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.

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The characteristics of the Fe(III)-chelate reductase activity have been investigated in mesophyll disks of Fe-sufficient and Fe-deficient sugar beet leaves. The Fe(III)-chelate reductase activity of mesophyll disks was light dependent and increased markedly when the epidermis was removed. Iron(III)-citrate was photo-reduced directly by light in the absence of plant tissue. Total reductase activity was the sum of enzymatic mesophyll reduction, enzymatic reduction carried out by organelles exposed at the disk edge and reduction caused by the release of substances both by exposed mesophyll cells and at the disk edge. Compounds excreted were shown by HPLC to include organic anions, mainly oxalate, citrate and malate. When expressed on a leaf surface basis, Fe deficiency decreased the total mesophyll Fe(III)-chelate reductase activity. However, Fe-sufficient disks reduced less Fe than the Fe-deficient ones when expressed on a chlorophyll basis. The optimal pH values for Fe(III) reduction were always in the range 6.0-6.7. In control leaves Fe(III)-citrate and Fe(III)-malate were the substrates that led to the highest Fe reduction rates. In Fe-deficient leaves Fe(III)-malate led to the highest Fe reduction rates, followed by Fe(III)-EDTA and then Fe(III)-citrate. K:(m) values for the total reductase activity, enzymatic mesophyll reduction and enzymatic reduction carried out by organelles at the disk edge were obtained.