Effect of rootstock on photoperiodic control of elongation growth in grafted ecotypes of Salix

Scions of a southern (59° 40'N Lat.) and a northern (69° 39'N Lat.) ecotype of Salix pentandra L. were grafted on clonal rootstocks of the same ecotypes. Effects of photoperiod on elongation growth of the 4 combinations (south/south, south/north, north/south and north/north) were studied in a phytotron at 18° and 15°C. The photoperiodic response was primarily dependent on the ecotype of the scion, but this response was significantly modified by the rootstock. Cessation of apical growth was advanced by a northern clone and delayed by a southern clone as a rootstock. The results indicate that the critical photoperiod for cessation of apical growth could be slightly decreased by a northern and increased by a southern rootstock.