WATER-SENSITIVE PERIODS DURING THE REPRODUCTIVE GROWTH-PHASE OF GLYCINE-MAX L .1. REGRESSION-MODEL FOR ESTIMATING DAILY LEAF WATER POTENTIAL

To establish the exact duration of critical periods for plant water st ress during the reproductive growth stage of soybeans it is necessary to draw correlations between daily leaf water potential values and cro p growth and yield. Different sets of daily leaf water potential value s can be induced by different sets of irrigation applications. It is, however, imperative that water stress values are available for each da y over the growing season of a large number of irrigation treatments. Since these values were not available, it was decided to calculate it by establishing a relationship between leaf water potential as determi ned by potential evapotranspiration and soil water content as well as soil water potential for various layers of the root zone. The most rel iable regression formula was found to be: Y = 10.6466 X(1) + 0.0037 (X (2))(2) + 41.3662 (R(2) = 0.877) where Y = leaf water potential as a p ositive value (kPa) at 1300 h X(1) = soil water depletion (mm) from 15 0-1800 mm soil depth X(2) = potential evapotranspiration (mm)