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 (Römheld, 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.

Effects of Cd and Pb toxicity were investigated in sugar beet ( Beta vulgaris L.) grown in hydroponics under growth-chamber-controlled conditions. Chemical speciation calculations were used to estimate the chemical species in equilibrium. Cd, used as chloride salt or chelated to EDTA, decreased fresh and dry mass of both root and shoot, and increased root / shoot ratios. Plants developed few brownish roots with short laterals. Cd decreased N, P, Mg, K, Mn, Cu and Zn uptake, and facilitated Ca uptake. Leaves of plants treated with 10 or 50 µM Cd–EDTA and 10 µM CdCl2 developed symptoms of Fe deficiency. These symptoms included decreased leaf chlorophyll (Chl) and carotenoid concentrations, increased carotenoid / Chl and Chl a / b ratios, de-epoxidation of violaxanthin cycle pigments, and decreased photosynthetic rates and PSII efficiency. Plants treated with 50 µM CdCl2, however, had decreased growth but did not show marked leaf Fe-deficiency symptoms. All Cd treatments increased Fe(III)-chelate reductase activity in root tips, although Fe concentrations in shoots were similar to those found in control plants. Pb chelated with EDTA induced visual symptoms only at concentrations of 2 mM. Leaves of Pb-treated plants remained green and their edges were rolled inwards. Pb increased root fresh and dry mass with no changes in shoot mass, therefore increasing the root / shoot ratio. Changes in plant nutrient concentrations with Pb were only minor, although leaf Cu levels approached critical deficiency levels. No symptoms of Fe deficiency were apparent in leaves. Root tips of Pb-treated plants, however, had increased Fe(III)-chelate reductase activities.

The characteristics of the Fe reduction mechanisms induced by Fe deficiency have been studied in intact plants of Beta vulgaris and in purified plasma membrane vesicles from the same plants. In Fe-deficient plants the in vivo Fe(III)-ethylenediaminetetraacetic complex [Fe(III)-EDTA] reductase activity increased over the control values 10 to 20 times when assayed at a pH of 6.0 or below (“turbo” reductase) but increased only 2 to 4 times when assayed at a pH of 6.5 or above. The Fe(III)-EDTA reductase activity of root plasma membrane preparations increased 2 and 3.5 times over the controls, irrespective of the assay pH. The Km for Fe(III)-EDTA of the in vivo ferric chelate reductase in Fe-deficient plants was approximately 510 and 240 [mu]M in the pH ranges 4.5 to 6.0 and 6.5 to 8.0, respectively. The Km for Fe(III)-EDTA of the ferric chelate reductase in intact control plants and in plasma membrane preparations isolated from Fe-deficient and control plants was approximately 200 to 240 [mu]M. Therefore, the turbo ferric chelate reductase activity of Fe-deficient plants at low pH appears to be different from the constitutive ferric chelate reductase.