Sgirdling (9,10,12). Factors that increase photosynthate available to the nodule increase N2 fixation, e.g., increased light intensity during the day and supplemental light (5, 10, 16), increased source size by grafting additional foliage and low planting density (3, 17), decreased demand of competitive sinks by pod removal and increased rates of photosynthesis by CO2 enrichment of the foliar canopy (3, 6, 9, 16). The most dramatic demonstration that production of photosynthate is a major limiting factor for N2 fixation in field-grown soybeans was obtained from a 3-fold CO2 enrichment of the soybean canopy during the period of reproductive growth. The amount of N2 fixed was increased from 75 to 425 kg/ha while the amount of N obtained from the soil was decreased from 220 to 85 kg/ha (3, 6). We have attributed the major effect of CO2 enrichment to an increase in net production of photosynthate made possible by a decrease in photorespiration produced by an elevated C02/02 ratio.Lowering the 02 content of the atmosphere will also greatly decrease photorespiration and the direct 02 inhibition of photosynthesis, and thereby increase photosynthetic CO2 fixation as shown by our recent long term studies (13,14). These studies with intact plants grown to maturity under low 02 concentrations showed that the growth rate of C3 species like soybean is increased while the growth rate of C4 species like sorghum is not affected markedly. In the N2 fixation experiments we have altered the C02/02 ratio by manipulation of P02 and compared the effects with those produced by manipulation of pCO2 in order to evaluate further the relationship between N2 fixation and production of photosynthate.MATERAILS ANDIMETHODS Plant Material. Soybeans (Glycine max [L.] Merr. cv. Wye) were grown in 18-cm plastic pots containing a Jiffy-mix (Jiffy Products of America, West Chicago, Ill.)-sand mixture (1 :1, v/v) and watered daily with a N-free modified Hoagland nutrient solution (7). Plants were grown at various 02 and CO2 concentrations using techniques previously described (13, 4). Plants were grown to maturity in a controlled environmental growth room (6000 ft-c, 12-hr photoperiod, 75% relative humidity, 29 C day, 18 C night). The aerial portion was exposed to air or a modified gas mixture containing 5, 10, or 30%02 balanced with 300 ,ul C02/1 and N2 or 21 %02 enriched to 1200 ,ul C02/l in chambers made of Lucite abrasion resistant sheet of 145-liter capacity with urethane foam gaskets around the stem entrance, while the roots were exposed to air. Gas mixtures were prepared in a continuous flow system and purged continuously with needle valves and flow meters at a rate of 15 1 /min. The plants were grown in the chambers from 18 to 83 days, the period from early seedling growth to maturity. At the conclusion of the experiment the plants were removed from the chambers, and dry weights of stems, leaves, roots, nodules, pods, and seeds were determined. 761www.plantphysiol.org on April 4, 2019 -Published by Downloaded from