The design and operation of hybrid physical/chemical and biological life support systems for space application is a complex and difficult process. This paper describes the approach and results of an effort to characterize wheat growth, under various environmental conditions, at the Johnson Space Center's (JSC) Ambient Pressure Growth Chamber (APGC). Using a designed experiment, a test plan was developed for varying environmental parameters during a wheat growth experiment. The test plan was developed using a Central Composite approach to experimental design. As a result of the experimental runs, an empirical model of both the transpiration process and carbon dioxide assimilation for wheat growth over specified ranges of environmental parameters has been developed. The environmental parameters include carbon dioxide concentration, ambient chamber temperature, vapor pressure deficit, and air velocity. In addition, this model is sensitive enough to predict transpiration and carbon dioxide assimilation for different ages of the plant canopy.