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  Section: Plant Nutrition » Micronutrients » Zinc
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Absorption and Function of Zinc in Plants

Early Research on Zinc Nutrition of Crops
Absorption and Function of Zinc in Plants
Zinc Deficiency
Zinc Tolerance
Trunk Injection
Zinc in Soils
Phosphorus–Zinc Interactions
Tryptophan and Indole Acetic and Synthesis
Root Uptake
Foliar Absorption
  Influence of Humidity on Foliar Absorption
Role of Zinc in DNA and RNA Metabolism and Protein Synthesis
Zinc Transporters and Zinc Efficiency

Zinc is taken up predominantly as a divalent cation (Zn2+), but at high pH it is probably absorbed as a monovalent cation (ZnOH+) (7). Zinc is either bound to organic acids during long distance transport in the xylem or may move as free divalent cations. Zinc concentrations are fairly high in phloem sap where it is probably complexed to low-molecular-weight organic solutes (8). The metabolic functions of zinc are based on its strong tendency to form tetrahedral complexes with N-, O-, and particularly S-ligands, and thus it plays a catalytic and structural role in enzyme reactions (9).

Zinc is an integral component of enzyme structures and has the following three functions: catalytic, coactive, or structural (9,10). The zinc atom is coordinated to four ligands in enzymes with catalytic functions. Three of them are amino acids, with histidine being the most frequent, followed by glutamine and asparagine. A water molecule is the fourth ligand at all catalytical sites. The structural zinc atoms are coordinated to the S-groups of four cysteine residues forming a tertiary structure of high stability. These structural enzymes include alcohol dehydrogenase, and the proteins involved in DNA replication and gene expression (11). Alcohol dehyrogenase contains two zinc atoms per molecule, one with catalytic reduction of acetaldehyde to ethanol and the other with structural functions. Ethanol formation primarily occurs in meristematic tissues under aerobic conditions in higher plants. Alcohol dehyrdrogenase activity decreases in zinc-deficient plants, but the consequences are not known (7). Flooding stimulates the alcohol dehydrogenase twice as much in zinc-sufficient compared with zinc-deficient plants, which could reduce functions in submerged rice (12).

Carbonic anhydrase (CA) contains one zinc atom, which catalyzes the hydration of carbon dioxide (CO2). The enzyme is located in the chloroplasts and the cytoplasm. Carbon dioxide is the substrate for photosynthesis in C3 plants, but no direct relationship was reported between CA activity and photosynthetic CO2 assimilation in C3 plants (13). The CA activity is absent when zinc is extremely low, but when even a small amount of zinc is present, maximum net photosynthesis can occur. Photosynthesis by C4 metabolism is considerably different (14,15) than that occurring in C3 plants. For C4 metabolism, a high CA activity is necessary to shift the equilibrium in favor of HCO3- for phosphoenolpyruvate carboxylase, which forms malate for the shuttle into the bundle sheath chloroplasts, where CO2 is released and serves as substrate of ribulosebisphosphate carboxylase.


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