HOW MUCH SODIUM ACCUMULATION IS NECESSARY FOR SALT TOLERANCE IN SUBSPECIES OF THE HALOPHYTE ATRIPLEX-CANESCENS

Two sympatric subspecies of the xerohalophyte Atriplex canescens Pursh. (Nutt.) were compared for 840 in outdoor salinity trials in their native coastal desert environment in Sonora, Mexico. Subspecies linearis grows naturally on sea water in the high intertidal zone of estuaries while subspecies canescens grows on dunes. In lysimeter pot experiments, ssp. linearis exhibited 50% growth reduction when the mean root zone salinity reached 1160 mol m(-3) NaCl compared to just 760 mol m(-3) for ssp. canescens. When irrigated with sea water in a flood plot, ssp. linearis had 50% higher growth rates than ssp. canescens. The specialization of ssp. linearis for a saline environment was associated with greater net transport of Na+ from root to shoot, greater Na+ accumulation in the leaves and a higher Na:K ratio in the leaves compared to ssp. canescens. On the other hand, the two subspecies achieved approximately the same degree of osmotic adjustment in the leaves, equal to two to three times the external salinity, and had similar water use efficiencies. Even at relatively low salinities, both subspecies accumulated larger quantities of Na+ for osmotic adjustment than K+. The results suggest that breeding for Na+ accumulation rather than exclusion might be the more effective strategy for improving salt tolerance of conventional crop plants.