Nitrogen and bicarbonate relationships with iron nutrition in plants

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Abstract
Bicarbonate levels in soils or in irrigation waters have often been implicated as a most important parameter in the induction of line‐induced chlorosis. If borderline deficiency is present, the addition of nitrogen also induces the deficiency. A physiological disorder induced by excess bicarbonate relating to the onset of iron deficiency has been extremely difficult to elucidate. One of the more obvious effects of bicarbonate is the retention of calcium in roots and failure of its translocation to shoots when high levels of it are present in nutrient solutions. Bicarbonate physiologically is metabolized ‐to organic acids in roots. High levels of nitrate nitrogen are also conducive to the synthesis of organic acids in roots and also to increased basicity in the rhizosphere which can be interpreted as result of bicarbonate. Net result in either case is decreased uptake and translocation of iron. In an experiment with PI‐54619–5‐l soybeans, bicarbonate and nitrate gave similar patterns of chlorosis and diminished Fe uptake. Bicarbonate interferes with uptake of iron from chelating agents in both soil and solution culture. With added P and N, with Fe to the PI‐54619–5–1 soybeans, however, bicarbonate was without effect. Without Fe and N, P in control decreased yields due to severe Fe chlorosis. Bicarbonate is difficult to main‐tain in aerated nutrient solutions because of the loss of CO2 with formation of hydroxide from the bicarbonate with resulting pH increases. Similar reactions occur in soil and this may be the major reason for induction of iron chlorosis by bicarbonate. Bicarbonate may inhibit the chelate‐splitting mechanism. In experimental studies, nitrate and bicarbonate both appeared to have the same effect on iron response, i.e., these effects were additive.