Investigations of the Role of Iron in Chlorophyll Metabolism I. Effect of Iron Deficiency on Chlorophyll and Heme Content and on the Activities of Certain Enzymes in Leaves

Abstract

For over 100 years it has been known that iron is essential for the growth of higher plants and that it is involved in some manner with chlorophyll metabolism. Iljin (17,18,19) demonstrated that iron deficiency in plants resulted in decreased dry weight, salt, and protein nitrogen contents and increased contents of amino and organic acids. Extensive studies (8, 9, 25, 33) have confirmed Iljin's observations, but despite the keen interest in iron chlorosis, no clear evidence of the precise role of iron in chlorophyll metabolism has been obtained. Evans (11) and DeKock et al. (7) found a positive correlation between the iron content of the nutrient medium and the chlorophyll and heme contents of leaves. The activities of enzymes containing heme prosthetic groups, including catalase, peroxidase, and cytochrome oxidase, have been reported (7,11, 33) to be less in iron-deficient than in normal tissues. There is no evidence for a direct role of the heme enzymes in chlorophyll metabolism; however, there is strong evidence that the porphyrin moieties of heme and chlorophyll are formed by the same biosynthetic system (2). The fact that the activities of heme-containing enzymes and the contents of both heme and chlorophyll in leaves are influenced in a similar manner by the iron content of nutrient solutions that some step in chlorophyll and heme synthesis is dependent on an adequate iron supply. Studies (22, 32) on porphyrin biosynthesis by microorganisms and animals have provided evidence suggesting a role of iron in this process. Small amounts of iron (32) have been reported to stimulate porphyrin and heme formation in the blood of irondeficient ducks. According to Lascelles (22) the formation of porphyrins in Rhodopseudomonas spheroides also is stimulated by iron additions. Studies show that insufficient iron results in a decreased rate of conversion of coproporphyrinogen III to protoporphyrin (23,28). Thus iron appears to be important in porphyrin metabolism in several types of organisms. Our experiments were initiated in an effort to identify in iron-deficient plants metabolic abnormalities that may be related directly or indirectly to chlorophyll metabolism. Experiments were conducted to determine the influence of the iron supply on the activities of certain enzymes and on the content of heme and protoporphyrin in leaves. The postulation was tested that the activities of PPNR5 and of cytochromes are decreased by iron deficiency, thus decreasing the energy supply which may be involved in chlorophyll biosynthesis (24).