The use of lanthanum to characterize cell wall permeability in relation to deep supercooling and extracellular freezing in woody plants

Summary

Current theories suggest that the diameter of the pores within xylem cell walls plays a major role in defining the ability of a cell or tissue to exhibit deep supercooling. These proposals have been only generally defined and based on either theoretical calculations or model systems. The present study utilized the apoplastic tracer, lanthanum nitrate, to contrast the permeability of cell walls in stem tissues of species that deep supercool (P. persica andC. florida) with one that exhibits equilibrium freezing (S. babylonica). Results indicated that the distribution of lanthanum crystals was similar in all species regardless of their freezing behavior. Although the primary cell walls of cortical tissue were very permeable, in general, both primary (except portions associated with pit membranes) and secondary walls of xylem cells exhibited low permeability to lanthanum ions (La³⁺). The pit membranes, between all cell types, composed of both the middle lamella, primary walls of adjoining cells, and the protective layer (in xylem parenchyma) were very permeable. Based on these results it is suggested that, in the species examined, the size of the pores in the pit membrane, rather than the entire cell wall, play the major role in defining the freezing behavior of a tissue. Additionally, we speculate that the overall composition of the tissue, particularly the number of libriform fibers vs. other xylem cell types, may also play a role in determining the freezing behavior of a tissue.