DIURNAL FLUCTUATION IN ROOT PRESSURE

Abstract

Karl A. Grossenbacher \tDIURNAL FLUCTUATION IN ROOT PRESSURE \tPlant Physiol. 13: 669-676.Diurnal cycles in root activity under constant conditions seem to have beenfirst described by HOFMEISTER in 1862 (7). Other investigators have confirmedhis findings (1, 11), but in recent years it has been concluded thatcycles in pressure and exudation were due to temperature fluctuations (HEm,5). On the other hand, studies in this laboratory (9) have substantiatedHOFMEISTER'S original results, and WmTE (10) has reported a cycle inexudation of excised tomato roots (though he did not control the conditionsunder which he was working). Complete proof of the existence of autonomiccycles has recently been obtained by the author with plants grown and testedunder controlled conditions (4).Several theories have been developed to explain the general phenomenaof root pressure and exudation. For the purpose of this discussion it issufficient to point out that osmotic mechanisms have generally been used.SABININ (8) holds the view that a simple osmotic mechanism is sufficient.The general problem is confused by the controversy over the parts playedby the phloem and xylem in translocation and root pressure. Since thesemeasurements extended over periods of several days it seems probable thatsieve tubes were plugged (CRAFTS, 3) and that the phloem played little partdirectly in maintaining the observed pressures. Here it is assumed that thehydrostatic-pressure system involved is the xylem-manometer system.