Comparative investigation on the susceptibility of faba bean (Vicia faba L.) and sunflower (Helianthus annuus L.) to iron chlorosis

Abstract

Comparative physiological studies on iron (Fe) chlorosis of Vicia faba L. and Helianthus annuus L. were carried out. High internal Fe contents in Vicia cotyledons (16–37 μg) were completely used for plant growth and Fe chlorosis was not inducible by the application of nitrate (with or without bicarbonate). In Helianthus, low quantities of Fe in the seeds (4 μg) were insufficient for normal growth and without Fe in the nutrient solution, Fe chlorosis was obtained in all treatments. This chlorosis was an absolute Fe deficiency. Also, the treatment with 1 μM Fe in the nutrient solution and nitrate (with or without bicarbonate) led to severe chlorotic symptoms associated with low leaf Fe concentrations and high Fe concentrations in the roots. In contrast, Helianthus grown with NH₄NO₃ and 1 μM Fe had green leaves and high leaf Fe concentrations. However, with NO₃ supply (with or without bicarbonate), Fe translocation from the roots to the upper plant parts was restricted and leaves were chlorotic. Chlorotic and green sunflower leaves may have the same Fe concentrations, leaf Fe concentration being dependent on Fe translocation into the leaf at the various pH levels in the nutrient solution. At low external pH levels (controlled conditions) more Fe was translocated into the leaf leading to similar leaf Fe concentrations with higher chlorophyll concentrations (NH₄NO₃) and with lower chlorophyll concentrations (NO₃). This indicates a lower utilization of leaf Fe of NO₃ grown sunflower plants. Utilization of Fe in faba bean leaves is presumably higher than in sunflower leaves. In Vicia xylem sap pH was not affected by nitrate. In contrast, the xylem sap pH in Helianthus was permanently increased by about 0.4 pH units when fed with nitrate (with or without bicarbonate) compared with NH₄NO₃ nutrition. The xylem sap pH is indicative of leaf apoplast pH. From our earlier work (Mengel et al., 1994; Kosegarten und Englisch, 1994) we therefore suppose that in Helianthus, Fe immobilization occurs in the leaf apoplast due to high pH levels when grown with nitrate (with or without bicarbonate).