Environmental Control of Cold Acclimation in Salix pentandra

Acclimation in seedlings and young plants of various ecotypes and crosses between ecotypes of Salix pentandra L. were studied under natural climatic conditions at 69 degrees 39' N lat. and in a phytotron. Rapid cold acclimation was initiated after cessation of apical growth and outdoors it occurred about four weeks earlier in a northern (from 69 degrees 39' N lat.) than in a southern (from 59 degrees 40' N lat.) ecotype. F-1-populations from crosses between a northern and a southern ecotype gave an intermediate response. Experiments with different morphogenetic light treatments indicated that these treatments affected cold acclimation mainly through their effects on cessation of growth. There was, however, an indication that blue light, given during a daylength extension period, slightly prevented acclimation in seedlings of a northern ecotype. After budset cold hardening was effectively triggered by low temperatures (3 degrees, -3 degrees C), but also seedlings that were kept in short day conditions at a high temperature (12, 15 degrees C) developed a significant level of frost hardiness (about -40 degrees C) during a period of eight weeks.