RESPONSE OF THE WATER STATUS OF SOYBEAN TO CHANGES IN SOIL-WATER POTENTIALS CONTROLLED BY THE WATER-PRESSURE IN MICROPOROUS TUBES

Water transport through a microporous tube-soil-plant system was inves tigated by measuring the response of soil and plant water status to st ep change reductions in the water pressure within the tubes, Soybeans were germinated and grown in a porous ceramic 'soil' at a porous tube water pressure of -0.5 kPa for 28 d. During this time, the soil matric potential was nearly in equilibrium with tube water pressure, Water p ressure in the porous tubes was then reduced to either -1.0, -1.5 or - 2.0 kPa. Sap flow rates, leaf conductance and soil, root and leaf wate r potentials were measured before and after this change, a reduction i n porous tube water pressure from -0.5 to -1.0 or -1.5 kPa did not res ult in any significant change in soil or plant water status, A reducti on in porous tube water pressure to -2.0 kPa resulted in significant r eductions in sap flow, leaf conductance, and soil, root and leaf mater potentials, Hydraulic conductance, calculated as the transpiration ra te/Delta psi between two points in the water transport pathway, was us ed to analyse water transport through the tube-soil-plant continuum. A t porous tube water pressures of -0.5 to -1.5 kPa soil moisture was re adily available and hydraulic conductance of the plant limited water t ransport. At -2.0 kPa, hydraulic conductance of the bulk soil was the dominant factor in water movement.