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It was shown previously that light-dependent germination of turions of Spirodela polyrhiza (Lemnaceae) is mediated by the photoreceptor phytochrome [Appenroth & Augsten (1990) Photochemistry and Photobiology 52,61-65].In the present study, we found that this photoresponse depends on nitrate in the surrounding medium both during after-ripening (under natural conditions occurring in winter) and during germination after light-induction (in spring). The action of nitrate in the germination response is neither related to the induction of nitrate reductase nor to the rate of uptake of ^^NOa". Moreover, two-factor analysis (phytochrome, nitrate) revealed a multiplicative coaction, i.e. independent action of both factors in mediation of germination. The notion that nitrate is a nutritional prerequisite in phytochrome-mediated germination of turions, is supported by the following facts: (1) Nitraterequirement during germination was strongly increased by nitrate starvation during after-ripening prior to germination. (2) Ammonium could substitute for nitrate. (3) Nitrate uptake by the turions was unaffected by phytochrome and very pronounced even at low concentrations (0-07 mol m"^) in the medium. With regard to the phytochrome-induced chain of events, it is concluded that nitrate is a prerequisite during a specific developmental phase. Nitrate is not a regulatory element within the chain. In an ecological sense, however, nitrate contents of the aquatic system regulate the germination of turions.

Etiolated turions of Spirudela polyrhiza are positively photoblastic and show a phytochromemediated low fluence germination response. The far-red light (FR) reversibility decreased with the delay of F R irradiation (lag phase 1.06 t 0.03 days after red light irradiation; half-maximal response 1.9 days). The action of the far-red-absorbing form of phytochrome (Pfr) was only realized by a germination response if exogenoudy applied Ca2+ was present. Calcium step-down (from 1 mM to 0.9 p M Ca") and Ca" step-up (from 0.9 pM to 1 mM ca'+) experiments were carried out to determine the Ca2+-sensitive phase. There was no time gap between the two phases determined by the step-down and step-up experiments but a clear coincidence of both curves. Pulse treatments (24 h) with Ca2+ (1 mM) showed the upper part of this common curve to represent the most Ca"-sensitive phase, The Ca2+-sensitive phase was within the Pfr-requiring phase. After reversion of Pfr by F R pulses there was only a negligible response to the high Ca2+-concentration, independent of the delay between the red light (R) and F R pulses. These results are compatible with the assumption of CaZ+ acting as a second messenger of Pfr. However. the Ca'+-insensitivity in the first 12 h after the R pulse points against this hypothesis.

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