The effect of CO2 enrichment on the growth of nodulated and non-nodulated isogenic types of soybean raised under two nitrogen concentrations

To find the effects of CO2 enrichment on plant development and photosynthet ic capacity of nodulated (line A62-1) and non-nodulated (line A62-2) isogen ic lines of soybean (Glycine max Merr.), we examined the interactions among two CO2 treatments (36+/-3 Pa = AC and 70+/-5 Pa = EC), and two nitrogen c oncentrations [0 g(N) m(-2)(land area)= ON; 30 g(N) m(-2)(land area) = 30N] . Nodules were found in both CO2 treatments in ON of A62-1 where the number and dry mass of nodules increased from AC to EC. While the allocation of d ry mass to root and shoot and the amount of N in each organ: did not differ between the growth CO2 concentrations, there was larger N allocation to ro ots in ON than in 30N for A62-2. The CO2-dependence of net photosynthetic r ate (P-N) for A62-1 was unaffected by both CO2 and N treatments. In contras t, the CO2-dependence of P-N was lower in ON than in 30N for A62-2, but it was independent of CO2 treatment. P-N per unit N content was unaffected by CO2 concentrations. The leaf area of both soybean lines grown in 30N increa sed in EC. But in ON, only the nodulated A62-1 showed an increase in leaf a rea in EC. Nitrogen use efficiency of plants, NUE [(total dry mass of the p lant)/(amount of N accumulated in the plant)] in 30N was unaffected by CO2 treatments. In ON, NUE in EC was lower than in AC in A62-1, and was higher than that at AC in A62-2. Hence, the larger amount and/or rate of N fixatio n with the increase of the sink-size of symbiotic microorganisms supplied a dequate N to the plant under EC. In EC, N deficiency caused the down-regula tion of the soybean plant.