Localization of phytosiderophore release and of iron uptake along intact barley roots

Under iron deficiency the release of so-called phytosiderophores by roots of barley plants ( Hordeum vulgare L. cv. Europa) was greater by a factor of 10 to 50 compared to iron-sufficient plants. This enhanced release occurred particularly in apical zones of the seminal roots and in the lateral root zones. Under iron deficiency, uptake rates for iron, supplied as FeIII phytosiderophore, increased by a factor of ca 5 as compared to iron-sufficient plants. This enhanced uptake rate for iron was also much more pronounced in apical than in basal root zones. In contrast, with supply of the synthetic iron chelate, FelII EDDHA (ferric diaminoethane-N, N-di- o -hydroxyphenyl acetic acid), the Fe deficiency-enhanced uptake rates for iron were only small and similar along the roots, except for the lateral root zones. The high selectivity of barley roots for uptake and translocation of FeIII phytosiderophores compared with FeIII EDDHA is reflected by the fact that, at the same external concentration (2 μ M ), rates of uptake and translocation of iron from FeIII phytosiderophores were between 100 (Fe-sufficient) and 1 000 times higher (Fe-deficient plants) than from FeIII EDDHA. The relatively high rates of uptake and particularly of translocation of iron supplied as FeIII EDDHA in the zone of lateral root formation strongly suggest an apoplastic pathway of radial transport of the synthetic iron chelate into the stele in this root zone.
The results demonstrate that apical root zones are the main sites both for Fe deficiency-enhanced release of phytosiderophores and for uptake and translocation of iron supplied as FeIII phytosiderophores.     

Relationships between structural development and the absorption of ions by the root system of Cucurbita pepo

Summary In both the seminal axis and lateral roots of Cucurbita pepo L. the formation of large central xylem elements and the commencement of secondary cambial activity occur 10–20 cm from the root tip. Concomitant with or slightly preceding these developments there are changes in the structure of the walls of endodermal cells where the lignified casparian band spreads along the radial wall and substances staining with Sudan IV are deposited in both radial and tangential walls. At distances more than 30 cm from the tip of primary roots the radius of the stele increases considerably causing splits in the cortex. The endodermis is stretched and the suberin becomes organized in a lamellar form.Against this background of anatomical change certain of the transport capabilities of the root are retained while others are lost. Using an apparatus for measuring the uptake of tracers by segments of intact roots it was found that neither the uptake nor translocation of potassium seem to be affected by the suberization of the endodermis or by secondary thickening, while the translocation of calcium is virtually eliminated when these processes begin. As the root ages its ability to absorb phosphate declines although the translocation of the phosphate absorbed is much less affected by structural development than that of calcium.The observed rates of potassium uptake by complete root systems could be predicted quite accurately from the average of segment uptake data suggesting that the method used gives reliable results.     

Sites of absorption and translocation of iron in barley roots: tracer and microautoradiographic studies

Absorption and translocation of labeled Fe were measured at various locations along the length of intact seminal axes and lateral roots of iron-sufficient (+Fe) and iron-stressed (−Fe) barley (Hordeum vulgare) plants. In seminal axes of +Fe plants, rates of translocation were very much higher in a zone 1 to 4 cm from the root tip than elsewhere in the root. Lateral roots of high rates of translocation were also restricted to a narrow band of maturing or recently matured cells. In −Fe plants the patterns of uptake and translocation were essentially the same as in +Fe plants but the rates were 7- to 10-fold higher. The amount of labeled Fe bound to the root itself was not increased by Fe stress and its distribution along the root seemed inversely related to the ability to translocate Fe.

Microautoradiographic studies showed that most of the iron bound to roots was held in an extracellular peripheral band in which iron seemed to be precipitated. This process may be assisted by microbial colonies but did not depend on them since it was seen, although to a lesser extent, in sterile roots. In zones from which iron was translocated there was evidence that internal root tissues became labeled readily, but as translocation declined with distance from the root tip, radial penetration of Fe appeared to become progressively less. The results are discussed in relation to possible changes in the pH or redox potential of the surface of the root.

     

Nutrient Supply and the Growth of the Seminal Root System in Barley: III. COMPENSATORY INCREASES IN GROWTH OF LATERAL ROOTS, AND IN RATES OF PHOSPHATE UPTAKE, IN RESPONSE TO A LOCALIZED SUPPLY OF PHOSPHATE

Responses to a localized supply of phosphate were studied inbarley grown in continuous flow solution culture. Root systemswere either uniformly supplied with 50 µM phosphate (controls)or the same solution was supplied to only a 4 cm or 2 cm lengthof a seminal root (localized supply), the remainder of the rootsystem receiving a nutrient solution lacking phosphate. Little development of laterals occurred on those parts of theroot system receiving no phosphate from the external solution,while an increase in the number and extension of laterals tookplace in the 4 cm zone enriched with phosphate. Compared withsimilar zones on controls, the total length of laterals wasincreased 15-fold in 21 d plants. In addition, rates of ³²P-phosphateuptake and translocation to shoots per unit root weight werehigher than in controls by a factor of 2?5–5?0. Furtherincreases in the growth of lateral roots, and rates of phosphateuptake, were induced when the segment initially supplied withphosphate was restricted to only 2 cm. These localized modifications to root growth and uptake of phosphatelargely compensated for the deficient supply of phosphate tothe remainder of the root system. After an early period of retardedgrowth and phosphate stress, the relative growth rate of plantsand the concentration of phosphate in shoots were restored tolevels similar to that of the controls. The manner in which the supply of phosphate may control rootdevelopment, and the nature of the co-ordination between rootgrowth, phosphate uptake, and shoot growth, are discussed.     

Uptake and Translocation of Benthiocarb Herbicide by Plants

The uptake and translocation of ¹⁴C-benthiocarb labelled at benzyl methylene by rice plant, bamyardgrass, wild amaranth, smart weed and lambsquarters were investigated, ¹⁴C-Benthiocarb was absorbed through the roots and the radioactivity was translocated into whole plants. The rate of absorption and translocation varied by the kind of plants. The translocation was occurred not only from roots into leaves, but from a leaf into other leaves, and even into roots of some kinds of plant. The absorption and translocation was more easy in barnyard-grass than in rice plant. Benthiocarb was rapidly absorbed by seeds and accumulated mostly in the embryo. The uptake of benthiocarb by seedlings decreased with the order of mesocotyl (bamyardgrass only), coleoptyl, root and leaf. Benthiocarb was degraded rapidly in plants.     

Behaviour of fertilizer-N absorbed through root and fruit in peanut

A greenhouse experiment was conducted to study the uptake and translocation of N applied at different rates of¹⁵N fertilizer to the fruiting and rooting zones of peanut plants. Higher N level treatments in the fruiting zone resulted in higher N concentrations in the shell and gynophore with fruit and lower N concentration in the testa when compared with the results of lower level treatments in the fruiting zone. Regardless of N levels applied in both rooting and fruiting zones, about 60–65% of¹⁵N application to the rooting zone was absorbed through the root, of which 30–35% was found in the seed. With fertilizer application to the fruiting zone and regardless of levels applied to both zones, 35–40% of the¹⁵N supplied was absorbed through the shell, and 65% of this remained in the fruit parts while 35% of it was translocated to the vegetation and roots. The percentage of N in the vegetative and root parts, derived from the fertilizer-¹⁵N through the shell, was lower in the root and nodules than in the shoot and gynophore without fruit. The translocation of N, absorbed through the roots, to the fruit as well as the amount of symbiotically fixed N were decreased by additions of N to the fruiting zone.     

Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley)

Plants of Hordeum vulgare (barley) were grown initially in a solution containing 150 M phosphate and then transferred on day 6 to solutions with (+P) and without (-P) phosphate supplied. After various times plants from these treatments were supplied with labelled phosphate. Analysis of plant growth and rates of labelled phosphate uptake showed that a general enhancement of uptake and translocation was found, in plants which had been in the-P solution, several days before the rate of dry matter accumulation was affected. Subsequently a detailed analysis of phosphate uptake by segments of intact root axes showed that the enhancement of phosphate uptake by P-stress occurred first in the old and mature parts of the seminal root axis and last in the young zones 1 cm from the root apex. During this transition period there were profound changes in the pattern of P absorption along the length of the root. Most of the additional P absorbed in response to P-stress was translocated to the shoot, particularly in older zones of the axis. Enhancement of phosphate uptake in young zones of nodal axes occurred at an earlier stage than in seminal axes. The results are related to the P-status of shoots and root zones and discussed in relation to the general control by the shoot of phosphate transport in the root.     

Net sodium fluxes change significantly at anatomically distinct root zones of rice (Oryza sativa L.) seedlings

Casparian bands of endodermis and exodermis play crucial roles in blocking apoplastic movement of ions and water into the stele of roots through the cortex. These apoplastic barriers differ considerably in structure and function along the developing root. The present study assessed net Na⁺ fluxes in anatomically distinct root zones of rice seedlings and analyzed parts of individual roots showing different Na⁺ uptake. The results indicated that anatomically distinct root zones contributed differently to the overall uptake of Na⁺. The average Na⁺ uptake in root zones in which Casparian bands of the endo- and exo-dermis were interrupted by initiating lateral root primordia (root zone III) was significantly greater than that at the root apex, where Casparian bands were not yet formed (root zone I), or in the region where endo- and exo-dermis with Casparian bands were well developed (root zone II). The measurement of net Na⁺ fluxes using a non-invasive scanning ion-selective electrode technique (SIET) demonstrated that net Na⁺ flux varied significantly in different positions along developing rice roots, and a net Na⁺ influx was obvious at the base of young lateral root primordia. Since sodium fluxes changed significantly along developing roots of rice seedlings, we suggest that the significantly distinct net Na⁺ flux profile may be attributed to different apoplastic permeability due to lateral root primordia development for non-selective apoplastic bypass of ions along the apoplast.     

Water Uptake by Different Regions of the Barley Root. Pathways of Radial Flow in Relation to Development of the Endodermis

The water uptake by different lengths of lateral roots and 1.0cm or 5.0 cm lengths of the seminal axes from differentregionsof the root were measured in potometers with the shoot in airat two humidity regimes. A model of the contribution by thesedifferent regions of the root to the total water absorptionby the plant agreed well with measurements of water uptake bythe whole root system. According to this model, about one halfof the water taken up by the main axis came from the older suberizedregions further than 10 cm from the tip, and together with itsassociated lateral roots this region provided 75% of the totalwater transpired. The development of State III endodermal cellswas correlated with decreases in both the water uptake by theolder regions of the root and the translocation of calcium.Thus in the younger regions of the root where water uptake ismaximal, the flow of water is principally apoplastic althoughthere is also likely to be flow via the symplast Despite a 43%difference in transpirational demand between the two humiditytreatments, the leaf water potentials remained constant, implyinga change in root resistance. This change in resistance mightbe explained if there were an apoplastic pathway within thesuberin lamellae of State III endodermal cells. The responseto the increased transpirational demand is met by the olderregions of the root, in particular by the zone of lateral emergencewhere an apoplastic pathway is known to exist as the Casparianband in the endodermis breaks down with the emergence of thelateral roots. Key words: Endodermis, Pathways, Water Uptake     

Effect of phosphate fertiliser and plant density on phosphate inflow into ryegrass roots in soil

Summary Ryegrass plants were grown in pots of Horotiu sandy loam, either singly or as a dense sward, with a range of P fertiliser rates and regular harvests. Plants were non-mycorrhizal. P inflows into roots increased with P fertiliser rate. Sward plants absorbed up to 14.3% of the P fertiliser added, and single plants up to 7.6%. Sward plants absorbed most of the fertiliser P available to them within two weeks of germinating. After that, root growth ceased except at the highest P fertiliser rate used, and P inflow into roots decreased from 10^–15 to 10^–16 mole/cm/sec. Single plants, however, had continuous root growth and P uptake throughout the two month experiment, except for those at the two lowest fertiliser rates. Singly grown plants absorbed much more P than each plant in the dense swards. In single plants, root length/root weight ratio increased with increasing P fertiliser. Plant growth was dependant on continued P uptake by the roots, which only occurred if new roots were continally produced and new volumes of soil tapped for P. Non-growing root systems absorbed very little P. re]19760201     

Regulation of nitrate metabolism through anion polycompartmentation in plant roots

A new concept illustrated by a corresponding mathematical model of nitrate metabolism regulation is proposed. The model is based on root nitrate compartmentation in several functional pools: storage, metabolic and a mobile pool which is intended for translocation to shoots. Data on nitrate uptake, compartmentation, reduction in intact roots and translocation to shoots were obtained on steady-state wheat seedlings grown at 25 and 12 degrees C in the root zone. The net uptake, influx/efflux ratio, mobile pool size and translocation changed depending on the medium temperature. The oscillations of the net uptake rate, nitrate tissue concentration were revealed and the effect of temperature on these changes was demonstrated. The scheme of regulation is based on the idea that net uptake through nitrate influx/efflux is under the control of the nitrate the mobile pool whose size was dependent on the nitrate translocation into shoots. The mathematical model is represented by a system of ordinary differential equations simplified according to the time hierarchy of reactions. It has a limit cycle at definite values of the parameters. The model postulates the mechanism of a positive feed-back regulation of the transfer of newly absorbed nitrate into translocated pool formed in the root cortex. Theoretical results are verified experimentally.     

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     

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl(2). Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature 'transition' zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83-95%) in response to HgCl(2) was observed in cortical cells. Water flow through roots and plants was reduced less (40-74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue.     

Water uptake and structural plasticity along roots of a desert succulent during prolonged drought

Desert succulents resume substantial water uptake within 1–2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the monocotyledonous leaf succulent Agave deserti Engelm. substantial root mortality occurred only for lateral roots near the soil surface; nearly all main roots were alive at 180 d of drought. New main roots were initiated and grew up to 320 mm at soil water potentials lower than – 5·0 MPa, utilizing water from the shoot. The hydraulic conductivity of distal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored following 45 d of drought but not after 90 and 180 d. The production of new lateral roots and the renewed apical elongation of main roots occurred 7–11 d after rewetting following 180 d of drought. Hydraulic conductivity was higher in the distal region than at midroot and often increased again near the root base, where many endodermal cells lacked suberin lamellae. Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity along a root allows A. deserti to capitalize on intermittent or heterogeneous supplies of water.     

The influence of phosphate nutrition on H ion efflux from the roots of young rape plants

Changes in pH around the roots of young rape plants were studied using a nutrient film technique which allowed either part or all of the root system to be subjected to specific nutrient treatments. The rapidity and direction of change of pH was assessed by embedding absorbing roots in a thin layer of agar containing bromocresol purple. Measurements were also made with a pH microelectrode placed next to the roots. Phosphate-fed plants were deprived of phosphate when 14 days old. Patterns of pH changes round the deprived roots were the same as with phosphate-fed plants until the plants had been deprived of P for three days, when H ion efflux started in the terminal portions of the roots. The lengths of root producing acid and amounts of H ion both increased as the plants became more P deficient. Both P fed and P deprived roots produced HCO₃ ions but the net amount of HCO₃ ion produced by the P deficient roots fell as did nitrate uptake rates. Cation-anion balances measured at the end of the experiment showed that uptake of all anions and K decreased in the P deprived plants but uptake of Ca and Mg were little altered. This resulted in a smaller ratio of anions to cations absorbed which was reflected in the reduced HCO₃ ion efflux.     

Transcriptional regulation and functional properties of <Emphasis Type="Italic">Arabidopsis</Emphasis> Pht1;4, a high affinity transporter contributing greatly to phosphate uptake in phosphate deprived plants

Phosphate mobilization into the plant is a complex process requiring numerous transporters for absorption and translocation of this major nutrient. In the genome of Arabidopsis thaliana, nine closely related high affinity phosphate transporters have been identified but their specific roles remain unclear. Here we report the molecular, histological and physiological characterization of Arabidopsis pht1;4 high affinity phosphate transporter mutants. Using GUS-gene trap and in situ hybridization, Pht1;4 was found mainly expressed in inorganic phosphate (Pi) limiting medium in roots, primarily in the epidermis, the cortex and the root cap. In addition to this, expression was also observed at the lateral root branch points on the primary root and in the stele of lateral roots, suggesting a role of Pht1;4 in phosphate absorption and translocation from the growth medium to the different parts of the plant. Pi-starved pht1;4 plantlets exhibited a strong reduction of phosphate uptake capacity (40). This phenotype appears only related to the pht1;4 mutation as there were no obvious changes in the expression of other Pht1 family members in the mutants background. However, after 10 days of growth on phosphate deficient or sufficient medium, the Pi content in the mutants was not significantly different from that of the corresponding wild type controls. Furthermore, the mutants did not display any obvious growth defects or visible phenotypes when grown on a low phosphate containing medium. The work described here offers a first step in the complex genetic dissection of the phosphate transport system in planta.     

Effect of different soil and nitrogen fertilizer conditions on 32P-labelled phosphate uptake by two grass species

In two years of trials, roots of ryegrass took up more ³²P-labelled phosphate than roots of fescue. Application of 672 kg N ha^-1 increased phosphate absorption compared with application of 112 kg N ha^-1. Roots in mineral soil absorbed more phosphate than those in peat soil. In both soils uptake decreased as depth of phosphate injection increased from 5 to 30 cm. An interaction occurred whereby roots in the intermediate depth (10–22-5 cm) in peat absorbed less phosphate than in mineral soil and this was apparently unrelated to the exchange or sorption properties of the soil.     

The absorption and translocation of diclofop-methyl and amitrole in wheat and oat roots

The absorption and translocation of diclofop-methyl (methyl 2-[4(2',4'-dichlorophenoxy)phenoxy]propanoate) was examined by using a specially designed treatment apparatus that separated excised roots or roots of seedlings into four zones. [¹⁴C]-Diclofop-methyl was absorbed along the entire root length of both wheat ( Triticum aestivum L.) and oat ( Avena sativa L.). In both species, absorption was greatest in the apical region of the root. Absorption by the apical region of wheat roots was more than three times greater than the basal portions, and more than twice as great as the apical region of oat roots. Less than 5% of the absorbed diclofop-methyl was translocated in both wheat and oat roots. Diclofop-methyl and diclofop(2-[4(2',4'-dichlorophenoxy)phenoxy]propanoic acid) were the predominant translocated forms. The absorption and translocation of amitrole (3-amino-1,2,4-triazole) were also examined. Amitrole was absorbed along the entire length of wheat roots and translocated primatily in the basipetal direction. The usefulness of the specially designed apparatus for biochemical and physiological studies is discussed.     

Nitrate Uptake and Partitioning by Corn Root Systems : Differential Effects of Ammonium among Genotypes and Stages of Root Development

The relative effects of ammonium on nitrate uptake and partitioning during induction were compared among decapitated seedlings of three corn (Zea mays L.) genotypes at two developmental stages. This study tested the hypothesis that root systems efficient at translocating products of ammonium assimilation away from sites of nitrate uptake or reduction would exhibit less inhibition of nitrate uptake by ammonium compared to root systems with inefficient N translocation efficiency. Inhibition of nitrate uptake by ammonium was relatively slight at day 5 ranging from 0% to 20% among the three genotypes, as compared to greater inhibition, from 20% to 37%, at day 8. Five-day-old roots exhibited negligible xylem translocation capacity in comparison with those grown for 8 days. Thus, although the capability to translocate ammonium assimilates out of the root increased between days 5 and 8, inhibitory effects of ammonium also increased. In the absence of ammonium, nitrate uptake per unit root mass increased between days 5 and 8. This increased activity of the uptake system was proportionally more sensitive to ammonium.

Partitioning of entering nitrate into the reduction process was positively correlated with lateral root development of the inbred root systems at 5 and 8 days. This is supportive of a localization of a major portion of nitrate reduction occurring in root apical regions. Nitrate reduction was the partitioning process most severely inhibited by ammonium in all cases, ranging from 39% to 55% inhibition. In contrast, ammonium-inhibition of nitrate accumulation in the root tissue and translocation via xylem vessels varied with genotype and root age.

Two mechanisms of ammonium-inhibition of nitrate are implicated, one which directly affects nitrate reduction and the uptake system associated with it, and another which may involve potassium as an intermediate regulator of nitrate accumulation in the root tissue and nitrate translocation out of the root tissue.

     

Diffusion of phosphate to plant roots in soil

Summary Autoradiographs of rape (Brassica napus L.) seedlings growing in a Begbroke Sandy Loam treated to different P levels showed P accumulations near root apices of primary and lateral roots, without corresponding depletion from the adjacent soil, indicating marked translocation.Laterals less than 2 days old did not deplete the soil despite considerable P accumulations in them. Their growth and P uptake were enhanced when the growth of the primary root was checked. The length of root hairs decreased markedly with increasing P supply.The P depletion zones developed in the same way at all points along the primary axis (except for a short length behind the apex). At the highest P level the concentration of exchangeable P at the root surface was lowered by about 30% on day 2, by about 40% on day 4 and rose slowly after day 8.Whereas in P treated soils the depletion from within the root hair cylinder was fairly uniform, in the low P soil there was a continuous decrease in P concentrations toward the root surface, within the root hair zone.Soil Science Laboratory, Department of Agricultural Science, University of Oxford