Phytochrome-regulated growth of excised coleoptile tips of Triticum aestivum induced by blue, red and far-red light

The elongation growth of 5-mm tip sections of 3-day-old etiolatedwheat coleoptiles was promoted by light flashes of 436, 650and 750 nm in comparison with the dark control. As short as7/1,000 sec red light (R) of 14,420 W/m² led to saturation ofthe R-induced growth response. At quantum-identical far-redlight (FR) irradiation, a plateau developed at about 9/1,000sec which, however, was interrupted at longer irradiation periods.Varying dark periods between R or FR activation of growth anda second FR flash, a light-independent period with a half-lifeof 45 to 60 sec was found. Only after this light-independentperiod could R or FR activation of growth be reversed by FR.A second light-independent period was found by the followingirradiation schedule: 1 sec R/100 sec darknees(D)/l sec FR/varied D/l sec R. Our experimental conditions enabled us (a) to distinguish betweenactivation and inactivation by FR, (b) to clarify the reactionchain of different phytochrome forms and (c) to determine thehalf-life of light-independent phytochrome reactions.