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Water Relations of Field-Grown Soybean under Drought¹

Two strategies for decreasing plant vulnerability to drought are increased access to the stored soil water and increased amount of osmotic adjustment in tissue to preserve turgor. Two indeterminate soybean [Glycine max (L.) Merr.] cultivars that had been identified in preliminary studies as having differing responses to drought were studied to aid in the evaluation of these strategies. Drought stress was imposed on the cultivars during 2 yr at differing ontogenetic stages of growth. Soil water extraction was determined with Bouyoucos gypsum blocks. Stress during vegetative growth was associated with a failure of the drought-susceptible cultivar to extract water from the deeper soil depths. Components of leaf water potential were determined from pressure-volume curves obtained with a pressure chamber. These data showed that drought stress resulted in a decrease in leaf osmotic potential of 0.4 to 0.5 MPa in both cultivars. Since, as compared to the more drought-tolerant cultivar, the total leaf water potential was as much as 0.9 MPa lower in the droughtsusceptible line, comparable osmotic adjustment did not prevent it from experiencing less turgor. Therefore, relatively higher drought tolerance was not associated with greater osmotic adjustment. It appears from the comparison of these two cultivars that maintaining the water supply to the plant tissue was of much greater benefit than tissue osmotic adjustment.

Key Words: Glycine max (L.) Merr. • Soil water potential • Leaf water potential • Leaf osmotic potential

² Research associate (current address: Research Institute for Forestry and Landscape Planning, P.p. Box 23, 6700 AA Wageningen, The Netherlands); plant physiologist, USDA-ARS and adjunct professor, Agronomy Dep., Univ. of Florida, Gainesville, FL 32611.

Received for publication October 15, 1985.