A fairly recent review on the role of nitrate in
terminating dormancy of seeds can be found in Karssen
and Hilhorst (1992). Consequently, we will refer to
it only brie¯y in this paper.
Nitrate is a requirement for dormancy termination
in a number of species. Because nitrate concentration
in the soil ¯uctuates seasonally (i.e. the higher the
temperature, the higher the mineralisation rate of soil
organic matter and consequently, the higher the actual
nitrate concentration), the possibility that the requirement
for nitrates could be an environmental signal
determining the seasonal emergence of some weeds
has been considered. No correlation was found
between the period of emergence of weeds requiring
nitrates to completely exit from dormancy, however,
and soil nitrate concentration at different times of the
year (Karssen and Hilhorst, 1992). Rather, this
requirement has been proposed as a gap-detection
mechanism, since mineralisation rates in open gaps
are higher than in the soil under a vegetation cover
(Pons, 1989).