Question

Using different K salts in the uptake media results in different H+ flux from the barley roots. Why?

The K salts in question are KCl and KSO4.

Answer 1

It is widely accepted that plant tissues are endowed with transport systems that extrude H+, hyperpolarize the membrane, and ensure the energetic coupling of K+ uptake (Hanson et al., 1977). Biochemical and Physiological studies have revealed that these systems are probably driven by vanadate sensitive Mg ATPase at the plasma membrane level.

The coupling between K+ and H+ transports may be either direct or indirect. The direct coupling has been proposed, based on indications of a net uphill K+ transport in some conditions. Indirect coupling or electrically driven K+ uniport was suggested by the finding that the stimulation of H+ extrusion by the fungal toxin FC' needed the presence of an electrical shunt, which possibly could be obtained by other means than a K+ influx. The electrogenic effect of a proton pump may, in theory, be regulated via the activity of the pump itself, or via the control of the K+ shunt (Sze, 1984).

On the basis of a mathematical analysis of the effects of K+ concentration on K+ influx and membrane potential, Cheeseman et al. concluded that the concentration-dependent hyperpolarization results from a progressive replacement of an active H+-K+ antiport with variable stoichiometry by an active H+ uniport associated with a passive K+ uniport, i.e., that control is via the rates of H+ and K+ pumping. This conclusion implies that part of the K+ influx is active on thermodynamic grounds. It also implies that the active K+ channel would close at high K+ concentration since it does not seem to mediate the exergonic influx. The increase of the K+ concentration or the membrane depolarization may be the signal which activates the control of the electrogenicity. Cheeseman et al. assumed a direct kinetic control by the K+ concentration. On another hand, the evidence for indirect (electrical) coupling between H+ and K+ transports favors the hypothesis of an electrical signal.

In a classical paper titled, “Adaptation of Barley Roots to Low Oxygen Supply and its Relation to Potassium and Sodium Uptake” by MG. Pitman in 1969, effects of H+ influx on the ratio of Potassium and Sodium has been studied extensively in Barley roots. Roots of barley seedlings can become adapted to low oxygen in their immediate environment by the development of an extensive air path that facilitates gas exchange with the air via the intercellular spaces of the shoot. Consequently, the seedlings are able to grow in solutions whether aerated or not. It was shown that uptake into plants growing in a nutrient or salt solution is strongly preferential for K+ whether aerated or not.

Comparison of the ratio of K+: Na+ and H+ release show that low K+: Na+ occurs when H+ release is high. Thus, the rate of H+ release decreased during salt accumulation by aerated roots; at the same time, K+: Na+ increased. Again K+: Na+ was much lower and H+ loss more rapid from aerated than non-aerated roots. Hydrogen ions released from the cell membrane must diffuse from the site of formation to the external solution. Due to the diffusion resistance, high rates of H+ release should lead to large local concentrations of H+ ions. As the cell wall has the properties of a Donnan system of negative charges with pKa of about 2.8, high H+ concentrations would reduce the local density of fixed changes and so reduce K+ concentrations at the cell wall/membrane interface. Both H+ and K+ ions have effects on membrane permeability and/or ion transport.

Barley is a good example of the way plant roots may become adapted morphologically and physiologically to local differences in the environment. Plants growing in the soil can experience fluctuations in their environment. As synthesis (or activation) of enzymes is a relatively rapid process, adaptational responses of cell metabolism could occur within 12 hr of an environmental change. A study of adaptations of cell metabolism to, for example, low water availability, high salt levels, or flooding should be a necessary part of the study of plant nutrition.

For references:-

1) Pitman, 1969; Adaptation of Barley Roots to Low Oxygen Supply and its Relation to Potassium and Sodium Uptake

2) Newman, 2001; Ion transport in roots: measurement of fluxes using ion-selective microelectrodes to characterize transporter function

3) Thibaud et al., 1986; H+ and K+ Electrogenic Exchanges in Corn Roots