Changing sensitivity to light and nitrate but not to gibberellins regulates seasonal dormancy patterns in Sisymbrium officinale seeds

Abstract

Seeds of Sisymbrium officinale (L.) Scop, that are buried under natural conditions in soil pass annually through a seasonal pattern of changes 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 and nitrate were remarkably similar. Sensitivity increased when both primary and secondary dormancy were alleviated, and it was reversed during induction of secondary dormancy. During alleviation of primary dormancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very-Iow-flueuce-range and in the low-fluence-range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurred in two distinct ranges as well. From interpretation of curve parameters, it is postulated that dormancy is regulated by changes in the number of phytochrome and nitrate receptors, in shifts in the hinding characteristics of the receptors and/or in shifts in the response chain initiated by the ligand-receptor interaction. Somewhere in this response chain, biosynthesis of gibbereiiins (GAs) is stimulated. By use of the GA biosynthesis inhibitor tetcyclasis, * it was indirectly proven that the capacity to synthesize GAs indeed varied with the seasons. Sensitivity to GAs gradually increased from burial onwards and was not particularly related to changes in dormancy. Thus, except for the first few months of burial, GA sensitivity may not be regarded as a limiting factor in controlling dormancy in this species.

Seed dormancy and germination

“…Furthermore it has been shown that the rate of increase of sensitivity to environmental signals was not fixed, seeds produced in different years had a different response. This is consistent with the fact that the depth of dormancy is not only determined genetically, but also by the ambient environment during seed formation (Donohue, 2005).Non-dormant seeds that are exposed for some time to unfavourable germination conditions (imbibed seeds kept at relatively high temperature in darkness for example) may enter a state of dormancy again, which is called secondary dormancy (Cone and Spruit, 1983;Derkx and Karssen 1993b; Hilhorst, 2007).A challenge in dormancy and germination research is to identify the nature of the crucial regulator(s) that prevent(s) the onset of germination (dormancy), that trigger(s) the germination process and their mutual interaction. Furthermore, it is important to know how the environmental factors such as light and cold affect the endogenous factors that control germination.…”

“…Non-dormant seeds that are exposed for some time to unfavourable germination conditions (imbibed seeds kept at relatively high temperature in darkness for example) may enter a state of dormancy again, which is called secondary dormancy (Cone and Spruit, 1983;Derkx and Karssen 1993b;Hilhorst, 2007).…”

Redefining Seed Dormancy: An Attempt to Integrate Physiology and Ecology

“…However, they do not really compete, because full induction of dormancy in buried seeds takes several months, while germination is completed in a few days. Derkx & Karssen (1993a) showed that when Sisymbrium officinale seeds are buried in darkness germination is prevented. However, the temperature they are exposed to regulates the changes in the requirements for light, nitrate and temperature for subsequent germination (i.e.…”

“…regulates dormancy). Changes in dormancy occur in spite of continuous darkness and are not correlated with changes in endogenous nitrate content (Derkx & Karssen 1993a). From experiments in which Sisymbrium officinale seeds were incubated in darkness, Derkx eta!.…”

“…Dormancy is only related to the width of the temperature range for germination, not to the question whether or not the current temperature is inside this range. Derkx & Karssen (1993a) changes in dormancy not only comprise changes in temperature requirements for germination, but also in its requirements for nitrate and light. Sensitivity to light and nitrate, both necessary stimuli for the germination of this species, increased when dormancy was broken and decreased when dormancy was induced.…”