The role of calcium ions in phytochrome-mediated germination of spores of Onoclea sensibilis L.

Phytochrome is confirmed to be the photoreceptor pigment in the germination response of Onoclea sensibilis L. by demonstrating red-far-red (R-FR) photoreversibility. External Ca(2+) is required for this response with a threshold at a submicromolar concentration. Ethylene glycol-bis(β-amino-ethyl ether)-N,N,N',N'-tetraacetic acid, La(3+) and Co(2+) reversibly inhibit germination. Lanthanum only inhibits germination when applied before or during irradiation, indicating that the external Ca(2+) requirement is transient, although in the absence of Ca(2+) the R-stimulated system remains maximally poised to accept the ion for over 4 h after irradiation. The ability to respond to Ca(2+) 4.1 h after R-irradiation is not reversed by FR-irradiation, indicating that Ca(2+) transport has been uncoupled from phytochrome. Barium and Sr(2+), but not Mg(2+) can substitute for Ca(2+). Artificially increasing the concentration of intracellular free Ca(2+) with the ionophore A 23187 stimulates germination in the dark. The Ca(2+)-calmodulin antagonists, trifluoperizine and chlorpromazine, reversibly inhibit germination. Calcium is required in phytochrome-mediated fern spore germination; it may be acting as a second messenger.