Latitudinal cline of requirement for far-red light for the photoperiodic control of budset and extension growth in Picea abies (Norway spruce).

To test for the effects of far-red light on preventing budset in Picea abies, seedlings of six populations originating from latitudes between 67°N and 47°N were grown for 4-8 weeks in continuous incandescent (metal halogen) light at 300 µmol m s and 20°C and then transferred, at the same temperature, to a daily regime of 8 h incandescent light (300 µmol m s ) followed by 16 h cool white fluorescent light (40 µmol m s ). (Cool white lamps are deficient in far-red light, with a R/FR ratio of 7.5 compared with 2.