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Effect of Water Stress on Carbon Assimilation and Distribution in Soybean Plants at Different Stages of Development¹

The effects of water stress on carbon dioxide (CO₂) exchange rates (CER) and assimilate distribution were studied in chamber-grown soybean plants (Glycine max (L.) Merr., ‘Fiskeby’) in the vegetative, flowering, and pod-filling stages of development. When water was withheld, the decrease in CER was correlated with the increase in stomatal resistance at leaf water potentials as low as –21 bars. Under the above conditions, CER and stomatal resistance recovered to predesiccation levels within 24 hours after rewatering. When leaf water potentials were allowed to drop below –21 bars, recovery of CER lagged behind the return of stomatal resistance to predesiccation levels, suggesting that additional factors may be involved. Whole plants were exposed to labeled CO₂ at the three different stages of development and the percent distribution of ¹⁴C was determined. At leaf water potentials of –15 to –20 bars, plants displayed alterations in ¹⁴C distribution among plant parts corresponding to alterations in dry weight distribution. Compared to well-watered plants, relatively more ¹⁴C was found in the roots of water-stressed plants before the pod-filling stage. Less ¹⁴C was retained in the leaf blades of these plants, but this response was reversed during pod-filling. Reduction in ¹⁴C in nodules was associated with water stress at all growth stages. Percentage of radioactivity increased in leaves and reproductive structures of plants rewatered and exposed to ¹⁴CO₂ following return of CER to predesiccation levels. Our results suggest that drought decreased CER and altered assimilate distribution, producing growth modifications that favor efficient use of the limited supply of fixed carbon.

Key Words: Carbon exchange rate • Assimilate distribution • Leaf water potential • Stomatal resistance • Glycine max L. • Carbon-14

² Former research associate, presently plant physiologist, USDAARS, Light and Plant Growth Laboratory, Bldg. 046-A, BARCWest, Beltsville, MD 20705; assistant professor of crop production, Dep. of Agronomy, Univ. of Illinois; soil scientist, USDAARS, and professor of soil physics, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801.

Received for publication January 3, 1977.