Iron (Fe) is a very important element for plants, since it is involved in many biochemical processes and,
often, for the low solubility of the natural Fe sources in soil, plants suffer from Fe e deficiency, especially
when grown on calcareous soils. Among the numerous plant growth-promoting rhizobacteria (PGPR)
that colonize the rhizosphere of agronomically important crops, Azospirillum brasilense has been shown
to exert strong stimulating activities on plants, by inducing alterations of the root architecture and an
improvement of mineral nutrition, which could result from an enhancement of ion uptake mechanisms
as well as by increased bioavailability of nutrients. Some studies have also established that A. brasilense
can act as biocontrol agent, by preventing the growth and/or virulence of phytopathogens, most likely
through the production of microbial siderophores that sequester Fe from the soil. Despite microbial
siderophores complexed with Fe could be an easily accessible Fe source for plants, the possible
involvement of A. brasilense in improving Fe nutrition in plants suffering from the micronutrient deficiency
has not been investigated yet. Within the present research, the characterization of the physiological
and biochemical effects induced by Fe starvation and PGPR inoculation in cucumber plants
(Cucumis sativus L. cv. Chinese Long) was carried out. The analyses of root exudates released by hydroponically
grown plants highlighted that cucumber plants respond differently depending on the nutritional
status. In addition, following the cultivation period on calcareous soil, also the root exudates found
in the extracts suggested a peculiar behaviour of plants as a function of the treatment. Interestingly, the
presence of the inoculum in soil allowed a faster recovery of cucumber plants from Fe-deficiency
symptoms, i.e. increase in the chlorophyll content, in the biomass and in the Fe content of leaves.
These observations might suggest a feasible application of A. brasilense in alleviating symptoms generated
by Fe-limiting growth condition in cucumber plants.