This dissertation reports two investigations. One was conducted at Ames, Iowa, during 1980 and 1981, with the aim of evaluating the response of soybean Glycine max (L.) Merr. stem termination types (indeterminate, semideterminate, determinate) to supplemental irrigation during seed growth. The second was done in Costa Rica from February to May (field) and from August to November (glasshouse), 1982. The objective of the second study was to evaluate the response of water stress of tropical determinate and indeterminate soybean cultivars. Because of their ability to continue growth during early reproduction, indeterminates should recover from stress better and respond better to irrigation;The nonirrigated Iowa soybean crop had lower predawn and midday water potentials during seed filling and significantly lower yield than the irrigated soybean crop. However, there was no difference in response of the stem termination types to supplemental irrigation. Average yield response to irrigation was 16.4%. Seed yield increase in response to irrigation of determinates and semideterminates was mostly due to increased number of pods per node, whereas, with indeterminates, it resulted partly from increased pods per node and partly from increased nodes. Indeterminates grew more and lodged more under irrigation than did semideterminates; the determinate types did not lodge at all;The study conducted in Costa Rica showed that field-grown soybean plants under stress had predawn water potentials within -0.1 to -0.2 MPa of well-watered controls after water had been withheld one week. Stomata showed the greatest decrement in water vapor conductance at water potentials between -1.3 and -1.4 MPa. Under moderate to severe stress, stomata on the adaxial surface completely closed early in the drying cycle;Indeterminate cultivars were better able to recover from water stress than were determinate ones, as indicated by better seed yield under field conditions. This response was associated with the number of pods per node rather than with other basic yield components, or with leaf area index or leaf area duration;Environmental conditions in the glasshouse induced determinateness in the indeterminate cultivar; consequently, it responded to water stress the same as the determinate cultivar.