Effect of scale of Cd heterogeneity and timing of exposure on the Cd uptake and shoot biomass, of plants with a contrasting root morphology

A pot experiment was conducted to investigate the influence of spatial heterogeneity of Cd distribution in soil on shoot biomass, shoot metal concentration and total shoot Cd uptake by lettuce (Lactuca sativa, variety Tom Thumb) and Indian mustard (Brassica juncea). Five different soil treatments had similar overall concentration of Cd per pot, but different scales of heterogeneity and also timing of plant exposure during the growth cycle. The presence and scale of heterogeneity and timing of exposure were found to have significant effects on shoot biomass for both plants (with one exception). The mean values of Cd mass taken up were significantly affected by the presence of heterogeneity and timing only for lettuce. Only the scale of heterogeneity affected the uptake of Cd by Indian mustard, presumably because of its larger root system (∼18 cm, compared with ∼5 cm for lettuce). These findings have important implications for phytoremediation, and for human health risk assessment where leafy vegetables are grown in situations with highly elevated Cd concentrations.