Solution culture studies have demonstrated that apoplasmic iron
(Fe) deposited in the roots of dicotyledonous and
graminaceous plants can be mobilized to improve plant iron status in conditions
of limited Fe supply. The present
study investigated the formation of the apoplasmic Fe pool in dicotyledonous
(soybean and cucumber) and
graminaceous (wheat) plants in a pot experiment. The pots had three compartments
such that plants could take
up Fe and other nutrients from two calcareous soils treated with different
Fe forms without their roots touching
with soil directly. In this way overestimating Fe accumulation in root
apoplasm was avoided. The results showed
that while the root d.wt of wheat did not vary when soils were supplied
with different Fe resources, the root
d. wt of soybean and cucumber supplied with FeEDTA decreased compared with
the control (without Fe
treatment). Supplying FeEDTA in the side compartment increased shoot d.
wt and Fe concentration in shoots of
all species. However, supplying Fe(OH)3 had no effect on shoot
d. wt or Fe concentration in the shoots of any
species. Soybean and cucumber accumulated little or no Fe in the root apoplasm
in controls or in Fe(OH)3
treatments. By contrast, a large amount of Fe was deposited in the root
apoplasm of wheat grown in similar
conditions. Remarkably, when FeEDTA was supplied in the soils, large apoplasmic
iron pools were formed in the
roots of all three species. Therefore, in dicotyledonous plants grown on
calcareous soils, little or no apoplasmic
iron pool forms, because there is not enough available Fe in the soil solution
and the plants have little ability to
mobilize Fe3+ in the soil. By contrast, a larger apoplasmic
iron pool could form in graminaceous plants at lower
concentrations of available soil-Fe possibly by enhancing the release of
phytosiderophores which could mobilize
Fe3+ in the soil and then transfer the Fe3+-complexes
to the root apoplasm.