Aluminium toxicity has been recognized as a major limiting factor for plant productivity on acid soil, which comprises about 40% of the arable land in the world. In India, 49 million hectares of land is affected by soil acidity of which 24 million hectares have pH below 5.5.
The problem of aluminium toxicity is particular serious in low pH acidic soils (pH<5.5) that is difficult to reclaimed with lime. At low pH, dissolution of Al-containing compounds is enhanced and the release of toxic Al3+ ions into the soil solution that can rapidly inhibit root growth.
The major symptom of aluminum toxicity is rapid inhibition of root growth. Some plant species have developed different mechanisms to minimize the harmful effects of Al-toxicity. The most documented mechanisms of Al3+ resistance is the secretion of anions of organic acids from the roots.
It is possible to detoxify Al3+ in surface soil by liming to pH 5.5 or above. However, liming is not a remedy for subsoil acidity and it is not always economically feasible.
Therefore, the most appropriate strategy to overcome Al-toxicity is to select or breed genotypes with tolerance to aluminium toxicity. For this, sources of tolerance and pattern of inheritance need to be identified.
In order to investigate the genetic mechanism of tolerance it is necessary to screen and measure the tolerance of aluminium toxicity in large number of genotypes by using rapid, effective and reproducible screening techniques.
Several screening techniques have been employed in evaluating Al3+ tolerance in plants such as root length, root re-growth, staining of roots with hematoxylin, root re-growth after staining.
Score of hematoxylin staining is effective technique in wheat, pea, chickpea, pigeonpea and okra. However, score of hemotoxylin staining is failed to discriminate aluminium tolerant genotypes in tomato and lentil. Root re-growth after hematoxylin staining has been suggested as the best method to discriminate genotypes for aluminium tolerance at seedling stage in tomato and lentil.
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Genetic mechanisms governing Al3+ tolerance in crop plants are poorly understood. Genetics of aluminium tolerance was reported to be controlled by a single gene in pea, chickpea, barley and two genes in maize, pigeonpea and okra. In contrast, the genetic system controlling aluminium tolerance in wheat appears to be complex.
However, information on the nature of genetic control of Al3+ tolerance in the Tumpluk-Adi (landrace) and Pant Bahar genotypes is not yet understood. It is important todetermine the genetic control of the Al3+ tolerance andexplore the possibility of utilizing the trait in future breeding programmes.
The data recorded for root re-growth showed that the parents ‘Tumpluk-Adi (landrace)’ and ‘Pant Bahar’ had long root re-growth (3.99 cm and 2.68 cm, respectively) compared with sensitive genotype (Selection-18) which had short root re-growth of 0.37 cm under controlled nutrient solution study.
The F1 progeny of the cross ‘Tumpluk-Adi x Selection 18’ showed similar response to the tolerant parent (Tumpluk-Adi). Reaction of F2 plants to aluminium stress showed segregation with a 3 (tolerant): 1 (sensitive). It indicates that Al-tolerance is controlled at a single locus in the Tumpluk-Adi (landrace) and its cross.
The single locus control of aluminium tolerance was further confirmed from segregation pattern in F3 generation. The F3 families (derived from ‘Tumpluk-Adi x Selection -18’) segregated in 1 (non-segregating tolerant): 2 (segregating): 1 (non-segregating sensitive).
This again confirmed that a single dominant gene governs aluminium tolerance in the Tumpluk-Adi. The present results are in agreement with the previous results on other crops. In contrast, complex resistance has also been reported in wheat, rice etc.
Genetic control of Al3+ was unravelled on the basis of root re-growth after staining. Root re-growth after staining is amenable for screening germplasm in large numbers. Since aluminium tolerance of ‘Tumpluk-Adi (landrace) is monogenic, it could be easily transferred to high yielding genotypes.
This study also allows developing Al3+ tolerance of tomato, investigating physiological and molecular mechanisms underlying Al3+ tolerance.
Dr. Dharmendra Singh
Senior Scientist, Division of Genetics,
Indian Agricultural Research Institute, New Delhi-110 012