Autumn senescence in aspen is not triggered by day length

Autumn senescence in mature aspens, grown under natural conditions, is initiated at almost the same date every year. The mechanism of such precise timing is not understood but we have previously shown that the signal must be derived from light. We studied variation in bud set and autumn senescence in a collection of 116 natural Eurasian aspen (Populus tremula) genotypes, from 12 populations in Sweden and planted in one northern and one southern common garden, to test the hypothesis that onset of autumn senescence is triggered by day length. We confirmed that, although bud set seemed to be triggered by a critical photoperiod/day length, other factors may influence it. The data on initiation of autumn senescence, on the other hand, were incompatible with the trigger being the day length per se, hence the trigger must be some other light-dependent factor.