Absorption and transport of nutrient cations and anions in maize roots

Abstract

In most plant species rates of absorption of nutrient cations and nutrient anions are unequal on an equivalent basis; consequently, processes within the plant are active to keep both the plant interior as well as the external root medium electrically neutral. Although excess cation over anion absorption initiates synthesis and accumulation of organic anions in the plant [4], reversal conditions are much more complicated. For these conditions Dijkshoorn et al. [3] and Ben Zioni et al. [1] proposed a K⁺ recirculation hypothesis in the plant. Part of the K⁺, transported by xylem upward to the upper plant parts, will be redirected via the phloem to the roots in, association with malate ions synthetised by nitrate reduction in the upper plant parts. In the roots malate will be decarboxylated and excreted from the root as OH⁻, while K⁺ will be transferred into the xylem again. According this K⁺ cycle excess absorbed negative charge (mainly NO ^–₃ ) can be eliminated.