PHASE‐SPECIFIC EFFECT OF NITRATE ON PHYTOCHROME‐MEDIATED GERMINATION IN SPORES OF Dryopteris filix‐mas L.*

Induction of germination in spores of Dryopteris filix‐mas L. shows an absolute requirement for light with phytochrome as photoreceptor. The effectiveness of a red light (R) irradiation is affected by the ionic composition of the culture medium (Haupt, Planta 164 ,63–68, 1985). In this paper NO 3 − is identified as the component mediating this effect whereas ammonium or organic nitrogen‐containing compounds, tested so far, are ineffective. Enhancement by NO 3 − can be observed at concentrations 1 μ M and an optimum is found around 1 m M. The kinetics of NO 3 − action are elucidated by restricting the presence of NO 3 − to defined periods after sowing, and they are compared with kinetics of Pfr action. As a result, the enhancement of NO 3 − can be attributed to a specific phase in the signal‐transduction chain: the promotive effect of nitrate is only obtained if the Pfr presence coincides with or slightly preceeds the presence of NO 3 − . If Pfr can act for 15 or 20 h and is reversed thereafter by far‐red light (FR), NO 3 − added during this period or immediately after FR is effective and this enhancement vanishes during 6 h following FR. In contrast, NO 3 − has no effect as long as it is applied only during a period before Pfr is formed. Moreover, a modulation of the effectiveness of the photoconversion Pr to Pfr by NO 3 − can be excluded. These results are interpreted by the action of NO 3 − during early steps in the Pfr‐induced signal‐transduction chain. However, an additional effect of NO 3 − on the “coupling” of Pfr itself to subsequent dark reactions cannot be excluded yet.