Factors Controlling the Germination Date of Winter Annuals

Abstract

A winter annual is a plant which germinates in the autumn, passes the winter in the vegetative state, and flowers, sets seed and dies in the following spring or summer. Thus the seeds are normally liberated in late spring or summer, but they do not germinate until the autumn. This paper is concerned with the reasons for this failure to germinate in the summer, and with the factors controlling the precise date of germination in the autumn. Several workers have conducted laboratory experiments on the germination of winter annual species, though often without any particular interest in the germination in natural conditions. In most species investigated there is a change in ability to germinate as the seed ages: after a period of storage the seeds give a higher total germination and/or a faster rate than do fresh seeds under the same conditions. In most species this change in germination ability is related to physiological changes in the seed (after-ripening), but in some Leguminosae it is due to the softening of a hard seed coat. In some species the germination ability at some germination temperatures changes more than at others, so that as a result of after-ripening a change (always a rise) occurs in the optimum and/or the maximum temperature for germination. This will be referred to as change in temperature response with storage. It can be illustrated by the germination in the dark of Bromus tectorum (data from Hulbert 1955). The total germination of fresh seed at 100 C was 53 %, at 20? C 1 %; after 7 weeks' storage germination was about 95 % at either temperature, but faster at 20? C. Among the species which have been studied, five sorts of change with storage can be distinguished.