Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden. X. Role of nitrate in the phytochrome‐mediated response

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Abstract
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 15NO3−. 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) Nitrate‐requirement 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−3) 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.

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