RELATIONSHIP BETWEEN PHYTOCHROME PHOTOCONVERSION and RESPONSE

Abstract— The action of phytochrome on appearance of NADH-dependent glutamate synthase (NADH-GOGAT) and ferredoxin-dependent glutamate synthase (Fd-GOGAT) was studied in the cotyledons of the mustard ( Sinapis alba L.) seedling. It was found that the [P_fr]-response curves are composed of two branches with two very different slopes (a₁ a₂). This explains the biphasic fiuence response curves reported previously. While a, is the same with both enzymes, a, is much higher in NADH-GOGAT than in Fd-GOGAT. However, the transition from the high (a₁) to the low (a₁) slope occurs at the same P_fr/P_tot ratio irrespective of the steepness of a₁ While the appearance of NADH-GOGAT is very sensitive to small amounts of P_rr (high a₁), the response is not sensitive to the "High Irradiance Reaction" (HIR) of phytochrome. On the other hand, appearance of Fd-GOGAT (relatively low a₁) exhibits the usual HIR. It is concluded that the presently available models of phytochrome action, including the dimeric model, are not adequate to account for the actual data. At present it appears that a multiplicity of primary actions of phytochrome exist, and that it is the availability of the primary reactant (X, _1,2,3) which determines transduction or non-transduction of the phytochrome signal.