WATER-USE EFFICIENCY OF SWEET SORGHUM UNDER WATER-STRESS CONDITIONS GAS-EXCHANGE INVESTIGATIONS AT LEAF AND CANOPY SCALES

Photosynthetic water-use efficiency of leaves (WUEl) and canopy (WUEc) of sweet sorghum (Sorghum bicolor [L.] Moench) was investigated to ga in insight into the crop ability to optimize water use and to provide explanations for the high values of biomass WUE reported in the litera ture. The Keller cultivar was grown for two consecutive years (1993 an d 1994) in open field in southern Italy. Two consecutive water stress cycles were imposed on the crop in each year. In the investigation, em phasis was placed on the leaf scale during the first year, and on the canopy scale during the second year. Leaf area index (LAI), predawn le af water potential (Psi(b)), leaf and canopy gas-exchange by a portabl e photosynthesis system and by a Bowen-ratio/energy-balance method wer e the major measurements carried out in the field. The variation in so il-water status, estimated by Psi(b), ranged from -0.1 to -0.8 MPa in 1993 and from -0.2 to -1.1 MPa in 1994. The investigation at the leaf scale revealed that the ratio of leaf internal CO2 partial pressure (c (i)) to ambient CO2 partial pressure (c(a)) was maintained generally w ithin a range 0.2-0.27, with the leaf conductance ranging from 0.08 to 0.35 mol m(-2) s(-1); and was about constant (average = 0.24) over a range of leaf-to-air Vapor pressure difference (VPDl) between 2.3 and 5.8 MPa. Due to this fairly stable c(i), WUEl was inversely proportion al to VPDl and its value, normalized for VPDl, was constant at 11.8 mu mol(CO2) mmol(H2O)(-1) kPa. In terms of gas-exchange responses, a par allel behavior was observed at both the leaf and canopy scale. This wa s particularly evident when comparing diurnal trends of WUEl and WUEc. The resulting value of WUEc, normalized for the evaporative demand of the atmosphere (E-o), was 1.56 mol(CO2) m(-2). While WUEl of sweet so rghum compared well with that of other C-4 crops, values at the canopy scale were higher than other C-4 crops, especially maize and grain so rghum, and were consistent with the high biomass WUE observed in the f ield by other authors. A possible explanation for a high WUE may be fo und in the differences in dark respiration of the whole canopy between sweet sorghum and other C-4 crops. (C) 1997 Elsevier Science B.V.