Ecology of Germination and Flowering in the Weedy Winter Annual Grass Bromus japonicus

Abstract

From the time of seed maturity in late June and early July until December when temperatures drop to near freezing, habitat temperatures are within the range of those required for germination of seeds of the winter annual Bromus japonicus. However, a large proportion of the seeds in a given seed crop fail to germinate in the autumn of the year in which they are produced because they are not dispersed until winter. A high percentage of of the winter-dispersed seeds is induced into dormancy and must undergo a period of afterripening the following summer before germination can occur the next autumn. Thus, many of the plants that become established at a population site in autumn are from the previous year's seed crop. Plants overwinter in the field as "rosettes" and require long days for flowering. Nonvernalized plants exposed to natural short photoperiods of late autumn and winter flower under long days in spring, but plants flower much sooner if they are subjected to both low temperatures (vernalization) and short photoperiods during winter.