Course of Cation Absorption by Plant Tissue

Characterization and comparison of nitrate fluxes in Tamarix ramosissima and cotton roots under simulated drought conditions

Tamarix ramosissima Ledeb., a major host plant for the parasitic angiosperm Cistanche tubulosa, and known for its unique drought tolerance, has significant ecological and economic benefits. However, the mechanisms of nitrogen acquisition by the T. ramosissima root system under drought have remained uncharacterized. Here, uptake of nitrate (NO3-) in various regions of the root system was measured in T. ramosissima using Non-invasive Micro-test Technology at the cellular level, and using a 15NO3--enrichment technique at the whole-root level. These results were compared with responses in the model system cotton (Gossypium hirsutum L.). Tamarix ramosissima had lower net NO3- influx and a significantly lower Km (the apparent Michalis-Menten constant; 8.5 μM) for NO3- uptake than cotton under normal conditions. Upon simulated drought conditions, using polyethylene glycol (PEG), NO3- flux in cotton switched from net influx to net efflux, with a substantive peak in the white zone (WZ) of the root. There were no significant NO3- influx signals observed in the WZ of T. ramosissima under control conditions, whereas PEG treatment significantly enhanced NO3- influx in the WZ of T. ramosissima. The effect of PEG application on NO3- fluxes was highly localized, and the increase in net NO3- influx in response to PEG stimulation was also found in C. tubulosa-inoculated T. ramosissima. Consistently, root nitrogen (N) content and root biomass were higher in T. ramosissima than in cotton under PEG treatment. Our study provides insights into NO3- uptake and the influence of C. tubulosa inoculation in T. ramosissima roots during acclimation to PEG-induced drought stress and provides guidelines for silvicultural practice and for breeding of T. ramosissima under coupled conditions of soil drought and N deficiency.

Equilibrium and Ion Exchange Characteristics of Potassium and Sodium Accumulation by Barley Roots

Potassium ion and Na(+) influx and efflux rates into and from excised barley roots are compared with the maximum capacity of accumulation. Potassium ion and Na(+) influx and efflux involve a cation exchange that is independent of simultaneous exchange of the accompanying anion. These exchange fluxes depend on the concentration and cation composition of the solutions from which they originate. Selective differences between K(+) and Na(+) fluxes are sufficient to account for a cationic distribution within the roots that differs markedly from that of the external solution and that persists for extended time periods. The accumulation maximum is a cation exchange equilibrium with the cation influx and efflux rates approaching equality. The equilibrium level is independent of the individual cation fluxes and the external solution concentration. It is a finite quantity which appears to be determined by the internal anion concentration including accumulated as well as endogenous anions.

The kinetics of calcium uptake by roots

The time course of the uptake of labelled calcium by roots of Hordeum vulgare L., Trifolium subterraneum L., and Phaseolus aureus Roxb. was found to have two distinct phases: an initial rapid adsorption phase of about 1 h duration (Phase I) and a slower accumulation which proceeded at a constant rate for at least 12 h (Phase II). Ions taken up by the mechanism of Phase I, but not Phase II, could be removed by rinsing the roots in cold solutions of unlabelled CaCl2 or KCl.The concentration dependence of calcium uptake by barley roots over the range 0.001 to 10.0 mN was analysed with the aid of Hofstee plots. Phase I consisted of two Donnan adsorption processes. The half-saturation concentrations were 0.065 mN and 0.002 mN and the respective capacities were 1.56 and 0.36 μeq g(-1) (fresh weight). Phase II displayed Michaelis-Menten kinetics for a single mechanism with a Km=0.077 mN and V max=0.15 μeq g(-1) (fresh weight) h(-1).

Light-enhanced potassium absorption by corn leaf tissue

The rate of absorption of potassium by slices of corn leaf in the light was about twice the rate in the dark. When the light was turned on or off, changes in the rate of absorption took place some minutes after the change in illumination. Experiments with the antimetabolites, 2,4-dinitrophenol and cyanide, indicated that the source of energy for active accumulation of potassium by green tissue in the light was different from that in the dark. In the light, energy was closely linked to photosynthetic reactions; in the dark, it was linked to respiratory processes.