Germination Ecology of Phacelia dubia Var. dubia in Tennessee Glades

Abstract

The germination characteristics of a population of the winter annual Phacelia dubia (L.) Trel. var. dubia from the middle Tennessee cedar glades were investigated in an attempt to define the factor(s) regulating germination in nature. Factors considered were changes in physiological response of the seeds (after-ripening), temperature, age, light and darkness, and soil moisture. At seed dispersal (late May to early June), approximately 50 % of the seeds were non-dormant but, would germinate only at low temperatures (10–15 C). As the seeds aged from June to September, there was an increase in rate and total percent of germination at 10, 15, and 20 C, and the maximum temperature for germination increased to 25 C. Little or no germination occurred at the June, July, and August temperatures in 0- to 2-month-old seeds, even in seeds on soil that was kept continuously moist during this 3-month period. At the September, October, and November temperatures 3- to 5-month-old seeds germinated to high percentages. In all experiments seeds germinated better at a 14-hr photoperiod than in constant darkness. Inability of 0- to 2-month-old seeds to germinate at high summer temperatures allows P. dubia dubia to pass the dry summer in the seed stage, while increase in optimum and maximum temperatures for germination during the summer permits seeds to germinate in late summer and early fall when conditions are favorable for seedling survival and eventual maturation.