The chlorosis paradox: Fe inactivation as a secondary event in chlorotic leaves of grapevine

Abstract

Iron chlorosis is a wide‐spread disorder of plants, in particular of those on calcareous soils. Elevated bicarbonate concentrations in the soil solution are considered as a main cause of this chlorosis. In nutrient solution culture experiments a supply of bicarbonate results in inhibited Fe acquisition and a subsequent decreased concentration of Fe in the leaf dry matter. This is indicated by a close positive relationship between chlorophyll and total Fe concentration in the upper leaves. In contrast to nutrient solution experiments, in some pot experiments particularly with calcareous soil, and in field experiments under certain conditions, no such close correlation can be observed and a higher Fe concentration can even be found in young chlorotic leaves than in green leaves. This phenomenon is called “the chlorosis paradox” and it has thus been concluded that, Fe chlorosis might be caused by an Fe inactivation in the plant, in particular in the leaf apoplast, e.g. by an alkalinization process. Reconsideration of published data on this phenomenon of enhanced Fe concentrations in chlorotic leaves, coupled with investigations of grapevine grown on calcareous soil reveal that “the chlorosis paradox” can only be observed in soil culture where severe shoot growth inhibition is already present at an early stage; presumably as a consequence of changes in phytohormone metabolism in connection with inhibited root growth. Obviously, the higher Fe concentration in chlorotic leaves with inhibited expansion growth is a consequence of the diminished dilution of normal high Fe concentrations in young leaves. This high concentration in chlorotic leaves can be observed in spite of a distinct lower Fe content in the individual leaves. The declined concentration of HCl‐extractable Fe in chlorotic leaves is presumably not the cause, but rather the consequence, of Fe chlorosis.