EFFECTS OF ELEVATED ATMOSPHERIC CO2 ON WATER RELATIONS OF SOYA BEAN

Soya bean (Glycine max (L.) Merr. 'Bragg') plants were grown in large containers in open-top field chambers under five atmospheric CO2 concentrations (349-946-mu-l-l-1) and two water regimes. Rate of soil water depletion for the high CO2 treatments started to decrease under well-watered conditions during anthesis and by early pod formation under water-stressed conditions. During reproductive growth, normal and stressed plants at 349-mu-l-l-1 (ambient level) received irrigation water 29 and 12 times, respectively, compared with 21 and 9 times, respectively, at 946-mu-l-l-1 CO2. At both anthesis and pod fill, plants grown under CO2 enrichment exhibited greater leaf area. Nevertheless, water use per plant either remained constant (stressed plants at anthesis) or else declined (well-watered plants at pod fill; both moisture levels during pod fill) in response to CO2 enrichment. At pod fill, leaves of CO2-enriched plants generally displayed a higher stomatal resistance, except near the end of the sampling period when a sudden increase in resistance was observed under low CO2 owing to low soil water availability. Midday xylem potential for well-watered plants was greater than values for stressed plants and was unaffected by CO2 treatment. Under low moisture conditions, elevated CO2 had no effect on xylem potential at anthesis; however, during pod fill potential increased significantly with increasing CO2 concentration, as elevated CO2 decreased water use rates, lowering soil water stress. Alleviation of water stress during critical reproductive phases was strongly suggested.