A model for the water relations, photosynthesis, and expansive growth of crops

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Abstract
A conceptual model was developed for studying the integrated effects of soil, crop, and climatic conditions on the expansive growth, photosynthesis, and water use of agricultural crops. The plant was represented by water storage capacity, leaf area index, and osmotic, turgor, and total leaf water potentials. Water loss from the leaves was based on Monteith's combination equation for vapor flux. The resulting gradient between leaf and root water potentials caused a liquid water flux through the xylem resistance. The water potential gradient between root xylem and bulk soil causes water flux as the system departs from thermodynamic equilibrium. Resistances to water flux into the root depended on soil hydraulic conductivity, root density, and root radial resistance as defined by Taylor and Klepper (1978). Failure of the root system to supply water rapidly enough to replace water lost by the leaves resulted in decreased turgor and osmotic potentials. Stomatal control and expansive growth of leaves was a function of leaf turgor pressure. The model was used to simulate 10‐day drying conditions for sand and clay soils and for various combinations of root densities, root zone depths, and potential evapotranspiration rates. Diurnal and total daily values of transpiration and photosynthesis predictions suggest that soil root zone depth and soil hydraulic conductivities are very important factors in determining the maintenance of favorable turgor potential in the plant. Results also suggest that the relationship between daily transpiration and soil water potential is not unique but highly dependent on soil properties. The model is intended for developing simpler relationships for specific soils for describing the effects of various atmospheric conditions on plant growth processes such as transpiration, photosynthesis, and growth.