Interspecific Analysis of Xylem Freezing Responses in Acer and Betula

Abstract

Temperate woody plants have evolved two methods for coping with seasonal exposure to subzero temperatures. Supercooling is a freeze-avoidance strategy in which cells can avoid intracellular freezing below subzero temperatures. Nonsupercooling is a freeze-tolerance strategy in which the growth of extracellular ice crystals is promoted and intracellular water is withdrawn. Thus, nonsupercooling species have also evolved adaptations to tolerate intracellular dehydration, which results from the formation of extracellular ice. The goal of our study was to provide the first broad characterization of freezing response within two representative woody genera (Acer and Betula) in relation to the evolution of the freezing response trait. Although all of the examined Acer species (nine) exhibited xylem supercooling response, only five of 14 Betula species were identified as supercooling species. When the characterized freezing responses were overlaid onto a phylogenetic tree, the supercooling freezing response was revealed as an ancestral trait in Betula.