QUANTITATIVE CORRELATION BETWEEN SPECTROPHOTOMETRIC PHYTOCHROME ASSAY AND PHYSIOLOGICAL RESPONSE

Abstract

Abstract— Apparent synthesis* of the enzyme lipoxygenase in the cotyledons of the mustard seedling (Sinapis alba L.) is controlled by phytochrome (Pfr ground state)† through a threshold (all‐or‐none) mechanism. This response was used to determine physiologically the photostationary states, Λ that is, the [Pfr]/[Ptot] ratios established by different wavelengths in the red and far‐red range of the spectrum, including the standard red and far‐red sources used in this laboratory (Mohr, 1966). Under the premises (for which justification has been given on previous occasions) that the [Pfr]/[Ptot] ratio for standard red light is 0.8, and that the decay of Pfr is a first‐order process with a half‐life of 45 min, the [Pfr]/[Ptot] ratios determined physiologically by means of the lipoxygenase response agree with the [Pfr]/[Ptot] ratios determined spectrophotometrically by Hartmann and Spruit (cf. Fig. 9 in Hanke et al., 1969) in hypocotyl hooks of mustard seedlings. In the hook the fr, that is, the [Pfr]/[Ptot] ratio for standard far‐red, is found to be 0.023. In the cotyledons, this ratio is several times higher (Schafer et al., 1972). The conclusion that apparent lipoxygenase synthesis in the cotyledons is controlled by phytochrome located in the hook has been substantiated by further spectrophotometric (Schäfer et al., 1973) and physiological experiments (H. Oelze‐Karow and H. Mohr, in preparation). The minimum steepness of the threshold was determined. An increase of the Pfr level from 118 (relative units) to 130 leads to an instantaneous and total suppression of apparent lipoxygenase synthesis; a corresponding decrease from 130 (relative units) to 118 leads to an immediate resumption of apparent LOG synthesis at full speed. It is concluded that an explanation of the experimental facts requires a cooperative effect on the level of Pfr, a high degree of synchrony on the cellular and organismic level and rapid communication between the hypocotyl hook and the cotyledons.*The term ‘apparent synthesis’ is used operationally in the present paper to denote any increase of enzyme activity, although de novo synthesis of lipoxygenase has not so far been rigorously demonstrated. The usual inhibitor experiments (cf. Oelze‐Karow et al., 1970) have led to the conclusion that intact RNA and protein synthesis is required for an increase of lipoxygenase activity.