Effects of bicarbonate, nitrate and other environmental factors on iron deficiency chlorosis. A review

The influence of several environmental factors on the occurrence of iron deficiency chlorosis on calcareous soils, also called bicarbonate induced chlorosis, is described. Bicarbonate in the soil solution is a strong pH buffer, mainly in the presence of calcium carbonate. Since bicarbonate is quite mobile, and CO2 diffusion is a slow process, the pH decrease in such soils after proton release by plants is small. Also, the ferric reductase activity of plant roots declines sharply at high pHs. The chemical Fe(III) reduction depends on the pe+pH; so, the lower the pH, the more favored is the formation of Fe(II) from the Fe(III) in the rizosphere. Nitrate can be acquired by the roots along with a proton cotransport that increase the pH outside the root plasmalemma, but a redox effect may be also important. In the rizosphere, the conditions for the reduction of nitrate (N(V)) to nitrite (N(III)) and ammonia (N(-III)) are attained before than the conditions for the Fe(III) reduction from a solid phase. So, from the thermodynamic point of view, the electrons released by the plant can be taken directly by nitrate instead of Fe(III), or the Fe(II) formed can be reoxidized by the nitrate. Furthermore, if nitrate and nitrite reductions occurs in the rizosphere, microsite pH increases at the root surface. Since controversial data have been reported on the pH variations in the xylem sap in the presence of an excess of bicarbonate in the medium, a slower transport of iron does not appear to be the main reason for bicarbonate induced chlorosis. The iron paradox is well known (Romheld, 1987): sometimes, in soil culture, chlorotic leaves can have higher iron concentration than green ones. Some authors suggest that bicarbonate may affect the Fe distribution within the leaves. Apoplastic pH would control the entry of iron to the cytosol in Leaves in a similar way than occurs in the root. Bicarbonate in the nutrient solution could act as a buffer in the leaf apoplast. Nitrate uptake in the meristem also produces a pH increase, with the same effect than the bicarbonate. Also, nitrate is an oxidant that could reduce the availability of the electrons for the Fe reduction for the membrane transport.