Plant requirements for microelements have only been elucidated over the past 50-60 years. Before the end of the last century, it had been realised that too little iron caused chlorophyll deficiency in plants, but the importance of other elements took many years to prove conclusively. Maze, for example, used hydroponic techniques during the years 1914-1919 to show that zinc, manganese and boron improved the growth of maize plants. Sommer and Lipman (1926) also showed the essentiality of boron, and Sommer (1931) of copper, but uncertainty over which elements were really indispensible to growth still existed in 1933 when Hoagland and Snyder proposed two supplementary nutrient solutions for water culture which in total contained 26 elements. It took several further years to prove that molybdenum (Arnon and Stout, 1939) and cobalt in very small amounts, were most important for healthy plant growth. Early plant tissue culture work was to both profit from, and contribute to the findings of previous hydroponic studies. Our understanding has been enhanced by investigations into the biochemical role of minor elements.
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