A Comparison of the Uptake and Translocation of Some Organic Herbicides and a Systemic Fungicide by Barley: I. ABSORPTION IN EELATION TO PHYSICO-CHEMICAL PROPERTIES

A comparative study has been made of the uptake by and translocationfrom roots of intact barley plants of six herbicides and a systemicfungicide (four triazines, diuron, 2,4-dichloro-phenoxyaceticacid (2,4-D) and ethirimol). Relationships between uptake andtranspiration rate are discussed in the light of the physico-chemicalproperties of these compounds, notably their partition coefficientsin oil/water systems and their dissociation constants. Apartfrom 2,4-D, sorption of these compounds appears to be a passiveprocess. At pH4 the uptake of 2,4-D seems to be influenced bymetabolism; not only may the concentration of this compoundin the transpiration stream be considerably greater than thatin the medium surrounding the roots but absorption by rootsis markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonablywell with their partition coefficients in olive oil/water orn-dodecane/water systems; likewise the concentration in thetranspiration stream is greater for lipophilic than for lipophobicsubstances. Whereas the hydrogen ion and calcium concentrations of the ambientmedium appear to have no effect on the uptake of compounds withlow pK's, the uptake of those substances which protonate betweenpH4 and pH6 is affected by them. These findings are discussedfrom the viewpoint that the pathways of transport of lipophilicand lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots canlimit transport to shoots, there is no simple inverse correlationbetween the total concentration of the different substancesin the roots and that in the transpiration stream. This questionis discussed in a subsequent paper.     

Transpiration does not control growth and nutrient supply in the amphibious plant Mentha aquatica

Mentha aquatica L. was grown at different nutrient availabilities in water and in air at 60% RH. The plants were kept at 600 mmol m^?3 free CO₂ dissolved in water (40 times air equilibrium) and at 30 mmol m^?3 CO₂ in air to ensure CO₂ saturation of growth in both environments. We quantified the transpiration-independent water transport from root to shoot in submerged plants relative to the transpiration stream in emergent plants and tested the importance of transpiration in sustaining nutrient flux and shoot growth. The acropetal water flow was substantial in submerged Mentha aquatica, reaching 14% of the transpiration stream in emergent plants. The transpiration-independent mass flow of water from the roots, measured by means of tritiated water, was diverted to leaves and adventitious shoots in active growth. The plants grew well and at the same rates in water and air, but nutrient fluxes to the shoot were greater in plants grown in air than in those that were submerged when they were rooted in fertile sediments. Restricted O₂ supply to the roots of submerged plants may account for the smaller nutrient concentrations, though these exceeded the levels required to saturate growth. In hydroponics, the root medium was aerated and circulated between submerged and emergent plants to minimize differences in medium chemistry, and here the two growth forms behaved similarly and could fully exploit nutrient enrichment. It is concluded that the lack of transpiration from leaf surfaces in a vapour-saturated atmosphere, or under water, is not likely to constrain the transfer of nutrients from root to shoot in herbaceous plants. Nutrient deficiency under these environmental conditions is more likely to derive from restricted development and function of the roots in waterlogged anoxic soils or from low porewater concentrations of nutrients.     

Export of Abscisic Acid, 1-Aminocyclopropane-1-Carboxylic Acid,Phosphate, and Nitrate from Roots to Shoots of Flooded Tomato Plants (Accounting for Effects of Xylem Sap Flow Rate on Concentration and Delivery)

We determined whether root stress alters the output of physiologically active messages passing from roots to shoots in the transpiration stream. Concentrations were not good measures of output. This was because changes in volume flow of xylem sap caused either by sampling procedures or by effects of root stress on rates of whole-plant transpiration modified concentrations simply by dilution. Thus, delivery rate (concentration x sap flow rate) was preferred to concentration as a measure of solute output from roots. To demonstrate these points, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid, phosphate, nitrate, and pH were measured in xylem sap of flooded and well-drained tomato (Lycopersicon esculentum Mill., cv Ailsa Craig) plants expressed at various rates from pressurized detopped roots. Concentrations decreased as sap flow rates were increased. However, dilution of solutes was often less than proportional to flow, especially in flooded plants. Thus, sap flowing through detopped roots at whole-plant transpiration rates was used to estimate solute delivery rates in intact plants. On this basis, delivery of ACC from roots to shoots was 3.1-fold greater in plants flooded for 24 h than in well-drained plants, and delivery of phosphate was 2.3-fold greater. Delivery rates of abscisic acid and nitrate in flooded plants were only 11 and 7%, respectively, of those in well-drained plants.     

Effect on Transpiration and Water Uptake by Rapid Changes in the Osmotic Potential of the Nutrient Solution

The sudden changes in the rates of transpiration and water uptake which occurred when the osmotic potential of the nutrient solution surrounding the roots of young wheat plants was rapidly changed were studied. The transpiration was measured by the aid of the microwave hygrometer and the water uptake by a recording poto-meter specially built for this investigation. When the osmotic potential of the nutrient solution was rapidly increased by adding mannitol, there was a temporary transpiration increase. The maximum increase was greater but the total time of the temporary increase shorter when a higher mannitol concentration was used. The quantity of water transpired by the shoots due to the temporary transpiration increase seemed to be fairly constant irrespectively of the mannitol concentration. The water transport to the shoots was immediately reduced when the osmotic potential was rapidly increased. The immediate reduction was greater when a higher mannitol concentration was used. After the immediate reduction the rate of water transport increased without delay. When the osmotic potential of the nutrient solution was rapidly decreased by withdrawing mannitol there was a temporary transpiration decrease, and the water transport to the shoots was immediately increased. After this increase the rate of water transport started to decrease at once. When, however, the mannitol concentration had been 0.30 M or higher, the transpiration rate increased progressively, and the change of the rate of water transport was small. The results indicate that the primary effect of the rapidly changed osmotic potential is localized to the root surface. The rapidly reduced water transport to the shoots after adding mannitol brings about the temporary transpiration increase. The course of events after withdrawing mannitol is just the reverse to that when adding mannitol.     

Mineral nutrition and the water relations of plants

Summary When young tomato plants were transferred from nutrient solution to mineral-free water, reductions in transpiration, water content of the shoots and stomatal aperture were not accompanied by a reduction in the relative water content or an increase in the suction pressure of the leaves. The relative water content of the leaves was increased and the suction pressure was little affected.Following transfer of the plants to mineral-free water, the mineral content of the shoots and the osmotic pressure of expressed leaf sap were reduced. It was concluded that mineral salts were necessary for maintaining the osmotic pressure of the leaf cell sap and that this was achieved, at least in part, by maintaining the mineral concentration of the sap. The amount of water that could be taken up by leaves and their turgor pressure were related to the osmotic pressure of the sap and calculations of turgor pressure showed that it was less in the leaves of plants with their roots in mineral-free water than in the leaves of plants in nutrient solution.Evidence was obtained that in leaflets detached from plants with their roots in mineral-free water, stomatal closure could occur at a higher water content than in leaflets detached from plants in nutrient solution, indicating a further role of minerals in leaf water relations. It is suggested that this role may be related to the properties of the cell walls.     

Uptake and transport of lead by perennial ryegrass from flowing solution culture with a controlled concentration of lead

Summary Perennial ryegrass was grown in flowing solution culture in a glasshouse, and during February lead was added to the nutrient solution and held at a constant concentration; uptake and transport of lead were followed in conditions of low intensity daylight or higher intensity artificial light. Uptake of lead by the roots was most rapid during the first 4 days after addition to the nutrient solution. After this time there was a steady increase in uptake per g dry weight of root with plants grown in artificial light having a much higher rate of uptake than plants grown in daylight. Roots always contained more lead than the corresponding shoots and concentration was always greater in the roots than in the shoots. The concentration in both roots and shoots increased with time but that in plants grown in artificial light was higher than that in plants grown in daylight. Two phases of uptake were identified, an initial rapid phase which is probably an exchange phenomenon, and a slow sustained phase which may be under metabolic control. A lower proportion of the total lead taken up remained in the roots of plants grown in artificial light than in those grown in daylight. This difference may have resulted from differences in (i) the production of organic carriers and/or (ii) transpiration. re]19750930     

The Effect of Root Temperatures on Water and Sulphate Absorption in Intact Sunflower Plants

The effect of 15, 25, and 35°C root temperature on waterabsorption, transpiration, and sulphate uptake by the rootsand transport to the shoots of intact sunflower plants has beenstudied using 0.5, 5.0, and 50.0 mM sulphate concentrationsat two rates of transpiration induced (1) by light and low relativehumidity and (2) by darkness and high relative humidity. Root temperatures and sulphate concentrations did not significantlyaffect the water absorption and transpiration and both theseprocesses were approximately similar at the different treatments.There was a nearly twofold increase in water absorption andtranspiration in the light and low relative humidity as comparedto the dark and high relative humidity irrespective of the roottemperatures and sulphate concentrations. The A.F.S. uptake in the roots was found to be independent ofthe root temperatures, sulphate concentrations, and transpirationrates, and amounted to 15 to 21 per cent based on the root weight.Sulphate accumulation in the roots was not significantly influencedby the root temperatures at 0.5 and 5.0 mM sulphate concentrations,but nearly doubled with temperature at 50.0 mM sulphate concentrationof the external solution. The slow nature of accumulation ofsulphate, the high sulphate status of the experimental plants,and the short duration of the experiments are considered aslikely reasons for the absence of a clear effect of temperatureson accumulation of sulphate at the two lower concentrationsof the external solution. Effects of high concentration on permeabilityand metabolism of the cells are suggested as the reasons forthe decreased accumulation with an increase in temperature at50.0 mM sulphate concentration. Accumulation of sulphate inthe roots was not significantly influenced by the transpirationrates. Unlike root accumulation, sulphate transport to the shoots increasedwith increasing transpiration. However, a major part of thesulphate transport (70 to 75 per cent at 0.5 and 5.0 mM sulphateconcentrations and 80 to 85 per cent at 50.0 mM sulphate concentration)appeared to have occurred at the low transpiration. The similarityof this transport to the accumulation of sulphate in the rootsindicates that it was due to an active transport process sensitiveto root temperatures and sulphate concentrations. A low concentrationof sulphate in the xylem and an increased permeability of theroot cells to ion movement induced by an increased suction inthe xylem are considered as reasons for a small increase inthe sulphate transport at high transpiration rate. The evidencefor the existence of a barrier—probably endodermis—preventingthe passive diffusion of sulphate and sensitivity of the TranspirationStream Concentration to root temperatures and sulphate concentrationsfavour that the increased transport with increased transpirationwas due to an active process.     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

The Development of Waterlogging Damage in Young Wheat Plants in Anaerobic Solution Cultures

Anaerobic (anoxic) solution cultures were used to investigatethe effect of a restricted oxygen supply to roots on the developmentof symptoms of waterlogging damage in young wheat plants, especiallyeffects on growth and nutrient uptake by the shoots. Anaerobicconditions produced by bubbling solutions with oxygen-free nitrogengas caused premature senescence of the lower leaves, slowedshoot fresh weight accumulation, and arrested the growth ofthe seminal roots. However the shoot dry weight initially increasedabove that of the aerobic controls. Nutrient accumulation bythe shoot was severely inhibited by anoxia, the uptake of nitrate,phosphate, and potassium being more affected than that of calciumand magnesium. The calculated concentrations in the xylem streamof all these ions (except nitrate) were equal to, or less than,those in the external solution, suggesting that the slow butcontinuous accumulation of nutrients in the shoot could haveoccurred passively by the mass flow of solution across damagedroots in response to transpiration. Aerenchymatous nodal rootsextended into the anoxic solutions to a maximum length of 12cm but there were few produced, and the size of the root systemremained small and may have limited shoot growth. Inclusionof carbon dioxide (10 kPa partial pressure) in the nitrogengas stream had little additional effect on plants to that causedby anoxia alone. All the responses of wheat to the anaerobic solutions were similarto those observed previously in waterlogged soil, indicatingthat many of the early symptoms of waterlogging damage to wheatcan be caused simply by the direct effects of inadequate oxygensupply to the roots. The results are discussed in relation tocurrent views of the mechanisms contributing to waterloggingdamage to plants.     

Artificially Induced Water and Sulfate Transport through Sunflower Roots

A new experimental method is used to determine simultaneously the quantity and composition of the sap exuded by a detopped root system at the same time that a pressure deficit of desired magnitude can be applied to the stem stump. The technique was used in a study of the transport of radioactive sulfate through the roots of young sunflower plants placed on complete nutrient solutions labelled with ³⁵S. The complications by the time factor on the composition and rate of the sap stream in experiments of this type were observed and discussed. The time of detopping the roots was very critical as the conditions of sulfate transport were greatly changed some time after the excision. A rectilinear connection existed between the rate of sulfate transport in the sap and the water flow at sap flow velocities comparable with transpiration rates. When the transport of water was very slow, the rate of sulfate transport became constant and independent of the water stream. It was suggested that diffusion or water flow could act as motive force for the ion transport in some non-metabolic phase of transfer in the roots. The addition of 2,4-DNP to the test solution severely interfered with the water and sulfate transport conditions in the roots.     

Acquisition of N by external hyphae of an arbuscular mycorrhizal fungus and its impact on physiological responses in maize under drought-stressed and well-watered conditions

This study examined the uptake of nitrogen by external hyphae of an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck &; Smith) and its impact on physiological responses in maize plants subjected to well-watered or drought-stressed conditions. Plants were grown in compartmented boxes divided by a nylon mesh (40?μm) into a root compartment and a hyphal compartment. Maize plants (Zea mays cv. 'Tuxpeño sequia' selection cycle C0) were exposed to 2 weeks of drought 56 days after sowing. A ^[15]N tracer was applied as K^[15]NO_[3] to the hyphal compartment at a distance of 5?cm from the root compartment. Root and shoot samples were then analyzed for ^[15]N atom % excess (APE), glutamine synthetase (GS) activity, protein concentration and nutritional status. Evapotranspiration rate and stomatal resistance were monitored daily to determine the degree of drought stress. The APE values for AM shoots and roots were 32% and 33% higher than non-AM shoots and roots, respectively, under drought conditions. This provides clear evidence that the external mycelium of AM fungus transports considerable amounts of ^[15]NO_[3]^[– ]to the host plant under drought conditions. Drought-stressed AM roots had 28% higher GS activity, possibly as a consequence of higher hyphal acquisition of NO_[3]^[–] ions. Mycorrhizal colonization significantly increased the host plant P status regardless of soil moisture regime. In addition, the N status of drought-stressed AM shoots and roots was slightly higher than stressed non-AM shoots and roots. The improved nutritional status may assist AM plants to exploit available soil moisture more efficiently and to maintain higher leaf relative water content under moderate drought conditions.     

Short-term leaf elongation kinetics of maize in response to salinity are independent of the root

The essentiality of roots to the short-term responses of leaf elongation to salinity was tested by removing the roots of maize (Zea mays L.) from the shoots and comparing the initial short-term response of leaf elongation to that with intact plants. Eightday-old seedlings growing in solution culture were treated with 80 millimolar NaCl and their leaf elongation rate (LER) was monitored with a linear variable differential transformer connected to a computerized data aquisition system. Initially, LER of intact plants was sharply reduced by salinity, then rose rapidly to reach a new steady-state rate about 1.5 hours after salinization. The new steady-state rate of salinized intact plants was about 80% of the control rate. When the roots of nonsalinized plants were excised under the surface of the nutrient solution, excision did not disturb the steady-state LER. When these shoots were salinized, they responded in a manner nearly identical to that of intact plants, indicating that roots are not essential for the modulation of short-term LER of salt-stressed plants.     

Alleviation of Injury to Young Wheat Plants in Anaerobic Solution Cultures in Relation to the Supply of Nitrate and other Inorganic Nutrients

We have investigated how far the supply of nitrate and otherinorganic nutrients can alleviate the effects of anaerobic (anoxic)conditions around the roots of 13 d old wheat plants in solutionculture. Anaerobic treatments of 2 weeks duration to the totalroot system caused inhibition of growth of roots and shoots,interference in ion accumulation by shoots, and premature leafsenescence. These responses, and the recovery of plants uponreoxygenation of their roots, could be modified greatly dependingon the concentration of nitrate and other ions in the nutrientsolution supplied either during or before the anaerobic treatment.Improvements in shoot growth and lessening of leaf senescencedepended more on the shoot nutrient status than on the nitrateconcentration in the nutrient solution during the anaerobictreatment. The growth of roots in the anaerobic solution wasunaffected by changes in its ionic composition. A total alleviationof shoot injury was achieved by maintaining a single seminalroot in aerated solution containing the full complement of inorganicnutrients. This effect was reversed by omitting nitrate fromthe aerated solution. We suggest that although nitrate is unlikelyto act as an alternative electron acceptor to molecular oxygenwhen the latter is deficient it can, together with other ions,enter the shoots and there act as a nitrogen source for metabolism.We discuss the possibility that ions cross the oxygen-deficient,injured roots by mass flow thereby accounting for the requirementfor a large external concentration to maintain adequate suppliesto the shoot.     

Growth Regulators in Populus tremula IV. Apical Dominance and Suckering in Young Plants

Experiments with small plants of Populus tremula L. growing in solution culture indicate that polarly transported auxin is an important factor in the control of axillary bud growth. If the auxin supply from the growing apex is eliminated, the number of buds released is influenced by factors translocated in the transpiration stream from the roots. Suckers may be induced to develop from aspen roots, the age of which is six weeks or more. Removal of the growing apex and the axillary buds or stoppage of shoot growth by short day treatment were effective in inducing abundant suckering in small aspen plants. Some mature leaves had to be maintained, indicating the dependence of sucker formation on carbohydrate supply. These treatments are known to decrease auxin production in the shoots. Extraction and biological assay showed a decrease in the content of auxin in the roots as a consequence of removal of growing shoot parts. The results indicate that suckering in roots of intact aspen plants is prevented by auxin transported into the roots from growing shoot parts.     

Nutrient Uptake by Different Parts of the Intact Roots of Plants

An apparatus is described for studying the uptake of ions byshort segments of intact root systems grown in water culture. When the entire root systems of young cereal plants are suppliedwith o'I ppm, P or Sr the quantities of both ions accumulatedin segments 3–5 mm long, or translocated from them toother tissues, are considerably smaller than those which movelongitudinally in the cortex for short distances. This process,which is under metabolic control, causes ions to be releasedto the external solution from parts of the root a few mm distantfrom the site of entry. The contribution, to the nutrition of barley plants 3–4weeks old, of different parts of the root system has been investigated.Between seminal axes, nodal axes, and laterals total uptakeper unit length of root varies largely, though not entirely,with volume. The ratio in which phosphate and strontium areabsorbed is not constant throughout the root system, the absorptionof phosphate being relatively greater by laterals. Little translocationoccurs from the apical 3 mm of roots and the fraction of theabsorbed ions translocated to shoots from older root segmentsis considerably greater for nodal axes than for seminal axesor laterals. The significance of the distribution of absorbing power throughoutthe root system is considered in relation to the nutrition ofplants grown in soil, especially when the rate of diffusionto the root surface may limit nutrient uptake.     

Effect of Waterlogged Soil Conditions on the Production of Ethylene and on Water Relationships in Tomato Plants

The roots of tomato plants (Lycopersicon esculentum Mill., cv.Moneymaker) were exposed to low concentrations of oxygen bywaterlogging the soil or by growing the plants in nutrient solutionflushed with nitrogen gas. After 24 h, the rate of ethyleneproduction by the petioles, main stem, and shoot apex was increasedby 4–6-fold and the petioles developed epinastic curvatures.Removing the roots did not reproduce these responses. The amountsof ethylene produced by shoot tissues in response to physicalwounding was greatly increased by waterlogging the soil. The production of ethylene by roots was suppressed by the absenceof oxygen. When the roots were transferred back to an aerobicenvironment ethylene production quickly exceeded that observedin roots maintained continuously in aerobic conditions. The enhanced rate of ethylene production in the shoots occurredin the absence of increased water stress as measured with aleaf pressure chamber; leaf water potentials were increasedrather than decreased by waterlogging for 30 h or more. Thiswas associated with stomatal closure and reduced transpiration.Resistance to water flow through the plant increased as transpirationdecreased in response to waterlogging. However, at similar ratesof transpiration, resistance was normally lower in waterloggedplants than in controls.     

Lead uptake from solution by perennial ryegrass and its transport from roots to shoots

Summary The uptake of lead by roots and its transport to the shoots was examined with perennial ryegrass in solution cultures. Root uptake as measured by the decrease in concentration of lead in an aqueous solution containing 1 mg Pb/l as Pb (NO₃)₂ was rapid, almost complete, and unaffected by removing the shoots or killing the roots. Lead bound in the roots was not released by exchange with Ca or Ba ions. The distribution of lead within the plant was examined at intervals after a single, 3-day exposure to various levels of lead added to a nutrient solution. The total uptake, or lead burden, increased with increasing rates of addition and ranged from 281 to 9969 g/Pb per 3 plants. The proportion of the lead reaching the shoots at the first harvest (7 days after adding lead) was 3.5 to 22.7 per cent of total uptake, the lower value being for plants with the greatest burden. Transport to the shoots continued throughout the experimental periods of 21 and 28 days but did not exceed 28.9 per cent of total uptake. The concentration of lead in shoots at the first harvest ranged from 0.2 to 58.4 ppm and that in the corresponding roots from 5.5 to 5310 ppm. At later harvests, and after cutting, the concentration in the shoots decreased; an exception was in plants with the greatest lead burden. It is concluded that roots of actively growing ryegrass provide a barrier which restricts the movement of lead to the above-ground parts of plants, and so to animals or man.     

Changes in Transpiration, Net Carbon Dioxide Assimilation and Leaf Water Potential Resulting from Application of Hydrostatic Pressure to Roots of Intact Pepper Plants

Hydrostatic pressures varying from 0 to 6.0 bar were applied to roots of intact Capsicum annuum L. cv. California Wonder plants growing in nutrient solution and the rates of transpiration, and net CO₂ assimilation, apparent compensation point and leaf water potential measured. Increasing the pressure on the roots of plants with roots in solution with either -0.5 or -5.0 bar osmotic potential with 1 bar increments resulted in a decrease in transpiration. With the application of 1 or 2 bar pressure the rate of transpiration returned to near or above the original rate. An application of 3 or 4 bar pressure reduced the rate of transpiration of all plants. The transpiration of plants with roots in solution with -0.5 bar osmotic potential remained at the reduced rate for as long as these pressures were maintained. The transpiration of plants with roots in solution with -5.0 bar was only temporarily suppressed at these pressures. Changing the applied pressure from 3 or 4 bar to 0 resulted in a rapid increase in transpiration which lasted approximately 15 minutes. This was followed by a decrease in transpiration to a rate lower than before the pressure was applied. The pattern of response was similar for plants at low or high light intensity or at normal or low CO₂ concentrations. When leaf diffusive resistance was 6.0 s cm^?1 or greater, changes in net CO₂ assimilation were similar to those of transpiration. The apparent CO₂ compensation point increased as pressure was applied and decreased with a release in pressure. Leaf water potential increased with an increase in pressure and decreased with a decrease in pressure. The changes in leaf water potential were frequently but not always proportional to changes in pressure. It is postulated that the respouses noted were due to changes in resistance to flow of water from xylem terminals through the mesophyll cells and stomatal cavities to the atmosphere.     

Nutrient availability moderates transpiration in Ehrharta calycina

Transpiration-driven 'mass-flow' of soil-water can increase nutrient flow to the root surface. Here it was investigated whether transpiration could be partially regulated by nutrient status. Seeds of Ehrharta calycina from nine sites across a rainfall gradient were supplied with slow-release fertilizer dibbled into the sand surrounding the roots and directly available through interception, mass-flow and diffusion (dubbed 'interception'), or sequestered behind a 40-microm mesh and not directly accessible by the roots, but from which nutrients could move by diffusion or mass-flow (dubbed 'mass-flow'). Although mass-flow plants were significantly smaller than interception plants as a consequence of nutrient limitation, they transpired 60% faster, had 90% higher photosynthesis relative to transpiration (A/E), and 40% higher tissue P, Ca and Na concentrations than plants allowed to intercept nutrients directly. Tissue N and K concentrations were similar for interception and mass-flow plants. Transpiration was thus higher in the nutrient-constrained 'mass-flow' plants, increasing the transport of nutrients to the roots by mass-flow. Transpiration may have been regulated by N availability, resulting in similar tissue concentration between treatments. It is concluded that, although transpiration is a necessary consequence of photosynthetic CO(2) uptake in C(3) plants, plants can respond to nutrient limitation by varying transpiration-driven mass-flow of nutrients.