The major input for crop growth is the amount of radiation intercepted by the crop canopy and this may be a limiting factor for optimum productivity. The effectiveness of the crop to chemically convert the intercepted radiation energy to organic molecules (radiation use efficiency) determines the amount of dry matter produced. The partitioning of the dry matter between the vegetative and reproductive units of the plants determines the final yield of the crop (harvest index). Yield variability in cotton from year to year or in different environments (geographic locations) is well documented (Oosterhuis, 2002). Higher cotton yields have been recorded in drier environment as California, than more humid environments such as Arkansas. However, the effect of environmental factors such temperature and relative humidity on radiation use efficiency and dry matter partitioning in cotton has not been described in contrasting environments. In addition, plant growth regulators (PGRs) are a common and widely used practice in cotton production for controlling plant growth, increasing yield and improving management efficiency. Most of the PGRs used have an effect on plant growth, both vegetative and reproductive, and dry matter partitioning. However, there have been no reports of effects of PGRs on radiation use efficiency. It is logical to assume that any chemical that affects canopy dynamics will change radiation use efficiency. Therefore, research was conducted to determine the effect of PGRs on dry matter partitioning and radiation use efficiency.