In the shoots of photoperiod-sensitive deciduous trees, including poplar, short-day and non-freezing lowtemperature conditions induce bud dormancy and its break, respectively, and these conditions also induce shoot cold acclimation. In a previous study, levels of organic and inorganic components, including proteins, increased in the xylem sap of Populus nigra in winter, suggesting seasonal changes in root functions. Here, analysis of a major xylem sap protein (XSP24) of P. nigra in winter by mass spectrometry together with the whole genome sequence of P. trichocarpa and transcript abundance in roots under short-day conditions identified PtXSP24 to be a germin-like protein of the cupin superfamily, which was reported to be associated with various stresses and to have oxalate oxidase and/or superoxide dismutase activities in the cell wall. Expression of XSP24, which corresponds to PtXSP24 in P. maximowiczii, a potentially useful Japanese native poplar in the same phylogenetic clade as P. trichocarpa, was enhanced under short-day and non-freezing lowtemperature conditions, as well as by application of abscisic acid. These results suggest that XSP24 is involved in tolerance to environmental stresses in autumn and early winter.