Acid tolerance mechanisms in soil grown plants

Acid soil covers 30% of ice-free land in the world, mainly distributed in the tropical and subtropical regions where agriculture is the dominant business. Soil acidity, particularly aluminium (Al) and manganese (Mn) toxicity, often constitutes one of the critical constraints limiting crop production on these acid soils. Research on acid-tolerance mechanisms of soil grown plants will have important implication in facilitating soil and crop management in the tropical and subtropical regions. Soil becomes acidic due to natural soil acidification process. In acidic soils, Al toxicity is common due to solubilisation of Al3+ ion which is toxic for plants. The main symptom of Al3+ toxicity is the inhibition of root growth. After Al3+ toxicity, Mn toxicity ranks as the second major concern for plant growth in acid soils. Unlike Al, Mn is an essential plant nutrient and is toxic when taken up in excessive quantities by plants. In addition to Al and Mn toxicity, low pH also affects plant growth in highly acid soils. Low pH stress facilitates H+ influx into root tissues resulting in poor plant growth. However, phosphorus (P) deficiency is also common in acid soils. Phosphorus leads to an Al-phosphate complex in acid soils that result in less P availability in these soils. Different Al tolerance mechanisms such as an increase in rhizosphere pH, plant tissue tolerance, exudation of organic acid and role of apoplast are discussed in this review. This review concludes that P may play an important role in Al tolerance. Thus, understanding acid tolerance mechanisms in soil grown plants has important implications for sustainable agriculture in the tropical and subtropical regions. This review suggests that introduction of Al-tolerant crops in acid soils will improve crop production in these soils