The Nutrition of Excised Wheat Roots

From an investigation of the nutrient requirements of Hilgendorf wheat roots, glucose emerged as the only suitable source of carbon and energy, and thiamine as the only vitamin necessary. The roots had the ability to utilise nitrogen from calcium nitrate, urea, alanine, glutamine, allantoin, and hydrolyzed casein.     

Potassium Fluxes in Excised Barley Roots

The method of the modified compartmental analysis for excisedroots has been adopted for measuring K⁺-fluxes and compartmentationin barley (Hordeum distichon) roots. Efflux of ⁴²K and ⁸⁶Rbindicated that more than two intracellular compartments wereinvolved in the tracer exchange; the ⁴²K data clearly showedthe components. On the basis of the efflux behaviour of theapical and more basal tissues of the roots, the three componentsof efflux were attributed to the cytoplasm of differentiated(fast) and meristematic tissues (intermediate) and to the vacuoles(slow exchange) of the roots. A model is proposed on the basisof which, the fluxes corresponding to the meristematic and differentiatedtissues of the root can be estimated. Additionally, fluxes ofthe differentiated root tissues were determined by using effluxdata obtained with root segments without apical tissues. Thedata obtained in both ways compare reasonably well and agreeto independent chemical measurements. Comparison of the ⁴²K and ⁸⁶Rb efflux data show strong discriminationof K^– in favour of Rb⁺ and indicate that ⁸⁶Rb is not suitableas a tracer for K⁺ in efflux measurements, at least with barleyroots.     

Boron Uptake by Excised Barley Roots

Active uptake of boron (B) by excised barley roots is linear with time for at least 1.5 h. Although no evidence was found for accumulation of B against a concentration gradient. this component of B uptake does satisfy other criteria for an active transport process. Transport is inhibited by 0.05 mM 2,4-dinitrophenol, 0.05 mM azide, 5 mM arsenate and 5 mM dicoumarol. Also, uptake is temperature-sensitive, being nil at 2°C and maximal at 34 to 38°C. Boron uptake by barley roots increases with time when they are washed in aerated 0.5 mM CaSO₄ solution. A double reciprocal plot of the B uptake data manifests a series of phases separated by sharp transitions or “jumps”, and is compatible with the concept of multiphasic uptake mechanisms. Kinetic constants and transition points for the various phases were calculated accordingly. The fit of these data was compared statistically to three other relevant models, viz, the dual model, the “single + diffusion” model (a Michaelis–Menten term and a diffusion term), and the negative cooperativity model. In each case, the data were better represented by the multiphasic model.     

Selenium Absorption by Excised Astragalus Roots

Absorption of selenate and selenite by excised roots of Astragalus Crotalariae, a selenium accumulator, and of A. lentiginosus, a non-accumulator, was favored by CaCl(2) and a pH of 4.0. The uptake of selenate and possibly selenite, is metabolically linked. Roots of a number of Astragalus species were examined, and in all cases selenate entered the roots much faster than selenite. In these short-term experiments there was no relation between uptake of the 2 ions and classification of a species as selenium-accumulator or non-accumulator.     

Manganese Uptake by Excised Oat Roots

Uptake of ⁵⁴Mn by excised oat roots from dilute manganese chloridesolutions has been investigated. The time-course of uptake hasbeen analysed into the customary but somewhat arbitrary fastand slow phases. Uptake is not metabolic in either of these.The fast phase (‘exchangeable’ manganese) is essentiallycomplete in about 30 minutes and represents the attainment ofequilibrium in a process of ion-exchange. It is shown that analysesappropriate for enzyme kinetics cannot be applied in this situation,and an alternative formulation is based on Donnan equilibration,taking account of the selectivity of the ion exchanger towardsdifferent counter-ions; the predictions of this latter theoryare compared with the experimentally determined uptake. Theslow phase (‘absorbed’ manganese) may also involveexchange sites, either chemically different from, or more difficultof access than, those involved in the fast phase, or both. Equilibriumwas certainly not reached in three hours in this slow-phaseprocess. Release of manganese, taken up by the roots from manganese chloridesolutions, into calcium chloride solutions does not seem tobe simply the reversal of uptake, particularly with very dilutesolutions. This is particularly shown by the kinetics of uptakeand release, uptake being a much faster process than release.Manganese may transfer from the first phase to the second phase,but there is no evidence that uptake by the roots proceeds inseries from first to second phase. It is considered more likelythat the two phases function independently, linked by the surroundingsolution.     

Uptake of Iodide by Excised Bean Roots

Mesophyll cells of leaf slices of bean (Phaseolus vulgaris L.) absorb six to ten times more K⁺ than Rb⁺ from 0.1 mM single chlorides of these cations. Absorption of ⁴²K⁺ from 0.1 mM⁴²KCl is much more inhibited by low concentrations of Rb₂SO₄ than by K₂SO₄. The isotherm for K⁺ absorption is biphasic in the range 0.1–1.1 mM, and K⁺ is more effective than Rb⁺ in causing transition from phase 1 to phase 2.     

The Presence of Gibberellins in Excised Tomato Roots

Substances having similar physiological properties to the gibberellins(located by the ‘Meteor’ dwarf pea bio-assay) havebeen detected in extracts from excised tomato root. The chromatographicbehaviour of the most active zone is similar to that of gibberellinA₁. Experiments using the d-1 and d-5 mutants of mazie did not indicatethe presence of substances with differential effects on thesetwo mutants.     

Effects of Exudation on Ion Relationships of Excised Roots

In excised tomato roots submerged in solution, mannitol at –2.8 atm increased the phosphorus uptake and decreased the loss of previously absorbed phosphorus. Separate collection of the xylem exudate demonstrated that these mannitol effects were due entirely to a reduced phosphorus flow via exudation. For example, in the case of previously absorbed phosphorus, the high loss of phosphorus at –0.3 atm could be contributed to phosphorus transport via the exudation stream, which was higher at –0.3 than at –2.8 atm. In contrast, loss from the root surface to the external medium was identical for the two different water potentials. The neglect in measuring ion flow in the exudate might have confounded ion transport studies by other workers. Some particular cases were re-examined, such as chloride uptake at high external concentrations and ion toss from different cell compartments.