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Growth and yield responses of a semi‐dwarf spring wheat (Triticum aestivum L.; cv. GWO 1809) to two CO2 concentrations and two water stress regimes were studied in controlled environment chambers of the Duke Univ. Phytotron. Groups of plants in low (350 ppm), or high (1,000 ppm) CO2 environments were subjected to water stress By withholding irrigation starting at the 10th day after the beginning of anthesis. A second drying cycle beginning 5 days after termination of the first cycle was also given to some of the plants. Water potential of the flag leaves of the main stem of the plants in each CO2 environment reached a minimum of −13 bars at the end of the first drying cycle and −17 bars at the end of the second cycle.Under well‐watered conditions high CO2 enhanced the rate of tiller production by 43% and significantly increased grain yield, total dry matter, and number and size of the grains.As water stress developed, the osmotic potentials of the high CO2. plants decreased at a faster rate and resulted in maintenance of higher turgor pressures at the end of each stress cycle compared to the low CO2 plants. Osmotic potentials of the leaves of both high and low 2 plants decreased faster in the second drying cycle than in the first.Significantly fewer and smaller grains were produced on the plants grown under water stress in both CO2 concentrations compared to unstressed plants. In general, high CO2 plants under water stress conditions had a grain yield and total dry matter production equal to the unstressed, low CO2 plants. Thus, CO2 enrichment increased the yield potential of the water limited wheat plants due probably to osmotic adjustment by an increased concentration of solutes in their leaves.

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Most previous studies on the influence of CO2 concentration on plant growth have investigated the effect of atmospheric CO2 enrichment alone. Little attention has been given to possible interactions between CO2 enrichment and nutrient supply. The objective of this study was to characterize the long‐term effects of atmospheric CO2 enrichment on growth components of wheat (Triticum aestivum L.) grown under different nutritional levels. Growth and yield responses of spring wheat (cv. GWO1809) to two (350 and 675 ppm) CO2 and four (full strength, 1/2, 1/8, or 1/16 strength Hoagland's solution No. 1) nutritional levels were measured in controlled environment chambers at day/night temperature of 26/20 C. Plants were grown from seed and were irrigated with respective nutrient solutions three times daily until maturity. Plants grown in a 675 ppm CO2 atmosphere produced more total dry matter at each nutrient level compared to those grown in 350 ppm CO2. The root:shoot ratio decreased with increased nutritional level in both CO2 concentrations. Total weight and number of seeds produced in high CO2 were greater than those produced in low CO2 under similar nutrition. In low CO2, seed weight and number increased with each increase in nutrient concentration up to the one‐half strength level and then decreased with full strength. In high CO2, however, increasing the nutritional level from one‐half to full strength did not decrease seed weight and number significantly. As the plants grew older, the increments of increase in total plant dry weight during harvest intervals were always greater in plants grown in 675 ppm CO2 than those grown in 350 ppm CO2.

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