The Effect of Light Intensity and Relative Humidity on Growth Rate and Root Respiration of Plantago lanceolata and Zea mays

Plants of Plantago lanceolata L. and Zea mays L., cv. ‘Campo’were grown at two levels of light intensity. Especially in theroots, the rate of dry matter accumulation decreased at lowlight intensity. The carbohydrate content of both roots andshoots of P. lanceolata was not affected by light intensity.The relative contribution of SHAM¹-sensitive respiration, thealternative chain, to total root respiration of both P. lanceolataand Z. mays, was not affected by light intensity during thedaytime. The alternative pathway was somewhat decreased at theend of the dark period, but not in the root tips (0–5mm) where it still contributed 56% in respiration. It was, therefore,concluded that photosynthesis is not a major factor in regulationof root growth in the species investigated. To see whether the effect of light intensity on root growthrate was via transpiration, plants of Z. mays were grown atdifferent air humidities. Both high humidity and low light intensityaffected the root morphology in such a way that the distancebetween the apex and the first laterals on the primary rootaxis increased. It is suggested that this effect on root morphologyis due to transpiration and the subsequent removal of root-producedinhibitors of lateral root growth; although light intensityalso affected the rate of dry matter accumulation of roots andthe rate was not affected by the humidity of the air. It is,therefore, concluded that the effect of light intensity on therate of dry matter accumulation of roots of Z. mays is not viaan effect on transpiration.     

Rapid decarboxylation of the products of dark fixation of CO2 in roots of pisum and Plantago

Seedlings of Pisum sativum and excised roots of Plantago major and P. lanceolata were given, in the dark, a pulse of ¹⁴CO₂ in air followed by a chase in ¹²CO₂ in air. A very substantial proportion of the ¹⁴C fixed into organic compounds in the pulse was lost from the tissues in the chase. The activity of NAD malic enzyme in extracts of roots of all three species exceeded their rate of respiration. Azide, 2-n-butylmalonate, and salicylhydroxamic acid each inhibited CO₂ fixation by excised roots of pea. The first two compounds inhibited respiratory gas exchange, but the third stimulated it. Arguments are presented for the widespread diversion of phosphoenolpyruvate from glycolysis to oxaloacetate and thence to malate in the cytosol followed by transport of the malate into the mitochondria for conversion to pyruvate via NAD malic enzyme. No differences, in the above respects, were found between the two species of Plantago.     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

Lipid Composition of the Roots of Plantago Species: Response to Alteration of the Level of Mineral Nutrition and Ecological Significance

In a study of the adaptation of Plantago species to their specific environment the lipid composition of the roots of several species: Plantago major L. ssp. major, Plantago major L. ssp. pleiosperma Pilger, Plantago lanceolata L., Plantago media L., Plantago maritima L., Plantago coronopus L. was studied as well as the effect of the nutritional regime. Upon exposure to low-salt conditions Plantago major L. ssp. major L. and Plantago maritima L. maintained the level of free sterols in the roots, despite a depressed level of total sterols, and the root lipids were more saturated than under high-salt conditions. Both factors may reduce nutrient leakage from the roots to the low-salt condition. Upon exposure to low-salt conditions, all Plantago species showed a decreased level of galactolipid (exception: Plantago coronopus) and a decreased level of sitosterol (exception: Plantago maritima); the latter being compensated by an elevated level of cholesterol + tocopherol in Plantago major ssp. pleiosperma, Plantago lanceolata and Plantago media. Plantago coronopus was the only species which under low-salt conditions showed an increased level of free sterols, among which cholesterol was the most important; thus indicating a high degree of regulation of membrane permeability under alternating nutritional conditions. The level of sulfolipid was kept constant in all Plantago species, with the highest level observed in Plantago maritima. The role which various lipids may play in maintenance of membrane integrity under alternating nutritional conditions is discussed.     

Influence of chromium(lll) on root-associated Fe(lll) reductase in Plantago lanceolata L.

The application of low chromium concentrations (5 M CrCI₃)decreased chlorophyll content of iron-deficient plants, butdid not cause visual toxicity symptoms in iron-sufficient plants.No significant plant growth differentials were obtained in responseto the various treatments. Chromium application to iron-deficientPlantago lanceolata L. roots increased the activity of root-associatedFe(lll) reductase. This effect was evident only with acceptorsof the turbo reductase and was not observed in iron-sufficientplants. Chromium accumulated in the roots, but was poorly transportedto the leaves. In split-root experiments, which allowed onlya part of the root system to receive chromium, while the otherportion was grown in iron-free medium, roots subjected to eithertreatment showed an intermediate FeEDTA reductase activity withrespect to non-split control plants. It is concluded that amobile factor from the leaves modulates the degree of expressionof the iron stress response. Possible mechanisms by which chromiumaffects Fe metabolism are discussed. Key words: Plantago lanceolata, ferric iron reduction, chromium, iron nutrition, shoot-to-root communication     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

Establishment of Plantago lanceolata L. and Plantago major L. among grass

Summary Establishment of Plantago lanceolata and P. major ssp major among grass was studied in a field experiment in which survival and selection on date of seedling emergence and plant size was investigated in relation to the vegetation structure. P. major — in contrast to P. lanceolata — was not able to establish itself in grass because of its lower competitive ability caused by later germination, smaller seedling size, and shorter leaves. In both species there was selection for early germination. For P. lanceolata a significant correlation was found between the strength of selection and the light climate, determined by the structure of the grass sward. Plants that germinated early were at an advantage because they were larger, especially the leaves, when compared with plants that germinated late. It seems likely that selection was mainly by competition for light. Contrary to expectation P. major-seedlings had a higher shade tolerance than those of P. lanceolata. The performance of both species is discussed in relation to their different life strategies.Grassland species research group publication no 142     

Distribution of Radioactivity from Exogenously Supplied [1-14C]Indol-3-yl-acetic Acid and [3, 4-3H (N)] Gibberellin A, in Geotropically-stimulated Picea abies (L.) Karst. Roots

The distribution of exogenously-supplied radioactive labelledindol-3-yl-acetic acid (IAA) and gibberellin A₁ (GA₁) in geotropicallystimulated roots of Norway spruce (Picea abies (L.) Karst.)has been demonstrated. Seedlings were positioned with theirroot tips in 2.1 x 10^–6 M [¹⁴C]IAA or 1.3 x 10^–8m ³H-GA₁ for 4 and 20 h, respectively. After geotropic stimulationfor 90 min in the horizontal position the root tips were cutlongitudinally in 50 µm thick sections, using a freeze-microtome.The radioactivity in the ¹⁴C-IAA treated roots occurred in higherconcentration in the lower than in the upper halves (ratio 1.25:1). A similar trend was observed in the [³H]GA₁-treated rootswhere the ratio lower: upper halves was 2.04: 1. The ratio ofradioactivity in right and left halves of vertical roots wasapproximately the same in roots supplied with [¹⁴C]IAA and [³H]GA₁(1.09: 1). The supplied radioactive compounds were analysed chromatographicallyafter extraction in methanol of 6 mm apical root segments. Onlya small fraction (7–8 per cent) of the supplied [¹⁴C]IAAwas revealed unchanged in the segments. The major part of thechromatographed, labelled compound has not been identified,but on basis of its R_F value it is suggested that it may beindol-3-acetyl-aspartic acid (IAAasp). The chromatographic analysis of the [³H]GA,-treated segmentsshowed that only small fractions of this gibberellin has beenconverted to other compounds. These results have been discussed and correlated with knowledgeof plant growth regulators and their participation in root geotropism. Picea abies, spruce, geotropism, gibberellin A₁, indol-3-yl-acetic acid, growth regulators, redistribution in roots     

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna     

The Specificity of the Carrier-Mediated Uptake of ABA by Root Segments of Phaseolus coccineus L.

Earlier work has established that the saturable component ofuptake of RS-[2¹⁴C]ABA by bean (Phaseolus coccineus L. cv. Prizewinner)root segments can be attributed to the action of a carrier.We now show that the carrier-mediated uptake is unaffected byRS-2-trans-ABA and lunularic acid and the unnatural R-ABA alsoappears to be ineffective. The specificity for S-ABA requiresthe halving of the K_m value for ABA determined previously (2.6mmol m_-3 for RS-; 1.3 mmol m_-3 for S-ABA). The RS-1', 4'-cis-dioland RS-1'-deoxy ABA reduce the uptake of RS-[2-₁₄C]ABA aboutas strongly as does unlabelled ABA, the K¹ for 1'-deoxy ABAwas similar to the K_m for ABA. The K₁ for RS-1', 4'-trans-diolwas 15.7 mmol m_-3. Consideration of the stereochemistry of thesecompounds suggests that the face of the ring of ABA away fromthe 1'-hydroxyl group interacts with the carrier site. Labelled material diffused out of undamaged root surfaces whichhad absorbed RS-[³H]ABA through an apical cut, suggesting thatABA is present in the apoplast. A simplified hypothesis is presented that can account for polartransport of ABA based on a gradient of a carrier in a tissuebut where the carrier is distributed uniformly on the apicaland basal ends of each cell. Key words: Uptake carrier, Abscisic acid, 1', 4'-Diol, Lunularic acid, Phaseolus coccineus, Polar-transport, Deoxyabscisic acid     

14CO2 Pulse-Chase Labelling in Clusia minor L.

Conditions and maintenance of growth were chosen so that plantsof Clusia minor L. were obtained which showed the C₃- and CAM-modes of CO₂-exchange, respectively. C. minor is known to accumulateconsiderable amounts of citric acid in addition to malic acidduring the dark-phase of CAM. ¹⁴CO₂-pulse-chase experiments were performed with these plants.Patterns of labelling during the pulse and redistribution oflabel during the chase in the C₃-mode were as expected for C₃-photosynthesis.Pulse-labelling in the CAM-mode during the last hour of thelight period, during the first part of the dark period and duringthe last hour of the dark period always led to an almost exclusiveincorporation of label into malate. Redistribution of labelfrom malate after the pulse at the end of the dark period duringthe chase in the subsequent light period followed the patternexpected for light-dependent reassimilation of CO₂ remobilizedfrom malate in CAM during the light period. During the chasesin the dark period, label was transferred from ^l4C-malate tocitrate. This suggests that during accumulation of citric acidin the dark period of CAM in C. minor, citrate is synthesizedin the mitochondria from malate or oxaloacetate after formationof malate via phosphoenolpyruvate carboxylase. The experiment also showed that no labelled compounds are exportedfrom leaves in the CAM-mode during the dark period. In plantsof the C₃-mode the roots proved to be strong sinks. Key words: Clusia minor, labelling, pulse-chase, ¹⁴CO₂     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

Trans-Root Electrical Potential in Roots of Plantago media L. as Affected by Hydrostatic Pressure: The Induction of an O2 Deficient Root Core

Excised roots of Plantago media L. were subjected to small hydrostaticpressure gradients. Pressure applied with a displacement pumpeasily extracted gas present in cortical lacunae consequenton the gas saturation deficit of the bathing solution. The temporarypressure treatment resulted in an irreversible decrease in theO₂ diffusion rate through the root tissue and consequently inO₂ deficient root cores. Therefore electrogenic pumps located at the symplast/xylem interface,i.e. in the centre of the root, were inhibited by pressure. The longitudinal distribution of the xylem pumps could be computed:in Plantago media they are located in the basal parts of theroot. The effect of pressure on the electrogenic xylem pumpsdemonstrated here should be taken into account when the processof ion transport from the root to the shoot is studied whilethe root is pressurized. (Received December 5, 1983; Accepted March 26, 1984)     

The Growth and Physiology of Pinus strobus L. Seedlings as Affected by Various Nutritional Levels of Nitrogen and Phosphorus

Potted white pine (Pinus strobus L.) seedlings were grown ingravel either in outdoor cold frames or in growth chambers.They were watered every second day with a salt solution containingdifferent amounts of nitrogen and phosphorus. After 13 weeksof growth individual seedlings were illuminated separately for8 h in the presence of ¹⁴CO₂ and the rates of their apparentphotosynthesis, respiration, and translocation of recent ¹⁴C-photosynthateto their roots were observed. Roots were extracted with 80 percent ethanol and the nature of various ¹⁴C compounds in theextract was determined by paper chromatography. The best over-all growth of plants, mycorrhizal development,apparent photosynthesis, and translocation of recent photosynthateto the roots were observed in plants grown at the intermediatelevels of N and P nutrition. Sucrose was always the dominant form in which recently translocated¹⁴C occurred in the roots, although with increased nitrogensupply there was increased hydrolysis of sucrose to hexosesand appearance of ¹⁴C in the amino- and organic acids.     

Influence of restoration on arbuscular mycorrhiza of Biscutella laevigata L. (Brassicaceae) and Plantago lanceolata L. (Plantaginaceae) from calamine spoil mounds

The arbuscular mycorrhizal status of two plant species, Biscutella laevigata L. and Plantago lanceolata L., was investigated on calamine spoil mounds in Boles^aw (southern Poland). Although B. laevigata is a member of the Brassicaceae, a family generally accepted as non-mycorrhizal, this species formed AM symbioses on both heavy metal-contaminated and non-contaminated sites. Besides vesicles and coils, arbuscules were also observed, especially in roots collected prior to seed maturity. Relative mycorrhizal root length and relative arbuscular richness were usually much higher in P. lanceolata than in B. laevigata but not absolute arbuscule richness. Roots of P. lanceolata showed higher colonisation than B. laevigata. Although roots were collected from plants in close proximity, no correlation in mycorrhizal parameters was found between the two species.     

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     

Sulphate supply and its regulation of transport in roots of a tropical legume Macroptilium atropurpureum cv. Sirato

During a period of sulphate deprivation, roots of Macroptiliumatropurpureum responded by increasing their uptake capacityat the plasma membrane. This effect was apparent both in intactplants and in tissues excised prior to uptake. In experiments using excised root systems previousy labelledwith ³⁵SO₄^2- the rate of tracer transport to the xylem was muchgreater in roots subsequently deprived of external sulphatethan in those supplied with unlabelled sulphate. Removing theexternal sulphate to the external solution. Additionally, compartmentalanalysis of tracer exchange kinetics showed that the flux ofsulphate from the cytoplasm to the xylem(     

Synthesis of L(+) Tartaric Acid from 5-Keto-D-gluconic Acid in Pelargonium

5-Keto-D-[1-¹⁴C]gluconic acid, the most effective precursorof L(+)tartaric acid among all labeled compounds which haveever been tested in grapes, was found to be a good precursorof L(+)tartaric acid in a species of Pelargonium. The synthesisof labeled L(+)tartaric acid from D-[1-¹⁴C]glucose in Pelargoniumwas remarkably depressed when a 0.5% solution of D-gluconateor 5-keto-D-gluconate was administered continuously to leavestogether with D-[1-¹⁴C]glucose. Our results provide strong evidence that D-[1-¹⁴C]glucose ismetabolized in Pelargonium to give labeled L(+)tartaric acidvia (probably D-gluconic acid and) 5-keto-D-gluconic acid withoutpassing through L-ascorbic acid. Labeled L-idonic acid was found in young leaves of Pelargoniumwhich had been labeled with L-[U-¹⁴C]ascorbic acid. The synthesisof the labeled L-idonic acid increased when a 0.1% solutionof L-threonate was administered continuously to leaves togetherwith L-[U-¹⁴C]ascorbic acid. Specifically labeled compounds, recognized as the members ofthe synthetic pathway for L(+)tartaric acid from L-ascorbicacid via L-idonic acid in grapes, were administered to youngleaves of Pelargonium. Each compound (2-keto-L-[U-¹⁴C]idonicacid, L-[U-¹⁴C]idonic acid, 5-keto-D-[1-¹⁴C]gluconic acid and5-keto-D-[6-¹⁴C]gluconic acid) was partly metabolized, as ingrapes. The metabolic pathway starting from L-ascorbic acidto L(+)tartaric acid via L-idonic acid, however, did not actuallycontribute to the synthesis of L(+)tartaric acid in Pelargoniumprobably because the activity of each metabolic step was muchlower than that observed in grapes. (Received May 28, 1984; Accepted July 30, 1984)     

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 comparison of nitrate transport in four different rice (Oryza sativa L.) cultivars

As rice can use both nitrate (NO₃^-) and ammonium (NH₄⁺), we have tested the hypothesis that the shift in the pattern of cultivars grown in Jiangsu Province reflects the ability of the plants to exploit NO₃^- as a nitrogen (N) source. Four rice cultivars were grown in solution culture for comparison of their growth on NO₃^- and NH₄⁺ nitrogen sources. All four types of rice, Xian You 63 (XY63), Yang Dao 6 (YD), Nong Keng 57 (NK) and Si You 917 (SY917), grew well and produced similar amounts of shoot biomass with 1 mmol/L NH₄⁺ as the only N source. However, the roots of NK were significantly smaller in comparison with the other cultivars. When supplied with 1 mmol/L NO₃^-, YD produced the greatest biomass; while NK achieved the lowest growth among the four cultivars. Electrophysiological measurements on root rhizodermal cells showed that the NO₃^--elicited changes in membrane potential (ΔE_m) of these four rice cultivars were significantly different when exposed to low external NO₃^- (<1 mmol/L); while they were very similar at high external NO₃^- (10 mmol/L). The root cell membrane potentials of YD and XY63 were more responsive to low external NO₃^- than those of NK and SY917. The ΔE_m values for YD and XY63 rhizodermal cells were almost the same at both 0.1 mmol/L and 1 mmol/L NO₃^-; while for the NK and SY917 the values became larger as the external NO₃^- increased. For YD cultivar, ΔE_m was measured over a range of NO₃^- concentrations and a Michaelis-Menten fit to the data gave a K_m value of 0.17 mmol/L. Net NO₃^- uptake depletion kinetics were also compared and for some cultivars (YD and XY63) a single-phase uptake system with first order kinetics best fitted the data; while other cultivars (ND and SY917) showed a better fit to two uptake systems. These uptake systems had two affinity ranges: one had a similar K_m in all the cultivars (0.2 mmol/L); the other much higher affinity system (0.03 mmol/L) was only present in NK and SY917. The expression pattern of twelve different NO₃^- transporter genes was tested using specific primers, but only OsNRT1.1 and OsNRT2.1 expression could be detected showing significant differences between the four rice cultivars. The results from both the physiological and molecular experiments do provide some support for the hypothesis that the more popular rice cultivars grown in Jiangsu Province may be better at using NO₃^- as an N source.