Phototransformation of oat phytochrome with millisecond and submillisecond flashes: The level of the photostationary P_fr/P_tot ratio is modified by photoreversible intermediates

Abstract

Photoconversion of the red‐absorbing form of phytochrome (Pr) to the far‐red‐absorbing form of phytochrome (Pfr) and vice versa has been measured spectrophotometrically at 10°C in immobilized and soluble phytochrome (118 kdalton), prepared from 5‐day‐old etiolated oat seedlings (Avena saliva L. cv. Sol II). The photostationary equilibrium φ= PfrPtot (with Ptot= total amount of phytochrome Pr+ Pfr) for red light depends on whether it is established by repetitive pulses (≥ 5 s) or by repetitive flashes (≥ 4 ms). In the wavelength region around 660 nm, a lower φ is reached with flashes as compared to that with pulses. This difference becomes negligible if the wavelength is shortened to the 600 nm region, and it also disappears if the fluence of each individual flash is reduced. In contrast, in long‐wavelength red light and short‐wavelength far‐red light, a higher φ is reached with flashes than with pulses. We relate the differences in φ for flash and pulse irradiation to photochromic systems between Pr and photoreversible intermediates in the phototransformation pathway Pr→ Pfr. Thus, light absorption by phytochrome intermediates can be limiting for the quantitative relationship between light signal and Pfr formed.

INTERMEDIATES IN THE PHOTOCONVERSION OF FUNCTIONAL PHYTOCHROME IN FERN SPORES OF Dryopteris‐I DEMONSTRATION AND QUANTITATIVE CHARACTERIZATION OF THE PHOTOCHROMIC SYSTEM Pr⇔ Ii700USING NANOSECOND‐LASER PULSES*,†

“…3). The response now approaches that for a saturating irradiation with low-fluence rate, in agreement with in vitro measurements carried out with large oat phytochrome and flashes in the ms-range (Scheuerlein et al, 1985). Therefore, the low cp-values, characteristic for saturating single-flash irradiation under highfluence-rate conditions, are interpreted by the establishment of a photochromic system P, o Goo, which is completed much faster than Ifl, can thermally react to Pf,.…”

INDUCTION OF CHLOROPLAST MOVEMENT IN THE ALGA Mougeotia BY POLARIZED NANOSECOND DYE‐LASER PULSES*†

“…Flashes of 690 nm in no case induce a response. Similar to the case of lettuce-seed germination (Scheuerlein and Braslavsky, 1985), the establishment of a photochromic system between P, and the very early intermediates I;,,,, (= lumi-R) during the 15-11s laser pulse can explain the results. At 690 nm, the photochromic system is shifted more extensively to P, than at shorter wavelengths; thus, P,, formation is strongly diminished.…”

“…, 1987). Photochromism occurs also in vivo as shown by Scheuerlein and Braslavsky (1985), using a 19ns'pulse for the induction of lettuceseed germination, and by Kadota et al, 1986 during the study of Adiantum prptonemata polarotropism.…”

“…Holzwarth et a f . , 1984), we assume, similar to the Lactuca system (Scheuerlein and Braslavsky, 1985), that the photochromic system P, 0 g(w, is established during the 15-11s laser pulse. This results in the reduction of Pf, formation and prevents the building-up of an active Pfr gradient in the cell.…”

“…, 1984; Heihoff et a f . , 1987) as well as in vivo (Inoue and Furuya, 1985;Scheuerlein and Braslavsky, 1985;Kadota et a f . , 1986).…”