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Kinetics and photocontrol of hypocotyl elongation in etiolated mustard (Sinapis alba L.) Kristie, David Nickolos


The kinetics of hypocotyl elongation in darkness, and the responses of elongation to brief and prolonged irradiations were investigated in white mustard ( Sinapis alba L.) seedlings and in other species. A growth measuring apparatus consisting of a linear displacement tranducer coupled to a signal differentiator was developed to provide continuous high resolution measurements of the rate of hypocotyl elongation. In darkness, the growth rate of Sinapis as well as other species often underwent sustained and highly regular oscillations with a period of 3-30 minutes. A long period oscillation (>50 min) generated by nutational bending was also observed in the growth rate traces of many seedlings. The effects of monochromatic light were tested at a number of wavelengths between 380 and 780 nm waveband. Based on differences in the kinetics and spectral dependence of the responses to light, five distinct types of photoresponses could be distinguished in Sinapis. (1) Brief or prolonged irradiations with 380-500 nm light inhibited elongation after a lag of ca. 1 minute. (2) Phototropic bending was induced by low fluence rates of blue light, which did not cause a rapid inhibition of growth, indicating that these two photoresponses were distinct. (3) Brief 550-710 nm light pulses caused a 40-50% inhibition of growth after about a 5 minute lag. (4) Prolonged 660-710 nm irradiations caused a fluence rate-dependent inhibition, with kinetics distinct from the effects of brief light pulses. (5) Brief or prolonged irradiations with 720-780 nm light caused a small transient inhibition following the irradiation, or had no effect. The inhibitory effects of brief or prolonged irradiations with red or far red light were rapidly reversed by a longwave far red light pulse. The inhibitory effects of 710 nm light were lost after a red light pretreatment. It was concluded that the rapid effects of UV-blue light were mediated by a photoreceptor distinct from phytochrome. Hypocotyl elongation was also, rapidly and reversibly controlled by a phytochrome threshold reaction, with a threshold level of ca. 5% Pfr required for inhibition. The phytochrome threshold reaction was a prerequisite to the response to continuous irradiations.

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