Seasonal variation in dormancy and light sensitivity in buried seeds of eight annual weed species.
Germination of 8 annual weed species was recorded in 3 different light environments (light, dark, and after a short light exposure). Seeds were buried outdoors at the end of November 1994 and exhumed monthly from March 1995 to April 1996. All species exhibited substantial seasonal changes in dormancy level, and the patterns suggest that seeds of Papaver rhoeas germinate strictly in the autumn; Capsella bursa-pastoris, Descurainia sophia, Spergula arvensis and Urtica urens germinate mainly in the autumn; Chenopodium suecicum germinates strictly in the spring; and Matricaria perforata germinates mainly in the spring. Lapsana communis showed inconsistent dormancy changes over the year. All species had acquired a light requirement for germination after being in the soil, and in many cases the short light exposure (1050 µmol m-2) was enough to fulfil this requirement. The demonstrated seasonal changes in light sensitivity in several of the species will have to be taken into account in attempts to photocontrol weeds. By using the short-light treatment, seasonal dormancy changes were detected that would not have been obvious by testing for germination in only light and darkness. Hence, light is not a simple dichotomous factor in its effect on germination.