Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden VIII. Calcium dependence and β-amylase activity.

Red-light-induced germination (via phytochrome) of turions of Spirodela polyrhiza (L.) Schleiden, strain SJ, showed an absolute requirement for exogenously applied calcium (Appenroth et al. 1990b, Biochem. Physiol. Pflanzen 186, 209-219). With calcium step-up experiments (from 0.9 μM to 1 mM Ca(2+)) the influence of Ca(2+) on the escape from far-red-light reversibility was investigated. Delaying the time of Ca(2+) application after the red light pulse resulted in an increased reversibility. This confirmed that the calcium-requiring phase is within the phytochrome-dependent period. Ten days after a red light pulse was applied the activity of β-amylase (EC 3.2.1.2) was increased about tenfold. The starting point was similar in the germination and enzymatic responses, but half-maximal response times according to the Mitscherlich function (2.5 d and 6.9 d, respectively) and times of saturation (about 6 d and 9 d, respectively) were different. Increase of β-amylase activity was completely inhibited by irradiation with far-red light (demonstrating phytochrome as the photoreceptor), as well as by inhibitors of translation (cycloheximide, puromycin) and transcription (cordycepin). Once formed, the active enzyme protein was stable for at least 4 d after cycloheximide application. After application of cordycepin a small amount of translation activity was detected until complete inhibition was reached after 2 d. This phytochrome-modulated, de-novo synthesis of β-amylase was only slightly dependent on exogenous Ca(2+). Therefore, it can be concluded that the phytochrome-mediated signal-transduction chain branches either in its reversible part or as a consequence of diversity of the primary phytochrome reaction.