Effects of water stress on phosphorus transport to the xylem

Summary Tomato plants were treated for one hour in nutrient solutions at-10.4 atm. Roots were excised, transferred to solutions at-0.4 atm and put into a pressure chamber to induce rates of water flow similar to those in transpiring plants.For roots continuously at-0.4 atm, the xylem sap had much higher phosphorus concentrations than the external solution, which contained 6 p.p.m. phosphorus.Roots previously treated at-10.4 atm had much lower concentrations in the sylem sap than in the external solution and the amount of phosphorus transported and the water flow were linearly related. This phosphorus transport was due to passive movement as shown by measuring transport of both ³²P and ¹⁴C mannitol. Thus transport to the xylem mediated by active processes was abolished even though uptake by the roots remained substantial. These results obtained after plasmolysis support the view that radial transport to the xylem includes uptake into and movement through the symplast.     

Relation between Anion Transport and Water Flow in Tomato Plants

Lowering the water potential of culture solutions from ?0.4 to ?5.4 atm reduced both phosphorus and bromide transport to the shoot, hut the content in the roots was not affected. Reductions in phosphorus transport to the shoot were measured during the first four hours of treatment and were related to concurrent decreases in water flow and not to an impairment of active phosphorus transport. The effect of low water potential on phosphorus transport to shoots was similar at external phosphorus concentrations between 0.6 and 15 mg/l. Phosphorus transport was greater in the dark at ?0.4 atm than in the light at ?5.4 atm even when these treatments gave the same overall rates of water flow; this is attributed to a different pattern of water flow through the various root zones. The results suggest that the main effect of water flow on anion transport to shoots occurred after the ions had been actively adsorbed by the roots and was not due to mass flow increasing ion delivery to sites of active uptake.     

Accumulation and radial transport of ions from potassium salts by cucumber roots

Accumulation of K(+) is insensitive to the anion supplied with it at a solution concentration below 1 mm. Rates of K(+) transport to the xylem from the same solutions are, however, dependent upon the anion present and decrease in the order NO(3) (-) > Cl(-) > SO(4) (2-). Parallel effects on rates of exudation and anion transport result from kind and concentration of anion supplied and time of exposure to the solution. When high K salt concentrations are used, only linear relationships are found between solution concentrations and transport rates. However, ion concentration in the exudate increases more than external solution concentration, while exudation rate is unaffected. It is suggested that some of the ions transported are from compartments within the cells. At high solution concentrations KNO(3) results in more exudation and in higher ion concentration in the exudate than is found with KCl.     

Effects of low water potential on ion uptake and loss for excised roots

Summary In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.Lowering the water potential greatly increased the rate of loss of potassium and bromide from the cytoplasm, but the increases in loss from the vacuoles were much smaller. The results suggest that the mechanisms of ion uptake are not affected by low water potential and that the decrease in ion accumulation is caused by the increased leakage from the cells.     

Quantitative evaluation of the role of organic acid exudation in the mobilization of rock phosphate by rape

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.     

Effects of Water Deficit on Phosphorus Nutrition of Tomato Plants

Measurements were made of phosphorus uptake by intact tomato plants from solutions labelled with ³²P. The plants were exposed to low water potentials by the addition of mannitol to culture solutions. The amounts of labelled phosphorus in the roots and in the shoots wore determined after a one- or two-hour period. Down to -5.4 atmospheres, the amount of labelled phosphorus in the roots remained constant, hut the amount transported to the shoots was reduced. However, potentials of -10.4 atm reduced the amount of labelled phosphorus in both the root and the shoot. Similar results were obtained when plants were tested immediately after water stress was imposed and when tested after water potentials had been lowered gradually. Plants were treated for one hour at low water potentials and then returned to control solutions (?0.4 atm). For a considerable time, these plants had a much lower phosphorus uptake than plants which had remained continuously at ?0.4 atm. These data support the idea that a disturbance in mineral nutrition is partly responsible for reduced growth in plants which experience a moderate water deficit.     

Evidence for Symplasmic Ion Transport in Maize Roots

Excised maize roots, placed in saturated water vapour to limitthe external ionic supply, continued to produce exudates attheir basal ends for at least 24 h. The mean rate of fluid exudationfrom roots in water vapour was about 28 per cent of the correspondingrate in ‘control’ roote placed in a solution containing0.1 mil CaCl₂ and 1 mM KC1. Moreover, the net fluxes (mean ±S.E.)of potassium and calcium ions into the exudate were reducedfrom (35.8±3.2) x 10^– and (4.37±0.39) xlO^–9 mole cm^–2 h^– for roots in solution to(10.9±0.6) x 10^–9 and (l.00±0.06)x 10^–9molecm^–2 h^–1 respectively for roots in vapour. It isconsidered that the observation of a prolonged exudation ofwater and ions from the roots placed in water vapour demonstratesthe existence of an alternative ionic supply within the roottissue itself and that this parallel route of ion transportto the exudate constitutes the cortical symplasmic pathway. Pre-treatment of the excised roots with 0.8 M mannitol beforeexudation studies in water vapour and solution led to a significantreduction in the rates of fluid and ion exudation which hadbeen observed in untreated roots under similar conditions. Itis concluded that the plas-molysis, induced by mannitol, disruptedthe symplasmic connections between root cells and that thisperturbation significantly reduced the operation of the symplasmicmode of ion transport into the exudate.     

Selectivity and Kinetics of Ion Uptake by Barley Plants Following Nutrient Deficiency

Barley plants grown without an external supply of phosphorus,sulphur, chlorine or nitrogen subsequently absorbed these nutrients,as phosphate, sulphate, chloride and nitrate, more rapidly thandid nutrient, sufficient control plants under similar conditions.With phosphorus, sulphur and chlorine, increased absorptionwas restricted to the nutrient which had been deficient, orto close chemical analogues of it, the uptake of other anionsbeing unaffected or decreased. The selectivity of enhanced nitrateuptake by nitrogen-deficient plants was not examined. The differencesin the rates of phosphate, sulphate and chloride absorptionby plants of differing nutrient status were due principallyto changes in the maximum transport capacity for these anionsper unit weight of root, although in plants grown without externalchloride there was some evidence that the roots also developedan increased affinity for that ion. Hordeum vulgare, barley, mineral nutrient deficiency, ion absorption, kinetics of ion uptake, phosphate, sulphate, chloride, arsenate, bromide, selenate     

Spontaneous phloem exudation accompanying abscission in Lupinus mutabilis (Sweet)

Like some of the Mediterranean members of the genus Lupinus, the New World lupin, Lupinus mutabilis (Sweet; cv. Inti), exhibits prolonged (20-40 min) exudation of phloem sap from incisions made in stems, in the raceme and at the tips and sutures of developing fruits. Just prior to or immediately following abscission of flowers of L. mutabilis there was also spontaneous exudation from the proximal face of the abscission zone at the base of the pedicel. This is not a recorded feature of other lupins. Analysis of solutes in this exudate was consistent with its having been derived directly from phloem. The major solutes were sucrose (0.940.04 M), amino acids (18811 mM, 45% as asparagine and 15% as glutamine), K ion (52 mM), and total phosphorus (17 mM). Microscopic examination of the proximal face of the pedicel abscission zone at or following abscission showed little or no breakage of the cells at the zone. The major solutes of spontaneous exudate were similar to those in exudates collected from incisions made in the supporting raceme, upper stem and branches, at the tips and sutures of developing fruits and in the mid- and basal stem regions. However, there were significant compositional differences among minor constituents. The spontaneous exudate had a higher level of Ca ion and, consequently, a narrower Mg/Ca ratio (2.8) than exudate from incisions in the adjacent raceme (9.3) or fruits (15.7). There were also higher concentrations of trace elements (Mn, Zn and Cu) but lower concentrations (3 ng m^-1) of cytokinins compared to exudates collected from incisions (20 ng ml^-1). The relative contents of K and Na ions in exudates from incisions at different sites on the plant showed evidence of selective phloem loading and downward translocation of Na ion and selective loading and upward translocation of K ion.     

Potassium transport in salt-stressed barley roots

Salinization of the medium inhibits both K⁺ uptake by excised barley (Hordeum vulgare L.) roots and K⁺ release from their stele, as measured by short-term ⁸⁶Rb uptake and xylem exudation, respectively. Although inhibition was not specific to chloride, mannitol caused a different response from that of inorganic sodium salts, indicating that inhibition was at least partly the result of an ion effect. In roots previously exposed to low levels of NaCl, NaCl stress directly affected stelar K⁺ release, whereas in low-sodium roots stelar K⁺ release was much less salt-sensitive than K⁺ uptake.Abbreviation chCl choline chloride     

The origin and composition of cucurbit "phloem" exudate

Cucurbits exude profusely when stems or petioles are cut. We conducted studies on pumpkin (Cucurbita maxima) and cucumber (Cucumis sativus) to determine the origin and composition of the exudate. Morphometric analysis indicated that the exudate is too voluminous to derive exclusively from the phloem. Cold, which inhibits phloem transport, did not interfere with exudation. However, ice water applied to the roots, which reduces root pressure, rapidly diminished exudation rate. Sap was seen by microscopic examination to flow primarily from the fascicular phloem in cucumber, and several other cucurbit species, but primarily from the extrafascicular phloem in pumpkin. Following exposure of leaves to 14CO2, radiolabeled stachyose and other sugars were detected in the exudate in proportions expected of authentic phloem sap. Most of this radiolabel was released during the first 20 s. Sugars in exudate were dilute. The sugar composition of exudate from extrafascicular phloem near the edge of the stem differed from that of other sources in that it was high in hexose and low in stachyose. We conclude that sap is released from cucurbit phloem upon wounding but contributes negligibly to total exudate volume. The sap is diluted by water from cut cells, the apoplast, and the xylem. Small amounts of dilute, mobile sap from sieve elements can be obtained, although there is evidence that it is contaminated by the contents of other cell types. The function of P-proteins may be to prevent water loss from the xylem as well as nutrient loss from the phloem.     

Effect of cycloheximide, gamma irradiation, and phosphorus deficiency on root pressure exudation in tobacco

Cycloheximide drastically reduced the rate of root pressure exudation in detopped tobacco (Nicotiana tabacum L.), and the effect was more pronounced for nitrate salts in the external solution than for some other salts or when the roots were in water. Diurnal periodicity was greatly decreased, and its phase was changed. Effects began within an hour. Cation uptake was reduced by relatively low levels of cycloheximide. The effects of cycloheximide seemed to be reversible. Mild phosphorus deficiency resulted in decreased exudation rates from detopped tobacco and in no response to nitrate. Periodicity of exudation was not greatly affected by phosphorus deficiency, however. Gamma ray irradiation with a ⁶⁰Co source at levels (up to 40-50 kiloroentgens) which are considered disruptive of moderately large molecules had relatively little effect on the exudation rate. Higher levels of irradiation, which disrupt most protein molecules, decreased exudation and obscured periodicity. The results indicate either that new protein (or peptide) synthesis is needed for the rapid nitrate transport or that the deficiency and inhibitor disrupt cellular membranes. Phosphorus deficiency increased the sensitivity of the plants to inhibition by irradiation of the exudation process.     

Radial salt transport in corn roots

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The ³²P uptake, O₂ consumption, and dehydrogenase activity of the root cortex and stele also were studied.

These roots produced copious root pressure exudate containing 4 to 10 times the concentration of ³²P in the external solution. Freshly separated stele from 5-day-old roots accumulated ³²P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated ³²P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, ³²P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.

These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.

     

Mannitol-enhanced, fluid-phase endocytosis in storage parenchyma cells of celery (Apium graveolens; Apiaceae) petioles

We recently demonstrated the occurrence of a sucrose-enhanced, fluid-phase endocytic (FPE) mechanism of nutrient uptake in heterotrophic cells. In the present work, the possible enhancement/induction of FPE by photoassimilates other than sucrose was investigated by measuring the incorporation of the fluorescent endocytosis marker d-TR (dextran-Texas red, 3000 mw) into celery (Apium graveolens) petiole storage parenchyma (CSP), a tissue that transports and accumulates mannitol. Mannitol uptake in these cells is biphasic, with a hyperbolic phase at concentrations below 20 mM and a linear phase above 20 mM external solute concentration. In the absence of mannitol, or in its presence at concentrations within the hyperbolic phase, CSP cells accumulated low levels of d-TR. Conversely, d-TR accumulation by CSP cells was greatly enhanced in the presence of mannitol at concentrations within the linear phase. At high external mannitol concentration, d-TR accumulation was prevented by the endocytic inhibitors LY294002 and latrunculin B. In addition, d-TR uptake was temperature dependent under high mannitol concentration. Microscopic observations revealed that d-TR accumulated in the vacuole. These data support the occurrence of an FPE mechanism in CSP cells that participates in trapping and transport of photoassimilates to the vacuole. The FPE mechanism is enhanced by high mannitol concentrations.     

A kinetic study of phosphorus absorption by excised maize roots from flowing solutions with different phosphorus concentrations

The effect of two different mechanisms of phosphorus ion transport from the nutrient solution volume to the surface areas of excised maize roots was studied under concentrations ranging from 0.01 mM to 50.0 mM KH₂PO₄. A modified technique of study of kinetic ion absorption was used. In the control series, the roots were placed in absorption solution without flow (the dominant mechanism of ion transport to the roots being diffusion), while in the experimental series the absorption solution was flowing round the roots at a rate of 0.162 cm s^?1 (the dominant mechanism of ion transport to the roots being mass flow). The rate of phosphorus absorption by the roots from flowing solutions was highly significantly increased at all concentrations of absorption solution except for the 50.0 mM KH₂PO₄ concentration. The increase in phosphorus absorption in the case of 50.0 mM KH₂PO₄ concentration was non-significant due to the fact that the high concentration of phosphorus together with the diffusion of phosphorus ions ensured a sufficient supply of phosphorus to the roots, covering the requirement for their uptake. The results point to the need for an analysis of environmental factors to be carried out in studying ion absorption kinetics, and reveal the inadequacy of methods usually employed in such investigations, in particular with respect to the homogeneity of the nutrient solution in the whole of its volume and especially round the roots.     

l-Malate as an Essential Component of the Xylem Fluid of Corn Seedling Roots

Corn seedling xylem exudate has a pH of 5.30 +/- 0.05 due to the presence of 10 millimolar malate which has a pK(a) of 5.13. This concentration of malate tends to buffer the xylem fluid at this pH. Exogenous treatment of corn seedling roots with CaCl(2) caused a concentration-dependent decrease in the pH of the xylem fluid as well as a decrease in the volume of fluid secreted into the xylem. Exogenous 50 millimolar CaCl(2) decreased exudate volume to 8% of control within 0.5 hour. Nitrate and malate deposition into the xylem was prevented by pretreatment in 5 millimolar CaCl(2); nitrate deposition was resumed shortly after resumption of malate deposition. Fifty millimolar l-tartrate, an inhibitor of the Cl(-), malate-activated ATPase (in vitro) of the tonoplast, also decreased exudate volume as well as slightly lowering exudate pH. The osmolality of the exudate was found to be constant at 70 +/- 11 milliosmomoles per kilogram in all treatments. Dixon plots (log of exudation rate versus pH of exudate) indicated a pK(a) of 5.11 for the exudation process which is very close to the pK(a) of l-malate (5.13). In addition, a Dixon plot of the l-glutamine deposition (l-glutamine being the major form of reduced nitrogen in the exudate) versus pH also indicated a pK(a) near 5.15.The pH optimum for glutamine transport into the xylem was 5.5. Deposition of glutamine into the xylem may be regulated by the xylem pH (5.30 +/- 0.05) which in turn may be regulated by the presence of 10 millimolar malate. It is proposed that the transport of glutamine into the xylem may provide the driving force for the exudation process.     

Nitrogen pretreatmentsvs nitrate treatments after detopping on xylem exudation in tobacco

Summary Nitrate salts resulted in a large stimulation of exudation from detopped tobacco and also increased cation transport to the exudate, but only if nutrient solutions applied previously to detopping had become depleted in salts. Such plants were not necessarily nitrogen deficient. Pregrowth in ammonium nitrogen resulted in the same effect as pregrowing with low amount of salts. Pregrowth with high levels of salt other than nitrate, however had the same effect as pregrowing with a high level of nitrate. There was no evidence that synthesis of nitrate reductase was necessary for the stimulation of exudation by nitrate. The nitrate response was usually apparent within 1 h. Glucose added with nitrate at 10°C resulted in prolonged and increased exudation for 32 days following detopping. Mg (NO₃)₂ as a single salt was toxic in terms of exudation production. Only one cycle of response to nitrate on exudation could be obtained from detopped plants. A cycle could be obtained when nitrate was added many days after detopping if nitrate had not been added previously since detopping.     

Characteristics of Radial Solution Flow in Roots of Cucumber (Cucumis sativus L.)

Low salt roots of Cucumis sativus L. cv. Burpeeana Hybrid weresubjected to iso-ionic treatments in which the external solutionconcentration of K⁺ was maintained at 14 mM. Solution concentrationof varied from 0 to 14 mM, other anions compensating. When Cl^– was the compensating ion, its concentrationin the exudate increased during the first 4 h and thereafterwas nearly the same as that of the external solution in alltreatments containing I mM or more. After 8 h of equilibration the concentration in the exudate increased almost exactly as its concentrationin the external solution. Rates of exudation and K⁺ transportwere almost constant between I and 14 mM KNO₂. More Cl^–was transported from solutions of similar Cl– concentrationwhen was also present. When water transport was inhibited with mannitol in treatments containing both KNO₃and KCI, exudate concentrations of K⁺ and were increased, but exudate concentration of Cl^– was notsignificantly affected except at the highest Cl     

Low-temperature pretreatment of the root system of Brassica rapa L. plants: effects on the xylem sap exudation and on the nitrate absorption rate

Brassica rapa plants were exposed for a 52 h period (as pretreatment) to a differential temperature (DT) between roots (5°C) and shoots (20°C), while control plants were maintained with both shoot and roots at 20°C (warm grown = WG). Measured at 20°C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration flows were similar in pretreated and control plants. Both transpiration and exudation flows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate flow from the pretreated roots was associated with a two-fold increase in nitrate uptake rate. The relationship of the cold-induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.