Planta 151, 135 140 (1981) Planta ~ Springer-Verlag 1981 High Irradiance Response Promotion of a Subsequent Light Induction Response in Sinapis alba L. C.J. Beggs, W. Geile, M.G. Holmes, M. Jabben, A.M. Jose, and E. Schfifer Biologisches Institut II der Universitfit. Sch~nzlestral3e1, D-7800 Freiburg, Federal Republic of Germany Abstract. Relative quantum responsivity curves for inhibition of hypocotyl elongation in Sinapis alba L. seedlings previously grown in white light confirm that a marked "end of day" inhibition response can be induced by a monochromatic light treatment (30 rain) at the end of the light period. In dark grown seedlings, however, no growth inhibition can be induced by a 30 rain monochromatic light treatment. A prerequi- site for an induction response appears to be a pretreat- ment with continuous light. Far red light is most effective with blue and red light showing a lesser effec- tiveness. The light pretreatment also shows a marked fluence rate dependency with respect to its ability to allow an induction response to manifest itself. The pretreatment required shows all the characteristics of a classical "HIR" response. The appearance of the effect in plants treated with the herbicide SAN 9789 seems to exclude chlorophyll as being the photorecep- tor. Key words: High irradiance response Hypocotyl elongation Photomorphogenesis - Phytochrome - Sinapis. Introduction It is possible to influence elongation responses in green plants by changes in the quality of a short light pulse given at the end of the daily light period. Downs et al. (1957) for example, showed that for both Abbreviations." SAN 9789=4-chloro-5-(methylamino)-2-(~, ~, ~- trifluoro-m-tolyl)-3(2H)-pyridazinone; RG9 light=long wave- length far red light (Schott RG9 colour glass); FR=far red light; WL=white light; BL-blue light; RL=red light; D--darkness; Pto~=total phytochrome; Pfr=far red absorbing form of phyto- chrome; HSR=high irradiance response Phaseolus vulgaris L. and two species of Ipomoea in- ternode elongation was stimulated if the daily light period was followed by a short period of far red light. This effect of far red light could be reversed by a subsequent pulse of red light and they therefore concluded that the response was ascribable to the pigment now known as phytochrome. Kasperbauer (1971) was able to repeat these observations for Nico- tiana tabaccum L. and also drew attention to the pos- sibility that this effect might be of importance in the response of plants to shading. Recently Wildermann et al. (1978) extended these observations to young seedlings of Sinapis alba L. where they found that in light-grown seedlings an "end of day" far red light pulse led to an increase in hypocotyl growth when compared to control plants grown in white light and transferred to darkness after a red light pulse. Mohr (1957), however, had investi- gated the effect of short light pulses on S. alba seed- lings which had been grown entirely in darkness. He was unable to detect any differences in hypocotyl growth between those plants which had received a short pulse of either red or far red light. Bertsch and Mohr (1965) were able to show that for light- induced anthocyanin synthesis in S. alba a red pulse was only slightly effective in inducing anthocyanin synthesis but if the seedlings had first received 12 h far red light the effectiveness of a subsequent red pulse was greatly increased. It appears, therefore, that a plant often required a light pretreatment before an inductive pulse becomes fully effective. This paper seeks to investigate this problem further and to char- acterise both the effect and the nature of the required pretreatment. Materials and Methods Sinapis alba L. seeds (harvest 1975) were obtained from Asgrow Co., Freiburg-Ebnet,FRG, and selectedand sown on chromatogra- 0032-0935/81/0151/0135/$01.20