Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Russell E. Mullen

Second Advisor

Richard M. Shibles


Control, moderate, and severe drought stresses were imposed on pot-grown soybean plants in the greenhouse at air temperatures (AT) of 20, 26, 27, 29, 33, and 35 C. Stress intensity was quantified by summing stress degree days (SDD), the daily difference between AT and leaf temperature (LT), during seed fill;As SDD increased from 46 to 141 at 26 C, leaf resistance increased 220%, the transpiration rate decreased 45%, and the photosynthetic rate decreased linearly 69%. Yield was reduced linearly 49% by drought and 29% by the stressfully-high AT. Seed mass was reduced 20% by severe drought at 28 C, and 47% at 34 C;The germination percentage was reduced 10% by severe drought, 3% by high AT, and 29% by a SDD x AT interaction. Seedling axis dry weight (SADW) decreased, and conductivity increased proportionately because of stress, indicating reduced vigor. Viability and vigor exhibited a correlation of 0.89 with seed mass. The production of fewer seeds because of moderate stress quenched the reductions in viability and vigor. Severe stress reduced seed number, mass, viability, and vigor;Protein and oil content exhibited an inverse curvilinear relationship with AT. Maximum oil and minimum protein content were 24.6 and 38.0, respectively, at a critical AT of 29 C. Severe drought reduced the oil content linearly by 3%, and increased the protein content by 5%. Linolenic and linoleic acid content were reduced 19 and 5%, and oleic acid increased 13%, by high AT. Fatty acid composition was little affected by drought;Phospholipid class composition, and the fatty acid composition of each class, are altered by the AT during development optimizing membrane fluidity. A cool germination environment may confer nonfluid and leaky membranes of seeds that developed in warm environments;The effect of pod position on seed mass, viability, and vigor was dependent upon growth habit, whereas seed chemical composition was dependent upon pod temperature. Pods from top nodes were 1.4 C warmer than those from bottom nodes between 900 and 1700 hours.



Digital Repository @ Iowa State University,

Copyright Owner

David L. Dornbos, Jr.



Proquest ID


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File Size

178 pages