Seeds of Gossypium hirsutum L. were planted in a 16 acre field with an under-ground drip irrigation system at the University of Arizona Is Maricopa Agricultural Center, Maricopa, Arizona, April 23, 1990. The field was divided into four replicate blocks. Each block was split into two sub-blocks, one of which was well-watered (ww) and the other was water-stressed (WS). Two rings of 22 m diameter each spanned the border between sub-blocks to provide one ambient-CO₂ control and one free-air-CO₂-enrichment (FACE) treatment. The distance between rings in each direction was 100-150 m. The CO₂ was injected in FACE plots beginning May 23, 1990, until September 12, 1990, from 06:00 h through 19:00 h at a set point of 550 + 75 µmol Mol^-1. The dispensing and sampling of carbon dioxide was regulated with infra-red gas analyzers coupled to computers. The sampling of ambient CO₂ was also done continually during entire growing season. All plants were well-watered until about 70 days after emergence when the water stress treatment was imposed on half the plots. Periodic application of herbicides and insecticides were made to control weeds and insect pests in the field.

Xylem potentials, osmotic potentials, and relative water contents were measured in ambient and FACE plots during pre-dawn (5:00-7:00 h) and mid-afternoon (12:30 14.45 h) beginning June 6, 1990, until September 12, 1990. One method used to measure xylem potential was the pressure bomb technique (PMS International Company, Corvallis, Oregon), which was done right in the plots (within 2-2.5 meter distance from the selected plants). At each measurement period, one person was responsible for selection of plants and sampling of leaves, a second person was responsible for operation of the bomb. A portable N₂ cylinder was carried with the bomb to the field sites.