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Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO 3 − nutrition or due to changes in rhizosphere pH as a result of NO 3 − uptake. This paper investigated the mechanism of NO 3 − -enhanced Zn hyperaccumulation in N.caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH 4 ) 2 SO 4 , NH 4 NO 3 or Ca(NO 3 ) 2 . The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO 3 − -fed plants than in NH 4 + -fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO 3 − , whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca coincreased and was found co-localized in leaf cells of NO 3 − -fed plants. We conclude that NO 3 − directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.

Nitrate has been shown to enhance Zn hyperaccumulation in the shoots of Noccaea caerulescens (formerly Thlaspi caerulescens) (Prayon); however, the mechanisms beyond the effect of nitrogen form are unknown. This study used synchrotron X-ray absorption near-edge spectroscopy (XANES) on alive and intact plants at room temperature to examine whether enhanced Zn hyperaccumulation in nitrate-fed plants was associated with differences in Zn speciation, and to correlate Zn species with mechanisms of Zn uptake, translocation and hyperaccumulation. The higher Zn concentration in plants supplied with nitrate compared to ammonium, or with high Zn exposure (300 μΜ), was not due to differences in Zn speciation. The importance of carboxylates for Zn hyperaccumulation in the shoots was supported by a predominance of Zn-malate or Zn-citrate. Zinc-phytate was detected for the first time in this species and may assist Zntolerance in the roots. The feasible presence of Znhistidine in the roots but not in the xylem sap suggests a mechanism for Zn binding and non-toxic transport through the cytoplasm and release of aqueous Zn into the xylem vessels. Zinc was translocated in the xylem as Zn-malate and weakly complexed or aqueous Zn forms. Zinc speciation in roots, shoots and xylem did not differ between nitrate-and ammonium-fed plants.

A pot culture experiment was used to determine the differences in uptake characteristics of a cadmium hyperaccumulator Solanum nigrum L. discovered in China, an ecotype from Melbourne, Australia and a non-hyperaccumulator Solanum melogena Australian ecotype was not significantly different to the China ecotype. In particular, Cd concentration in leaves and shoots of S. nigrum collected from Australia were 166.0 and 146.3 mg kg(-1) respectively when 20 mg kg(-1) Cd spiked, and were not significantly different to the ecotype imported from China which had 109.8 and 85.3 mg kg(-1) respectively, in the stems and leaves. In contrast, the tolerance of the eggplant to Cd was significantly less than the two S. nigrum ecotypes. Although some morphological properties of S. nigrum collected from Australia were different from that of the plants collected from China, Cd hyperaccumulator characteristics of two ecotypes were similar. The results suggested that the tolerance and uptake of Cd may be a constitutive trait of this species.

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