RELATION BETWEEN CARBON DIOXIDE AND ELEMENTAL NITROGEN ASSIMILATION IN LEGUMINOUS PLANTS

Abstract

Clover plants inoculated with Rhizobium trifolii 209, a good strain, were grown on agar and sand in 64 oz. clear flint-glass bottles; plant cultures were set up in a closed system and provided with atmospheres containing 0.03, 0.1, 02, 0.4, and 0.8% of CO3. The effects of increased CO2 tension on general growth and, in particular, on N fixation were determined by appropriate measurements. In general, as the atmospheric pCO2 (CO2 potential) is raised, the leaf area, length of tops, stockiness of plants, and excess of carbo-hydrates are increased, but the increase is not proportional to the atmospheric pCO2 used. The most pronounced change in these characteristics of development is observed when the CO2 content is changed from 0.03 to 0.1%. Further increments in pCO2 cause relatively small further increases in development. If the atmospheric CO2 supplied to clover plants in agar is raised to 0.1% or higher, both dry weight of plants and quantity of N fixed are effectively increased in comparison with controls provided with air. However, the gain in N is relatively less than that in dry weighty so that % of N in plants receiving additional CO2 is lower than it is in controls grown in air. With plants on sand, this lag in N fixation is not observed until the atmospheric CO2 reaches about 0.4%. This difference in response to small increases in pCO2 between plants in agar and those in sand is believed to be related to the lowered rate of gas exchange of roots in agar. Other changes induced by increase in atmospheric pCO2 are in the number, size, and distribution of nodules. Addition of CO2 to the atmosphere causes the number of nodules to increase 100% or more, and many of those formed are 2-3 times as large as on controls grown in air. A feature even more striking than either of these changes, however, is observed in the distribution of the nodules, viz., lateral roots are invaded so that the distribution is definitely typical of the type usually associated with a poor strain of the organism. Increase in number of nodules occurs when the CO2 content is raised to 0.1%, but the change in size and distribution is observed only on plants grown in atmospheres containing 0.4% or more of CO2. The major findings obtained with clover grown in a closed system were verified for clover in bottles open to the air. and for alfalfa plants in a closed system.