CHARACTERIZATION OF A CALCIUM-REQUIRING PHASE DURING PHYTOCHROME-MEDIATED FERN-SPORE GERMINATION OF DRYOPTERIS-PALEACEA SW

Abstract— For phytochrome‐mediated fern‐spore germination in Dryopteris paleacea Sw., initiated by a saturating red light (R) irradiation 20 h after imbibition, an almost absolute requirement for extracellular Ca2+ is found. To investigate the kinetics of this Ca2+ requirement spores were sown on a Ca2+‐free medium (Ca2+ < 10−8M) and Ca2+ was raised to 1 mM at defined periods after R. Alternatively, spores were sown on a Ca2+‐containing medium (1 mM) and transferred to a Ca2+‐free medium. A clearly defined period for the Ca2+ requirement is found between 30 and 50 h after R, while Ca2+ had to be present in the medium for at least 9 h to obtain half‐maximal germination. However, since the kinetics of deprivation and delayed addition of Ca2+ do not provide significantly different results, only a relative short presentation time for extracellular Ca2+ has to be expected at the level of a single cell. Ca2+ sensitivity is determined by the timing of the R irradiation, i.e. the timing of Pfr formation, which has been concluded from the observation that the variation of the imbibition time does not affect the kinetics of Ca2+ requirement and that the temporal application of polyethylene glycol (PEG), which is assumed to interrupt Pfr action, shifts the Ca2+ requirement to delayed intervals. These observations, as well as the fact that the requirement of Ca2+ has been observed for a limited period are interpreted as indirect evidence that Ca2+ action plays the role of a specific link in the phytochrome‐mediated signal‐transduction chain. The “Ca2+ kinetics” are compared with the kinetics of escape from reversibility by far‐red (FR) light and with kinetics of basic cell physiological processes occurring during germination. “Escape kinetics”, indicating “coupling” of Pfr to subsequent dark reactions, are observed at significant earlier intervals, and a gap of about 15 h is found for both the action of Pfr and of Ca2+. Thus, the direct interaction of Pfr with external Ca2+ as a first transduction step can be excluded experimentally. The kinetics of chlorophyll formation are found to be only slightly delayed, whereas the kinetics of mitosis are shifted by approximately 30 h. Almost the same slope is obtained for all kinetics investigated so far and one reaction spans a period of approximately 35 h. Obviously, variability found in the spore population is due to the coupling of Pfr whereas subsequent reactions proceed with almost identical velocity. Copyright © 1990, Wiley Blackwell. All rights reserved