DAILY TRANSPIRATION OF FIELD SOYBEANS AS RELATED TO HYDRAULIC CONDUCTANCE, ROOT DISTRIBUTION, SOIL POTENTIAL AND MIDDAY LEAF POTENTIAL

The present study aims at characterizing plant water status under field conditions on a daily basis, in order to improve operational predictions of plant water stress. Ohm's law analog serves as a basis for establishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: T = G (psi(Esoil) - psi(Eplant)). The daily transpiration flux, T, is estimated from experimental evapotranspiration data and simulated soil evaporation values. The difference, psi(Esoil) - psi(Eplant), named the effective potential gradient, is derived from i) the midday leaf potential of the uppermost expanded leaves and ii) an effective soil potential accounting for soil potential profile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roots. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extraction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is found to be equal to -1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradient remains constant (1.2 MPa), implying that, on a daily basis, transpiration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on Morgan's theory (1984). Possible generalization of the results' to other crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.