snail Biomphalaria glabrata

• 1.1. Isolated midguts of the freshwater snail Biomphalaria glabrata were mounted in an incubation chamber in saline containing 2 mM glucose and perfused with the same solution. External and internal media were continuously gassed with carbogen gas (95% O₂, 5% CO₂). In order to measure the flux rates of glucose [¹⁴C]glucose was applied in the perfusion medium or in the incubation medium. Net fluxes of glucose were calculated as the differences between unidirectional in- and effluxes.
• 2.2. A directed net flux from the mucosal to the serosal side of the intestine was demonstrated (mucosal to serosal = 50 ± 10 nmol cm⁻²hr⁻¹(N = 6) serosal to mucosal 7 ± 1 nmol cm⁻²hr⁻¹ (N = 6), net flux = 43 nmol cm⁻²hr⁻¹).r
• 3.3. The active transport of glucose was reduced by the presence of metabolic inhibitors, cyanide (1 mM) and dinitrophenol (1 mM) on the mucosal as well as on the serosal side. Ouabain (1 mM) inhibited the transport rate only when it was added on the serosal side. Amiloride (1 mM) had no effect on the transport rate whether added on the mucosal or on the serosal side.
• 4.4. Inhibition of glucose transport by oubain, a specific inhibitor of Na⁺/K⁺-ATPase, suggests that glucose transport is secondary active and coupled to Na⁺-transport.

     

Xylem sap flow as a major pathway for oxygen supply to the sapwood of birch (Betula pubescens Ehr.)

The role of xylem sap flow as an aqueous pathway for oxygen supply to the wood parenchyma of Betula pubescens saplings was investigated. Using micro‐optode sensors the oxygen status of the sapwood was quantified in relation to mass flow of xylem sap. Sap flow was gradually reduced by an increasing oxygen depletion in the root space. The effect of sap flow on radial O₂ transport between stem and atmosphere was assessed by a stoichiometrical approach between respiratory CO₂ production and O₂ consumption. Restriction of sap flow set in 36.5 h after the onset of O₂ depletion, and was complete after 71 h. Interruption of sap flow drastically increased the O₂ deficit in the sapwood to 70%. Sap flow contributed about 60% to the total oxygen supply to the sapwood. Diurnal O₂ flow rates varied between 3 and 6.3 nmol O₂ m^?2 leaf area (LA) s^?1 during night‐ and daytime, respectively. Maximum O₂ flow rates of 20 nmol O₂ m^?2 LA s^?1 were reached at highest sap flow rates of 5.7 mmol H₂O m^?2 LA s^?1. Sap flow not only affected the oxygen status of the sapwood but also had an effect on radial O₂ transport between stem and atmosphere.     

Caspase-like enzymatic activity and the ascorbate-glutathione cycle participate in salt stress tolerance of maize conferred by exogenously applied nitric oxide

Salinity stress causes ionic stress (mainly from high Na⁺ and Cl^- levels) and osmotic stress (as a result of inhibition of water uptake by roots and amplified water loss from plant tissue), resulting in cell death and inhibition of growth and ultimately adversely reducing crop productivity. In this report, changes in root nitric oxide content, shoot and root biomass, root H₂O₂ content, root lipid peroxidation, root cell death, root caspase-like enzymatic activity, root antioxidant enzymatic activity and root ascorbate and glutathione contents/redox states were investigated in maize (Zea mays L. cv Silverking) after long-term (21 d) salt stress (150 mM NaCl) with or without exogenously applied nitric oxide generated from the nitric oxide donor 2,2′-(Hydroxynitrosohydrazano)bis-ethane. In addition to reduced shoot and root biomass, salt stress increased the nitric oxide and H₂O₂ contents in the maize roots and resulted in elevated lipid peroxidation, caspase-like activity and cell death in the roots. Altered antioxidant enzymatic activities, along with changes in ascorbate and glutathione contents/redox status were observed in the roots in response to salt stress. The detrimental effects of salt stress in the roots were reversed by exogenously applied nitric oxide. These results demonstrate that exogenously applied nitric oxide confers salt stress tolerance in maize by reducing salt stress-induced oxidative stress and caspase-like activity through a process that limits accumulation of reactive oxygen species via enhanced antioxidant enzymatic activity.     

Oxygen stress in Salvinia natans: Interactive effects of oxygen availability and nitrogen source

The ability of Salvinia natans (L.) All. to tolerate growth in oxic, hypoxic and anoxic nutrient solutions when supplied with either NH₄⁺ or NO₃^? were studied in the laboratory to test the hypothesis that inorganic N-source affects the response of the plants to O₂ deprivation. The relative growth rate (RGR) was significantly reduced in the anoxic treatment, but in the hypoxic treatment RGR was only slightly affected. The NH₄⁺ fed plants generally had a higher shoot to root ratio than the NO₃^? fed plants, and highest in the anoxic treatment. Plants had more roots and larger leaves when supplied with NH₄⁺ as compared with NO₃^?, particularly in the oxic treatment, and root length was most affected by O₂ deprivation for NO₃^? fed plants. Cell walls in the endodermis, the bundle sheath and the cortex adjacent to endodermis developed thickened sclerenchymatous walls when deprived of O₂, and more so in plants supplied with NO₃^?. Plants lost chlorophylls, had lower rates of photosynthetic electron transport (ETR_max) and lower quantum yields (Fv/Fm ratios) when grown in anoxic solutions, and the negative effects were mildest for NO₃^? fed plants suggesting that NO₃^? may be used as an alternative e^?-acceptor in non-cyclic electron transport in the chloroplasts. Overall S. natans grew best on NH₄⁺, but it also grew well on NO₃^?, and the O₂ stress symptoms differed somewhat between NH₄⁺ fed and NO₃^? fed plants. However, because N-form itself significantly influenced morphology and cell metabolism, it was impossible to conclusively identify the role of N-form for the O₂ stress reactions. S. natans is not well-adapted to grow in O₂ deficient waters and will not tolerate completely anoxic conditions as will prevail in waters receiving high loadings of organic pollutants such as livestock wastewater.     

The effect of temperature and oxygen on the CO2 compensation point of the marine alga Ulva lactuca

Abstract The CO₂ compensation point of Ulva lactuca frond sections has been measured in artificial seawater using a sensitive gas-chromatographic method. Under nitrogen the compensation point remained relatively constant at 3–6 cm³ m⁻³ at temperatures from 10 to 30°C while in air-saturated medium (0.3 kg m⁻³ O₂) the compensation point rose from 5 cm³ m⁻³ at 10°C to 11 cm³ m⁻³ at 30°C. These responses of the compensation point to temperature and oxygen concentration indicate that there is little photorespiratory CO₂ loss in this marine macroalga, and the low values of these compensation points indicate that inorganic carbon is actively accumulated by the plant.     

Segmental distribution in potentials of lateral root budding and oxygen uptake of plant hairy roots

The positional distributions in potential of lateral root budding and oxygen uptake rate were examined using the segments of madder and horseradish hairy roots with a length of 5.0×10⁻³ m obtained at different mean distances from the root tips of l=7.5×10⁻³–47.5×10⁻³ m. The average rate of lateral root budding and oxygen uptake rate of the roots with smaller l values were higher and both the rates gradually decreased with increase in l value. Positive relations were observed between the rates of lateral root budding and oxygen uptake of both the hairy roots. The relation indicated that the potential of lateral root budding was suppressed at the oxygen uptake rates of 0.15×10⁻⁵ and 0.32×10⁻⁵ mol O₂/(h m) for madder and horseradish hairy roots, respectively.     

NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2

The effect of elevated atmospheric CO₂ on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (¹H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO₂-enriched air at 650 cm³ m^?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO₂ (350 m³ m^?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO₂ does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO₂ environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.     

Root Growth and Oxygen Relations at Low Water Potentials. Impact of Oxygen Availability in Polyethylene Glycol Solutions

Polyethylene glycol (PEG), which is often used to impose low water potentials (ψ_w) in solution culture, decreases O₂ movement by increasing solution viscosity. We investigated whether this property causes O₂ deficiency that affects the elongation or metabolism of maize (Zea mays L.) primary roots. Seedlings grown in vigorously aerated PEG solutions at ambient solution O₂ partial pressure (pO₂) had decreased steady-state root elongation rates, increased root-tip alanine concentrations, and decreased root-tip proline concentrations relative to seedlings grown in PEG solutions of above-ambient pO₂ (alanine and proline accumulation are responses to hypoxia and low ψ_w, respectively). Measurements of root pO₂ were made using an O₂ microsensor to ensure that increased solution pO₂ did not increase root pO₂ above physiological levels. In oxygenated PEG solutions that gave maximal root elongation rates, root pO₂ was similar to or less than (depending on depth in the tissue) pO₂ of roots growing in vermiculite at the same ψ_w. Even without PEG, high solution pO₂ was necessary to raise root pO₂ to the levels found in vermiculite-grown roots. Vermiculite was used for comparison because it has large air spaces that allow free movement of O₂ to the root surface. The results show that supplemental oxygenation is required to avoid hypoxia in PEG solutions. Also, the data suggest that the O₂ demand of the root elongation zone may be greater at low relative to high ψ_w, compounding the effect of PEG on O₂ supply. Under O₂-sufficient conditions root elongation was substantially less sensitive to the low ψ_w imposed by PEG than that imposed by dry vermiculite.     

Changes in root system structure,leaf water potential and gas exchange of maize and triticale seedlings affected by soil compaction

The physiological reasons associated with differential sensitivity of C₃ and C₄ plant species to soil compaction stress are not well explained and understood. The responses of growth characteristics, changes in leaf water potential and gas exchange in maize and triticale to a different soil compaction were investigated. In the present study seedlings of triticale and maize, representative of C₃ and C₄ plants were subjected to low (L – 1.10 g cm⁻³), moderate (M – 1.34 g cm⁻³) and severe (S – 1.58 g cm⁻³) soil compaction level. Distinct differences in distribution of roots in the soil profile were observed. Plants of treatments M or S in comparison to treatment L, showed a decrease in leaf number, dry mass of stem, leaves and roots, and an increase in the shoot to root ratio. A drastic decrease in root biomass in M and S treatments in the soil profile on depth from 15 to 40 cm was observed. Any level of soil compaction did not influence the number of seminal and seminal-adventitious roots but decreased their length. The number and total length of nodal roots decreased with compaction. Changes of growth traits in M and S treatments in comparison to the L were greater for maize than for triticale and were accompanied by daily changes in water potential (ψ) and gas exchange parameters (P_N, E, g_s). Differences between M and S treatments in daily changes in ψ for maize were in most cases statistically insignificant, whereas for triticale, they were statistically significant. Differences in the responses of maize and triticale to soil compaction were found in P_N, E and g_s in particular for the measurements taken at 12:00 and 16:00. The highest correlation coefficients were obtained for the relationship between leaf water potential and stomatal conductance, both for maize and triticale, which indicates the close association between stomata behavior and changes in leaf water status.