Investigations were conducted to elucidate the key factor behind Cd2+-toxicity-induced proline accumulation in Indian mustard (Brassica juncea) by raising seedlings, independently in distilled water (DW) and mineral growth medium (MGM) in the presence of 0-500 μM CdCl2. Invariably, Cd2+-induced toxicity, measured in terms of growth, was significantly more prominent in seedlings raised in DW than those raised in MGM. Cd2+ brought about a significant reduction in growth and photosystem II activity with a concomitant increase in proline levels, in a concentration-dependent manner. Interestingly, the level of iron in shoots of seedlings decreased proportionately with increase in Cd2+ toxicity. Cd2+-promoted proline accumulation was significantly higher in seedlings raised in DW than those raised in MGM. Depletion of essential cations (viz. Ca2+, Mg2+, K+, and Fe2+) from MGM one at a time revealed that depletion of Fe2+ leads to maximal proline accumulation under Cd2+ toxicity. Interestingly, proline level in seedlings raised under Cd2+ toxicity in DW supplemented with Fe2+ was similar to that recorded in seedlings raised in MGM. Our results convincingly demonstrated that Cd2+-induced iron deficiency promotes proline accumulation.
Keywords: Brassica juncea; Cadmium toxicity; Iron; Photosystem II; Proline.