Effect of photoperiod and temperature on apical growth cessation in two ecotypes of Salix and Betula

Apical growth cessation as affected by photoperiod and temperature has been studied in seedlings of two latitudinal ecotypes of Salix and Betula. The critical photoperiod for apical growth cessation at constant temperatures of 15 and 21°C was about 22 h for a northern (69°C39′N) and about 15–16 h for a southern (59°C40′N) ecotype of Salix pentandra. Fluctuating day/night temperatures (21°C/9°C, 15°C/6°C) induced apical growth cessation in northern ecotypes even at 24–h photoperiod. Disagreements in critical photoperiods found in various studies are discussed.