EFFECT OF PRECHILLING ON THE DORMANCY OF BETULA-PENDULA SEEDS

The effect of prechilling on the germination response to temperature w as investigated by germinating prechilled (at 5 degrees C) and unchill ed seeds of Betula pendula Roth (silver birch) at temperatures of 10, 15, 20, 25, and 30 degrees C in darkness. Unchilled and prechilled see ds produced 50% germination at temperatures of 26.7 and 15.3 degrees C , respectively. Two experiments were organized to study the effects of a range of prechilling temperatures (0-12 degrees C) on release of do rmancy. The level of dormancy in the seeds was tested at 12 degrees C using 6- and 18-h photoperiods. The optimal prechilling temperatures w ere between 1.0 and 3.3 degrees C. As the prechilling temperature incr eased, the effect of prechilling decreased so that the upper effective limit was between 8.7 and 12.0 degrees C. The effect of prechilling t ime on dormancy release was studied in an experiment where seeds prech illed at 2.4, 5.5, and 12.4 degrees C for 6, 12, 21, and 42 days were germinated at 12 degrees C using 6- and 18-h photoperiods. The germina tion probability (in legit scale) was directly related to the logarith m of the number of prechilling days. In seeds prechilled at 2.4 degree s C the requirement for a long photoperiod for germination was removed in 21 days, whereas 42 days were needed in seeds prechilled at 5.5 de grees C.