The relative importance of some aspects of iron nutrition for the
distribution of six Plantago species (P. maritima
L., P. coronopus L., P. major ssp. major L.,
P. serpentina All., P. media L. and P. lanceolata
L.) with different
habitat requirements for soil pH and moisture was evaluated. Iron efficiency
and Fe tolerance of hydroponically-grown plants were assessed by
determining the decrease in relative growth rates caused by suboptimal
and
supra-optimal external Fe concentrations. Marked interspecific differences
were observed in visual symptoms of Fe
deficiency and Fe toxicity and in the external Fe concentrations leading
to
50% inhibition of maximal relative
growth rates (rgr50%). Whereas P. serpentina
displayed a clear preference for low external Fe concentrations,
growth rates of P. maritima and P. media were found to
be increased at higher concentrations. Severe growth
restriction at both low and high Fe concentrations was evident in P.
lanceolata and P. major. A broad optimum
was observed in P. coronopus exhibiting a low external Fe
requirement and a high tolerance to supra-optimal Fe
concentrations. Iron efficiency and Fe tolerance differed in a way
which was only partly correlated with the
expected Fe availability at the natural habitat of the species,
suggesting that Fe concentrations are of minor
importance for their distribution, or that some of the mechanisms that
render Fe oxides available for uptake in situ
are masked in solution experiments.
To gain insight into the impact of the physiological and morphological
characteristics of the species on Fe
efficiency, the effect of Fe status on shoot[ratio ]root ratio, relative
root surface area, root Fe(III) reductase and proton
extrusion capacity were investigated. Roots of all species showed
increased Fe chelate reduction activity upon
growth at suboptimal Fe concentrations; marked differences were
observed with respect to the kinetic parameters
of the reductase. Maximal velocity of the reduction was positively
correlated with relative growth rate and was not
related to the Fe efficiency of the species. By contrast,
Km corresponded to Fe efficiency ranking and
can therefore
be regarded as an important parameter for the uptake of Fe at low
availability. Enhanced ability to acidify the
rhizosphere was only observed in P. major. From the morphological
characteristics investigated, root surface area
appears to be the most important parameter in the uptake of Fe at
suboptimal external concentrations.