Sources and mechanisms of tolerance to acid soil among wild annual Cicer germplasm and their symbiosis

Shahana Sultana

Legume growth, nutrient uptake, nodulation, and symbiotic nitrogen fixation can be hampered by soil acidity (pH CaCl2 < 5.0). Chickpea (Cicer arietinum L.) is sensitive to acidic soils, but the mechanism of inhibition and relative sensitivity of the symbiosis is not defined. Moreover, there is limited genetic diversity among chickpea for a range of traits including acid tolerance. This thesis explored whether wild Cicer species contained germplasm with greater acid soil tolerance than cultivated chickpea. A range of solution and soil experiments was carried out to investigate nutrient uptake, nodulation pattern and N2 fixation in acidic conditions, using two cultivated chickpea (Cicer arietinum) cultivars and two wild Cicer species (C. reticulatum (C. reti) and C. echinospermum (C. echi). Aluminium toxicity screening experiments with cultivated chickpea and Cicer genotypes showed that the haematoxylin stain of roots was unable to distinguish sensitive from more tolerant germplasm. In dilute nutrient solution at pH 4.2 and low Al concentrations (7.5, 15, 30 µM), C. echi contained higher concentrations in root and shoot of Al, P and S, but marginally lower Mg, K and Ca. By contrast, C. reti that maintained higher relative root length at 15 and 30 µM Al contained lower Al in roots but higher shoot Ca concentration. In solution culture, inoculated PBA HatTrick, PBA Striker and C. reti produced nodules at pH 4.2 but at pH 5.2 nodulation and N2 fixation increased. Genotype PBA Striker performed better in nodule formation, while both PBA Striker and C. reti contained higher shoot N concentration at pH 5.2 than pH 4.2. In an acid soil, Mesorhizobium inoculated plants formed nodules in both wild Cicer species at pH 5.6 while nodule formation was inhibited at pH 4.0 and 4.5. When soil and solution pH were > 5.0, PBA Striker and C. reti effectively forms symbiosis with Mesorhizobium while N-treated plants absorb higher shoot N concentration. This study has shown there is genotypic variation between wild Cicer in acid tolerance in plant growth, nutrient uptake, and symbiotic interaction with Mesorhizobium.

Sources and mechanisms of tolerance to acid soil among wild annual Cicer germplasm and their symbiosis

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