Active proton efflux, nutrient retention and boron-bridging of pectin are related to greater tolerance of proton toxicity in the roots of two Erica species.

BACKGROUND AND AIMS: Tolerance to soil acidity was studied in two species of Ericaceae that grow in mine-contaminated soils (S Portugal, SW Spain) to find out if there are interspecific variations in H+ tolerance which might be related to their particular location.

METHODS: Tolerance to H+ toxicity was tested in nutrient solutions using seeds collected in SW Spain. Plant growth and nutrient contents in leaves, stems and roots were determined. Viability tests and proton exchange were studied in roots exposed, short-term, to acidic conditions. Membrane ATPase activity and the cell-wall pectic polysaccharide domain rhamnogalacturonan-II (RG-II) were analysed to find out interspecific differences.

RESULTS: Variation in survival, growth and mineral composition was found between species. The H+ -tolerant species (Erica andevalensis) showed greater concentration of nutrients than E. australis. Very low pH (pH 2) produced a significant loss of root nutrients (K, P, Mg) in the sensitive species. Root ATPase activity was slightly higher in the tolerant species with a correspondingly greater H+ efflux capacity. In both species, the great majority of the RG-II domains were in their boron-bridged dimeric form. However, shifting to a medium of pH 2 caused some of the boron bridges to break in the sensitive species.

CONCLUSIONS: Variation in elements linked to the cell wall-membrane complex and the stability of their components (RG-II, H+ -ATPases) are crucial for acid stress tolerance. Thus, by maintaining root cell structure, active proton efflux avoided toxic H+ build-up in the cytoplasm and supported greater nutrient acquisition in H+ -tolerant species.