Predicting the cold hardiness of willow stems using visible and near-infrared spectra and sugar concentrations

Closely related, fast-growing clones of willows from northern/continental and southern/maritime origins were assessed for their levels of cold hardiness. Assessments were made during active growth and, subsequently, during cold hardening at mean temperatures of 3°C (the COLD regime) and 8°C (the MILD regime). The onset of hardening was triggered simultaneously in all clones by administering a drastic day length reduction on the first day. The northern/continental clones showed consistently higher rates of hardening than the southern/maritime ones. This was particularly true under the COLD regime, suggesting that their hardening was less sensitive to low temperatures. The stems' visible and near-infrared absorption spectra, and concentrations of ten major soluble sugars, were also determined. Multivariate analysis revealed that spectral data could predict up to 96% of the variation in cold hardiness, when the analysis was restricted to the MILD regime and the data corrected for irrelevant systematic information. Possible direct links between spectral changes and chemical changes are discussed. Multivariate analysis also revealed that sugar concentrations could be used to predict up to 73% of the variation in cold hardiness. Different sugars displayed different patterns of variation during hardening. Concentrations of mannose and myo-inositol both decreased, whereas concentrations of galactose, sucrose, maltose, raffinose and stachyose all increased, but at different times. Dry matter increased markedly during hardening, so expressing the concentrations of sugars relative to dry matter does not provide an accurate measure of the amounts present.