Causes of variation in cold hardiness among fast-growing willows (Salix spp.) with particular reference to their inherent rates of cold hardening

Abstract

Causes of variation in cold hardiness in the autumn were assessed among closely related, fast-growing clones of willow of northern/continental and southern/maritime origins, under controlled regimes and natural conditions. Cold hardiness was assessed by controlled freezing followed by injury analysis, based on measurements of chlorophyll fluorescence (stems) and electrolyte leakage (leaves). During growth at a given temperature, the cold hardiness of the clones' stems was negatively correlated with their rate of growth. This apparently phenotypic variation was independent of temperature and, hence, the absolute growth rate. At later stages, cold hardiness of stems varied mainly with respect to genetic differences in the timing and rate of cold hardening. Cold hardening began up to 7 weeks earlier in northern/continental clones, and their rates of hardening in cool temperature regimes were up to three times higher than in southern/maritime clones. Ranking of clones with respect to rates was essentially the same whether natural or abrupt reductions of day length were used to trigger cold hardening. Results closely agreed with those of a previous field trial. Comparisons of rates at cool and warm temperatures suggest that cold hardening became increasingly dependent on cool temperatures with time. Increasing sucrose-to-glucose ratios, and especially dry-to-fresh weight ratios, paralleled early cold hardening. Before leaves were shed in the autumn they underwent cold hardening in parallel with stems, eventually allowing them to tolerate temperatures down to -10°C.