ESTIMATION OF BASE AND OPTIMUM TEMPERATURES FOR SEED-GERMINATION IN COMMON CRUPINA (CRUPINA-VULGARIS)

Thermal time models for predicting phenological development require an estimate of base temperature, an attribute not previously defined for common crupina, a Mediterranean winter annual introduced in western N orth America. The stage of seed germination was selected for estimatin g base temperature, because facilities were available for experiments over a range of constant temperatures and base temperature is relative ly constant throughout the life cycle in other species. Achenes from t hree populations of common crupina, including two varieties, typica an d brachypappa, were produced under uniform conditions. Cumulative germ ination was recorded at 12 h intervals for achenes in darkness and opt imum moisture at 23 constant temperatures from 4 to 17 C. The time cou rse of germination was best described by a logistic growth curve from which time to 50% germination was estimated. A parabolic model provide d the best fit in a regression of germination rate (reciprocal of time to 50%) against the temperature gradient, yielding base and optimum t emperatures of 1 and 10.5 C, respectively. Bootstrap confidence interv als indicated no significant difference in base and optimum temperatur es in germination between the two varieties nor between two population s of var. typica of common crupina introduced in the northwestern Unit ed States.