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Effect of Atmospheric CO₂ Enrichment on Growth, Nonstructural Carbohydrate Content, and Root Nodule Activity in Soybean ¹

Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55108

The objective of this study was to determine whether the supply of current photosynthate was limiting root nodule activity. Both short-term (36 hours) and long-term (16 days) periods of CO₂ enrichment were imposed on vegetative, growth chamber-grown soybean plants (Glycine max. [L.] Merr. cv. `Clay') to increase the supply of current photosynthate and to observe the effects on photosynthate partitioning in the plants, plant growth, and root nodule activity.

Neither total nor specific nodule activities were increased during exposure to short-term (36 hours) CO₂ enrichment. Dry weight of the leaves increased after 12, 24, and 36 hours of CO₂ enrichment and dry weight of the stems plus petioles increased after 36 hours of CO₂ enrichment. Dry weights of the roots and nodules were not altered by short-term CO₂ enrichment. Short-term CO₂ enrichment increased the total nonstructural carbohydrates in the leaves and stems plus petioles, but not in the roots and nodules. Analyses of the separate pools of carbohydrate reserves indicated that the majority of the additional carbohydrate provided by short-term CO₂ enrichment was stored as leaf starch with relatively little being partitioned to the roots and nodules.

Long-term CO₂ enrichment (16 days) did not enhance specific nodule activity. Shoot, root, and nodule dry weights were increased 109, 34%, and 56% respectively. Total nodule activity per plant was significantly enhanced only after 16 days of treatment and was related to increased nodule mass. These results indicate that the increased total nodule activity in response to CO₂ enrichment is a consequence of a general growth response of the plant.

Results of both studies indicate that nodule activity was not directly limited by current photosynthesis but rather by the partitioning and utilization of photosynthate in the plant.

¹ Contribution from the Agricultural Experiment Station, University of Minnesota, St. Paul, MN 55108. Paper No. 11,510 of Scientific Journal Series. Supported in part by Cooperative State Research Service Grant 701-15-28.

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