CHANGING SENSITIVITY TO LIGHT AND NITRATE BUT NOT TO GIBBERELLINS REGULATES SEASONAL DORMANCY PATTERNS IN SISYMBRIUM-OFFICINALE SEEDS

Seeds of Sisymbrium officinale (L.) Scop. that are buried under natura l conditions in soil pass annually through a seasonal pattern of chang es in dormancy. Dormancy is broken in autumn-winter and re-induced in summer. To elucidate dormancy regulation in this species under natural conditions, a detailed analysis of the changes in sensitivity to some relevant germination factors was carried out. Germination data fitted as logistic dose response curves showed that sensitivity to light and nitrate, both indispensable stimuli for germination of this species, varied with the seasons. Patterns of shifts in requirement for light a nd nitrate were remarkably similar. Sensitivity increased when both pr imary and secondary dormancy were alleviated, and it was reversed duri ng induction of secondary dormancy. During alleviation of primary dorm ancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very-low-fluence-range and in the low-fluence-range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurre d in two distinct ranges as well. From interpretation of curve paramet ers, it is postulated that dormancy is regulated by changes in the num ber of phytochrome and nitrate receptors, in shifts in the binding cha racteristics of the receptors and/or in shifts in the response chain i nitiated by the ligand-receptor interaction. Somewhere in this respons e chain, biosynthesis of gibberellins (GAs) is stimulated. By use of t he GA biosynthesis inhibitor tetcyclasis, it was indirectly proven tha t the capacity to synthesize GAs indeed varied with the seasons. Sensi tivity to GAs gradually increased from burial onwards and was not part icularly related to changes in dormancy. Thus, except for the first fe w months of burial, GA sensitivity may not be regarded as a limiting f actor in controlling dormancy in this species.