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Manipulation of crop architecture through the use of chemical bioregulators suggests the potential to improve plant water use and production efficiency. A bioregulator, mepiquat chloride (1,1‐dimethyl piperidinium chloride), was evaluated in field grown cotton (Gossypium hirsutumL.) during 1979, 1980, and 1981 to determine effects on several plant water relations parameters and crop development in a semiarid environment. All experiments were conducted on an Olton loam soil (fine, mixed, thermic family of Aridic Paleustolls). A broadcast, foliar application was made at the rate of 50 g active ingredient (a.i.)/ha during the early square (young floral bud) Stage. Increases in leaf water potential (Ψw), solute potential (Ψw), and turgor pressure (Ψp) in mepiquat chloride treated plants were recorded during midmorning and at daily minimum Ψw. The earliest Ψw increase wass measured 12 days after application. Changes in Ψw ranged from no significant differences to a 0.63 MPa increase in treated plants, 77 days after application under a high soil water level. Increased Ψs and Ψp in treated plants occurred most frequently during the midmorning period and increases Of 0.20 MPa were measured. Improvements in plant water status were not restricted to low soil water availability and occurred across a range of control Ψw from −1.43 to −3.43 MPa. Increased abaxial transpiration rate, associated with decreased diffusive resistance, was measured in treated plants, particularly during the afternoon. No difference in seasonal soil water extraction was measured between treatments, even in tests in which leaf area index (LAI) was reduced 33% by mepiquat chloride application. Leaf area index reductions were attributable to both fewer leaves per plant and reduced leaf size. Differences in the growth parameters between treated and untreated plants were most evident when conditions for growth were optimum. A maximum decrease in plant height of 25 cm was measured 43 days after application in a high soil water level experiment. Reductions of one to two mainstem nodes were measured in treated plants. The number of bolls per plant throughout the season was not altered by mepiquat chloride application, and there were no differences in lint yield as a result of treatment.