Cold in the common garden: comparative low-temperature tolerance of boreal and temperate conifer foliage

Because they maintain green foliage throughout the winter season, evergreen conifers may face special physiological challenges in a warming world. We assessed the midwinter low-temperature (LT) tolerance of foliage from eight temperate and boreal species in each of the genera Abies, Picea, and Pinus growing in an arboretum in Trondheim, Norway, using relative electrolyte leakage (REL) as an index of cell injury. Relatively LT sensitive species came from temperate coastal and Mediterranean environments and displayed a well-defined sigmoidal response to LT stress, with LT50 ranging from -27 to -38 degrees C. Species originating from boreal regions were not lethally stressed by slow freezing to temperatures as low as -80 degrees C, while species from temperate mountains and continental interiors displayed intermediate responses, with LT50s ranging from -33 to -44 degrees C. Further evaluation of one sensitive and one insensitive species in each genus showed that boreal species can survive quenching in liquid nitrogen at -196 degrees C provided they are first slowly cooled to -30 degrees C or lower. Quantitative image analysis of color changes resulting from LT stress followed by exposure to light showed that foliage from nonlethally stressed boreal species developed mild to moderate chlorosis while more sensitive species developed a mixture of chlorosis and necrosis, with significant necrosis occurring mainly at temperatures resulting in REL of 50% or more. Sensitive and insensitive trees differed significantly in total raffinose, sucrose, and total sugar concentrations, and raffinose and sucrose correlated significantly with LT50 within the sensitive group.