Differential distribution of cadmium in lettuce (Lactuca sativa L.) and maize (Zea mays L.)

Large genotypic variation in shoot Cd concentrations has been reported in literature for several plant species including lettuce ( Lactuca sativa L.) and maize ( Zea mays L.). The objective of this thesis was to elucidate the physiological andlor morphological basis of this differential behaviour using lettuce and maize as test plants.Lettuce varieties were grown on soil and on nutrient solution culture with or without Cd. In contrast to published data, the genotypic variation in Cd uptake and distribution was small. A close similarity in shoot Cd concentrations was obtained, ranging from 8.2 to 10.7 μg Cd g -1dry wt. for the soil experiment and from 4. 0 to 8.4 μg Cd g -1dry wt. in the water culture. The absorbed Cd was predominantly transported to the shoots. Cadmium had a similar translocation behaviour as calcium. Compared to NO 3 -N, NH 4 -N nutrition significantly increased the Cd concentrations in shoots and roots at a constant pH of the solution. The Cd partitioning over the plant organs was independent of the form of N nutrition.The variation in shoot Cd concentrations among maize inbred lines was studied in experiments on soil and nutrient solution culture. The Cd concentrations in the shoots ranged from 0.9 to 9.9 μg Cd g -1dry wt. for plants grown on Cd-contaminated soil and from 2.5 to 56.9 μg Cd g -1dry wt. for the nutrient solution culture. This variation was mainly related to a differential shoot/root partitioning rather than to a differential total uptake. 'Shoot Cd excluders' retained the Cd taken up in the roots, whereas 'non-shoot Cd excluders' exhibited similar Cd concentrations in shoots and roots. This differential Cd distribution pattern persisted in a wide range of environmental conditions such as solution pH or level of Cd supply. Morphological parameters of shoots (e.g. specific leaf area or leaf area ratio) and roots ( e.g. specific root length or specific surface area) were not related to the Cd partitioning.The amount of Cd adsorbed to the roots was about twice as high for 'shoot Cd excluders' compared to 'non-shoot Cd excluders'. The intracellular partitioning of Cd in roots over a 'soluble pool' and an 'insoluble pool' was different in representatives of the two groups of inbreds with similar total root Cd concentrations. Both the amounts of Cd present in the 'soluble pool' and the total amounts of acid-soluble thiol compounds were twice as high for the 'non-shoot Cd excluder' compared to the 'shoot Cd excluder'. Probably the soluble Cd is complexed to metal- binding peptides (phytochelatins) that form part of the acidsoluble thiol compounds. Binding characteristics of Cd to internal structural root components were similar for the two groups of inbreds.It is suggested that the differential Cd distribution between roots and shoots of the maize inbreds is caused by the differential mobility of Cd in their roots, due to the higher amounts of soluble complexes of Cd and acid-soluble thiol compounds in 'non-shoot Cd excluders' compared to 'shoot Cd excluders'.