Osmoregulation in the Avena Coleoptile

Peeled Avena sativa coleoptile sections (i.e. sections from which the epidermis has been removed) have been used to study the control of solute uptake under conditions where the uptake is not limited by the cuticular barrier. In the presence of 2% sucrose, auxin enhances the rate at which the total osmotic solutes increase, but this appears to be a response to the increased growth rate, inasmuch as the auxin effect is eliminated when growth is inhibited osmotically. When sections are incubated in sucrose or in 20 millimolar NaCl, the osmotic concentration increases until a plateau is reached after 8 to 24 hours. Auxin has no effect on the initial rate of increase in osmotic concentration but causes the osmotic concentration to reach a plateau earlier and at a lower osmotic conentration value. This difference in steady-state osmotic concentration is, in part, a response to auxin itself, as it persists when auxin-induced growth is inhibited osmotically. The upper limit for osmotic concentration does not appear to be determined by the turgor pressure, inasmuch as a combination of sucrose and NaCl gave a higher plateau osmotic concentration than did either solute alone. We suggest that the rate of solute uptake is determined by the availability of absorbable solutes and by the surface area exposed to the solutes. Each absorbable solute reaches a maximum internal concentration independent of other absorbable solutes; the steady-state osmotic concentration is simply the sum of these individual internal concentrations.