KINETICS OF P_fr APPEARANCE IN Amaranthus caudatus‡

Abstract

Abstract— The kinetics of the far‐red absorbing form of phytochrome (Pfr) appearance from intermediates in the pathway from the red absorbing form of phytochrome (Pr) to Pfr that accumulate under high fluence rate white light have been investigated in 3‐day old dark grown Amaranthus caudatus seedlings. The appearance of P(r after a 5 s white light pulse was measured over the temperature range ‐8 to 25°C in samples flushed with O2 or N2. Over the whole temperature range under anaerobic conditions the kinetics of the slowest component of Pfr appearance are faster than in the presence of O2. Arrhenius plots are linear over this temperature range and indicate the activation energy for the slowest component of Pfr appearance is 44.05 ± 1.97 kJ mol−1 for O2 and 53.69 ± 4.86 kJ mol−1 for N2.

PHYTOCHROME ACTION AT HIGH PHOTON FLUENCE RATES: RAPID EXTENSION RATE RESPONSES OF LIGHT‐GROWN MUSTARD TO VARIATIONS INFLUENCE RATE and RED: FAR‐RED RATIO*

“…In order to maximise the proportion of phytochrome existing as intermediates, WL was used, allowing very high fluence rates to be generated. Plotting the data for the highest fluence rate on a logarithmic scale revealed two components, the faster of which had a half-time of ca 0.6 s. These observations compare with a half-time of ca 0.3 s observed by Kendrick et al (1985) for the equivalent process in vivo in Amaranthus seedlings. The halftime data obtained were used in computer simulations of Pr, Pfr and intermediate levels as described in the Appendix.…”

“…To estimate the half-life of Pfr formation from intermediates, the increase in absorbance at 730 nm was followed with time after the cessation of irradiation, essentially as carried out by Kendrick et al (1985). In order to maximise the proportion of phytochrome existing as intermediates, WL was used, allowing very high fluence rates to be generated.…”

“…It is possible, of course, that intermediates other than metu-Rb also accumulate at high fluence rates. As yet, direct quantitative estimates of the proportions of phytochrome present as intermediates in vivo have been difficult to obtain in etiolated tissues and impossible in chlorophyllous material, although the appearance of Pfr in darkness after red or white light treatments indicates that intermediate accumulation is significant at daylight fluence rates (Kendrick and Spruit, 1973;Kendrick et al, 1985). Discrepancies between measurements and calculations of phytochrome photoequilibrium in a range of tissues have been interpreted as evidence for intermediate accumulation (Gardner and Graceffo, 1982).…”

The function, action and adaptive significance of phytochrome in light‐grown plants

“…First, the aeeutnulation of meta-Rb, for the pool of phytochrotne controlling shade avoidatiee responses, may be much lower than the high values predicted by means of in vivo spectrophototnetry (e.g. Kendrick et al [1985] estimated that more than 50% of P would be present as intertnediates at sunlight fluence rates) or by measuring the amount of Pfr available for destruction (Smith, Jackson & Whitelam, 1988), This could be accounted for by the existence of a different environment surrounding the relevant pool of phytochrome (Spruit, 1982;Kendrick et al, 1985), or by the possibility that photoconversion intermediates do not accutnulate at high lluence rates in the form of phytochrome that predominates in light-grown plants (this matter is addressed below). Secondly, plants may tneasure Pfr/(Pr-|-Pfr) or Pfr/Pr, A third alternative is that plants may measure [Pfr] or Pfr/P, but at high fiuence rates the responses caused by a reduction in the values of these parameters are compensated by responses of opposite direction dependent on fluence rate (Casal & Alvarez, 1988;Casal, 1989;Casal & Smith, 1989b), Similar flueticerate-dependent responses are well-documented in etiolated seedlings (Moht-, 1972) but the picture is far from clear for de-etiolated seedlings.…”

Phytochrome, a family of photoreceptors with multiple physiological roles

“…Recently, this question has been approached by analysing the responses of mustard seedlings to fluctuations in R: FR at very high photon fluence rates (Smith, 1990). It is known that at high fluence rates short-lived photoconversion intermediates between Pr and Pfr accumulate to significant proportions (Kendrick & Spruit, 1972Kendrick, Kome & Jaspers, 1985;Smith, Jackson & Whitelam, 1988). This means that, at constant R: FR, the concentration of Pfr will be considerably lower at high, than at low, fluence rates.…”