Zinc

Chapman (1965) reviewed the early research on the essentiality of zinc (Zn) and noted the work of pioneers in the eld of basic plant nutrition and the work of investigators who studied crop nutrition. Recognition of the discovery of Zn as an essential element is credited to Sommer and Lipman (1926). The modern history of Zn as a nutrient started in 1918, in a pecan (Carya illinoinensis K. Koch) orchard in Georgia, United States (Skinner and Demaree, 1926; Storey, 2007). In this orchard, trees had increased in trunk diameter, but their tops had dieback or rosetting. Researchers and growers tried fertilizers and cover crops without success to overcome the dieback, but none of these means were successful. The common assumption among pecan growers and researchers at the time was that rosetting was due to iron deciency (Storey, 2007). In 1932, researchers discovered that Zn and not iron was the corrective element for rosetting (Alben et al., 1932). Alben et al. (1932) used solutions of FeCl2 or FeSO4 in their treatments for rosetting and obtained conicting results (Storey, 2007). Other treatments included injections into dormant trees, soil applications while the trees were dormant and after the foliage was well developed, and foliar applications. The only favorable results were obtained if the iron solutions were mixed in Zn-galvanized containers (Alben et al., 1932; Storey, 2007). Analysis of the successful solutions indicated that they contained

15.1 Historical Background .......................................................................................................... 537 15.2 Zinc as an Essential Element in Plant Metabolism .............................................................. 538

15.2.1 Zinc Involvement in Enzymatic Reactions ............................................................... 538 15.2.1.1 Zinc-Containing Enzymes ......................................................................... 538 15.2.1.2 Zinc-Activated Enzymes ............................................................................ 541

15.2.2 Zinc Involvement in Protein Synthesis ..................................................................... 541 15.2.3 Zinc Involvement in Carbohydrate Metabolism ....................................................... 541 15.2.4 Zinc Involvement in Indoleacetic Acid Synthesis .................................................... 542 15.2.5 Zinc Involvement in Membrane Integrity ................................................................. 542

15.3 Availability of Zinc to Plants ................................................................................................ 543 15.3.1 Zinc Content, Forms, and Dynamics in Soils........................................................... 543 15.3.2 Uptake and Transport of Zinc by Plants ................................................................... 543

15.3.2.1 Genetic Control .......................................................................................... 543 15.3.2.2 Zinc Uptake and Accumulation as Affected by Other Nutrients .............. 545

15.3.3 Zinc Accumulation in Plants .................................................................................... 547 15.3.4 Biofortication of Foods with Zinc ..........................................................................548