Potassium Fluxes during Potassium Absorption by Intact Barley Plants of Increasing Potassium Content

The presence of previously absorbed K in plants caused a marked reduction in the short term influx of ⁸⁶Rb-labeled K into roots of barley seedlings. The influx values agreed with net K absorption rates into intact plants, thus suggesting that K efflux was negligible in comparison with influx.     

The Interactions Between Monovalent Cations and Calcium During Their Adsorption on Isolated Cell Walls and Absorption by Intact Barley Roots

The adsorption of sodium, potassium, rubidium and calcium ionsat different concentrations was measured on cell walls frombarley roots (Hordeum vulgare L. cv. Union), prepared by detergenttreatment. It was found that sodium and calcium interacted verystrongly during their simultaneous adsorption, whereas potassiumdid not interfere with calcium. This has led us to concludethat calcium and sodium are adsorbed on identical sites in thecell wall, whereas potassium is adsorbed at another site. Rubidiumseems to be less specific for both sites and interferes onlymoderately with calcium. The adsorption on cell walls of thesecations was compared with their adsorption on intact roots at2 °C, where beside the cell wall, sites may be availableat the outer surface of the membrane, and further measurementswere made of absorption at 25 °C. The fact that sodium interactswith calcium and potassium alters the ratio of K to Na in thecell wall compared to their concentrations in the medium. Thepreferential shift towards potassium when calcium is presentcould be very important for the rates of initial uptake in lowsalt barley roots, since the membrane is in contact with a differentproportion of K to Na in the cell wall from the one suppliedin the medium. Hordeum vulgare L., barley, absorption of cations, adsorption, calcium, sodium, potassium rubidium     

An Anomaly in Potassium Accumulation by Barley Roots: II. Effect of Calcium Concentration and Rubidium-86 Labeling

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 CaSO₄. 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.     

Mechanism of Zinc Absorption by Intact Bean Plants

The time course of absorption of ⁶²Zn and ³²P by roots of intactbean plants (Phaseolus vulgaris L.), and the effects of azideand amytal have been studied. The evidence indicates that zincuptake occurs mainly by a non-metabolic process.     

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.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. II. The Effects of Abscisic Acid and Cytokinins

Guttation was used as a non-destructive way to study the flowof water and mineral ions from the roots and compared with parallelmeasurements of root exudation. Guttation of the leaves of barley seedlings depends on age andon the culture solution. Best rates of guttation were obtainedwith the primary leaves of 6- to 7-day-old seedlings grown onfull mineral nutrient solution. The growing leaf tissue becomessaturated with K⁺ below 1.5 mM K⁺ in the medium, whereas K⁺concentration in the guttated fluid still increases furtheras K⁺ concentration in the medium is raised. At 3 mM K⁺ averagevalues of guttation were 1.4–2.4 mm³ h^–1 per plantwith a K⁺ concentration of 10–20 mM; for exuding plantsthe flow was 4.2–7.6 mm³ h^–1 per plant and K⁺ concentration35–55 mM. Abscisic acid (ABA) at 10^–6 to 10^–4 M 0–2h after addition to the root medium increased volume flow ofguttation and exudation and the amount of K⁺ exported. Threeh after addition of ABA both volume and amount of K⁺ were reduced.There was an ABA-dependent increase in water permeability (Lp)of exuding roots shortly after ABA addition. Later Lp was decreasedby 35 per cent and salt export by 60 per cent suggesting aneffect of ABA on salt transport to the xylem apart from itseffect on Lp. Benzyladenine (5 x 10^–8 to 10^–5 M)and kinetin (5 x 10^–6 M) progressively reduced volumeflow and K⁺ export in guttation and exudation and reduced Lp. Guttation showed a qualitatively similar response to phytohormonesas found here and elsewhere using exuding roots. Hordeum vulgare L., barley, guttation, abscisic acid, cytokinins, benzyl adenine, kinetin     

Effects of Nitrogen Deficiency on the Absorption of Nitrate and Ammonium by Barley Plants

When young barley plants which had been supplied with nitratewere deprived of this source of N, an enhanced capacity forabsorption of either nitrate or ammonium ions developed, reachinga maximum in about 3 d under the particular experimental conditionsused. The net uptake rate of either nutrient was then approximatelythree times that in plants which had received nitrate throughout.Likewise, withholding external N from plants previously growingwith ammonium caused a 2.4-fold increase in their subsequentcapacity to absorb that ion, compared with control plants grownwith an uninterrupted ammonium supply. Accelerated nitrate uptakein N-starved plants was not accompanied by additional phosphateor sulphate absorption, but the plants had the capacity to absorbmore potassium, whether or not ammonium was also present inthe solution. Indirect evidence from analyses of root tissuesuggests that these responses to mild N-stress may depend onsome property of an N fraction which does not include nitrateor ammonium. Hordeum vulgare, barley, nitrogen, ammonium, nitrate, N-deficiency, absorption     

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.     

Effects of Shading the First Leaf on Growth of Barley Plants: II. Effects on Photosynthesis

The contributions made by photosynthesis in the first leaf toseedling growth have been examined by a variety of methods includinginfra-red gas analysis and the use of ¹⁴CO₂. The first leafis fully expanded by day 8 and maximal rates of photosynthesisare achieved about I day later. Up to day 8 growth of the seedlingsresults from the redistribution of seed reserves and once theseare exhausted growth is dependent upon the first leaf, beingreduced to very low levels if this is shaded. The second leafwhich begins to expand rapidly after day 10 is contributingto growth by day 14, and the contribution from the first leafbegins to decline after day 12. Apart from greatly reducing photosynthesis in treated leaves,shade also affects the development of photosynthetic capacity.When applied for 2 days or more from day 6, shade reduces thepeak level of carbon fixation achieved on days 9 and 10 by upto 35 per cent. It is shown that development of the first leafin terms of increased dry weight and photosynthetic capacity,is dependent on photosynthesis in the developing leaf itself.The mechanisms by which shading affects development are discussed.     

The Relationship between Transpiration and the Absorption of Inorganic Ions by Intact Plants

The relationship between the rate at which water and the rubidiumand phosphate ions are absorbed by intact plants, and transferredto their shoots has been investigated in water culture undervarying conditions of transpiration and nutrient supply. When the external concentration and the nutrient status of theplants are sufficient low, wide variations in the rate of transpirationhave little effect on the transfer of nutrients to shoots; whenlittle water is being lost by transpiration the concentrationin the transpiration stream may exceed that in the externalmedium by factors exceeding 100. In contrast when the externalconcenration and the nutrient status of the plants are highthe rate of transfer of ions to shoots may vary closely withthe rate of transpiration and the concentration in the transpirationstream may be similar to, or less then, that in the externalmedium. The occurrence of concentrations of ions in the roots is transpirationstream which greatly exceed those in the medium external tothe roots is regarded as evidence that ions not transferredpassively across the roots of intact plants to a significantextent.     

Nitrate Absorption by Barley: II. Influence of Nitrate Reductase Activity

The influence of protein synthesis and nitrate reductase activity on nitrate absorption by barley (Hordeum vulgare L.) was investigated. Cycloheximide decreased nitrate absorption. Pretreatment studies showed that cycloheximide affects either energy transfer or nitrate reductase activity or both.     

The Effect of Root Temperatures on Water and Sulphate Absorption in Intact Sunflower Plants

The effect of 15, 25, and 35°C root temperature on waterabsorption, transpiration, and sulphate uptake by the rootsand transport to the shoots of intact sunflower plants has beenstudied using 0.5, 5.0, and 50.0 mM sulphate concentrationsat two rates of transpiration induced (1) by light and low relativehumidity and (2) by darkness and high relative humidity. Root temperatures and sulphate concentrations did not significantlyaffect the water absorption and transpiration and both theseprocesses were approximately similar at the different treatments.There was a nearly twofold increase in water absorption andtranspiration in the light and low relative humidity as comparedto the dark and high relative humidity irrespective of the roottemperatures and sulphate concentrations. The A.F.S. uptake in the roots was found to be independent ofthe root temperatures, sulphate concentrations, and transpirationrates, and amounted to 15 to 21 per cent based on the root weight.Sulphate accumulation in the roots was not significantly influencedby the root temperatures at 0.5 and 5.0 mM sulphate concentrations,but nearly doubled with temperature at 50.0 mM sulphate concentrationof the external solution. The slow nature of accumulation ofsulphate, the high sulphate status of the experimental plants,and the short duration of the experiments are considered aslikely reasons for the absence of a clear effect of temperatureson accumulation of sulphate at the two lower concentrationsof the external solution. Effects of high concentration on permeabilityand metabolism of the cells are suggested as the reasons forthe decreased accumulation with an increase in temperature at50.0 mM sulphate concentration. Accumulation of sulphate inthe roots was not significantly influenced by the transpirationrates. Unlike root accumulation, sulphate transport to the shoots increasedwith increasing transpiration. However, a major part of thesulphate transport (70 to 75 per cent at 0.5 and 5.0 mM sulphateconcentrations and 80 to 85 per cent at 50.0 mM sulphate concentration)appeared to have occurred at the low transpiration. The similarityof this transport to the accumulation of sulphate in the rootsindicates that it was due to an active transport process sensitiveto root temperatures and sulphate concentrations. A low concentrationof sulphate in the xylem and an increased permeability of theroot cells to ion movement induced by an increased suction inthe xylem are considered as reasons for a small increase inthe sulphate transport at high transpiration rate. The evidencefor the existence of a barrier—probably endodermis—preventingthe passive diffusion of sulphate and sensitivity of the TranspirationStream Concentration to root temperatures and sulphate concentrationsfavour that the increased transport with increased transpirationwas due to an active process.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. I. Structural and Cytological Aspects

The structural details of the guttating tips of 7-day-old barleyleaves were studied as a basis for a subsequent report on thephysiology of guttation. The walls of the vessels at the tipsof leaves bear many pits and are rather thin, appearing neithercutinized nor lignified. This could facilitate a direct passageof solutes out of the xylary system through the leaf apoplastand out to the leaf surface via hydathode openings. The latterare formed by modified stomatal guard cells, and there are nospecially differentiated epithem, epithelium or gland hair likestructures that could serve an active elimination of guttation.Xylem parenchyma cells and the peculiar mesophyll cells withdense cytoplasm, numerous mitochondria, an extended ER systemand a considerable formation of small vesicles in the leaf tipcould modify the content of the guttated fluid along the routeof transport. Hordeum vulgare L., barley, hydathodes, guttation     

Influence of Phosvitin and Calcium Gluconate Concentration on Permeation and Intestinal Absorption of Calcium Ions

The effect of egg yolk phosvitin on the permeation and absorption of calcium was investigated in vitro in relation to calcium gluconate concentration. Obtained results indicate that phosvitin significantly reduces the intestinal calcium absorption from 1 and 10 mM of calcium gluconate solution. It is associated with the formation of the complex of Ca (II) ions with phosvitin. The process of calcium permeation increases under phosvitin influence when calcium gluconate concentrations rise up to 10 mM. At a higher concentration of calcium gluconate (20 mM), no effect of phosvitin was seen on permeation of calcium ions.