Aluminium, the third most abundant metal in the earth’s crust is not essential for plant growth; alternatively it poses serious threat to crop productivity in acidic soils which covers approximately 40 percent area of the earth’s arable land. It is considered as the main abiotic stress to cause 25–80 percent yield losses in crop plants grown in soils containing excessive Aluminium (Al). Many plants are sensitive even to micromolar concentrations of soluble forms of aluminium. Although, Al does not inhibit seed germination, but it impairs the growth of new roots and have comparatively high effect on young seedlings than older plants. The affected roots become short, stubby and lateral roots become peg like or fail to develop and thus the whole root systems fails to elongate and acquire brownish coloration. Inhibition of root elongation occurs due to disturbed mitosis in the apical meristem. Ultimately, the crop yield is affected as there is reduction in nutrient and water absorption by roots.
Shoot growth is considered a secondary visible symptom of aluminium toxicity and is often similar to deficiencies of phosphorus, calcium, magnesium and iron. Curling or rolling of young leaves and collapse of growing points of petioles are well known symptoms of Calcium deficiency induced by Al toxicity in the crops. Foliar symptoms resembling phosphorus deficiency like small dark green leaves, yellowing and death of leaf tips, stunted growth, delayed maturity, chlorosis, purple colour in petiole, abaxial leaf surface and stems are caused due to Al toxicity.
Aluminium toxic acidic soils can be amended by following any of the three broad approaches, which includes liming, use of farm yard manure and biological approaches. The first approach involves soil amelioration which is useful but an expensive process. In lieu of, continuous application of farm yard manure in aluminium toxic soils can decrease soluble aluminium and can significantly increase calcium, magnesium, phosphorus and potassium contents of plants. However, breeding crop varieties for Al tolerance through marker assisted selection is realised as a most promising, less resource consuming and economically and socially acceptable approach.
Aluminium toxicity is affected by several factors such as concentration of Al, pH, temperature and concentrations of phosphorus, calcium, magnesium among others. Several parameters are used to evaluate degree of aluminium resistance in plants which includes root growth method, shoot growth method, root re-growth and staining methods. Also, callose and organic acids concentration provide accurate measures of Al resistance and gives results similar to relative root length measurements. Further, increase in uptake of nutrients such as Ca, Mg, P, B etc. is also good parameter for aluminium tolerance in crops plants.
Generally, two types of mechanisms operate in plants to resist Al toxicity which have been classified into external and internal tolerance mechanisms. The main difference between these two mechanisms is the site of Al detoxification such as apoplasm (external) or symplasm (internal). In the former, the aluminium uptake is prevented by roots while later includes the chelation of aluminium in the cytosol by organic acids, its compartmentation in the vacuole, effect of aluminium binding proteins or increase in reactive oxygen species scavenging enzymes. Also, organic acids secreted by plant roots also play an important role in the external and internal neutralisation of aluminium.
Al tolerance in crop plants can be increased by utilisation of existing variation already present in crops, use of interspecific hybridisation and by generating tolerance in the high yielding crops by recurrent selection or tissue culture. Further, if genetic variability for tolerance to aluminium toxicity is inadequate or not available, then mutagenesis, somaclonal variation and recombinant DNA technologies also have great potential for introducing Al tolerance in the crop plants though. These techniques require detailed understanding of the physiological genetics and biochemistry of Al tolerance.
For effective use of acidic soils and to mitigate Al phytotoxicity problems in crop plants, there is a need to develop Al tolerant varieties, which can be achieved through selection and introgression of Al tolerance genes into high yielding cultivars. The tolerant varieties thus developed will directly benefit those farmers who are facing high yield losses due to aluminium toxicity in acidic soils.
(The author is a senior scientist at Division of Genetics, Indian Agricultural Research Institute, New Delhi)