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 (     

Dynamic measurements of roots hydraulic conductance using a high-pressure flowmeter in the laboratory and field

A new high-pressure flowmeter(HPFM)is described which is capableof rapid water-flow measurements. The HPFM permits dynamic determinationof hydraulic conductance of roots, K_r, and can be used in tehlaboratory or field. The base of a root is connected to theHPFM and water is perfused into the root system opposite tothe normal direction of flow during trnaspiration. The perfusionpressure is changed at a constant rate of 3–7 kPa s^–1while measuring the flow into the root every 2–4 s. Theslope of the plot of flow versus applied pressure is K_r. This paper describes the HPFM, presnents the theory of dynamicflow measurements, discusses sources of error, presnets evidencethat dynamic measurements of K_r in Ficus maclellandi (and sixother tropical species from Panama) yield the correct result,and demonstrates the use of the method under field conditionsin Panama on Cecropia obtusifolia and Palicourea guianensis. Key words: High-pressure flowmeter, root and shoot hydraulic conductance, Ficus maclellandi, Cecropia obtusifolia, Palicourea guianensis     

Radial Hydraulic Conductivity of Individual Root Tissues of Opuntia ficus-indica (L.) Miller as Soil Moisture Varies

The constraints on water uptake imposed by individual root tissueswere examined forOpuntia ficus-indicaunder wet, drying, andrewetted soil conditions. Root hydraulic conductivity (L_P) andaxial conductance (K_h) were measured for intact root segmentsfrom the distal region with an endodermis and from midroot witha periderm;L_Pwas then measured for each segment with successivetissues removed by dissection. Radial conductivity (L_R) wascalculated fromL_PandK_hfor the intact segment and for the individualtissues by considering the tissue conductivities in series.Under wet conditions,L_Rfor intact distal root segments was lowestfor the cortex; at midroot, where cortical cells are dead,L_Rforthe cortex was higher and no single tissue was the predominantlimiter ofL_R.L_Rfor the endodermis and the periderm were similarunder wet conditions. During 30d of soil drying,L_Rfor the distalcortex increased almost three-fold due to the death of corticalcells, whereasL_Rfor the midroot cortex was unaffected;L_Rforthe endodermis and the periderm decreased by 40 and 90%, respectively,during drying. For both root regions under wet conditions, thevascular cylinder had the highestL_R, which decreased by 50–70%during 30d of soil drying. After 3d of rewetting, new lateralroots emerged, increasingL_Rfor the tissues outside the vascularcylinder as well as increasing uptake of an apoplastic tracerinto the xylem of both the roots and the shoot. The averageL_Rforintact root segments was similar under wet and rewetted conditions,but the conductivity of the tissues outside the vascular cylinderincreased after rewetting, as did the contribution of the apoplasticpathway to water uptake. Opuntia ficus-indica; prickly pear; root hydraulic conductivity; endodermis; periderm; apoplast; lateral root emergence     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

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     

Hydraulic and osmotic properties of oak roots

Hydraulic and osmotic properties of root systems of 2.5–8-months-oldoak seedlings (Quercus robur and Q. petraea) were measured usingthe root pressure probe. Root pressures of excised roots rangedbetween 0.05 and 0.15 MPa which was similar to values obtainedfor herbaceous species. Root hydraulic conductivity (Lp_r; perunit of root surface area) was much larger in the presence ofhydrostatic than in the presence of osmotic pressure differencesdriving water flow across the roots. Differences were as largeas a factor of 20 to 470. Roots of the young seedlings of Q.robur grew more rapidly than those of Q. petraea and had a hydraulicconductivity which was substantially higher. Nitrogen nutritionaffected root growth of Q. robur more than that of Q. petraea,but did not affect root Lp_r of either species. For Q. robur,Lp_r decreased with root age (size) which is interpreted by aneffect of suberization during the development of fine roots.Root hydraulic conductance remained constant for both species.For Q. robur, this was due to the fact that the overall decreasein Lp_r was compensated for by an increase in root surface area.Root reflection coefficients (_sr) were low and ranged between_sr=0.1 and 0.5 for solutes for which cell membranes exhibitreflection coefficients of virtually unity (salts, sugars etc.).Solute permeability was small and was usually not measurablewith the technique. When root systems were attached to the rootpressure probe for longer periods of time (up to 10d), solutepermeability increased due to ageing effects which, however,did not cause a general leakiness of the roots as Lp_r decreased.Hence, values were only used from measurements taken duringthe first day. Transport properties of oak roots are comparedwith those recently obtained for spruce (Rdinger et al., 1994).They are discussed in terms of a composite transport model ofthe root which explains low root _sr at low solute permeabilityand reasonable rootLp_r The model predicts differences betweenosmotic and hydraulic water flow and differences in the transportproperties of roots of herbs and trees as found. Key words: Composite transport, hydraulic conductivity, nitrogen nutrition, Quercus, reflection coefficient, root transport, water relations     

Root Hydraulic Conductivity of Two Cactus Species in Relation to Root Age, Temperature, and Soil Water Status

The effects of root age, temperature, and soil water statuson root hydraulic conductivity (L_P) were investigated for twocactus species, Ferocactus acanthodes and Opuntia ficus-indica.The volumetric flux density of water was measured for excisedroot segments, either using negative hydrostatic pressures appliedto the proximal end or using reverse flow of water from theroot to the soil. For both species, L_P at 20 ?C increased withroot age, average values reaching a maximum of 3.9 ? 10^–7m s^–1 MPa^–1 for F. acanthodes and 5.2 ? 10^–7m s^–1 MPa^–1 for O.ficus-indica at 11 to 17 weeksof age; L_P subsequently declined with increasing root age forboth species. L_P was maximal at a temperature of about 10 ?Cfor the youngest roots (1–3 weeks), this optimum shiftingto 40 ?C for 8-week-old roots of both species. For older roots(up to 1.5-years-old), L_P increased with temperature from 0?C to 50 ?C, with a Q₁₀ of 1.3 between 20 ?C and 30 ?C. At asoil water potential (_soil) of –0.016 MPa, root L_P wasindependent of the direction of water flow for both species.Depending on root age, L_P declined 45- to 500-fold for F. acanthodesand 90- to 800-fold for O.ficus-indica as _soil was reduced from–0.016 to –1.06 MPa, consistent with a rectifier-likebehaviour with respect to water movement between soil and roots.Incorporation of such responses into water uptake models shouldlead to a better understanding of root function. Key words: Ferocactus acanthodes, Opuntia ficus-indica, water potential, tension, reverse flow     

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     

A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitationon hydraulic conductance are limited to unbranched stem or rootsegments with hydraulic conductances above c. 2 mmol s^–1MPa^–1. Lower conductances and/or highly branched systemsare encountered in seedlings, arid-land shrubs, herbs, and distalportions of shoot and root systems of trees. In order to quantifythe hydraulic impact of cavitation in such systems, existingtechniques have been modified. Branched shoot or root systemswere prepared for measurement by removal of leaves, or roottips, respectively. The shoot or root system was enclosed ina vacuum chamber with the proximal end protruding and suppliedwith perfusing solution. Flow through the xylem was inducedby chamber vacuum. Hydraulic conductance was determined fromthe slope of the flow rate versus pressure relationship. Xylemembolism was quantified from the increase in hydraulic conductancefollowing high pressure (100 kPa) perfusion of solution throughthe plant. Examples are provided of the application of the methodto cavitation studies in the cold desert shrub Artemisia tridentata. Key words: Hydraulic conductance, xylem cavitation, embolism, whole root/shoot system     

Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown inminus nitrogen culture were each partnered with a single seedlingof the root hemiparasite Olax phyllanthi. Partitioning of fixedN between plant organs of the host and parasite was studiedfor the period 4–8 months after introducing the parasite.N fluxes through nodules of Acacia and xylem-tapping haustoriaof Olax were compared using measured xylem flows of fixed Nand anatomical information for the two organs. N₂ fixation duringthe study interval (635 µg N g FW nodules^–1 d^–1)corresponded to a xylem loading flux of 0.20 µg N mm^–2d^–1 across the secretory membranes of the pencycle parenchymaof the nodule vascular strands. A much higher flux of N (4891µg mm^–2 d^–1) exited through xylem at the junctionof nodule and root. The corresponding flux of N from host xylemacross absorptive membranes of the endophyte parenchyma of Olaxhaustorium was 1.15 µg N mm^–1 d^–1, six timesthe loading flux in nodules. The exit flux from haustorium toparasite rootlet was 20.0 pg N mm^–1 d^–1, 200-foldless than that passing through xylem elements of the nodule.Fluxes of individual amino compounds in xylem of nodule andhaustorium were assessed on a molar and N basis. N flux valuesare related to data for transpiration and partitioning of Cand N of the association recorded in a companion paper. Key words: Olax phyllanthi, host-parasite relationships, N flux, Acacia, N₂ fixation     

Some Experimental and Theoretical Observations Concerning Mass Flow in the Vascular Bundles of Heracleum

The theory and practice of applying the thermodynamics of irreversibleprocesses to mass-flow theories is presented. Onsager coefficientswere measured on cut and uncut phloem and cut xylem strandsof Heracleum muntegazzimum. In 0.3 N sucrose + 1 mN KC1 theyare as follows. In phloem, L_EE = 5 ? 10–⁴ mho cm^–1,L_pE = 9 ? 10^–6 cm³ s^–1 cm^–2 volt^–1 cm,and L_PP = 0.16 cm³ s^–1 cm^–2 (J cm^–3)^–1cm. In uncut phloem strands L_EE is about 1 ? 10^–3 mhocm^–1. In xylem in 2 x 10^–3 N KCI, L_pp = 50 to 225,L_PE = 2 ? 10^–4, and L^EE = 4 ? 10^–3. The measurementsare tentative since the blockage of the sieve plates is an interferingfactor, but if they are valid they lead to the conclusion thatneither a pressure-flow nor an electro-kinetic mechanism envisaginga ‘long distance’ current pathway can be the majormotive ‘force’ for transport in mature phloem. Measurementsof biopotentials along conducting but laterally detached phloembundles of Heracleum suggest, nevertheless, that there may bea small electro-osmotic component of at least 0.1 mV cm^–1endogenous in the phloem.     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

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^–     

Shrinkage of Attached Roots of Opuntia ficus-indica in Response to Lowered Water Potentials--Predicted Consequences for Water Uptake or Loss to Soil

Attached 2-month-old roots of the succulent plant, Opuntia ficus-indica,shrank 0.4% radially during periods of maximal transpirationunder wet conditions. In contrast, reversible decreases in diameterof nearly 20% occurred for these roots as their ambient waterpotential () in the vapour phase decreased from –0.01to –10 MPa over 8 d, the changes being slightly more rapidat 40 °C than at 10 °C. Such substantial diameter changesbecame progressively less with root age, from a 43% decreasein diameter at 3 weeks to a 6% decrease at 12 months Root shrinkagewas slight when was decreased from –0.01 to –0.3MPa, the latter being similar to the root water potential.As was further decreased from –0.3 to –10 MPa,water movement out of cortical cells caused considerable rootshrinkage. The root hydraulic conductivity (L_p) decreased only30 to 60% for a change in from –0.01 to –10 MPacompared with a decrease of over 10⁶-fold for the soil hydraulicconductivity over this range. The overall conductivity of thesoil, the root-soil air gap, and the root was predicted to bedominated by L_p for _soil above –0.3 MPa. As simulated_soil decreased below –0.3 MPa, the root-soil air gap initiallybecame the primary limiter of water loss from the roots. Below–5 MPa for 1-month-old roots and below –2 MPa for12-month-old roots, the soil became the main limiter of waterloss. Thus, water uptake from wet soils apparently was mainlycontrolled by root properties Water loss to drying soils wascontrolled by the development of a root-soil air gap aroundshrinking roots during the initial phase of soil drying andby the reduction of the soil hydraulic conductivity at evenlower _soil. Root diameter, root hydraulic conductivity, root-soil air gap, soil hydraulic conductivity     

Agrobacterium rhizogenes-transformed roots of coffee (Coffea arabica): conditions for long-term proliferation, and morphological and molecular characterization

Background and Aims: The aims of this study were to set up proliferation conditionsfor hairy roots of Coffea arabica regenerated after transformationby Agrobacterium rhizogenes strain A4-RS, and to carry out themorphological and molecular characterization of hairy root clonesmaintained over the long term. Methods: Auxin supply, light conditions and sucrose concentration weremodified with the aim of establishing efficient root proliferationconditions. The morphological variability among 62 establishedhairy root clones was phenotyped by scanning the roots and analysingthe images using ‘whinRHIZO’ software procedures.PCR analysis of integration in transformed root cells of roland aux oncogenes from the T-DNA of the Ri plasmid was usedto study the molecular variability among clones. Key Results: Auxin supply was necessary to obtain and stimulate growth andbranching, and IBA applied at 0·5 µM was the mostefficient auxin. Significant differences were shown among the62 clones for total root length and for the percentage of fineroots. These variables were stable across subcultures and couldhence be used for efficient characterization of hairy root clones.The majority of hairy root clones (86 %) exhibited non-significantphenotype differences with non-transformed roots. Eight cloneswere significantly different from the non-transformed controlsin that they possessed a low proportion of fine roots. Two otherhairy root clones grew significantly faster than the other clones.The PCR analysis revealed a low variability in the integrationof rol and aux oncogenes in transformed root cells. The T_R-DNAwas never integrated as aux1 and aux2 genes were not found,although rolB and rolC genes from the T_L-DNA were always present. Conclusions: The discovery of low morphological variability among coffeehairy roots together with the identification of morphologicalvariables allowing easy identification of phenotypically alteredclones represent two important results. They make hairy rootsa possible, and efficient, tool for functional-genomic studiesof coffee root genes.     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Hydraulic Parameters Measured in 1-Year-Old Twigs of some Mediterranean Species with Diffuse-Porous Wood: Changes in Hydraulic Conductivity and Their Possible Functional Significance

We report measurements of hydraulic conductivity of Vitis oinifera,Oleo europaea and Populus deltoides 1-year-old twigs. Singleserial internodes were tested for the volume flow rate whichwas related to: (a) the xylem tissue cross-sectional area, (b)the vessel lumina cross-sectional area and (c) the leaf surfacearea supplied by a given stem section. From this, whole xylemhydraulic conductivity (L_x), vessel lumina hydraulic conductivity(L_xv) and leaf specific conductivity (LSC) were calculated.All the three parameters turned out to be linearly related toeach other. This is kcause: (a) the leaf surface area (A₁) waslinearly related to xylem cross-sectional area (A_x and (b) theratio of the vessel lumina cross-sectional area (A_xv) to xylemcross-sectional area (A_x) was approximately constant along thetwigs. Moreover, the hydraulic conductivity of twig segmentswhere buds grow most actively (distal internodes in V. viniferaand proximal ones in O. europaea) was much lower than in therest of the twigs. A possible role played by these ‘constricted’twig regions is discussed. Key words: Changes, Hydraulic conductivity, Stem     

The Relation Between the Nitrogen Concentration and Photosynthetic Capacity of Potato (Solanum tuberosum L.) Leaves

Measurements of the rate of light-saturated photosynthesis (P_max)were made on terminal leaflets of potato plants growing in cropssupplied with 0, 3, 6, 12, 24 and 36 g N m^–2. Measurementswere made between 100 and 154 d after planting. Two types ofleaf were selected—the fourth leaf on the second-levelbranch (L₄, _B1) and the youngest terminal leaflet that was measurable(L_YM). Later, the total nitrogen concentration of each leaflet(N_L) was measured. A linear regression between P_max and N_L,common to both leaf positions, explained 68.5% of the totalvariation. With L₄, _B1 leaves there was a significant improvementin the proportion of variation explained when regressions withseparate intercepts and a common slope were fitted to individualfertilizer treatments. These results suggest that an increasingproportion of leaf nitrogen was not associated with the performanceof the photosynthetic system with increasing nitrogen supply.This separation between nitrogen treatments was not as clearfor L_YM leaves. Stomatal conductance to transfer of water vapourwas neither influenced by leaf position nor directly by nitrogensupply. Rather conductance declined in parallel with the declinein photosynthetic capacity. Solanum tuberosum, potato, nitrogen, photosynthesis, stomatal conductance, leaf     

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