Variation in freezing resistance during different phenological stages in some Populus and Salix clones: Implications for clonal selection

Nineteen (19) clones of Salix and twenty-one (21) clones of Populus were ex amined for their variability in freezing resistance. A series of laboratory freezing tests were conducted, using visual assessment and electrolyte lea kage to detect freezing injury and survival. Clones were tested at predeter mined levels of freezing stress and during seven (7) phenological stages: d ormant (D), early spring (ES), spring (S), flushing of terminal buds (FTB), new axillary bud growth (NAG), growing (G) and early-fall (EF) stages. Sig nificant clonal variation in freezing resistance was detected at four (4) s tages: S, FTB, NAG, and EF. At the D and ES stages, when freezing resistanc e was greatest, no significant differences in clonal survival were detected . Similarly, at the G stage in which clones exhibited the highest susceptib ility to freezing stress, clonal variation for the estimated index of injur y was negligible, At the G stage, significant clonal differences were detec ted only at relatively mild stress (-3 degreesC) in Salix, which accounted for 10% of the total variation. At the EF stage, clonal differences were hi ghly significant for the index of injury and accounted for 34% of the varia tion in Salix, and 32% in Populus. For a subset of 14 Populus clones, all e stimated SPEARMAN's rank correlation coefficients between stages were signi ficant, except in the case between the S and EF stages. Implications of the results for clonal selection with respect to freezing resistance in Populu s and Salix are briefly discussed.