Arsenate (As V) is the predominant form of arsenic in soils under aerobic conditions and competes with the major plant nutrient phosphorus (P) in the form of phosphate (PV) not only for sorption sites on mineral surfaces in soil but also for root membrane transporters. Plants have evolved several mechanisms for the mobilization of PV in soils in response to P deficiency, such as the release of organic anions and protons. The aim of the present study was to test whether these mechanisms result in a simultaneous mobilization of arsenate and what would be the consequences for As transfer from soil to plant. The compartment system approach with Zea mays as model crop was chosen as an experimental setup. The system is equipped with micro suction cups and allowed us to investigate processes occurring in the vicinity of roots. As a case study, an artificial quartz substrate with well defined soil physical properties was fertilized, spiked with As V, and amended with increasing amounts of goethite (0, 1, and 4 g kg(-1) in treatments G-0, G-1, and G-4, respectively). The addition of goethite alleviated the As V-induced growth reduction and reduced As V transfer from the substrate to the plant but induced P deficiency at the same time. When low amounts of goethite (1 g kg(-1)) were added, plants mobilized PV but not As V, which might be related to differences in surface complexation reported for PV and As V. No mobilization of PV or As V was observed with the addition of higher amounts of goethite, probably because of decreasing competition between organic anions, PV, and As V for binding sites.

The results presented in this paper provide evidence for the role of phytochelatins (PCs) in the detoxification of arsenic in six nonhyperaccumulating plant species, Agropyron repens, Glecoma hederacea, Leonurus marrubiastrum, Lolium perenne, Urtica dioica and Zea mays, in a pot experiment with high phosphate treatment. These plants differed in their arsenic sensitivities and were selected to investigate whether PCs with longer chains in roots of arsenictolerant species are synthesized. Raised concentrations of total PCs were measured in plant species with a range of sensitivities to arsenic at equivalent levels of arsenic exposure, determined as the inhibition of root biomass. In addition, the production of PCs as a function of accumulated arsenic was studied. Longterm PC synthesis (over a 5-week period) was positively, but non-linearly correlated with arsenic, suggesting that probably not all As is bound by PCs. Moreover, it could be shown that the synthesis of different chain lengths of PCs is associated with differences in As tolerance. In the more tolerant grasses A. repens and L. perenne it was chiefly the dithiol PC2 which was measured. In contrast, the dominant PC species in the less tolerant plants U. dioica, G. hederacea, L. marrubiastrum and Z. mays was PC3, while PC2 and PC3 were detected as well.

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