EFFICIENCY OF H+-ATPASE ACTIVITY ON CADMIUM UPTAKE BY 4 CULTIVARS OF LETTUCE

Cadmium (Cd) uptake by lettuce (Lactuca sativa L.) was studied in a hydroponic solution study at concentrations approaching the total concentration in contaminated soil solutions. Four cultivars of lettuce were tested (Divina, Reine de Mai, Melina, and J.44). Ten 12-day old seedlings, pretreated in 0.5 mu M CdCl2 solution, were labelled with carrier free (109)CdCl2 (from 0.05 mu M to 5 mu M Cd in nutrient solution) in the presence and absence of metabolic inhibitors, DNP and DCCD. Cadmium taken up by the roots was determined after a 30 min desorption in unlabelled CdCl2 solution. In the absence of metabolic inhibitors and at 5 mu M Cd, root absorbed from 2.5 to 8 mg Cd/g root dry weight. Exchangeable Cd measured after desorption represented less than 1% of the total Cd absorbed by the root. Cadmium absorption in presence of DNP showed that approximately 80% of the Cd enters the cell through an active process. This mechanism seems to be directly associated with H+-ATPase as observed with DCCD inhibition. Varietal differences in shoot Cd uptake were only demonstrated at concentrations below 0.1 mu M Screening lettuce cultivars only by the Cd level in the tissue seems not to be possible for these cultivars except at concentrations close to that in the soil solution. But differences in relative contribution of uptake mechanisms in total Cd absorption were observed. High levels of Cd in roots were correlated with high contri- butions from H+-ATPase in the active process of Cd uptake.