WATER DEFICITS DURING REPRODUCTIVE GROWTH OF SOYBEANS .2. WATER-USE AND WATER DEFICIENCY INDICATORS

An experiment was carried out over 2 years with soybeans to study water use and performance of different indicators of water status under field controlled conditions, comparing an irrigated check with treatments with imposed water deficiencies. Statistically significant differences were found between treatments in predawn water potential (PWP), noon water potential (NWP) and leaf resistance (LR). PWP best reflects soil water availability or percentage of available water (PAW). The determination coefficient (R2) between PWP:PAW for both years was 0.77**. Water use was reduced when water deficiency was applied, and actual water use: potential evapotranspiration (WU:PET) relationship was drastically reduced for the dry treatments. (0l, l0, 00) compared with the control (II), but it recovered almost completely when the deficiency was removed. Total water used efficiency (TWUE), as dry matter: water use (DM:WU) and water use efficiency (WUE); as seed yield: water use (SY:WU), were significantly higher for the second year. A time-integrated deficiency index (Dl) was calculated as: Dl = 1 - integral-tb/te (PAW/AWT) dt, for all PAW < AWT, where tb and te are the beginning and the end of the period, and AWT is the PAW threshold (62%). Below that value, WU and DM are reduced. Dry matter production and seed yield were related to Dl as: DM = 5.216 - 1.353 in (Dl); R2 = 0.75** n = 32 GY = 3.187 - 0.349 in (Dl); R2 = 0.61** n = 32.