Characterization of cadmium uptake by plant tissue

The uptake of cadmium by excised root tissue of barley (Hordeum vulgare L. cv. Arivat) was investigated with respect to kinetics, concentration, and interactions with various cations. The role of metabolism in Cd absorption was examined using a range of temperatures, anaerobic treatments, and chemical inhibitors. The uptake and distribution of Cd in intact barley plants was also determined. A large fraction of the Cd taken up by excised barley roots was apparently the result of exchange adsorption and was displaced by subsequent desorption with unlabeled Cd, Zn, Cu, or Hg. Another fraction of Cd which could not be displaced by desorption in unlabeled Cd was thought to result from strong irreversible binding of Cd, perhaps on sites of the cell wall. The fraction of the Cd taken up beyond that by exchange adsorption by fresh roots was a linear function of temperature, and inhibited by conditions of low oxygen and by the presence of 2,4-dinitrophenol. It was concluded that this fraction of Cd entered excised barley roots by diffusion. Diffusion, when followed by sequestering, probably accounts for the accumulation of Cd observed in intact barley plants.     

Cation-Anion Balance during Potassium and Sodium Absorption by Barley Roots

Steady-state rates of potassium ion and sodium ion absorption by excised barley roots accompanied by various anions were compared with the rates of anion absorption and the concomitant H⁺ and base release by the roots. The cation absorption rates were found to be independent of the identities, concentrations, and rates of absorption of the anions of the external solution, including bicarbonate. Absorption of the anion of the salt plus bicarbonate could not account for the cation absorption. H⁺ is released during cation absorption and base during anion absorption. The magnitude by which one or the other predominates depends on the relative rates of anion and cation absorption under various conditions of pH, cation and anion concentration, and inhibitor concentrations. The conclusion is that potassium and sodium ions are absorbed independently of the anions of the absorption solution in exchange for H⁺, while anions are exchanged for a base. The H⁺ release reflects a specificity between K⁺ and Na⁺ absorption such that it appears to be H⁺ exchanged in the specific rate-limiting reactions of the cation absorption.     

Effect of Removal of a Part of the Root System on the Subsequent Growth of the Root and Shoot

Removal of up to 50 per cent. of the roots of barley and ryehas no effect on the growth-rate of the root which is the sameas in the intact plant. In contrast the growth-rate of the shootdecreases as more roots are removed. When more than 50 per cent.of the roots are removed, root growth declines but not so rapidlyas that of the shoot. Similar results are obtained by the removalof lateral roots of tomato but root growth begins to declinewhen 40 per cent. of the lateral roots are removed. The uptake of potassium by barley plants with proportions ofthe root system excised is closely proportional to the dry-matterincrease when the nutrient supply is not limiting. In conditionsof low nutrition the potassium uptake is less than the dry-matterincrease.     

Interactions in the Absorption of Monovalent Cations by Excised Radish Roots

Preferential absorption of potassium over sodium has been observedwith excised radish roots using a wide range of concentrationsin the bathing medium. This result is contrary to the situationobserved in most other plants which have been investigated,where it is found that at high external concentrations (>1·0mM) the uptake of potassium is less specific and the rate ofsodium absorption exceeds that of potassium. In radish rootscalcium does not interact with the monovalent cation absorptionin the higher range of concentration and the sodium absorptionis not sensitive to chloride-sulphate substitution. These resultsare discussed in relation to salinity-tolerance and potassium:sodiuminteractions.     

Calcium inhibition of potassium absorption in corn roots

Calcium (or magnesium) sulfate or chloride was found to inhibit energy dependent potassium transport in excised corn roots. This Ca²⁺ inhibition of K⁺ transport was most pronounced during the initial phases of transport. As the absorption periods were lengthened the effect of Ca²⁺ gradually changed from an inhibition to a typical promotion (after about 30-45 mins) of K⁺ transport. Kinetic analysis indicated the inhibition to be of a non-competitive nature.

Identical experiments with excised barley roots showed that CaSO₄ had no effect on K⁺ absorption whereas CaCl₂ had a typical stimulatory effect on K⁺ absorption. Kinetic analysis indicated that both corn and barley have efficient K⁺ transporting systems but barley roots are approximately 5 times more active (on a fr wt basis) than corn roots.

These results illustrate the hazards involved in applying results obtained with 1 (or even several) plant species to all species.

     

Further studies on cation uptake from bentonite suspensions by excised barley roots

Summary The net uptake of Na, K, Li, and Ca or Mg by excised barley roots was studied from bi-ionic and tri-ionic bentonite suspensions. The net uptake of Li from Li-Ca system progressed linearly with progressive Li levels and was related to the concentration of soluble lithium. Calcium in this system was taken up only at the 100 per cent Ca level. At lower Ca levels calcium was lost from the roots to the suspensions. In K-Mg and Na-Mg systems the net uptake of Na or K by the excised roots was related to the concentration of the cation in the solution phase. Magnesium uptake took place at 80 and 100 per cent Mg levels. It was much less than that of K or Na at similar levels. At lower levels of Mg the roots lost some of their initial Mg contents to the suspensions. In the Na-K-Mg system magnesium was not taken up by the excised roots. Sodium uptake was not practically affected by the Mg level, but K uptake was greatly enhanced by magnesium.     

An anomaly in potassium accumulation by barley roots: I. Effect of anions, sodium concentration, and length of absorption period

Excised barley roots accumulated 40 to 50% more K(+) from 0.04 mm than from 0.06 mm KCl when incubated for 24 hours in KCl solutions containing 0.2 mm CaSO(4). This phenomenon was not markedly influenced by the rate of absorption of the counteranion. The presence of Na(+) in the treatment solutions decreased total K accumulation but did not alter the K(+) concentration at which the accumulation peak occurred. Short interval studies indicated that this phenomenon is easily observable after 4 hours and begins to become apparent within 2 hours. In comparison with barley, accumulation of K(+) by excised wheat roots decreased as KCl concentration was increased from 0.02 to 0.06 mm; but K(+) accumulation curve for corn roots showed no peaks or depressions in the concentration range of 0.01 to 0.1 mm. A normal hyperbolic curve was noted for the accumulation of Na(+) from 0.01 to 1 mm NaCl by barley roots.     

The Relation between the Rate of Nutrient Uptake by Excised Barley Roots and their Content of Sucrose and Reducing Sugars

A re-examination of the results of previous experiments hasshown that the positive regression of the rate of nutrient uptakeby excised barley roots on sugar content is attributable tothe reducing sugar fraction. Partial regression coefficientsof the rates of uptake of potassium, nitrogen, and phosphoruson reducing sugar content were all positive; those on sucrosecontent were either not significant or negative. These results were confirmed for potassium by experiments inwhich the sugar content of the roots was varied by varying thenitrogen supply to the plants from which the roots were taken,by shading them or by supplying sucrose to the solution in whichthe excised roots were held.     

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.     

The Salt Absorption Isotherm

It is shown that the absorption isotherm of rubidium by excised barley roots can be explained either by two uptake mechanisms following Michaelis-Menten kinetics or by two mechanisms, one actively transporting salts into the tissue (the pump), the other one being more passive in nature (the leak), operating in either direction, depending on external and internal substrate concentration. Kinetic data are thus consistent with more than one transport model. It was further demonstrated for the pair K-Na, that a competitor not only reduces salt uptake but can also reverse the direction of net flux. This observation cannot be explained by classical enzyme kinetics, it is, however, consistent with the pump and leak system. Just as Michaelis-Menten kinetics, the pump and leak system can explain ion competition, in addition it offers a possible explanation of the Viets' effect and it can explain the time curve of absorption.     

ON THE EFFECTS OF DMSO IN CATION TRANSPORT BY EXCISED BARLEY ROOTS

Dimethyl sulfoxide (DMSO) in concentrations of up to 10% by volume stimulates the uptake of zinc by excised barley roots. In the same concentration it severely depresses uptake of sodium and of rubidium. It does not seem to affect the permeability of the membrane since roots treated with desorption solutions which were 10% in DMSO did not lose more of the preferred ion than did roots desorbed in solutions not containing DMSO. Oxygen utilization (measured in the Warburg respirometer) was reduced when DMSO was present. It is suggested that DMSO is a poisoning agent which interferes with cation transport by attacking some aspect of metabolism and not by influencing the permeability of the membrane.     

Influence of Calcium and Magnesium on Manganese Absorption

Mutual effects between Mn, Ca, and Mg were studied during steady-state absorption experiments with excised barley roots. Calcium appeared to enhance the rate of Mn absorption; whereas, Mg had a highly depressive effect. The combination of both Ca and Mg was even more inhibitory to Mn absorption than Mg alone. Manganese had no effect on the usual negligible Ca absorption by this tissue, but effectively inhibited the absorption of Mg. Although divalent cation absorption from the Ca-Mg-Mn system was essentially nil, K absorption was greatly stimulated in the presence of these cations.These mutual effects and others reported in the literature are explained by the hypothesis that selectivity in ion absorption results from cation-induced conformational changes in the structure of the carrier molecule.     

Al uptake kinetics in roots of Melastoma malabathricum L. – an Al accumulator plant

The mechanism of Al uptake in melastoma (Melastoma malabathricum L.), which accumulates Al in excess of 10 000 mg kg^–1 in its leaves and roots, was investigated. Al uptake kinetics in excised melastoma roots showed a biphasic pattern, with an initial rapid phase followed by a slow phase. It was indicated that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. On the other hand, Al uptake rate in roots of whole melastoma plant was almost double that in excised roots. The difference of Al uptake rate between excised roots and whole plant seems to be due to transpiration-depended Al uptake. Results from a long-term experiment showed that different characteristics of Al accumulation between melastoma and barley was caused by the difference in capacity to retain Al in root symplast, rather than by the difference in uptake rate into symplast. Concentrations of oxalate in root symplastic and apoplastic fractions, and total oxalate in shoots and roots, did not change greatly with time of Al exposure compared to Al concentration, although oxalate is considered as a main Al ligand in tissue of melastoma. On the other hand, oxalate exudation to root apoplast was induced within 24 h of Al exposure; the role of such exudation was discussed.     

Effect of calcium on the absorption and translocation of heavy metals in excised barley roots: Multi-compartment transport box experiment

Summary The effect of Ca on the absorption and translocation of Mn, Zn and Cd in excised barley roots was studied using a multi-compartment transport box technique. A radioisotope (⁵⁴Mn,⁶⁵Zn or^115mCd)-labelled test solution was supplied to the apexes of excised roots and the distribution pattern in the roots was examined in the absence or presence of Ca. Results obtained were as follows. Addition of Ca to the test solution reduced the absorption of Mn and inhibited drastically its translocation in excised roots. With increasing concentrations of Ca in test solutions, its inhibitory effects on the absorption and translocation of Mn became severe. Similar results were observed for the absorption and translocation of Zn. Ca in the test solution decreased the absorption and inhibited drastically the translocation of Zn; as in the case of Mn, higher concentrations of Ca had severe effects on these functions. It was also evident that the addition of Ca to the test solution reduced the absorption of Cd at all levels of Cd concentration (1, 10, and 100 μM). Cd absorption decreased with increasing concentrations of Ca in the test solution. However, Ca accelerated the translocation of Cd in excised roots supplied with test solutions containing up to 10μM Cd. At 100μM Cd, addition of Ca caused a negligibly small acceleration of Cd translocation. The accelerating effect of Ca on Cd translocation, especially “xylem exudation”, decreased markedly with the addition of 2,4-dinitrophenol, but not with the addition of chloramphenicol or p-chloromercuribenzene sulphonic acid. When barley plants were supplied with only CaSO₄ during the entire growing period, that is, plants were not supplied with nutrient solution on the last day of this period, Ca had no accelerating effect on Cd translocation in excised roots.     

The effect of Aging on Ion Uptake by Excised Barley Roots

When excised barley (Hodeum Vulgare L.) roots were aged, the rate of uptake of K and C1 increased raching a maximum in about 14 to 18 h. At its maximum the uptake rate was approxiamately twice that of the freshly excised root materila. Respiratory activity declined markedly during the aging period. Although excision was an essential requirement for uptake enhancement, washing was not. Both the uptake and aging processes were shown ot be unresponsive to the presence or absence of Ca in the tratment solutions. Because of cation exchange properties, the change in the total cation content of the root material was a more meaningful measure of the metabolically mediated cation uptake than was the change in content of the test cation itself. The pluggin of xylem vessels with protein and pectin-like substances was observed to increase with aging. It is proposed that the occlusion of the vessels may account for an apparent increaase in uptake since the obstruction could reduce a concurrent loss of ions through the cut ends of the root segments.     

Effects of silicon on enzyme activity and sodium,potassium and calcium concentration in barley under salt stress

Two contrasting barley (Hordeum vulgare L.) cultivars: Kepin No.7 (salt sensitive), and Jian 4 (salt tolerant) were grown in a hydroponics system containing 120 mol m^-3 NaCl only and 120 mol m^-3 NaCl with 1.0 mol m^-3 Si (as potassium silicate). Compared with the plants treated with salt alone, superoxide dismutase (SOD) activity in plant leaves and H⁺-ATPase activity in plant roots increased, and malondialdehyde (MDA) concentration in plant leaves decreased significantly for both cultivars when treated with salt and Si. The addition of Si was also found to reduce sodium but increase potassium concentrations in shoots and roots of salt-stressed barley. Sodium uptake and transport into shoots from roots was greatly inhibited by added Si under salt stress conditions. However, Si addition exhibited little effect on calcium concentrations in shoots of salt-stressed barley. Thus, Si-enhanced salt tolerance is attributed to selective uptake and transport of potassium and sodium by plants. The results of the present study suggest that Si is involved in the metabolic or physiological changes in plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cation uptake from bentonite suspensions by excised barley roots

Summary The net uptake of Na, K, or Ca by excised barley roots as influenced by the amount and kind of other cation was studied from bi-ionic and tri-ionic bentonite suspensions. The net uptake of Na or K from the Na-Ca or K-Ca system followed the concentration of soluble Na or K in the system. Calcium in both systems was taken up by the excised roots only at the 80 and 100 per cent Ca levels. At lower levels of Ca, the roots lost some of their initial calcium contents to the suspensions. The net uptake of Na or K from the Na-K system was in agreement with the concentrations of soluble Na or K in the system. At the various levels of Na or K, some of the initial calcium of the roots was lost to the suspensions. In the Na-K-Ca system, Na uptake gradually decreased with the increase of Ca level. Calcium was not absorbed by the roots at the various Ca levels. However, calcium greatly enhanced K uptake from this system. Part of a dissertation submitted by the senior author in partial fulfillment of requirements for the Ph. D. degree.     

On the mechanism of anion uptake by plant roots

Summary The effect of the valence of the associated cation on Cl^–-uptake by excised barley roots grown in CaSO₄ has been studied at 26°, 6° and 2°C. The uptake of Cl^– relative to that of the associated cation was found to increase in the order: trivalent > divalent > monovalent. This was explained on the expected effect of the cation on the negative charge and potential of root surfaces. A lyotropic order was observed in case of monovalent cations, whereas divalent cations showed no such order. The order observed in Cl^–-uptake from chloride solutions of monovalent cations is associated with the ability of the absorbed cation to remove Ca and Mg from the roots. Li⁺ behaved similar to divalent cations in affecting the relative Cl^–-uptake from LiCl.As to the effect of temperature on the uptake of Cl^– and associated cation, it appears that Cl^– is not taken up to any great extent at 2°C whereas cations are still adsorbed at this low temperature. This has been explained on the assumption of the presence of negative adsorption spots on the root surface which can hold cations but not anions. It appears that Cl^–-uptake by roots requires the expenditure of energy to overcome repulsion arising from the negative surface.This work is supported by AEC contract AT (11-1) — 34 project 55.     

Temperature and pH interaction in NO3 uptake

Summary The interaction effects of temperature, pH and the presence of the poly-valent cation aluminium on nitrate uptake by excised barley roots have been studied. Results indicate that nitrate uptake increased with temperature at the two pH values studied. NO₃ ⁻ uptake was higher from pH6 than from pH4 at all temperatures in the absence of Al⁺³. The addition of Al⁺³ to the external solution modifies this effect. Different theories are given to explain these interactions.     

Chloramphenicol Inhibition of Salt Absorption by Intact Plants

The influence of chloramphenicol on the accumulation of calciumand rubidium by intact barley plants has been investigated.The uptake of both ions during 24 hrs. was reduced by exposureto the inhibitor for 24 hrs. before the uptake period. The maineffect was on the fractions of the ions which reached shootsand on those present in roots in non-exchangeable forms. Chloramphenicolalso reduced transpiration, but to a considerably smaller extent. The primary effect of chloramphenicol on salt uptake appearsto be on the mechanisms of active transport which convey ionsacross the root. Since chloramphenicol does not affect the uptakeof oxygen, it appears that the active accumulation of saltsis not directly mediated by the electron transfer in respiration.The results are compatible with the existence of a linkage betweensalt absorption and protein synthesis; the nature of this linkageis at present unknown.