Physiology of trace element toxicity and its relation to iron stress

Evidence is presented which supports the view that cobalt toxicity induces iron deficiency in sugar beets. When excess cobalt was supplied to plants cultured hydroponically in half‐Hoagland's solution, less iron was transported to leaves, and chlorosis developed. Chlorosis caused by excess cobalt had characteristics similar to that obtained by withholding iron. Chlorophyll a/chlorophyll b ratios, carotene and xanthophyll contents, number and size of photosynthetic units, and the number and average volume of chloroplasts and leaf cells exhibited similar changes irrespective of how chlorosis was induced. Cobalt toxicity also decreased the rate of photosynthesis per unit leaf area in a manner comparable to iron deficiency, i.e. by reducing the number of photosynthetic units (and thereby photosynthetic electron transport capacity) per unit leaf area. A linear relationship between chlorophyll content and total leaf iron content was obtained with cobalt toxicity. This is because most (about 69 to 84%) of the iron in the leaf blade was strongly bound to the chloroplast lamellae which contained iron in a fixed ratio to chlorophyll. Of the cobalt in the leaf, half was located in the chloroplasts, and it is evident that cobalt traversed the plasmalemma and chloroplast envelopes. Thus, in addition to inducing iron deficiency, cobalt could have affected chloroplast enzymes directly.