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Barley, ryegrass, and fodder radish were grown in flowing nutrient solutions at four potassium concentrations, [K+], from 0-05 to 4 mg l-1. During the first 2 weeks after germination the response to [K+] (fodder radish > barley > ryegrass) depended on the potential relative growth rate, the ratio of root surface area to plant weight, and on the K+ flux into the roots. Subsequently, there was no effect of [K+] on growth rate within the range tested. The K+ flux decreased from 4-23 x 10"12 mol cm-2 s"1 in the first 2 weeks after germination, when it was concentration-dependent, to 2-5 x 10~12 mol cm-2 s"1 after 4-5 weeks, when it became independent of [K+] down to 0-05 mg 1_I. The results explain the importance of high [K+] and rapid root growth during the first 2 weeks after seed germination.

A system is described for growing plants in flowing solution in which the concentration (activity) of some nutrient ions is continuously monitored and held almost constant by means of ion-selective electrodes which control the operation of nutrient pumps. By recording the concentration of a given ion in the flowing nutrient solution and the amount of that ion which is delivered by the nutrient pumps, it is possible to follow nutrient uptake over long and short periods. A brief account is given of the operation of the system using a nitrate ion-selective electrode to examine aspects of NO?~ uptake by perennial ryegrass (Lolium perenne L.).