SAP FLOW MEASUREMENTS OF TRANSPIRATION FROM COTTON GROWN UNDER AMBIENT AND ENRICHED CO2 CONCENTRATIONS

Increasing atmospheric CO2 concentration has many implications for agr iculture and forestry, one of which is the effect it will have on tran spiration (T). The objective of this work was to quantify T of cotton (Gossypium hirsutum L.) grown in the field under ambient (370 mumol mo l-1) and enriched (550 mumol mol-1) CO2 concentrations. Measurements w ere made in 1990 and 1991 at the Maricopa Agricultural Center, Arizona . Constant-power sap flow gauges were used to measure T. In 1990, thre e plants and in 1991, 10 plants were simultaneously instrumented with gauges in each of the CO2 treatments. Leaf area of plants with gauges was measured. T measured by sap flow was compared with evapotranspirat ion (ET) calculated by water balance in 1990 and with T calculated by water balance in 1991. Soil evaporation was measured using microlysime ters in 1991, and was found to be essentially equal (approximately 0.8 mm day-1, or about 10% of T) in the two CO2 treatments. There were no consistent differences in leaf area of plants with gauges between the two CO2 treatments. Sap flow, for periods from 15 min to 2 weeks, was not significantly different between the two CO2 treatments in either year, except for a few days in 1990. In 1991, the coefficient of varia tion of daily sap flow across plants was the same (about 30%) for both CO2 treatments throughout the year. The water balance ET (1990) and T (1991) were similar to sap flow in both years, and also showed no eff ect of CO2 treatment. These results show that for this crop, grown und er well-watered and high-fertility conditions, there was no effect of CO2 on T, on a per unit ground area or per plant basis. These results are relevant for assessing the effects of increasing atmospheric CO2 c oncentrations on transpiration by cotton.