Impact of environmental factors on seed germination and emergence of wild poinsettia (Euphorbia geniculata Ortega)

Wild poinsettia (Euphorbia geniculata Ortega) is an annual noxious weed that is considered as a substantial problem in irrigated field crops and orchards in Israel. The effect of environmental factors on germination and emergence of a local wild poinsettia population was examined in growth chambers and greenhouse experiments, and its germination dynamics was modelled. Germination was not affected by photoperiodism. Maximum emergence, 43.3%, was obtained from seeds planted on the soil surface; from a depth of 1–7 cm, emergence decreased at a constant rate proportionate to depth, from 26.6 to 10.0%. Wild poinsettia germination rate was reduced by high osmotic potential and high NaCl concentrations, with decreased germination compared to controls starting at −0.3 MPa and 150 mM, respectively. Seeds germinated under all evaluated pH levels, with highest germination value (68%) at pH 7. A Weibull regression was applied for time and accumulated germination at temperatures between 15 and 40 °C. As reflected by the a and X₀ coefficients, the maximum germination percentage and rate were observed at temperatures between 25 °C (81.6% and 1.35 days, respectively) and 35 °C (76.5% and 1.84 days, respectively). Germination dynamics was modelled using two thermal time unit concepts, growing degree days (GDD) and beta function. The beta function better fit the data with a root mean square error (RMSE) value of 4.5 compared to 51.1 for GDD. R² values of 0.22 and 0.98 for the GDD and beta-function models, respectively, further supported superior goodness of fit for the beta-function model. The results presented here contribute to our knowledge of wild poinsettia biology and can promote the use of efficient control strategies.