Cadmium Tolerance in Mustard Cultivars: Dependence on Proline Accumulation and Nitrogen Assimilation

Cadmium [Cd] is a heavy metal, which reduces crop productivity even at low doses and reduces overall performance of the crop plants. Four mustard [Brassicajuncea L. Czern and Coss.] cultivars, Alankar, Pusa Bold, Rohini and SS2 differing in Cd tolerance were treated with different doses 0, 50 and 100 mg Cd kg⁻¹ soil. Cultivar SS2 [Cd sensitive] accumulated greater Cd in roots and leaves than Alankar [Cd tolerant]. Furthermore, SS2 also exhibited higher contents of thiobarbituric acid reactive substances [TBARS], hydrogen peroxide [H₂O₂] and electrolyte leakage. Photosynthesis and growth were adversely and maximally decreased by 100 mg Cd kg⁻¹ soil treatment in all four of the cultivars, but SS2 exhibited greater reductions. Increasing concentrations of Cd in the soil result in decreased net photosynthesis [Pn], stomatal conductance [gs] and intercellular CO₂ [Ci]_, but increase in proline content. Cultivars Alankar with higher proline content and greater nitrogen and nitrate reductase activity are efficient in controlling the Cd-induced oxidative stress as compared to least tolerant SS2, whereas tolerance capacity of Pusa Bold and Rohini lie in between the two. The tolerance index of Alankar was found to be higher among all the studied mustard cultivars, which indicate its higher tolerance to Cd toxicity. Tolerance index has been used as a good criterion to select the tolerant genotypes under high Cd stress. Leaf area and dry mass also decrease maximally at 100 mg Cd kg⁻¹ soil with highest reduction in SS2. Maximum reduction of water potential and osmotic potential occur in SS2 and least in Alankar.