Effect of seed moisture content during prechilling on the germination response of alder and birch seeds

The effect of prechilling duration on the germination performance of alder (Alnus glutinosa (L.) Gaertn.) and birch (Betula pubescens Ehrh.) at 15 degrees C and 20/30 degrees C was investigated. Seeds of two different seed lots of each species were adjusted to target moisture contents (TMC) of 25 or 30% (alder) and 30 or 35% (birch) and then chilled (4 degrees C) for 0, 6, 9, 12, 18, 24 and 36 weeks. Additional seeds of the same species were adjusted to full moisture content, as used in operational practice and then chilled for the same periods of time. Many fully imbibed seeds germinated prematurely after 12-18 weeks prechilling or deteriorated during the pretreatment period. Treatment effects in each species were consistent in each seed lot. In general, germination of the TMC seeds increased following up to 24 weeks of prechilling in alder and 12 weeks in birch. Thereafter, germination remained almost unchanged at both germination temperatures for TMC seeds that received up to 36 weeks of prechilling. The pattern of response to the prechilling treatment was similar for speed of germination following prechilling periods of 12 weeks in alder and 6 weeks in birch.