Salt accumulation in the root zone of the halophyte Atriplex nummalaria

[Truncated] Increasing salinity is a major environmental problem that limits productivity of many agricultural systems, particularly in rain-fed regions with a Mediterranean-type climate and in irrigated arid and semi-arid regions. Revegetation of saline land with halophytic plants is one solution for the productive use of salt-affected land. However, the growth of perennial plants on sites with shallow saline groundwater can lead to increasing salts in the plant root zone due to the absorption of saline groundwater and ‘exclusion’ of ions by the roots; this could threaten plant function, growth and survival. This thesis investigated the factors which affect the accumulation of salt in the root zone of the halophytic shrub Atriplex nummularia, such as transpiration, time, root distribution, groundwater salinity and seasonal change, and the effect of these factors on plant growth and water relations. The research was conducted at three different scales: in pots, soil columns and in the field.

In pots without drainage an experiment was conducted to examine the influence of leaf area (and therefore different amounts of plant transpiration) on Na⁺ and Cl^- accumulation close to the roots of plants grown with four NaCl concentrations in the soil solution (20, 50, 200 and 400 mM). The plants were watered to a target weight through a tube that delivered water to the bottom of each pot without causing ions to be leached. In this experiment, transpiration was manipulated using three leaf removal treatments (removal of none, 50%, or 100% of leaves). The soil was sampled at the end of the experiment in three fractions: soil closely or loosely adhering to roots and bulk soil. Relationships were found between leaf area and Na⁺ concentration in the rootzone; specifically, increases in leaf area were accompanied by the movement of Na+ from the bulk soil to the root surface (closely and loosely adhering soil). The average Na⁺ concentration in the closely and loosely adhering soil for the non-defoliated plants grown at 200 and 400 mM were higher by 13-20% and 30-40% than for plants with 50% or all leaves removed respectively. In addition, shoot water potentials and shoot dry mass decreased with the increased Na⁺ concentrations close to the roots. This experiment showed the strong influence of transpiration on ion accumulation in the root zone, and inferred a potential benefit of grazing in decreasing salt concentrations in the root zone of Atriplex nummularia.