PHOTOCONTROL OF PHYTOCHROME DESTRUCTION IN GRASS SEEDLINGS. THE INFLUENCE OF WAVELENGTH AND IRRADIANCE

Publisher Website
Google Scholar
Abstract
Abstract— –The kinetics of phytochrome destruction in vivo of coleoptiles and mesocotyls of etiolated grass seedlings (Avena sativa L., Zea mays L.) in continuous light were investigated using wavelength and irradiance as experimental variables. In contrast to dicotyledonous seedlings, the destruction reaction of these monocotyledons is saturated at very low levels of the far‐red absorbing form of phytochrome, Pfr (e.g. at 1% of total phytochrome, corresponding to the photostationary state established by 727 nm light, in 2.5‐day‐old dark‐grown Avena). On the other hand, the first‐order rate constant of monocotyledon destruction may be at least one order of magnitude larger than in dicots, as indicated by the zero‐order rate measured in the presence of saturating amounts of Pfr (τl/2 1.5 min in Avena). At sub‐saturation Pfr levels, the destruction rate was found to be determined by the rate constants of the photoreactions over a wide range of wavelengths and irradiances. These results can be interpreted in terms of a destruction enzyme with high catalytic efficiency but limited availability.Analysis of in vivo binding of phytochrome to a pelletable cell structure during destruction revealed that both the pelletable and the non‐pelletable fraction lose photoreversiblility with similar rates and thus provide no useful information with respect to a causal relationship between the two processes. However, due to the short half‐life of Pfr at sub‐saturation levels (which make the photoreactions and intermediary processes rate‐limiting for destruction even at relatively high irradiances) the existence of a similarly rapid dark‐reaction between the photoreactions producing Pfr and the destruction reaction could be demonstrated. This dark reaction displays the properties of Pfr binding to a receptor site.