SAP FLOW IN WHEAT UNDER FREE-AIR CO2 ENRICHMENT

The effects of elevated carbon dioxide (CO2) concentration on plant wa ter use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function o f the plant, seat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivun L. cv Yecora rojo) grown under norm al ambient conditions (control) and in a free-air CO2 enrichment (FACE ) system in Arizona with either high (control+high H2O=CW; FACE+high H 2O=FW) or low (control+low H2O=CD; FACE+low H2O=FD) irrigation regimen s, Over a 30d period (stem elongation to anthesis), combinations of tr eatments were monitored with 10-40 gauges per treatment, The effects o f increased CO2 on tiller water use were inconsistent in both the diur nal patterns of sap flow and the statistical analyses of daily sap flo w (F-tot), Initial results suggested that the reductions in F-tot from CO2 enrichment were small (0-10%) in relation to the H2O treatment ef fect (20-30%), For a 3d period, F-tot of FW was 19-26% less than that of CW (P=0.10). Examination of the different sources of variation in t he study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements, This varia tion was probably related to positional variation in subsurface drip l ines used to irrigate plots, A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effects of CO2 enrichment and the oth er sources of variation identified in this study, Despite the small an d often statistically non-significant differences in F-tot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than t hat of the CW treatment, Differences in sap flow between FW and CW com pared well with treatment differences in evapotranspiration. The resul ts of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production, in t he present climatic regimen of the south-western US, may be predicted to decreases lightly because of increasing atmospheric CO2.