Growth and biomass partitioning of populations of European Pinus sylvestris L. under simulated 50° and 60° N daylengths: evidence for photoperiodic ecotypes

Seedlings of 24 European populations of Scots pine (Pinus sylvestris L.) were grown from seed for 4 months in controlled environments under photoperiod conditions of 50° or 60° N, but with similar temperatures and total daily quantum flux in both photoperiods. During the experiment the photoperiod was continually changed to simulate the photoperiod from 1 May to 1 September at each latitude. Secondary needle length, plant height and dry mass at harvest were greater at 60° than the 50° N photoperiod. All populations allocated significantly greater proportions of their biomass to roots at 50° than 60° N photoperiod, yet central populations allocated less to roots than northern ones in both photoperiods. Northern populations (origin > 55° N latitude) stopped height growth up to 50 d earlier than the other populations when grown under 50° N photoperiod conditions. Under the 60° N photoperiod, all populations, except the most southern one, developed terminal leader buds and ceased growth in height between 87 and 105 d after sowing. Using cluster analysis, populations with similar growth and photoperiod responses were assigned to two major groups: northern (61–56° N from Sweden and USSR), and central (55–40° N from the USSR, Poland, Germany, Belgium, France, Czechoslovakia, Yugoslavia, and Turkey). The most southern population, from Turkey, was classified independent from the central group based on growth characteristics alone. No single mechanism of height growth cessation can explain the response of all populations of 1‐yr‐old seedlings of Scots pine. Our findings support the hypothesis that northern plants respond to a combination of increasing night length and accumulated degree‐days. However, the evidence suggests that central and southern populations do not respond to increased night length as they all ceased growth earlier in the 60° than the 50° photoperiod. As hypotheses for further testing, we propose that central and southern populations are either photoperiodically insensitive or respond to short nights or to accumulated degree‐days at their native latitudes.