Cotton (Gossypium hirsutum l.) plant water status as a function of soil water supply and evaporative demand in a semiarid environment
The Southern High Plains of Texas (SHPT) representing a major cotton iGossypium hirsutum L.) growing area of the United States and the world is characterized as a semiarid environment with inadequate rainfall to meet the high evaporative demand. Soil water content and evaporative demand interact with the plant to determine tissue water status. Leaf water potential (øL), the primary indicator of plant water status, is commonly used in crop simulation models to determine growth rates and yield potential. Functional relationships between cotton øL and soil water supply and evaporative demand have not been well-defined for cotton grown in the SHPT area. A field experiment was conducted for two years to determine the relationship between *FL of cotton plants as a function of Ts, plant size and atmospheric factors affecting evaporative demand. Three irrigation levels (0%, 50% , and 100%) of replacement of water used as evapotranspiration (ETa) were used to provide a range of soil water supplies in two soil types (Amarillo loamy sand and Pullman clay loam), øL, soil water supply as indicated by soil matric potential Q^s), air temperature, relative humidity, incident and absorbed solar radiation, and plant growth were monitored throughout the growing season. Correlation and stepwise multiple regression were used to investigate and develop relationships between øL and øs and the evaporative demand. The variables highly correlated with øL were øs, and daily (24 hrs) average air temperatures (Tavg) as the most effective parameters of the evaporative demand. The relative contribution of each parameter on the variation explained for øL was shown not to be sensitive of plant growth stage as indicated by LAI ranges, but was sensitive to existing soil water supply. The overall effect of soil water supply was more dominant than the evaporative demand across the entire period of data measurement and within all LAI ranges. Crop growth rate (CGR), mainstem elongation rate (MER) and leaf area growth rate (LAGR) declined sharply when øL declined from its non-limiting conditions of -1.2 MPa to -1.6 MPa. From -1.6 MPa to -2.5 MPa øL growth rates continued to decline but at a slower rate. Essentially zero growth of all parameters was observed when øL dropped below -2.7 MPa. Results also indicated no differential sensitivity of reproductive versus vegetative growth rates to declining øL. In general, for cotton grown in the SHPT area, plant water status should be maintained above -1.6 MPa for maximum growth.