TEMPERATURE-DEPENDENT MODEL FOR NONDORMANT SEED-GERMINATION AND RHIZOME BUD BREAK OF JOHNSONGRASS (SORGHUM-HALEPENSE)

Research was conducted to formulate a temperature-dependent population level model for johnsongrass seed germination and rhizome bud break, A nonlinear poikilotherm rate equation was used to describe developmen t rate as a function of temperature, and a temperature-independent Wei bull function was used to distribute development times for the populat ion, Seed germination and initiation of rhizome bud break of johnsongr ass were collected under constant temperature conditions to parameteri ze the model, Seed germination rate increased with temperature up to 3 6 C, then declined at 40 C, Rate of rhizome bud break increased with t emperature up to 32 C, then rapidly decreased,vith further temperature increases, Rate of rhizome bud break was higher than for seed germina tion at temperatures of 32 C or below, but lower at higher temperature s, Time to first germination or bud break event was longer for seed th an for rhizomes, but subsequent progression of development was higher for seed, A population level temperature-dependent model was developed by coupling the poikilotherm equation with the Weibull function, The model was validated against two independent seed germination and three independent rhizome bud germination data sets.