Plant responses to soil-borne ion toxicities

This chapter focuses on the constraints imposed on plants by soil-associated abiotic stresses, including toxicity of aluminum (Al), protons (H), and manganese (Mn) in acid soils; Mn and iron (Fe) in waterlogged and flooded soils; and sodium (Na) and chloride (or sulfate) in saline/sodic soils, emphasizing the mechanisms of adaptation and resistance. The two components of resistance (avoidance and internal tolerance) are highlighted. Resistance to Al is based mainly on exudation of malate, citrate, or other organic acid anions that complex Al into nontoxic forms in the rhizosphere. In waterlogged or flooded soils, anaerobic conditions are conducive to the reduction of Fe and Mn into their most mobile forms (Fe2+ and Mn2+), potentially exceeding the nutritional or even tolerable concentrations in the soil solution. The main constraints to plant growth in saline soils (in addition to ion toxicity) are low osmotic potential and ion imbalances (low K/Na ratio). Salt tolerance mechanisms can be divided into salt exclusion (reduced uptake, increased efflux) and salt inclusion (compartmentation into the vacuole, release of salts via salt glands, salt-tolerant enzymes). To avoid water loss, plants accumulate osmotically active compounds to retain water in the cells.