Water Transport in Isolated Maize Roots

A simple model has been devised which predicts the concentration,C^x_s, of salt (e.g. KCl) in the exudate from isolated roots asa function of the salt concentration, C⁰_s, in the medium. Thechief assumption, made in deriving the relationship betweenC^x_s and C⁰₅, is that the exudation of water, J, from the rootsconsists of two components (one being osmotic, Ø, inorigin and the other, Ø⁰, flowing in the absence of anosmotic gradient). The exudation of salt, J_s, calculated asJ C^x_s, was found to be dependent on C⁰_s. Our investigationson maize roots were concerned with estimations of L_p and Ø^vand determinations of C^x_s as a function of C⁰_s. Satisfactoryagreement between prediction and experiment was found in thesepreliminary studies. It is considered that water movement inisolated roots can be explained by a simple osmotic model withthe additional possibility that a relatively small non-osmoticwater flow occurs.     

A Finite-element Model of Radial and Axial Conductivities for Individual Roots: Development and Validation for Two Desert Succulents

A morphologically explicit numerical model for analysing wateruptake by individual roots was developed based on a conductornetwork, with specific conductors representing axial or radialconductivities for discrete root segments. Hydraulic conductivity(L_p; m s^–1 MPa^–1) was measured for roots of Agavedeserti Engelm. and Opuntia ficus-indica (L.) Miller by applyinga partial vacuum to the proximal ends of excised roots in solution.L_p was also measured for 40- to 80-mm segments along a root,followed by measurements of axial conductivity and calculationof radial conductivity. Predicted values of L_p for entire rootsbased on two to ten segments per root averaged 1.04±0.07(mean±s.e. mean for n = 3) of the measured L_p for A.deserti and 1.06±0.10 for O. ficus-indica. The modelalso closely predicted the drop in water potential along theroot xylem (^xylem); when a tension of 50 kPa was applied tothe proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica,the measured ^xylem to midroot averaged 30 kPa compared witha predicted decrease of 36 kPa. Such steep gradients in ^xylemsuggest that the driving force for water movement from the soilto young distal roots may be relatively small. The model, whichagreed with an analytical solution for a simple hypotheticalsituation, can quantify situations without analytical solutions,such as when root and soil properties vary arbitrarily alonga root. Agave deserti, electrical circuit analog, hydraulic conductivity, Opuntia ficus-indica, water potential     

Water Influx Characteristics and Hydraulic Conductivity for Roots of Agave deserti Engelm.

Various plant and environmental factors influence the hydraulicproperties for roots, which were examined using negative hydrostaticpressures applied to the proximal ends of individual excisedroots of a common succulent perennial from the Sonoran Desert,Agave deserti Engelm. The root hydraulic conductivity, L^p, increasedsubstantially with temperature, the approximately 4-fold increasefrom 0.5°C to 40°C representing a Q¹⁰ of 1.45. Suchvariations in L_p with temperature must be taken into accountwhen modelling water uptake, as soil temperatures in the rootzone of such a shallow-rooted species vary substantially bothdaily and seasonally. At 20°C, L_p was 2.3 x 10^–7 ms^{macron}1MPa^{macron}1for 3-week-old roots, decreasing to abouthalf this value at 10 weeks and then becoming approximatelyhalved again at 6 months. For a given root age, L_p for rainroots that are induced by watering as lateral branches on theestablished roots (which arise from the stem base) was aboutthe same as L_p for established roots. Hence, the conventionalbelief that rain roots have a higher L_p than do establishedroots is more a reflection of root age, as the rain roots tendto be shed following drought and thus on average are much youngerthan are established roots. Unlike previous measurements onroot respiration, lowering the gas-phase oxygen concentrationfrom 21% to 0% or raising the carbon dioxide concentration from0.1% to 2% had no detectable effect on L_p for rain roots andestablished roots. L_p for rain roots and established roots wasdecreased by an average of 11% and 35% by lowering the soilwater potential from wet conditions (_soil=0 kPa) to {macron}40kPa and {macron}80 kPa, respectively. Such decreases in L_p mayreflect reduced water contact between soil particles and theroot surface and should be taken into account when predictingwater uptake by A. deserti. Key words: Gas phase, rain roots, root age, soil, temperature, water potential     

Shoot-Dependent Regulation of Sodium and Potassium Fluxes in Roots of Whole Barley Seedlings

Unidirectional fluxes and the cytoplasmic and vacuolar contentsof potassium and sodium in root cells of intact barley seedlings(Hordeum vulgare L., cv. Villa) were determined by use of compartmentalanalysis. In addition, the net vacuolar accumulation J_cv andthe xylem transport ø_cx of K⁺ and Na⁺ were measured.Both of these data were needed for the evaluation of the effluxdata. Fluxes and compartmental contents of K⁺ and Na⁺ were comparableto data obtained with excised roots. The effect of the shoot-to-rootratio—as varied by partial excision of the seedlings seminalroots—on the fluxes and contents was investigated. Highershoot-to-root ratios induced an increase in xylem transport,in plasmalemma influx, and also in the cytoplasmic content ofK⁺ and Na⁺. With potassium the plasmalemma efflux was almostunaltered while the tonoplast fluxes and vacuolar content weredecreased (in presence of Na⁺). With sodium, on the other hand,the plasmalemma efflux and the tonoplast fluxes were also increasedin the plants having one root and a high shoot-to-root ratio.These changes occurred even under conditions of low humidity,when transpiration was low and guttation occurred. The latterwas also increased at the high shoot-to-root ratio. The observedchanges could be due to a relieved feedback control of ion fluxesby the shoot and mediated in part by a relatively higher supplyof photosynthates in the plants having one root In addition,hormonal signals were suggested to participate. In particulara possibly decreased level of cytokinins in the plants havingonly one root could contribute to the signal. The observed changesappear to be responses of the plant to an alteration that canoccur under natural conditions when the root system is damaged.     

Temperature-Dependent Water and Ion Transport Properties of Barley and Sorghum Roots : II. Effects of Abscisic Acid

Water flux through excised roots (J_v) is determined by root hydraulic conductance (L_p) and the ion flux to the xylem (J_i) that generates an osmotic gradient to drive water movement. These properties of roots are strongly temperature dependent. Abscisic acid (ABA) can influence J_v by altering L_p, J_i, or both. The effects of root temperature on responses to ABA were determined in two species differing in their temperature tolerances. In excised barley (Hordeum vulgare L.) roots, J_v was maximum at 25°C; 10 micromolar ABA enhanced J_v, primarily by increasing L_p, at all temperatures tested (15-40°C). In sorghum (Sorghum bicolor L.) roots, J_v peaked at 35°C; ABA reduced this optimum temperature for J_v to 25°C by increasing L_p at low temperatures and severely inhibiting J_i (dominated by fluxes of K⁺ and NO₃⁻) at warm temperatures. The inhibition of K⁺ flux by ABA at high temperature was mostly independent of external K⁺ availability, implying an effect of ABA on ion release into the xylem. In sorghum, ABA enhanced water flux through roots at nonchilling low temperatures but at the expense of tolerance of warm temperatures. These effects imply that ABA may shift the thermal tolerance range of roots of this heat-tolerant species toward cooler temperatures.     

Pressure-Induced Water and Solute Flow Through Plant Roots

Water and salt flows through detopped sunflower, tomato andred kidney bean roots under applied pressure were studied usinga pressure chamber. Values of J_v for these root systems weremeasured applying variable pressure on the root medium, andL_p calculated. The K, Na and Cl fluxes under applied pressure were comparedwith those in intact plants at the same water flow rates. Tento 100 times higher Na and Cl fluxes were observed through detoppedroots under pressure as compared to those in the unpressurized,intact plants. It is suggested that the roots under pressureare not completely analogous to intact plant roots, and thatpressure-induced flow may not be a reliable method of determiningcharacteristics of ion flow in roots in relation to water flow. Key words: Volume flow, Hydraulic conductivity, K selectivity     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

大豆初生根凯氏带对铜离子的通透性

采用在根内生成有色铜沉淀的方法研究大豆(Glycine max)初生根凯氏带对铜离子的通透性。用真空泵抽取浓度为200 μmol·L^–1 的CuSO₄溶液进入根中, 然后在重力作用下从根基部灌注400 μmol·L^–1的K₄[Fe(CN)₆]溶液, 两种物质在根内相遇即可产生棕色的Cu₂[Fe(CN)₆]沉淀, 根据沉淀的位置来确定铜离子所经过的途径。结果表明: Cu²⁺可以穿过内皮层凯氏带, 在木质部导管壁以及凯氏带至木质部之间的细胞壁处产生棕色沉淀, 侧根发生的部位也产生了大量的沉淀; 当抽取K₄[Fe(CN)₆]溶液后再灌注CuSO₄溶液, 发现Cu²⁺仍然可以穿过凯氏带, 并在凯氏带外侧以及外皮层细胞的细胞壁处产生棕色沉淀。研究结果证明凯氏带并不是一个可以完全阻止离子进出的完美屏障。     

NaCl Uptake in Roots of Zea mays Seedlings: Comparison of Root Pressure Probe and EDX Data

The extent by which salinity affects plant growth depends partlyon the ability of the plant to exclude NaCl. To study the uptakeof NaCl into excised roots of Zea mays L. cv. ‘Tanker’,two different techniques were applied. A root pressure probewas used to record steady state as well as transient valuesof root (xylem) pressure upon exposure of the root to mediacontaining NaCl and KCl as osmotic solutes. In treatments withNaCl, pressure/time responses of the root indicated a significantuptake of NaCl into the xylem. NaCl induced kinetics were completelyreversible when the NaCl solution was replaced by an isosmoticKCl solution. This indicated a passive movement of Na⁺-saltsacross the root cylinder. Root samples were taken at differenttimes of exposure to NaCl and prepared for X-ray microanalysis(EDX analysis). Radial profiles of ion concentrations (Na⁺,K⁺, Cl^–) were measured in cell vacuoles and xylem vesselsalong the root axis. Na⁺ appeared rapidly in mature xylem (earlymetaxylem) and living xylem (late metaxylem) before it was detectablein vacuoles of the root cortex. EDX results confirmed that thekinetics observed by the pressure probe technique correspondedmainly to an influx of Na⁺-salts into early metaxylem. In latemetaxylem, the uptake of Na⁺ was associated with a decline ofK⁺. The Na⁺/K⁺ exchange indicated a mechanism to reduce sodiumfrom the transpiration stream. Ion localization, ion transport, maize, root pressure, salinity, water relations, X-ray microanalysis, Zea mays     

A Study of the Exudation of Excised Maize Roots after Removal of the Epidermis and Outer Cortex

The fluid exudation rates and the ionic composition of the exudatesof isolated maize roots were compared with those of similarroots from which the epidermal and outer cortical cells hadbeen removed. The results indicate that in low ionic strengthmedia the salt fluxes into the exudation stream are severaltimes larger in the control than in the treated roots. As theionic strength of the bathing medium increases to 10 mM KClor greater, the salt fluxes become equal. A mechanism to accountfor this behaviour is discussed. The values of L_p, the hydraulic conductivity of the roots toradial water flow, are 0.26 x 10^–6 cm sec^–1 atm^–1for treated and 0.14 x 10^–6 cm sec^–1 atm^–1for control roots, but a statistical technicality precludeda significance test on this apparent difference. The kinetics of tritiated water exchange from external mediumto the exudation fluid in both control and treated roots werenot significantly different. The entry of labelled water seemsto be achieved by the fluid exudation rate in both cases; thusthe barrier to water movement to the exudate compartment remainsintact after removal of the epidermis.     

Vacuolar Na/K Exchange, its Occurrence in Root Cells of Hordeum, Atriplex and Zea and its Significance for K/Na Discrimination in Roots

Using excised low-salt roots of barley and Atriplex hortenslsthe transport of endogenous potassium through the xylem vesselswas studied It was enhanced by nitrate and additionally by sodiumions which apparently replaced vacuolar potassium which wasthen available in the symplasm of root cells for transport tothe shoot Vacuolar Na/K exchange also has been investigatedby measurements of longitudinal ion profiles in single rootsof both species. In Atriplex roots a change in the externalsolution from K⁺ to Na⁺ induced an exchange of vacuolar K⁺ forNa⁺, in particular in the subapical root tissues and led toincreased K⁺ transport and loss of K⁺ from the cortex. In inverseexperiments a change from Na⁺ to K⁺ did not induce an exchangeof vacuolar Na⁺; merely in meristematic tissues Na⁺—apparentlyfrom the cytoplasm—was extruded in exchange for K⁺. Inroots of barley seedlings without caryopsis, as in excised roots,a massive exchange of K⁺ for Na⁺ was observed in the continuouspresence of external 1.0 mM Na and 0.2 mM K. This exchange alsowas attributed to the vacuole and was most pronounced in theyoung subapical tissues. It did not occur, however, in the correspondingtissues in roots of fully intact barley seedlings. In these,the young tissues retained a relatively high K/Na ratio alsoin their vacuoles. Similarly, contrasting results were obtainedwith intact and excised roots of Zea mays L. Based on theseresults a scheme of the events that lead to selective cationuptake in intact barley roots is proposed. In this scheme acrucial factor of selectivity is sufficient phloem recirculationof K⁺ by the aid of which K⁺ rich cortical cells are formednear the root tip. When matured these cells are suggested tomaintain a high cytoplasmic K/Na ratio due to K⁺ dependent sodiumextrusion at the plasmalemma and due to recovery of vacuolarK⁺ by Na/K exchange across the tonoplast. Key words: Potassium/Sodium selectivity, Vacuolar exchange, Xylem transport, Hordeum, Zea, Atriplex     

Effects of in Vivo Treatment with Abscisic Acid and/or Cytokinin on Activities of Vacuolar H+ Pumps of Tonoplast-Enriched Membrane Vesicles Prepared from Barley Roots

We investigated the effects of in vivo treatment (1 day) ofbarley roots with abscisic acid (ABA) and/or a cytokinin (6-benzyladenine;BA) on the ATP- and PP_i-dependent H⁺ transport activities oftonoplast-enriched membrane vesicles prepared from the roots.Treatment with ABA significantly increased the two H⁺ transportactivities. By contrast, treatment with BA significantly decreasedPP_i-dependent H⁺ transport activity, while the change in ATP-dependentH⁺ transport activity was small. Increases in the two H⁺ transportactivities caused by treatment with ABA were suppressed duringtreatment with ABA and BA. Changes in the NO^–-inhibitableATPase activity and the Na⁺-inhibitable PP_iase activity of membranevesicles after treatment of roots with phytohormone(s) (ABA,BA, ABA + BA) were similar to changes in the ATP- and PP_i dependentH⁺ transport activities of the membrane vesicles, respectively.Immunoblot analysis with antibodies raised against the functionalcatalytic subunits of the vacuolar H⁺ pumps (H⁺- ATPase andH⁺-PP_iase) of mung bean revealed that only the level of thefunctional catalytic subunit of the H⁺-PP_iase of the membranevesicles was significantly increased by treatment with ABA aloneand in combination with BA. These results suggest that treatmentwith ABA has a stimulatory effect on the activities of the twoH⁺ pumps of the vacuolar membrane of barley roots, with increasein the level of the catalytic subunit of the H⁺-PP_iase, andthat treatment with BA has an inhibitory effect on the two H⁺pump activities of the vacuolar membrane without changes inthe levels of the catalytic subunits of either H⁺ pump, withthe limitation that treatment with BA has an inhibitory effectonly when the activity of the H⁺-ATPase has been increased bytreatment with ABA. ³Present address: Department of Biology, Faculty of Science,Hirosaki University, Hirosaki, 036 Japan     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Diurnal Rb+ Transport from Roots to the Laminar Pulvinus in Phaseolus vulgaris L.

The primary leaves of kidney bean (Phaseolus vulgaris L.) openunder light and close in the dark by the deformation of thepulvinus resulting from diurnal distribution changes of K⁺,Cl^–, organic acid (or H⁺) and NO₃^–. When Rb⁺ was added as a tracer of K⁺ to the seedlings throughtheir roots, it was transported to the pulvinus cells duringthe light period but not during the dark period. Transpirationoccurred vigorously in the light but almost stopped in the dark.We concluded that Rb⁺ absorbed by the roots was carried to thepulvinus by the transpiration stream. Phaseolus vulgaris L., pulvinus, Rb⁺, diurnal transport transpiration stream     

Short-term Effects of Ferulic Acid on Ion Uptake and Water Relations in Cucumber Seedlings

Ferulic acid (FA) is commonly found in soils and is consideredan allelochemical. Studies have suggested that FA and otherphenolic acids decrease plant growth in part by decreasing theabsorption of mineral nutrients and water. However, no studieshave examined these parameters in a single experimental systemto investigate how FA affected both ion uptake and plant-waterrelations in whole plants. Using intact cucumber (Cucumis sativusL. cv. Early Green Cluster) seedlings, we examined short-termeffects of FA on ion uptake kinetics, transport promoters andinhibitors, and water relations as indicated by a pressure-volumeanalysis. We found that after 3 h of treatment, 200 µMFA inhibited net ion uptake, particularly NO₃, and promotednet K⁺ efflux from seedling roots. The addition of fusicoccin,a K⁺ transport promoter, counteracted the inhibitory effectof FA on net K⁺ uptake. Concurrent treatment of seedlings withFA and tetraethylammonium, a channel-blocking salt, reducedaverage K⁺ efflux by 66%. Treatment of seedlings with FA alsodecreased leaf water potential (₁ and turgor pressure (P_T).However, decreased ₁ and P_T were not caused by changes in theosmotic properties of the symplast or stomatal conductance.A decrease in water absorption is a likely explanation for theloss of P_T observed. The results of our experiments indicatethat both ion uptake and plant-water relations can be adverselyaffected by FA. Key words: Cucumis sativus, ferulic acid, allelopathy, ion uptake, water relations     

Hydraulic and osmotic properties of spruce roots

Hydraulic and osmotic properties of roots of 2-year-old Norwayspruce seedlings (Plcea abiea (L.) Karst) were investigatedusing different techniques (steady flow, pressure probe, andstop flow technique). Root pressures were measured using theroot pressure probe. Compared to roots of herbaceous plantsor deciduous trees, excised root systems of spruce did not developappreciable root pressure (-0.001 to 0.004 MPa or -10 to 40cm of water column). When hydrostatic pressure gradients wereused to drive water flows across the roots, hydraulic conductivities(Lp_r) were determined in two types of experiments: (i) rootpressure relaxations (using the root pressure probe) and (ii)steady flow experiments (pneumatic pressures applied to theroot system or xylem or partial vacuum applied to the xylem).Root Lp_r ranged between 0.2 and 810^–8m s^–1 MPa^–1(on average) depending on the conditions. In steady flow experiments,Lpr depended on the pressure applied (or on the flow acrossthe roots) and equalled (0.190.12) to (1.21.7)10^–8m s^–1 MPa^–1 at pressures between 0.2 and 0.4 MPaand (1.51.3)10^–8 m s^–1 MPa^–1 at appliedpressures between 0.8 and 1.0 MPa. When pressures or vacuumwere applied to the xylem, Lp_r values were similar. The hydraulicconductivity measured during pressure relaxations (transientwater flows) was similar to that obtained at high pressures(and water flows). Although there was a considerable scatterin the data, there was a tendency of the hydraulic conductivityof the roots to decrease with increasing size of the root system.When osmotic gradients were used to drive water flows, Lp_r valuesobtained with the root pressure probe were much smaller thanthose measured in the presence of hydrostatic gradients. Onaverage, a root Lp_r=0.01710^–8 was found for osmotic andLp_r=6.410^–8 m s^–1 MPa^–1 in correspondinghydrostatic experiments, i.e. the two values differed by a factorwhich was as large as 380. The same hydraulic conductivity asthat obtained in osmotic experiments using the pressure probewas obtained by the 'stop flow techniquel. In this technique,the suction created by an osmoticum applied to the root wasbalanced by a vacuum applied to the xylem. Lp_r values of rootsystems did not change significantly when measured for up to5 d. In osmotic experiments with different solutes (Na₂S0₄,K₂S0₄, Ca(NO₃)₂, mannitol), no passive uptake of solutes couldbe detected, i.e. the solute permeability was very low whichwas different from earlier findings on roots of herbs. Reflectioncoefficients of spruce roots (O were low for solutes for whichplant cell membranes exhibit values of virtually unity (     

Regulation of K+ Uptake and Transport to the Xylem in Barley Roots; K+ Distribution Determined by Electron Probe X-ray Microanalysis of Frozen-Hydrated Cells

Intact roots of young barley plants (Hordeum vulgare L. cv.Proctor) were induced to transport K⁺ to the xylem at rapidor slow rates. Roots were then rapidly frozen in liquid nitrogenand fractured in the zone 70 mm behind the root tip to givetransverse faces for electron probe microanalysis. With SEMvisualization, analyses were made over the cytoplasm and vacuole(or lumen) of 14 cells types along the root radius between theouter cortex and stele, with particular emphasis on the xylem,xylem parenchyma, and phloem. Data were recorded in the formof colour-coded maps and also quantitatively. For both typesof roots, K⁺ concentration was lower over the xylem and phloemthan in the remainder of the root. The concentration of K⁺ wasgreater in the vacuole than in the cytoplasm, while for P itwas the reverse. Significantly, in roots induced to transportK⁺ rapidly the concentration of K⁺ was low in the early maturingmetaxylem and protoxylem, and in the sieve tubes of the metaphloemand protophloem. The concentrations of K⁺ in various cell typesare discussed in relation to regulation of K⁺ loading of thexylem in long-distance ion transport. Key words: Ion transport, nutrient deficiency     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

The Effect of Cycloheximide on Uptake and Transport of lons by Sunflower Roots

The effect of cycloheximide on uptake and transport of saltsby sunflower roots was investigated. Treatment with cycloheximideresulted in a reduction of uptake and transport of K⁺ and NO₃^–to the xylem. Cycloheximide stimulated O₂ uptake and appearedto act as an uncoupler of oxidative phosphorylation. The implicationsof these results regarding the use of cycloheximide as a meansof distinguishing between uptake and transport components ofion movement to the xylem are discussed.     

Hydraulic Conductivity and Anatomy for Lateral Roots of Agave deserti During Root Growth and Drought-induced Abscission

Hydraulic conductivity (L_p), radial conductivity (L_R), axialconductance (K_h), and related anatomical characteristics forlateral roots of Agave deserti were investigated during rootgrowth and drought-induced abscission. The elongation rate oflateral roots averaged 5 mm d^–1 under wet conditions andwas reduced 95% by 17 d of drought (