MODELING SORGHUM SEEDLING ESTABLISHMENT FROM SOIL WETNESS AND TEMPERATURE OF DRYING SEED ZONES

Existing crop simulation models predict emergence based on experimental results on the effects of soil temperature and water in systems sealed against evaporation. Under field conditions, however, the seed zone is open to evaporation. Seed zone water decreases with time and is often inversely related to soil temperature. Little is known about the combined effects of soil temperature and water on sorghum [Sorghum bicolor (L.) Moench] emergence rates in drying seed zones. Therefore, our objective was to study this and to model the effect to predict seedling stand establishment. Greenhouse experiments were conducted to study sorghum emergence at six constant temperatures (8.8, 15.9, 20.5, 25.2, 30.2, and 35.8-degrees-C) attained on a thermogradient plate, and three initial soil water matric potentials (psi(s)) (-0.03, -0.1, and -0.3 MPa), using a Pullman clay loam. Emergence and plant height were recorded daily for 2 wk and root length was measured at termination of the experiment. We developed a sorghum stand establishment model that utilizes heat units and indices of emergence or growth as input parameters to predict emergence and root and shoot growth of seedlings. Optimum sorghum emergence (>80%) was obtained at temperatures of 20.5 to 30.2-degrees-C, and -0.03 to -0.1 MPa psi(s). Furthermore, combinations of cool temperature (15.9-degrees-C) and psi(s) of -0.1 MPa as well as warm temperature (35.8-degrees-C) and psi(s) of -0.03 MPa produced satisfactory emergence (>80%). Sorghum emergence regressed with thermal emergence index yielded a highly significant (P < 0.0001) positive nonlinear correlation (Adj. R2 = 0.62). Similarly, main axis root length, lateral root length, and plant height regressed with thermal indices of main axis root length, lateral root length, and shoot growth yielded significant (P < 0.0001) positive, nonlinear correlations (Adj. R2 = 0.46, 0.70, and 0.77, respectively). The model should provide more realistic sorghum emergence predictions than models developed in systems closed to evaporation.