Effect of lead treatment on the distribution of essential elements in cucumber

Changes in the fresh and dry weight of leaf blades, petioles, internodes, a nd their ion concentrations as well as rates of root exudation were investi gated in cucumber grown in hydroponic culture and treated with physiologica l concentrations of lead (Pb). The growth of flowering cucumber supplied wi th FeCl3 shows a typical bell-shaped curve derived from leaf dry weights, w here the 4th, 5th, and 6th leaves are the largest ones. Lead inhibited grow th up to 20% (except the youngest leaves which were stimulated), but did no t cause morphological changes. Whereas root growth was also retarded by abo ut 20%, exudation was inhibited to 50%, that is, the root resistance to wat er transport was increased by Pb. In spite of that the dry matter yield of internodes and petioles hardly reached 10 and 5% of that of leaf blades, re spectively, their calcium (Ca), magnesium (Mg), and zinc (Zn) concentration s were almost the same. Similarly, the concentrations of these metals in th e specific leaf blades are equal despite their different growth characteris tics but the concentration of Mn is independent of the growth of the examin ed tissues. Lead was the only cation to accumulate in the largest leaves. T he inhibitory effect of ph on ion uptake was proportional to the growth inh ibition of the tissues except manganese (Mn), the accumulation of which was stimulated by Pb in all the analyzed tissues and copper (Cu) which increas ed only in the internodes. We could not explain the extremely different ion distributions observed during the growth of cucumber. However, it is evide nt that the measurement of ion concentrations at whole shoot level is not e nough to characterize ion interactions because only some ions accumulate at the same rate as tissues grow. These ions maintain stable concentration le vels in shoot parts whereas others decrease or increase independently from growth.