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Chlorosis-susceptible fruit trees growing on calcareous soils have been observed to recover in the presence of grass cover species. However, the physiological mechanisms behind this phenomenon are only scarcely understood. An investigation was carried out to verify whether citrus plants can use 59Fe solubilized from a sparingly soluble source by the phytosiderophores (PS) released from graminaceous species. Experiments were performed in hydroponics, using two citrus rootstocks differing in their sensitivity to Fe-deficiency in the field (Poncirus trifoliata · Citrus paradisi, citrumelo ‘‘Swingle’’, highly susceptible, and Citrus aurantium L., moderately tolerant). Barley (Hordeum vulgare L., cv Europa) was used as a model species for PS-releasing graminaceous plants. Fe-deficient citrus plants increased 59Fe uptake from 59Fe-hydroxide supplied inside a dialysis tube, when Fe-deficient barley plants or PS-containing barley root exudates were present in the uptake solution, this effect being particularly evident for the susceptible rootstock. 59Fe-uptake from 59Fe-hydroxide was also enhanced in Fe-deficient citrumelo ‘‘Swingle’’ in the presence of Fe-deficient Poa pratensis L. and Festuca rubra L., two perennial grasses normally grown in association with fruit trees; no effect was found when Fe-sufficient grasses were employed. The uptake of 59Fe by the susceptible citrus rootstock increased in proportion to the amount of 2¢-deoxymugineic acid (DMA), the major PS released by Fe-deficient F. rubra, present in the uptake solution. The beneficial effect of F. rubra or P. pratensis was evident from the leaf re-greening observed when Fe-deficient citrumelo ‘‘Swingle’’ plants were grown in association with the grasses in pots filled with a calcareous soil. Leaf regreening was not observed when citrumelo ‘‘Swingle’’ plants and yellow stripe 3 (ys3) maize (Zea mays L.) mutant plants, unable to release PS, were co-cultivated in pots filled with calcareous soil, unless exogenous PS were added to the soil. Results indicate that graminaceous cover species can improve the Fe-nutrition of fruit trees grown on calcareous soils by enhancing Fe-availability.

Aims To test if multi–surface models can provide a soil-specific prediction of metal mobilization by phytosiderophores (PS) based on the characteristics of individual soils. Methods Mechanistic multi-surface chemical equilibrium modeling was applied for obtaining soil-specific predictions of metal and PS speciation upon interaction of the PS 2’-deoxymugineic acid (DMA) with 6 soils differing in availability of Fe and other metals. Results from multi-surface modeling were compared with empirical data from soil interaction experiments. Results For soils in which equilibrium was reached during the interaction experiment, multi-surface models could well predict PS equilibrium speciation. However, in uncontaminated calcareous soils, equilibrium was not reached within a week, and experimental and modeled DMA speciation differed considerably. In soils with circum-neutral pH, on which Fe deficiency is likely to occur, no substantial Fe mobilization by DMA was predicted. However, in all but the contaminated soils, Fe mobilization by DMA was observed experimentally. Cu and Ni were the quantitatively most important metals competing with Fe for complexation and mobilization by DMA. Conclusion Thermodynamics are unable to explain the role of PS as Fe carrier in calcareous soils, and the kinetic aspects of metal mobilization by PS need to be closer examined in order to understand the mechanisms underlying strategy II Fe acquisition. Electronic supplementary material The online version of this article (doi:10.1007/s11104-014-2128-3) contains supplementary material, which is available to authorized users.