Requirement for far-red light to maintain secondary needle extension growth in northern but not southern populations of Pinus sylvestris (Scots pine)

Extension growth of secondary needles is under photoperiodic control in Pinus sylvestris . To test for the effects of far-red light on maintaining this extension growth, seedlings of six populations originating from latitudes between 57° and 67°N were raised for 11 weeks in continuous incandescent (metal halogen) light at 300 µmol m^?2 s^?1 and 20°C and then transferred at the same temperature to a daily regime of 8 h incandescent light (230 µmol m^?2 s^?1) followed by a 16 h day extension with cool white fluorescent light (40 µmol m^?2 s^?1, R/FR ratio 7.5) or with incandescent lamps (20 µmol m^?2 s^?1, R/FR ratio 2.0). For the seedlings from the three populations north of 64°, needle extension growth over 42 days in the FR-poor day extension treatment was lower by up to 40% than in the FR-rich day extension treatment, whereas for the seedlings from the three southern populations the needle extension growth was similar in both day extension treatments. The requirement for FR in day extensions is characteristic of 'light-dominant' photoperiodic control mechanisms. It appears that P. sylvestris changes from dark-dominant night timekeeping to light-dominant day timekeeping with increasing latitude, as with the photoperiodic control of budset in Picea abies .