The phytochrome B encoded by the HLG locus of Nicotiana plumbaginifolia is required for detection of photoperiod: hlg mutants show altered regulation of flowering and circadian movement

The sensitivity of the Nicotiana plumbaginifolia wild-type and phytochrome B (hlg) mutant to photoperiod was investigated. Leaf number at bolting and rosette-leaf movement were measured under conditions of different light quality and photoperiod. Bolting in wild-type plants was slightly delayed under short days (SD), compared to plants grown under long days (LD). The hlg mutant bolted later than the wild-type under LD, but not under SD. An endogenous rhythm of rosette-leaf movement in the wildtype was present under SD, but suppressed under LD. The hlg mutant behaved indistinguishably from the wild-type under SD conditions, but the movements were not suppressed under LD in the mutants. The endogenous rhythm appeared to be entrained normally in the mutants under all the conditions investigated. It was concluded that the B-type phytochrome absent in the hlg mutant is required for the measurement of photoperiod duration in N. plumbaginifolia, but is not required for entrainment of the circadian clock.