Research progress on responses of xylem of woody plants to freeze-thaw embolism

Abstract

Embolism induced by the freeze-thaw commonly occurs in the woody plant in mid- and high-latitude regions. In cold environments, the resistance capacity to freeze-thaw embolism of woody plants is critical for their growth and successful survival throughout winter, which directly determines their distribution in the earth. During freeze-thaw cycles, the freeze-thaw embolism of vessels is induced by the bubbles that are from dissolved frozen water because of the low solubility of ice. The resistance capacity to freeze-thaw embolism varies from plants with different anatomical structures. Plants can also reduce potential damages of freeze-thaw embolism through adjusting the xylem positive pressure by refilling embolism and metabolism, and/or increasing the sap solute contents to avoid the stress. Compared to embolism induced by drought, however, few studies have been conducted on embolism by freeze-thaw, and the underlying mechanisms of plant responses and regulation are poorly understood. In this paper, we first reviewed the process of formation and repair of freeze-thaw embolism of xylem, including the strategies of escaping freezing, tolerance, and repairing freeze-thaw cavitation. Then we summarized physiological characteristics of plant resistance to low-temperature stress, influencing factors and evaluating criteria, based on the multiple trade-offs between low-temperature resistance, drought resistance, and hydraulic efficiency. Finally, we proposed five priorities for future study in this field: (1) the threshold of freezing temperature for different plants; (2) the vulnerability segmentation of frost-induced embolism; (3) the relationships between freeze-thaw cavitation repair and metabolism exhaustion; (4) the potential trade-offs between low-temperature resistance, drought resistance and hydraulic efficiency; and (5) integration of the traits of freeze-thaw embolism resistance into the economic spectrum of plants.