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 degrees N and 47 degrees N were grown for 4-8 weeks in continuous incandescent (metal halogen) light at 300 mu mol m(-2) s(-1) and 20 degrees C and then transferred, at the same temperature, to a daily regime of 8 h incandescent light (300 mu mol m(-2) s(-1)) followed by 16 h cool white fluorescent light (40 mu mol m(-2) s(-1)). (Cool white lamps are deficient in far-red light, with a R/FR ratio of 7.5 compared with 2.0 for the incandescent lamps.) All the seedlings from 67 degrees and 80% of those from 64 degrees stopped extension growth and set terminal buds within 28 days of the change of regime. The seedlings from 61 degrees and further south continued growing, as did control seedlings from 67 degrees grown as above but with incandescent light at 20 mu mol m(-2) s(-1) replacing cool white illumination. To distinguish between a clinal and ecotypic pattern of variation, the interval between 64 degrees and 59 degrees was investigated by growing populations originating from that area in the same regimes as before. After 28 days in the cool white day-extension regime, the percentage budset was 86 for the population from 64 degrees, 0 for the population from 59 degrees and 25-50 for the intermediate populations; i.e. the populations showed a clinal variation in requirement for far-red light according to latitude. Thus northern populations of Picea abies appear to behave as 'light-dominant' plants for the photoperiodic control of extension growth and budset, whereas the more southern populations behave as 'dark-dominant' plants.