The riddle of root pressure. II. Root exudation at extreme osmolalities

To test the predictions of a recent theory of root pressure and exudation, exudation from detopped tomato seedlings was studied. Experimental findings were generally in qualitative accord with the theory. Two confirmed predictions are of particular interest. First, when a normally exuding stump was challenged by having its roots exposed to concentrated osmolyte, the exudation rate dropped quickly to zero but later often recovered to a higher level. Second, when exudation from a plant challenged by concentrated osmolyte did not recover, flushing the soil solution away with tap water usually resulted in a sudden resumption of exudation followed by a rapid falloff.