Some effects of urea and of soil moisture on solute translocation

Abstract

In detopped plants, stump exudation resulting from root pressure provides a measure of the quantity of solutes and of water being transferred to the xylem. In dry soil the transfer of water to the xylem stops when the force required to remove the water from the soil is equal to the osmotic pull of the solutes in the xylem. In a sandy loam soil mixture, detopped tomato plants (Lycopersicon esculentum Mill.) treated with urea maintained stump exudation to a soil moisture content as low as 12.1% while water-control plants ceased exudation at 15.0% soil moisture. By maintaining the transfer of solutes to the xylem, the application of urea enhanced the ability of the detopped root system to remove water from the dry soil.By watering plants to pot capacity at different times before and after detopping, a series of soil moisture levels were obtained with detopped roots of tomato. In this clay loam soil mixture stump exudation started at 10% soil moisture (about −3.0 bars potential). With increasing moisture the rate of exudation increased rapidly to reach a maximum exceeding 5 ml per hour at soil moisture contents between 20 and 25% (about −0.05 bars potential). The rate then decreased gradually to 2.5 ml per hour as the soil moisture content increased from 25 to 45%. Starting at 10% soil moisture and continuing up to 25%, solute uptake increased proportionately with increasing moisture content, and at these dry and medium soil moisture levels, the supply of water to the root determined the quantity of solutes being translocated to the stump. The depletion of oxygen and accumulation of carbon dioxide in soils containing more than 25% moisture reduced the translocation of solutes in the plants.