Fluence-response curves and action spectra for the very low fluence and the low fluence response for the induction of kalanchoë seed germination.

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 mu, 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.