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 fiel d conditions on a daily basis, in order to improve operational predict ions of plant water stress. Ohm's law analog serves as a basis for est ablishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: T = G (psi(Esoil) - psi(Eplant)). The d aily transpiration flux, T, is estimated from experimental evapotransp iration data and simulated soil evaporation values. The difference, ps i(Esoil) - psi(Eplant), named the effective potential gradient, is der ived from i) the midday leaf potential of the uppermost expanded leave s and ii) an effective soil potential accounting for soil potential pr ofile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roo ts. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extra ction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is foun d to be equal to -1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradien t remains constant (1.2 MPa), implying that, on a daily basis, transpi ration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on M organ's theory (1984). Possible generalization of the results' to othe r crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.