Water use efficiency response of sweet sorghum under progressing water stress conditions

Interest in sweet sorghum raised from its high efficiency in water use, and the lack of specific information about its response to water stress. This work aimed to characterize the water use efficiency (WUE) response of sweet sorghun at leaf (WUEl), and partially at canopy (WUEc), levels under progressing water stress, and to compare such response with maize, to infer the features explaining its apparent higher WUE. The field trial took place during summer 1993 at Rutigliano (BA), Italy. Field grown sweet sorghum was subjected to two water stress cycles, along which water vapour and CO2 fluxes were measured, at noon, on upper fully expanded leaves, with a portable photosynthesis apparatus operating as closed system. Predawn leaf water potential (PDLWP) was monitored as a measure of soil water status. Canopy gas exchanges were estimated using the Bowen-Ratio/Energy Balance/CO2 gradient (BREB+) method. Under water stress, WUEl remained almost constant around a value of 5 mmolCO2.molH2O, showing a dependence on leaf-to-air vapour pressure deficits (VPD) but not varying sensibly with decreasing PDLWP along the two stress cycles. Leaf conductance (gl) showed curvilinear responses to VPD and PDLWP, and a shift of the curve gl vs.PDLWP towards lower values of PDLWP on the second stress cycle probably due to osmotic adjustment. Internal leaf CO2 concentration (Ci) remained constant with variations in VPD and PDLWP. Parallel evolution of gas exchange parameters at leaf and canopy scales indicated an effective stomatal control over the whole canopy. WUEc remained roughly constant at about 6.5 mmolCO2.molH2O, a value higher than those of other crops confirming results found on biomass WUE. Reasons proposed to explain the sweet sorghum higher WUE are: stomatal response to VPD, ability to adjust osmotically, capability to maintain a high carboxilation capacity under water stress, and limited senescence within the productive season.