Clonal variation in temperature requirements for budburst and dehardening in Salix species used for biomass production

Differences in temperature requirements for budburst were investigated among Salix clones used for biomass production spanning a wide range of geographical origins. When assessed under controlled conditions, the threshold temperature for the accumulation of the temperature sum for budburst was not significantly different among clones, with six out of the seven clones having a value of 1.0-1.2degreesC. However, there were significant clonal differences in the temperature sum required for budburst, with values ranging from 110 to 191 ddegreesC. This variation was only weakly linked to the geographical origin of the clones. The decline during the spring in levels of cold hardiness and sugars was also investigated in field-grown plants. All clones could tolerate temperatures less than or equal to-15degreesC, according to freeze tests, until at least a couple of weeks before budburst. This finding, together with the fact that buds become frost sensitive when they burst, suggests that the clonal variation in susceptibility to spring frosts, as observed in field trials, is related to the clonal variation in the timing of budburst, largely determined by the temperature sum requirement. Stem contents of cryoprotective sugars (sucrose, raffinose and stachyose) declined in spring in parallel with dehardening at first, but reached summer levels before dehardening was completed.