Effects of bicarbonate, phosphate and high pH on the reducing capacity of Fe‐deficient sunflower and cucumber plants

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Abstract
Bicarbonate, phosphate and high pH are factors considered to induce or aggravate iron chlorosis. We have studied the effects of these factors on the ferric reducing capacity of the roots of plants grown with iron deficiency. Three kinds of experiments were performed with young sunflower (Helianthus annuus L., line RHA 274) and cucumber (Cucumis sativus L., cv “Burpee pickler"), plants grown in nutrient solution with the following results: i) Once the reducing capacity had been increased by iron deficiency, bicarbonate and high pH inhibited the reduction of ferric ethylenediaminetetraacetate to a similar extent, while phosphate had no effect. ii) When the considered factors were present during the period of growth without iron, the development of reducing capacity was inhibited more by bicarbonate than by a treatment with the same high pH value buffered with N‐[2‐Hydroxyethyl]piperazine‐N ‐[2‐ethanesulfonic acid] (Hepes). A negative effect of bicarbonate on the reducing capacity depended on its concentration and was greater in sunflower than in cucumber plants. Increased phosphate had no inhibitory effect alone, but increased the bicarbonate inhibition of the reducing capacity. The inhibitory effect of bicarbonate was partially suppressed if a low amount of iron was added to the nutrient solution. iii) In a split‐root experiment, bicarbonate applied to a part of the root did not affect the reducing capacity of the other part.