Mechanisms of Ion Transport in Halophytes: From Roots to Leaves

Abstract

The chapter describes peculiarities of ion transport in halophytic plants, aiming to help understand the mechanisms important for their tolerance of salt. An initial introduction to methods for studying ion transport is followed by analysis of ion transport from a broad thermodynamic point of view. Further detailed survey of ion channels and ion transporters in plants adds to the picture of ion transport pathways through cell membranes. A typical ‘generalised’ plant cell is depicted to illustrate the variety of ion transport systems known so far for all plants. This serves as a basis for a comparison of ion transport in salt-sensitive glycophytes and salt-tolerant halophytes. Next, there is a description of what we know of transport systems in halophytes, beginning from the thermodynamics of ion transport under salinity. In halophytes, low negative stable plasma membrane potentials and cytoplasmic Na+ concentrations that are often higher than in glycophytes are important for their life under salinity. Comparison of similar pairs of plants with contrasting halophytic and glycophytic habits allows us to find specific features of ion transport essential for high salinity tolerance. Mechanisms of high- and low-affinity sodium transport in halophytes are briefly characterised to explain and stress the increased accumulation of Na+ by halophytes compared to glycophytes. Description of ion channels and transporters in halophytes and pathways of ion transport from nutrient solution to their roots, then to the xylem and finally to leaves completes the chapter. Problems and unsolved questions are proposed for the future study of ion transport in halophytes.