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Development and Growth Regulation

Fluence-Response Curves and Action Spectra for the Very Low Fluence and the Low Fluence Response for the Induction of Kalanchoë Seed Germination ¹

Laboratory of Plant Physiology, State University of Gent, Ledeganckstraat 35, B-9000 Gent, Belgium, Department of Biology, University of Antwerpen, Universiteitsplein 1, B-2610 Wilrijk, Belgium

Germination of Kalanchoë blossfeldiana Poelln. seeds is absolutely light-requiring. Germination of one seed is the result of one out of three reactions, viz. the very low fluence response (VLFR), the low fluence response (LFR) and the high fluence response/high irradiance response. In order to demonstrate the involvement of phytochrome for both photoresponses, i.e. VLFR and LFR, action spectra for induction were determined. Fluence-response data are analyzed by means of probit analysis in order to calculate the seed population parameters, with special attention to µ, or the fluence for half-maximal induction, and B, the slope in the probit diagram. Laser light was used between 620 and 800 nanometers to analyze the VLFR. Phytochrome is responsible for both photoresponses: the VLFR action spectrum demonstrates an exponential decrease in apparent photoconversion cross-section (P_r P_fr) up to about 800 nanometers. Assuming that P_r:P_fr-X and P_fr:P_fr-X are the effectors for the VLFR and the LFR, respectively, we estimate an average induction threshold of about 0.003% P_r:P_fr for the VLFR and about 1% P_fr:P_fr for the LFR among individuals of the seed population.