Site, Scale and Time-course for Adjustments in Lateral Root Initiation in Wheat Following Changes in C and N Supply

The pattern of lateral root initiation in seminal roots of wheat(Triticum aestivumL. cv. Alexandria) and the location, scaleand time-course for adjustments in initiation were studied afterchanges in C and N supply. Macroscopically visible primordiaappeared in a non-acropetal sequence with the frequency (numberper unit length) increasing with distance behind the main rootapex to a maximum at 40–50 mm behind the root tip. Pruningthe root system to a single seminal axis increased the primordiafrequency by 23% within 15 h. After longer periods, the effectof root-pruning was greater. The enhanced primordia frequencywas first observed in tissue located 0–10 mm behind theapex at the start of treatment. Feeding glucose (50 mM) alsoincreased primordia frequency within 15 h, but to a greaterextent, and here additional primordia were initiated in tissuelocated 0–10and10–20 mm behind the apex at the startof treatment. Withdrawing NO₃^-from one part of a split-rootsystem, whilst maintaining the supply to the other, reducedprimordia frequency in the non-fed roots and, in some cases,a compensatory increase in the NO₃^--fed roots was observed.The location and scale of the adjustments were similar to thosefound with root-pruning and glucose-feeding, but were slightlyslower to appear. In spite of some differences in detail, therewas a broad similarity in site, scale and time-course for adjustmentsin lateral root initiation with these treatments, which is consistentwith the operation of a common mechanism. Whenever an increasein primordia frequency was observed, it was associated withan increase in the ethanol-soluble sugar content of the tissue.However, the reduction in frequency in NO₃^--deprived roots wasalso accompanied by an increase in sugar content. There wasno consistent relationship between total N content of the tissueand primordia frequency, but there was between primordia frequencyand the rate of net NO₃^-uptake. The possible mechanisms controllinglateral root initiation are discussed. Compensatory growth; correlative growth; glucose; initiation; lateral root; nitrate; primordium; split-root; Triticum aestivum; wheat     

The Use of Agar Nutrient Solution to Simulate Lack of Convection in Waterlogged Soils

Agar at 0.1% in nutrient solution (‘stagnant solution’)was used to prevent turbulence (convection), thus simulatingthe slow gas movements which occur in waterlogged soils. Wheat,aged between 6 and 16 d at the start of the treatment, was usedto test plant growth and development in this stagnant solutionfor 8–15 d. K-MES buffer at 5 mol m^-3was used to retainthe pH of the rhizosphere in the stagnant solution at pH 6.5. The prevention of convection reduced dissolved oxygen concentrationsin the bulk solution from 0.275 to below 0.05 mol m^-3after 1d, while ethylene accumulated over 10 d to 6.5x10^-6m³m^-3(ppm). Aerenchyma of nodal roots grown in stagnant solution comprised22% of the cross sectional area of the root 50 mm behind theroot tip; this was similar to values recorded earlier for nodalroots of wheat in waterlogged soil and contrasts with 7.6% forroots in non-flushed solution without agar (referred to in thispaper as ‘semi-stagnant solution’) and 2.4% in N₂-flushedsolution. Increases in dry weight and numbers of nodal roots with timewere larger for stagnant and N₂-flushed, than for semi-stagnantor aerated solution. In contrast, seminal roots did not growin stagnant solution, while seminal roots in N₂-flushed solutiongrew much less than in semi-stagnant or aerated solution. In the stagnant solution, relatively high concentrations ofN, K and P were required to avoid limitations in mineral uptakeinto the roots, due to the long diffusion pathway from the bulksolution imposed by the lack of convection. Nevertheless, ourdata show that the slow growth imposed by the lack of convectionwas due to factors other than low mineral nutrition. The mostlikely cause was the change in the dissolved gas compositionof the root media, particularly of the rhizosphere. In conclusion, in terms of anatomy and morphology the rootsgrown in the stagnant solution more closely resembled thosefrom waterlogged soil than did those grown in either semi-stagnantor N₂-flushed solution. Triticum aestivum; wheat; waterlogging; lack of convection; aerenchyma; root development; nutrient uptake     

Root-Tiller Interrelationships in Spring Barley (Hordeum distichum (L.) Lam.)

Root-tiller relations were investigated in spring barley grownin soil in deep pots. The total dry wt of the root system reachedits maximum 6 weeks from sowing, when the shoot weight was only50 per cent of its value at maturity. Seminal and nodal rootscomprised 40 and 60 per cent, respectively, of the total rootdry wt at maturity; the majority of the nodal root weight wasassociated with the main shoot. The main shoot had approximatelytwice as many nodal roots as either of the first two primarytillers (T1 and T2), and the primary and secondary tillers appearinglater were very poorly rooted. Some tillers, especially secondarytillers that died prematurely, produced no nodal roots. Theweight of the seminal roots and nodal roots attached to themain shoot continued to increase up to maturity but the drywt of nodal roots on tillers declined with time. This patternof growth was closely related to the pattern of ¹⁴C assimilateddistribution within the root system. A very small proportionof ¹⁴C assimilated by the main shoot and T1 and T2 was exported.The majority of the exported assimilate went to the seminalroot system and to nodal roots attached to the main shoot. Individualnodal and seminal roots seemed to have different roles in supplyingnutrients to the shoot system, with the former mainly providing³²P-phosphate to its tiller of origin and the latter generallysupplying the main shoot and primary tillers. Hordeum distichum. (L.) Lam., barley, root growth, nodal roots, seminal roots, tillering, assimilate distribution, ³²P-distribution     

Root Morphology and Nitrogen Uptake of Maize Simultaneously Supplied with Ammonium and Nitrate in a Split-root System

We studied the response of maize (Zea mays L. cv. Anjou 256)to a simultaneous, but separated supply of ammonium and nitrate(localized supply, LS). A split-root system was used to supplyhalf of the roots with ammonium and the other half with nitrate.A homogeneously distributed supply of both nitrogen forms (HS)was the control treatment. Seedlings were grown for 12 d fromthe two-leaf to the three-leaf stage in hydroponics at threepH levels (4, 5·5 and 7). The total N concentration was3 mol m^-3. The split-root system was established by removingthe seminal root system and using only four nodal roots perplant. Total root length and root surface area were recordedautomatically with a modified Delta- T area meter. Other morphologicalroot traits (such as main axis length and diameter, number,density, and length of laterals) were recorded manually. Uptakeof ammonium and nitrate was measured by the depletion of thenutrient solution. As compared with LS, HS was superior in shootand root DM, total root length and root surface area, ammoniumand nitrate uptake and shoot nitrogen concentration, irrespectiveof pH level. This indicates that, also under field conditions,mixed ammonium and nitrate fertilization is only beneficialto plant growth if both N forms are evenly distributed in thesoil. At both HS and LS, ascending pH increased the ammonium:nitrateuptake ratio. At LS, declining pH induced a considerable shiftin the distribution of root DM, root length, and root surfacearea the nitrate-fed compartment.Copyright 1993, 1999 AcademicPress Maize, Zea may L., ammonium, nitrate, pH, root morphology, split-root     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: IV. EFFECT OF PHOSPHATE CONCENTRATION ON THE GROWTH OF TRIFOLIUM REPENS L. SUPPLIED WITH NITRATE, OR DEPENDENT UPON SYMBIOTICALLY FIXED NITROGEN

Breeze, V. G. and Hopper, M. J. 1987. The uptake of phosphateby plants from flowing nutrient solution. IV. Effect of phosphateconcentration on the growth of Trifolium repens L. suppliedwith nitrate, or dependent upon symbiotically fixed nitrogen.—J.exp. Bot. 38: 618–630. Nodulated white clover plants were subjected to a range of phosphateconcentrations in flowing solution culture (0.32 to 8.0 mmolm^–3 P) at 41 d from sowing, either supplied with nitrateor dependent on symbiotically-fixed nitrogen. No effect of phosphateconcentration in solution on dry matter production, relativegrowth rate, root/shoot ratio, or water soluble carbohydrateconcentration of the plant tissue was observed after 24 d fromthe start of the experiment, although the plants supplied withnitrate yielded more than the others. Phosphate uptake throughoutthe experimental period was related to the solution concentration,but the source of nitrogen did not affect the phosphorus concentrationsof the shoots. However, the roots of the plants dependent onsymbiotically-fixed nitrogen had higher concentrations of phosphorusthan those supplied with nitrate, but this did not appear tobe due to an increased phosphorus requirement for nitrogen fixation,because the amount fixed was unaffected by the phosphate concentrationin solution. The cation-anion balance showed that plants dependenton nitrogen fixation had no larger requirement for calcium thanplants supplied with nitrate, but a requirement for hydroxylions equivalent to over 130 kg lime per tonne of dry shoot.It is suggested that the enhanced phosphate uptake by plantsdependent on nitrogen fixation is due to this need for a cation-chargebalancing anion. Key words: Phosphate uptake, nitrogen fixation, Trifolium repens L., repens L., cation-anion balance, flowing solution culture     

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     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Nitrogen Utilization in N-limited Barley during Vegetative and Generative Growth: I. GROWTH AND NITRATE UPTAKE KINETICS IN VEGETATIVE CULTURES GROWN AT DIFFERENT RELATIVE ADDITION RATES OF NITRATE-N

Growth and nitrate uptake kinetics in vegetatively growing barley(Hordeum vulgare L., cvs Laevigatum, Golf, and Mette) were investigatedin solution culture under long-term limitations of externalnitrogen availability. Nitrate was fed to the cultures at relativeaddition rates (RA) ranging from 0.02 to 0.2 d^–1. Therelative growth rate (RG, calculated for total plant dry weight)correlated well with RA in the range 0.02 to 0.07 d^–1.In the RA range from 0.07 to 0.2 d^–1 RG continued to increase,but an increasing fraction of nitrogen, added and absorbed,was apparently stored rather than used for structural growth.The RG of the roots was less affected by RA. V_max, for net nitrateuptake increased with RA up to 0.11 d^–1, but decreasedat higher RA. The decline in V_max coincided with a build-upof nitrate stores in both roots and shoots. V_max, expressedper unit nitrogen in the plants (the relative V_max, was higherthan required for maintenance of growth (up to 30-fold) at lowRA, whereas at higher RA the relative V_max decreased. Kineticpredictions of steady-state external nitrate concentrationsduring N-limited growth ranged from 0.2 to 5.0 mmol m^–3over the RG range 0.02 to 0.11 d_–1. It is suggested thatthe nitrate uptake system is not under specific regulation atlow RA, but co-ordinated with root protein synthesis and growthin general. At RA higher than 0.11 d_–1, however, specificregulation of nitrate uptake, possibly via root nitrate pools,become important. The three cultivars showed very similar growthand nitrate uptake characteristics. Key words: Barley, growth, nitrogen limitation, nitrate uptake, kinetics     

Effects of Salinity on Growth, Water Relations and Ion Accumulation of the Subtropical Perennial Halophyte, Atriplex griffithii var. stocksii

The effects of salinity on growth, water relations, glycinebetainecontent, and ion accumulation in the perennial halophyte Atriplexgriffithii var. stocksii were determined. The following questionswere addressed: (1) What effect does salinity have on growthresponses at different ages? (2) Is A. griffithii an ion accumulator?(3) Does A. griffithii accumulate glycinebetaine in responseto salinity? Atriplex griffithii plants were grown in pots at0, 90, 180 and 360  m M NaCl in sand culture in a plantgrowth chamber and plants were harvested after 30, 60 and 90d. Plant total dry weight was significantly inhibited at 360m M NaCl. Root growth showed a substantial promotion at 90 mM NaCl. The water potential and osmotic potential of shootsbecame more negative with increasing salinity and time of growth.The Na⁺and Cl^-content in both shoots and roots increased withincreases in salinity. Increased treatment levels of NaCl induceddecreases in Ca⁺, K⁺and Mg²⁺in plants. Atriplex griffithii accumulateda large quantity of ions, with the ash content reaching 39%of the dry weight in leaves. Inorganic ion accumulation is significantin osmotic adjustment and facilitates water uptake along a soil-plantgradient. Glycinebetaine concentration was low in roots, andin stems it increased with increases in salinity. Total amountsof glycinebetaine in leaves increased with increases in salinity,and its concentration increased substantially at 360 m M NaCl.Copyright 2000 Annals of Botany Company Atriplex griffithii, glycinebetaine, growth, ions, water relations.     

Uptake of zinc from chelate-buffered nutrient solutions by wheat genotypes differing in zinc efficiency

Zinc-efficient Triticum aestivum (cv. Warigal) and Zn-inefficientTriticum turgidum conv. durum (cv. Durati) were grown in chelate-buffered,complete nutrient solutions providing either deficient or sufficientZn supply. When transferred to fresh chelatebuffered nutrientsolutions containing a wide range of Zn supplies (0–1.28µmol m^–3 Zn²⁺ activity) for 24–48 h, bothgenotypes increased net Zn uptake linearly with an increasein solution Zn²⁺ activities. Zincefficient Warigal accumulatedZn at a greater rate than Zn-inefficient Durati. The greaterrate of net Zn uptake was observed by plants of both genotypeswhen pretreated at deficient Zn supply. Net loss of Zn to thesolution was higher in plants pretreated with sufficient Znand was inversely related to Zn²⁺ activity in the external solution.When continuously supplied with 40 nmol m^–3 Zn²⁺, netZn uptake by Zn-efficient Warigal was significantly greaterthan that of Zn-inefficient Durati, but the difference diminishedwith plant age. Shoot concentrations of Fe, Mn and Cu were higherwhen plants were grown at deficient than at sufficient Zn supply.The Zn-efficient genotype transported less Zn and Fe to shootsand had higher Fe concentrations in roots than the Zn-inefficientgenotype, supporting the hypothesis that Zn efficiency may beconnected with inefficient transport of Fe from roots to shootsand thus initiation of the Fe-deficiency response resultingin increased release of Zn- and Fe-binding phytosiderophores.It is concluded that differential Zn efficiency of wheat genotypesis at least partly due to a greater ability of efficient genotypesto accumulate Zn. Key words: Chelate-buffering, genotypes, micronutrients, Triticum spp., uptake, zinc efficiency     

Uptake and Distribution of Copper in Chrysanthemum morifolium

The effects of various levels of copper on the uptake and distributionof copper in Chrysanthemum morifolium grown in solution cultureand peat-sand have been examined. Whole plants growing in shortdays were sampled at regular intervals, divided into roots,stem, leaves and lateral shoots, and analysed for copper. Thepartitioning of copper between these tissues showed that a relativelylarge proportion (30–40 per cent) of the total plant copperwas accumulated in the roots of normal plants during the harvestingperiod, compared with approximately 10 per cent in the rootsof copper deficient plants. Whilst the copper content (ug g^–1) of leaves and stemfrom normal plants was negatively correlated with the amountof dry matter produced (P < 0·001), the correspondingcopper deficient tissues showed little variation in copper contentwith increases in tissue dry weight. A more detailed investigationof the copper content of leaves from normal plants showed thatgradients existed within the plant with respect to both leafposition and time of harvest which could be described by a singlecubic surface equation (P < 0·001).     

Growth and development of seminal and crown root systems in N-limited barley,and their contributions to nitrate acquisition during vegetative and generative growth

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation, controlled by the relative rate of nitrate-N addition (RA), in solution culture. The seminal and crown root systems were kept apart, but in contact with the same nutrient solution throughout culturing. Growth, nitrate uptake, and in vitro nitrate reductase (NR) activity in the different root parts were studied at plant ages from 40 (late vegetative stage) to 110 (mid grain-filling) days. The RA was during this time interval stepwise decreased from 0.08 day^–1 to 0.005 day^–1. The ratio between seminal root dry weight and total plant dry weight decreased drastically during post-anthesis growth, whereas the contribution by crown roots remained unchanged. Tissue nitrogen concentrations in seminal roots did not change with time, but decreased in crown roots after day 80. The NR activity decreased with age in both seminal and crown roots. The V_max for net nitrate uptake decreased throughout the experiment in the seminal root system, but not in the crown root system. The kinetic properties (V_max and K_M) were used to calculate the nitrate concentration required to maintain a relative rate of nitrate-N uptake that equals the relative addition rate. These concentrations (2 to 5 mmol m^–3) were found to closely match actually measured nitrate concentrations in the nutrient solution (1 to 6 mmol m^–3). From uptake kinetics, it was deduced that the contribution by seminal roots to total nitrate uptake at these concentrations decreased from more than 50% in vegetative plants, to about 20% just after main shoot anthesis, and to less than 5% during grain-filling. ei]Section editor: H Lambers     

Growth and Development of Young Roots of Barley (Hordeum vulgare L.) Genotypes

Barley (Hordeum vulgare L.) genotypes from countries with aMediterranean climate grown in temperature-controlled glasshousein nutrient solution to determine whether the co-ordinationof root branching and growth found by other workers appliedto a wider of up to 14 genotypes. There was substantial variationin the number of seminal axes produced by the genotypes, rangingfrom about seven for Hoshimasari and Swanneck to about fourfor Gerbel 'B'. The number of nodal axes was linearly relatedto the number of leaves and typically between one and two mainstemleaves were required before nodal axes appeared. There weresmall genotypic differences in the number of axes produced perleaf with values ranging from 1·5 to 2·3. Theproduction and growth of lateral roots were coordinated so thatthe mean length of laterals generally increased with time. Landraces(Arabic abiad and Arabic aswad) produced more lateral rootswith a faster rate extension compared with other genotypes.The length and number of primary and secondary lateral rootswere related linearly, but no genotypic differences in thisrelation were evident. Length of primary lateral roots increasedmore rapidly than that of secondary lateral roots throughoutthe three to five leaf stage. The ratio of root weight to totalplant weight decreased with time but there were only small differenceswithin this range of genotypes.Copyright 1995, 1999 AcademicPress Barley, seminal axes, nodal axes, primary lateral roots, relative extension rates, relative multiplication rates     

Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea mays L.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Two experiments were conducted to evaluate the effects of phenotypicrecurrent selection for high and low post-anthesis leaf-laminain vivo NRA on nitrate uptake, nitrate partitioning and in vitroNRA of seedling roots and leaves. In Experiment 1, intact plantsof cycle 0, 4, and 6 of the high and low NRA strains were grownon NH₄-N for 11 d, then exposed to 1.0 mol m^–3 KNO₃, andcultures sampled at 6 h and 28 h (induction and post-inductionperiods). Nitrate uptake, tissue nitrate concentration and invitro NRA were determined. The pattern of response to selectionin seedling leaf NRA was similar to that observed for in vivoNRA of field grown plants. Leaf NRA increased between 6 h and28 h. Root NRA was not affected by selection or sampling time.Treatments differed in total fresh weight but not in reductionor uptake of nitrate per unit weight, indicating a lack of correspondencebetween NRA and reduction and supporting the idea that concomitantreduction by NR is not obligatorily linked to nitrate influxin the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of thehigh and low NRA strains were cultured without N, detopped onday 6, transferred the following day to 0-75 mol m^–3 KNO₃and sampled at 6 h and 28 h. In contrast to Experiment 1, selectionpopulations differed in nitrate reduction and root NRA, whichby 28 h reached higher average levels than root NRA of intactplants. Translocation and reduction were inversely related amongstrains within each sampling time. The high level of translocationin detopped plants of the low NRA strain was difficult to reconcilewith its low leaf NRA level of Experiment 1. It is suggestedthat nitrate transport in detopped roots is altered relativeto the intact system in a way which permits greater NRA inductionand nitrate reduction. The results indicate that nitrate partitioningby detopped root systems should be interpreted with caution. Key words: Zea, nitrate reductase activity, nitrate uptake, nitrate reduction, nitrate partitioning, selection     

Effects of NO2 and O3 Alone or in Combination on Kidney Bean Plants (Phaseolus vulgaris L.): Growth, Partitioning of Assimilates and Root Activities

Ten-day old kidney bean plants (Phaseolus vulgaris L. cv. Shin-edogawa)were exposed to 2.0 and 4–0 parts 10^–6 NO2, and0.1, 0.2, and 0.4 parts 10^–6 O3 alone or in combinationfor 2, 4, and 7 d. The effects of these air pollutants wereexamined with respect to the growth, partitioning of assimilates,nitrogen uptake, soluble sugar content, and root respiration. Decreased dry matter production was significant with all treatmentsexcept 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃.Exposure to mixtures of the gases produced more severe suppressionof growth than exposure to the single gases. Root/shoot ratiowas significantly lowered at 7 d by the gas treatments otherthan 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃. Thetotal nitrogen content of plants was increased by all treatments;the higher percent of nitrogen found with O₃ exposure will resultfrom the growth retardation which increases the concentrationof nitrogen in the plants because the absorption of nitrogenby roots was unaffected. The combination of O₃ with NO₂ significantlydecreased the assimilation of NO₂ by the plants. The concentration of soluble sugars in roots was decreased bythe gas treatments. There was a strong positive correlationbetween soluble sugar content and dry weight of the roots harvestedat 7 d. Root respiration was relatively unchanged until 5 dand then decreased significantly at 7 d by 2.0 parts 10^–6NO₂ and 0–2 parts 10^–6 O₃. Retarded growth of theroots and the decreased root respiration may be due to diminishedtranslocation of sugars from leaves to roots caused by exposureto air pollutants. The uptake of soil nitrogen was not closelyrelated with root respiration in the case of O₃ exposure. Key words: NO₂, O₃, Phaseolus vulgaris, Growth, Sugars, Root respiration     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: II. EFFECT OF PLANT N-STATUS ON THE KINETIC PARAMETERS OF NITRATE INFLUX

Influx of nitrate into the roots of intact barley plants wasfollowed over periods of 1–15 min using nitrogen-13 asa tracer. Based on measurements taken over 15 min from a rangeof external nitrate concentrations (0·2–250 mmolm^–3), the kinetic parameters of influx, I_max and K_m, werecalculated. Compared with plants grown in the presence of nitrate throughout,plants that had been starved of N for 3 d showed a significantlygreater value ofI_max for ¹³N-nitrate influx (by a factor of1·4–1·8), but a similar value of K_m (12–14mmol m^–3). Pre-treating N-starved plants with nitratefor about 5 h further increased the subsequent rate of ¹³N-nitrateinflux, but had little effect in the unstarved controls. Allowingfor this induction of additional nitrate transport, the differencein rates of nitrate influx in control and N-starved plants wassufficient to account for the previously-observed differencein net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, butwith ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrateinflux were significantly decreased (by about one-half) comparedwith the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulationof net uptake of nitrate is discussed. Key words: Ion transport, nitrate, influx, kinetic parameters, N-deficiency     

Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus

Five-month-old Cleopatra mandarin (Citrus reticulata Blanco)(CM) and Volkamer lemon (Citrus volkameriana Ten. and Pasq.)(VL) seedlings were grown in a glasshouse in 2·3-1 potsof Candler fine sand. Plants were irrigated with either non-saline(EC_e = 0·23 dS m^-1) or saline (6·13 dS m^-1) waterusing 3:1 NaCl:CaCl₂ solution over a 4-week period. A singleapplication of K¹⁵NO₃ (19·64 atom % excess ¹⁵N) at 212mg N1^-1, was substituted for a normal weekly fertilization after3 weeks and plants were harvested 7 d later. The transpirationrate, uptake of nitrogen, growth and nitrogen-use efficiency(NUE) on a dry weight basis (mg d. wt mg^-1 N) of both specieswas reduced by salinity. Based on growth, water-use and chloride(Cl) accumulation in leaves, VL was more salt-sensitive thanCM, but ¹⁵N uptake was equally reduced by salinity in both species.Salinity reduced ¹⁵N uptake relatively more than shoot growthover the 7-d period, such that the ¹⁵NUE (mg d. wt µg^-115N) of new shoot growth of both species increased. There wasno evidence of Cl antagonism of nitrate (NO₃) uptake but totalplant ¹⁵NO₃ uptake was positively correlated with whole planttranspiration in both species. Thus, it appears that reductionsin NO₃ uptake are more strongly related to reduced water usethan to Cl antagonism from salt stress.Copyright 1993, 1999Academic Press Sodium, chloride, salinity, calcium, nitrate, ¹⁵NO₃ uptake, nitrogen allocation, nitrogen-use efficiency, water use, Citrus reticulata, Citrus volkameriana     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Uptake and Physiological Effects of Cadmium in Sugar Beet (Beta vulgaris) Related to Mineral Provision

Sugar beet seedlings (Beta vulgaris L. cv. Monohill) were grownfor 14 d on a nutrient solution based on the nutrient proportionsin healthy plants. Nutrients were supplied either once at relativelyhigh concentrations, or in small amounts with a daily incrementalincrease of 0?15 or 0?20 in accordance with an exponential growthrate. Cadmium (0, 0?6, 2?3, 50 or 20?0 µmol) was introducedeither by a single addition or in daily increments of 0?15 or0?20. Cadmium uptake, expressed as a percentage of total Cd²⁺supplied, decreased with increasing total C^d2+ content and withdecreasing availability of nutrients. With a daily supply ofcadmium, net uptake, transport and content per unit of dry weightin roots and shoots were related to the total Cd²⁺ supplied.Cadmium caused growth retardation, increased root/whole-plantratio, and decreased root-tip respiration and photosynthesis.At high initial nutrient concentrations, Cd²⁺ decreased thecontents of sucrose, glucose, fructose, and starch per unitof dry weight. The opposite was found if nutrients were addeddaily. In the latter case, the dry weight/fresh weight ratioalso increased. The effects of cadmium were related to [Cd²⁺]in proportion both to the root absorption area and to the nutrientconcentration. Key words: Sugar beet, mineral provision, cadmium uptake, sugar formation, growth     

Phosphate Uptake and Transport by Roots and Stolons of Intact White Clover (Trifolium repens L.)

The extent to which phosphate can be absorbed directly fromthe outer medium by stolon internodes and contribute to thetotal accumulation of phosphate by intact plants of white clover(Trifolium repens L. cv. Blanca) was assessed in hydroponicexperiments in a controlled environment room. The uptake ofphosphate by intact roots or stolons was measured by sealinga segment (6-0 mm long) across a flow-cell in which ³²P-labellednutrient solution was circulated for 24 h, the rest of the rootsystem receiving unlabelled nutrient solution. The rate of uptakeof phosphate (µmol g^–1 d^–1 dry wt. basis)by roots was more than 300 times that by intact stolons. Pretreatmentof stolons by gentle abrasion to remove cuticle, so as to simulatethe condition of stolons in the field, increased the uptakeof phosphate 7-fold compared with that of intact stolons. However,the potential of stolons to contribute to the P status of whitedover in the field was calculated to be small (5%). When an incision was made through the hypodermal layer of stolons,the rate of phosphate uptake greatly increased, attaining 71%of that by root segments. This increase, which was greater athigher phosphate concentrations, indicates that the suberi.zedhypodermis constitutes a major barrier to the influx of phosphatein the stolon. After withholding phosphate for different time intervals, thesubsequent rate of phosphate uptake by roots was increased 2-3-foldafter 2 d phosphate deprivation and 3-4-fold after 6 d or 13d phosphate deprivation. A higher proportion of absorbed phosphatewas transported to shoots in phosphate-deprived plants. After1 d of uptake following restoration of the phosphate supply,the concentrations of labelled phosphate in shoots were greaterthan in control plants, although the concentrations of labelin roots was less. However, the rate of uptake of phosphateby stolons, following deprivation, was not significantly increased.These results suggest that the mechanism regulating the enhancedrate of phosphate loading into the xylem, initiated by a periodof phosphate deprivation, is specific to roots and is not inducedin stolons. The results are discussed in relation to the growth and acquisitionof phosphate by white clover in the field. Key words: Nutrient deficiency, phosphate, stolons, transport (ions), Trifolium repens