Productivity and water use efficiency of sweet sorghum as affected by soil water deficit occurring at different vegetative growth stages

The growth and production of sweet sorghum [Sorghum bicolor (L.) Moench] crops under semi-arid conditions in the Mediterranean environment of southern Italy are constrained by water stress. The effects of temporary water stress on growth and productivity of sweet sorghum were studied during three seasons at Rutigliano (Bari, Italy). The aim of this research was to evaluate the sensitivity of phenological stages subjected to the same water deficit. In a preliminary study it was observed that stomata closed when pre-dawn leaf water potential (Psi(b)) became lower than -0.4 MPa. This criterion was used in monitoring plant water status in three different plots: one never stressed and two stressed at different phenological stages ('leaf' and 'stem') when mainly leaves or stems were growing, respectively. An evaluation of the sensitivity of phenological stages subjected to identical water stress was obtained by comparing the above-ground biomass and WUE of drought crops with those of the well-irrigated crop (up to 32.5 t ha(-1) of dry matter and 5.7 g kg(-1)). The sensitivity was greatest at the early stage ('leaf'), when a temporary soil water stress reduced the biomass production by up to 30% with respect to the control and WUE was 4.8 g kg(-1) (average of three seasons). These results help quantify the effects of water constraints on sweet sorghum productivity. An irrigation strategy based on phenological stage sensitivity is suggested. (C) 1999 Elsevier Science B.V. Al rights reserved.