Historical landmarks in progress relating to iron chlorosis in plants

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Abstract
Iron deficiency has long been considered to be the most difficult of nutrient deficiencies in plants to understand and to correct. After the landmark discovery in 1843 that lime‐induced chlorosis was a form of iron deficiency, progress was slow for the next 100 years. Many, but not all, of the scientific giants who made significant progress on understanding iron chlorosis and iron physiology in plants in the 1940's and 1950's have passed away. Thome was certain that ferrous iron was the key to understanding the disorder. Kliman had suggested uptake of Pe in the ferrous form. Iljin found that iron chlorotic leaves had high levels of free amino acids, high levels of citrate and total organic acids, were high in P and K and low in Ca concentrations. These are the criteria that generally identify iron chlorosis. Shive and his students found an Fe‐Mn interaction through which Fe deficiency can be induced. Warm found that lime‐induced chlorosis in grapes could be largely prevented if resistant rootstocks were chosen. Those workers represented a generation gone but it will not be forgotten. Leeper did critical evaluations which demonstrated that chlorotic leaves often— very often—contain more iron than do green leaves. Oserkowsky and Lindner and Harley found that green leaves contained more “active” iron than chlorotic leaves. The role of bicarbonate was better elucidated by many workers. While the nature of iron chlorosis was being subjected to many studies, same breakthroughs occurred in its control. Jacobson used iron chelate (EDTA) as the Fe source in nutrient solutions. Stewart and Leonard successfully took FeEDTA to the field for control of Fe deficiency on orange trees. Kroll synthesized a chelating agent with 108 higher stability with Fe than FeEOTA. It found many applications. There were many other pioneers in this period. The 1960's and 1970's saw modem biochemical techniques move into the field. Iron deficient roots excrete more protons than normal roots. Iron inefficient plants are genetically controlled (one gene in at least some situations). They have less reducing ability at the root surface and excrete fewer protons than Fe efficient plants. Citrate functions in the xylem transport of Fe (ferric yet). Iron deficiency can be identified through analysis of certain enzymes before the chlorophyll begins to disappear. The role of Fe in synthesis of chlorophyll is better elucidated. The modern pioneers include Brown, Evans, Marshner, DeKock, Chaney, Tiffin, Bar‐Akiva and many others. But the next pioneers probably will be the plant breeders who will wipe out the problem—at least in important plant species.