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

Abstract

Abstract— Extension growth rate of light‐grown mustard (Sinapis alba L.) seedlings was monitored continuously using a sensitive linear displacement transducer system. When high fluence rates (ca 2 mmol m−2 s_1) of mixed red and far‐red light were presented to the growing internodes from fibre optic probes, fluctuations in extension rate occurred during the first 30 min. High red: far‐red ratios (R: FR) caused growth deceleration, whilst low R: FR caused transitory growth acceleration. These changes in extension rate were not exactly as predicted from the proportions of Pr (the red‐absorbing form of phytochrome) and Pfr (the far‐red absorbing form of phytochrome) calculated to be established by the light sources. Nevertheless, the data demonstrate that phytochrome is able to control extension growth at fiuence rates approaching those of summer sunlight, thereby providing the capacity to sense the presence of neighbouring vegetation before shading seriously compromises photosynthesis.Varying fiuence rate over two orders of magnitude whilst maintaining R: FR constant evoked transient fluctuations in extension rate. At high R: FR, a 100‐fold step down in fiuence rate led, after a lag of ca 10 min, to a transient (i.e. 20 min) deceleration of extension that was followed by a marked transient (i.e. 20 min) acceleration. After a 100‐fold step up in fiuence rate, a transient (i.e. 20 min) acceleration only was observed, beginning after a lag of ca 10 min. When R: FR was low, neither a step‐down nor a step‐up in fluence rate resulted in appreciable fluctuations in extension rate.The data are discussed in relation to the possible role played by the accumulation of photoconversion intermediates using a simple computer model for simulating active phytochrome concentrations at high fluence rates. The possibility that the mechanism for the photoperception of light quality by phytochrome may be capable of rapid adaptation to fluence rate fluctuations is proposed.