SENSITIVITY TO AND REQUIREMENT FOR IRON IN PLANTAGO SPECIES

The relative importance of some aspects of iron nutrition for the dist ribution 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 pl ants were assessed by determining the decrease in relative growth rate s caused by suboptimal and supraoptimal external Fe concentrations. Ma rked interspecific differences were observed in visual symptoms of Fe deficiency and Fe toxicity and in the external Fe concentrations leadi ng to 50% inhibition of maximal relative growth rates (RGR(50%)). Wher eas P. serpentina displayed a clear preference for low external Fe con centrations, growth rates of P. maritime 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. ma jor. A broad optimum was observed in P. coronopus exhibiting a low ext ernal Fe requirement and a high tolerance to supra-optimal Fe concentr ations. Iron efficiency and Fe tolerance differed in a way which was o nly partly correlated with the expected Fe availability at the natural habitat of the species, suggesting that Fe concentrations are of mino r importance for their distribution, or that some of the mechanisms th at render Fe oxides available for uptake in situ are mashed in solutio n experiments. To gain insight into the impact of the physiological an d morphological characteristics of the species on Fe efficiency, the e ffect of Fe status on shoot:root ratio, relative root surface area, ro ot Fe(III) reductase and proton extrusion capacity were investigated. Roots of all species showed increased Fe chelate reduction activity up on growth at suboptimal Fe concentrations; marked differences were obs erved with respect to the kinetic parameters of the reductase. Maximal velocity of the reduction was positively correlated with relative gro wth rate and was not related to the Fe efficiency of the species. By c ontrast, K-m corresponded to Fe efficiency ranking and can therefore b e regarded as an important parameter for the uptake of Fe at low avail ability. Enhanced ability to acidify the rhizosphere was only observed in P. major. From the morphological characteristics investigated, roo t surface area appears to be the most important parameter in the uptak e of Fe at suboptimal external concentrations.