Pressure-flow characteristics of the roots ofZea mays

Abstract

Root pressure is generally considered to be osmotic in origin. This explanation is not held by all workers, however. An alternative view contends that while an osmotic component certainly exists, the xylem concentrations are inadequate to account for the total pressure and that therefore the remainder must result from some form of active water pumping (see for example [2,4]). In the ideal method of testing this claim one would allow the root pressure to attain its maximum value, collect a sample of the xylem sap and compare its osmotic pressure to that developed by the root. Unfortunately, this is impossible with present techniques, since to develop maximum pressures requires zero flow, in which case no sap can be collected. One can, however, reduce the flow very nearly to zero and still obtain a small sample for comparison. Such an experiment was performed by collecting 50 µl samples of exudate from each of 22 plants under 2.7 bar pressure and pooling them for analysis. Since the exudation flowrate in this case was reduced to an average of only 7% of the unimpeded rate, the results should represent a close approximation to the ideal case. The osmotic pressure of the exudate collected in this way was found to be 3.2 bar above that of the bathing solution, well in excess of the applied pressure. Thus in the present instance, there is no need to assume that root pressure is generated by any means other than osmotic.