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Water Stress Effects on CO₂ Assimilation, Photosynthate Partitioning, Stomatal Resistance, and Nodule Activity in Soybean¹

¹⁴CO₂ assimilation, photosynthate distribution, stomatal resistance, and acetylene reduction activity of vegetative soybean plants [Glycine max (L.) Merr., var. ‘Clay’] were examined 3 hours after pulse labeling the leaves with ¹⁴CO₂. The plants had previously been water stressed for 20 hours by polyethylene glycol additions to the culture solution. Photosynthetic CO₂ exchange rate (CER) and acytelene reduction activity of control and water stressed soybean plants were also monitored for 6 hours following polyethylene glycol induced water stress.

¹⁴CO₂ assimilation and specific nodule activity (SNA) decreased and stomatal resistance (r₈) increased with increasing water stress in the ¹⁴CO₂ labeling experiment. There was a significant negative correlation (r=–0.71) between SNA and r₈• Dry weights of leaves, of stems plus petioles, and of nodules were significantly decreased in the water stress treatments, while root dry weights increased. Water stress resulted in a redistribution of ¹⁴C with a greater proportion of the ¹⁴C being translocated to the roots and nodules at the expense of the leaves.

In the time course study, CER decreased after the imposition of an abrupt water stress (–12 bar), but total nodule activity was not affected. It is suggested that in. hibition of nodule activity under long term water stress was not caused solely by the decreased CER, but also by changes in photosynthate pool sizes.

Key Words: Glycine max (L.) Merr. • Polyethylene glycol • Acetylene reduction activity • Carbohydrate reserves

² Research assistant and professor of agronomy, Dep, of Agronomy, and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication September 12, 1979.