Abstract— The irradiance and wavelength dependence of phytochrome destruction in vivo was analysed in etiolated cotyledons of Cucurbita pepo L. and etiolated seedlings of Amaranthus caudatus L. In contrast to grass seedlings, the rate of Ptot destruction could only be saturated by light sources that establish relatively high Pfr levels (about 50% of total phytochrome, corresponding to the photostationary state established by 693 nm light). To explain the irradiance dependence of Ptot destruction in dicots at irradiances above 0.1 Wm-2, where the light reaction is at least one order of magnitude faster than Pfr destruction, we suggest there is a fast intercalary dark reaction between photoreaction and destruction. This dark reaction is probably—as in grass seedlings—the binding of Pfr to a receptor site. We conclude that the differences between dicots and grass seedlings with respect to the phytochrome system are of a quantitative rather than a qualitative nature.
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