Cotton seeds (Gossypium hirsutum L.) were planted in a 16 acre field with an under-ground drip irrigation system at the University of Arizona's Maricopa Agricultural Center, Maricopa, Arizona, April 16, 1991. The field was divided into four replicate blocks. Each block was subdivided 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 separated 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 carbon dioxide was injected in FACE rings beginning April 26, 1991 (when 50% percent of the seedlings emerged) and continued until September 12, 1991 from 06:00 h through 19:00 h at a set point of 550 ± 75 µmol mol^-1. The dispensing and sampling of CO₂ was regulated with infrared gas analyzers coupled to computers. The carbon dioxide concentration in the ambient plots was monitored continuously during entire growing period. All plants were well watered until May 19, 1991. The differential water treatment was imposed from May 20, 1991. The periodic application of herbicides and insecticides were made to control weeds and insect pests in the field.

Xylem potentials, transpiration and diffusive resistance were measured in ambient and FACE plots during pre-dawn to sunset on certain days in July, August and September, 1991. However, most of the xylem potential measurements were taken during pre-dawn through Midafternoon; transpiration rate and diffusive resistance measurements were taken during 9:00 to 14:30 h.