Solute active transport or exclusion by plants can be identified by the values of the Transpiration Stream Concentration Factor (TSCFxylem:solution solute concentration ratio). The aim of this study was to estimate this parameter for Ni uptake by the Ni-hyperaccumulator Leptoplax emarginata or the Ni-excluder Triticum aestivum cultivar oFidel'. The Intact Plant TSCF for nickel (IPTSCFNi) was calculated as the ratio between the nickel mass accumulation in the leaves and the nickel concentration in solution per volume of water transpired. Predominantly, Ni active transport occurred for L. emarginata, with IPTSCFNi values of 4.77.2 and convective component proportions of the root Ni uptake flow of only 1520% for a range of Ni concentrations in solutions of 216 mol Ni l(1), regardless of the growth period and the time of Ni uptake. Hyperaccumulator roots were permeable to both water and nickel (mean reflection coefficient for Ni, sigma(Ni), of 0.06), which was mainly attributed to an absence of exodermis. Results provide a new view of the mechanisms of Ni hyperaccumulation. By contrast, the wheat excluder was characterized by an extremely low mean IPTSCFNi value of 0.006, characterizing a predominantly Ni sequestration in roots. From a methodological viewpoint, the omicroscopic' TSCFNi, measured directly on excised plants was 2.4 times larger than its recommended omacroscopic' IPTSCFNi counterpart. Overall, IPTSCF and sigma determined on intact transpiring plants appeared to be very useful biophysical parameters in the study of the mechanisms involved in metal uptake and accumulation by plants, and in their modelling.