Iron chlorosis is a wide-spread disorder of plants, in particular of those
on calcareous soils. Elevated bicarbonate concentrations in the soil soluti
on 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 be
tween chlorophyll and total Fe concentration in the upper leaves. In contra
st to nutrient solution experiments, in some pot experiments particularly w
ith calcareous soil, and in field experiments under certain conditions, no
such close correlation can be observed and a higher Fe concentration can ev
en 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 concent
rations in chlorotic leaves, coupled with investigations of grapevine grown
on calcareous soil reveal that "the chlorosis paradox" can only be observe
d in soil culture where severe shoot growth inhibition is already present a
t an early stage; presumably as a consequence of changes in phytohormone me
tabolism in connection with inhibited root growth. Obviously, the higher Fe
concentration in chlorotic leaves with inhibited expansion growth is a con
sequence of the diminished dilution of normal high Fe concentrations in you
ng leaves. This high concentration in chlorotic leaves can be observed in s
pite of a distinct lower Fe content in the individual leaves. The declined
concentration of HCl-extractable Fe in chlorotic leaves is presumably not t
he cause, but rather the consequence, of Fe chlorosis.