Cu, Zn, and Cd acquisition by two spinach cultivars depending on P nutrition and root exudation

Within a spectrum of 11 spinach cultivars (cvs) differences in the Cu. Zn, and Cd contents of shoots had been noticed. The aim of this study was there fore to analyze in more detail the acquisition of Cu, Zn, and Cd by the mos t differing cultivars (Tabu and Monnopa) in dependence on P nutrition. The plants were grown in a low phosphorus Luvisol (pH 6.3: total contents Cu: 8 9, Zn: 297, Cd: 1.4 mg kg(-1)) with two phosphorus levels in pots under nat ural conditions. For the determination of inflow, root length/ shoot weight ratio and of the Cu, Zn, and Cd concentration in the soil solution (rhizos phere) plants were harvested 26 and 40 days after sowing. Root exudation of organic acids of the two cvs was measured 35 days after growing in quartz sand with different P supply. Both cultivars responded to P fertilizer by doubling their shoot weight. Wi th increased P supply (0.68-0.77% P in shoot-DM) both cultivars showed simi lar heavy metal contents in the shoot resulting from similar root length/sh oot weight ratios (RSR) and net uptake rates of the three elements as well as the same element concentrations in the rhizosphere soil solution. Under P deficiency, however, cv. Tabu (0.52% P in shoot-DM) showed in comparison with cv. Monnopa (0.48% P) higher Cu, Zn, and Cd contents of shoots althoug h its RSR was smaller than that of cv. Monnopa. However, the inflow for Cu was higher and for Zn and Cd significantly higher compared with cv. Monnopa . This result of cv. Tabu corresponded with higher concentrations of Cu, Zn , and Cd of its rhizosphere soil solution, and its higher exudation rates o f oxalate, citrate, and malate (3.9; 1.0; 0.7 nmol cm(-1) h(-1)). The corre sponding values for cv. Monnopa were: 1.7: 0.3; 0.4 nmol cm-l h-l. The mobi lization of Cu, Zn, and Cd by the excreted organic acids seems to be respon sible for the higher Cu, Zn, and Cd inflow of cv. Tabu.