Plant growth and physiology under heterogeneous salinity

Soil salinity is heterogeneous, and within the root-zone of single plants the salinity of the soil solution can vary widely. This review shows that water uptake by roots from the least saline part of the soil is the key factor driving shoot growth; plants with part of the root at low salinity (0-10 mM NaCl) had 3- to 10-fold higher shoot dry mass than plants with roots in uniformly saline (50-800 mM NaCl) media. Plants in heterogeneous salinity had shoot water potentials similar to those of plants growing in uniform low-salt media, and this was likely a result of uptake of low salinity water and reduced stomatal conductance. Under heterogeneous conditions, roots in saline media took up ions, resulting in higher shoot Na+ and Cl- concentrations compared with plants growing in low-salt media. Results from split-root experiments complement knowledge of plant responses to uniform salinities; the next challenge is to develop new protocols so that this understanding can be extrapolated to more complex soil- and field-based systems. More work is also required to understand the physiological mechanisms underlying changes in stomatal conductance and shoot ion regulation in plants under heterogeneous salinities and how these are linked to the saline parts of the root-zone.