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.

Screening for efficient cold hardening in a breeding population of Salix using near infrared reflectance spectroscopy

“…Plants of similar height were placed adjacent to each other so as to minimize shading. All the remaining plants were treated to drastic day length reduction (from 19 to 6 h), known to trigger growth cessation [13]. Thus, the rate of cold hardening could be determined from a single measurement at a later stage.…”

“…Freeze tests were performed on the calibration set using the protocol described in [13], except that plants were assessed individually. The 20-54 cm section below the stem tip was cut into fifteen 2 cm segments, leaving the central 4 cm segment for subsequent spectral analysis.…”

“…After thawing and subsequent recovery in darkness at 10°C for 13 d, freezing damage was determined by measuring the ratio of variable to maximal chlorophyll fluorescence, a method that was validated before by comparison with scorings of tissue browning and regrowth capacity [17]. Data for individual plants were fitted to a sigmoid function by regression analysis (SPSS software, Chicago, USA), to determine the inflection point corresponding to 50% injury (LT50), as detailed in [13]. For fifteen out of the 150 plants tested no LT50 value could be obtained.…”

“…In Salix, as with most other woody species, cold hardening does not begin until growth has stopped in response to the reduction in day length [17]. The small variations in cold hardiness that plants display while actively growing seem to be phenotypic, rather than genetic in origin [13]. Northern ecotypes of Salix cease to grow earlier -at longer day lengths -than the southern ones [12], as do other woody species [10].…”

“…This linkage between growth and hardening cycles imposes restrictions on the extent to which either of these factors can be improved in isolation [19,22]. However, we have previously presented evidence that cold hardiness is also affected by differences in the inherent rate of cold hardening [13,14]. Some northern and continental ecotypes have the ability to undergo cold hardening up to three times as quickly as typical southern and maritime clones.…”