Comparative analysis of natural cold acclimation and deacclimation of two Magnolia species with different winter hardiness

Winter injury is frequently observed in the rare species Magnolia wufengensis L.Y. Ma et L.R. Wang, but rarely in Magnolia denudata Desr in northern China. To improve the understanding of the causes for poor winter hardiness of M. wufengensis, the physiological and biochemical processes of M. denudata and M. Wufengensis during natural cold acclimation and deacclimation were investigated and compared. In early cold acclimation, the growth cessation of M. wufengensis was later than that of M. denudata, and the leaf senescence was also slower in M. wufengensis. Both the bud dormancy level and shoot freezing tolerance were lower in M. wufengensis throughout cold acclimation and deacclimation. A significant development in bud dormancy level and shoot freezing tolerance appeared long after growth cessation for M. wufengensis but soon for M. denudata. The increase rates of dormancy level and freezing tolerance during cold acclimation were lower in M. wufengensis, while the corresponding decrease rates of these two parameters during deacclimation were also lower in M. wufengensis. The interspecific differences in freezing tolerance highly correlated with the dehydration and accumulation of proline and certain soluble sugars. We conclude that weaker mid-winter cold hardiness and slower cold acclimation should be the reasons for poorer winter hardiness in M. wufengensis.