Nitrate nutrition enhances zinc hyperaccumulation in Noccaea caerulescens (Prayon)

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 NO3− nutrition or due to changes in rhizosphere pH as a result of NO3− uptake. This paper investigated the mechanism of NO3−-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 (NH4)2SO4, NH4NO3 or Ca(NO3)2. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO3−-fed plants than in NH4+-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO3−, 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 co-increased and was found co-localized in leaf cells of NO3−-fed plants. We conclude that NO3− directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.