PHOTOPHYSIOLOGY OF TURION GERMINATION IN Spirodela polyrhiza (L.) SCHLEIDEN–V. DEMONSTRATION OF A CALCIUM‐REQUIRING PHASE DURING PHYTOCHROME‐MEDIATED GERMINATION

Etiolated turions of Spirodela polyrhiza are positively photoblastic and show a phytochrome‐mediated low fiuence germination response. The far‐red light (FR) reversibility decreased with the delay of FR irradiation (lag phase 1.06 ± 0.03 days after red light irradiation; half‐maximal response 1.9 days). The action of the far‐red‐absorbing form of phytochrome (Pfr) was only realized by a germination response if exogenously applied Ca2+ was present. Calcium step‐down (from 1 mM to 0.9 μM Ca2+) and Ca2+ step‐up (from 0.9 μM to 1 mM Ca2+) experiments were carried out to determine the Ca2+‐sensitive phase. There was no time gap between the two phases determined by the step‐down and step‐up experiments but a clear coincidence of both curves. Pulse treatments (24 h) with Ca2+ (1 mM) showed the upper part of this common curve to represent the most Ca2+‐sensitive phase. The Ca2+‐sensitive phase was within the Pfr‐requiring phase. After reversion of Pfr by FR pulses there was only a negligible response to the high Ca2+‐concentration, independent of the delay between the red light (R) and FR pulses. These results are compatible with the assumption of Ca2+ acting as a second messenger of Pfr. However, the Ca2+‐insensitivity in the first 12 h after the R pulse points against this hypothesis.