Different root exudation patterns affect the mobilization of Fe from soil minerals.

Plants can cope with Fe deficiency by either acidifying the rhizosphere and enhancing the ferric chelate reductase activity (strategy I) or by releasing high affinity complexing compounds known as phytosiderophores (strategy II). In this research, tomato, barley and cucumber plants were grown hydroponically in an Fe free (–Fe) nutrient solution using the RHIZOtest. Fe-deficient (–Fe) plants were grown on a calcareous agricultural soil and root. Root exudates were determined quali-quantitatively using HPLC and colorimetric analyses. The accumulation of nutrients in plant tissues was measured by ICP-OES. Soil mineral modifications were assessed by XRD and SEM-EDX. Root exudates obtained by the hydroponic system were higher than those obtained from the soil extracts; plant uptake and adsorption by soil particles could be, at least in part, responsible of this result. After a 6-day soil contact the plants show a visible recovery from Fe deficiency symptoms at leaf level suggesting the efficacy, of root exudate release in the mobilization of Fe into soluble soil forms and its uptake. Moreover, significant soil mineral modifications were observed. At last, organic ligands released in Fe-deficient soil conditions show very complex exudation patterns both in monocotyledonous and dicotyledonous plants. For instance, barley plants besides the dominant release of phytosiderophores also significant amounts of amino acids and traces of organic acids were found. The results of this research will enable to better understand the soil influence on the release of root exudates, the dynamics of mineral weathering in the rhizosphere and the capability of plant species to mobilize and take up poorly soluble Fe forms.