Inhibition of iron-stress reactions in sunflower by bicarbonate

In pot culture experiments using a calcareous soil the growth rate of sunflowers was depressed by latent iron deficiency. Iron-stress reactions, reflected by enhanced uptake rate of Fe-59 after short-term supply of ⁵⁹FeEDDHA were observed under these conditions. These reactions, however, were delayed and much less distinct than those observed under latent iron deficiency in water culture experiments. Addition of MgCO₃ to this soil increased the iron deficiency of the plants and caused chlorosis, but prevented these iron-stress reactions. In contrast to the soil experiments, distinct iron-stress reactions could be observed with latent iron-deficiency in sand culture experiments in which varied amounts of inorganic Fe-III were supplied. These reactions were only observed, however, in absence of bicarbonate. Even 4 meq bicarbonate severely inhibited these iron-stress reactions which were almost totally inhibited at 10 meq bicarbonate. Bicarbonate depressed both short-term uptake of Fe-59 as well as total iron content of the leaves. There was no evidence for an additional “inactivation” of iron within the leaves due to bicarbonate treatment. Bicarbonate also strongly depressed the manganese content of the young leaves. The results demonstrate that the regulatory mechanism of so-called “iron-efficient” plant species like sunflower under iron-stress - increase in H⁺ efflux and reducing capacity of the roots and thus enhanced uptake of iron (and manganese) - is severely inhibited or even blocked by high bicarbonate concentrations. “Lime chlorosis”, caused by high bicarbonate concentrations on calcareous substrates is therefore also widespread in so-called “iron-efficient” species.