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

Abstract

Abstract— 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, Planta164,63–68, 1985). In this paper NO3−is identified as the component mediating this effect whereas ammonium or organic nitrogen‐containing compounds, tested so far, are ineffective. Enhancement by NO3− can be observed at concentrations 1 μM and an optimum is found around 1 mM. The kinetics of NO3− action are elucidated by restricting the presence of NO3− to defined periods after sowing, and they are compared with kinetics of Pfr action. As a result, the enhancement of NO3− 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 NO3−. If Pfr can act for 15 or 20 h and is reversed thereafter by far‐red light (FR), NO3− added during this period or immediately after FR is effective and this enhancement vanishes during 6 h following FR. In contrast, NO3− 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 NO3− can be excluded. These results are interpreted by the action of NO3− during early steps in the Pfr‐induced signal‐transduction chain. However, an additional effect of NO3− on the “coupling” of Pfr itself to subsequent dark reactions cannot be excluded yet.