The Cessation of Apical Growth in Latitudinal Ecotypes and Ecotype Crosses ofSalix pentandraL

Apical growth cessation in seedling progenies of three latitudinal ecotypes and ecotype crosses of Salix pentandra L. was investigated under controlled growth conditions and out-of-doors. The southern ecotype (59° 40′ N) was characterized by late growth cessation under short day conditions, short critical photoperiod for growth cessation, and high light sensitivity; the northern ecotype (69° 39′ N) by rapid growth cessation, long critical photoperiod, and low light sensitivity. The geographically intermediate ecotype (64° 28′ N) had an intermediate photoperiodic response. The hybrid progenies were, however, closer to the southern than to the northern parent ecotype indicating a tendency to favour inheritance of southern characters. Fluctuating temperatures (21 °C/9 °C and 18 °C/9 °C) induced growth cessation in the northern ecotype but not in the other ecotypes or ecotype crosses with the exception of a few plants of the intermediate ecotype and of two hybrid progenies. The time course of apical growth cessation in various progenies grown out-of-doors at 69° 39′ N was similar to that observed under SD conditions except that the differences between the northern ecotype and the other ecotypes and crosses were greater.