Water- and pyrophosphate-extractable humic substances fractions as a source of iron for Fe-deficient cucumber plants

Abstract

The capacity of Fe-deficient cucumber plants to utilise water-extractable and pyrophosphate-extractable humic substances as a source of Fe was investigated. Plants were grown for 13 days in nutrient solution in the presence or absence of Fe and during the last 7 days water-extractable and pyrophosphate-extractable humic substances were added to the solution at a final concentration of 5 mu g organic C ml(-1). The water-extractable humic fraction did not significantly modify leaf area and dry matter accumulation, leaf total Fe or chlorophyll content of cucumber plants adequately supplied with Fe. In contrast, pyrophosphate-extractable humic substances caused a slight but significant decrease of all the leaf parameters considered, with the exception of the chlorophyll content. Root Fe content of Fe-sufficient plants was decreased by more than 50% in the presence of each humified fraction. Addition of each humic fraction to Fe-deficient plants led to a partial disappearance of leaf chlorosis symptoms with a significant increase in chlorophyll and leaf Fe content. Fe content of roots was also significantly increased in Fe-deficient plants by the addition of humic substances to the nutrient solution. These results show that Fe-deficient cucumber plants can utilise Fe contained in the two fractions of humified organic matter. However, by calculating the amount of total Fe accumulated per plant in the presence of water-extractable or pyrophosphate-extractable humic substances, it could be seen that Fe contained in the water-extractable humic fraction was almost totally used by Fe-deficient cucumber plants, while that present in the pyrophosphate-extractable fraction could only be partially absorbed. The results strongly support a role of humified organic matter in Fe nutrition of plants and are discussed in terms of a possible interaction between soil humic substances and the biochemical mechanisms involved in the plant response to Fe deficiency

Review on iron availability in soil: interaction of Fe minerals, plants, and microbes

“…An analogous process has been described for flavonoids and citrate released by roots of white lupin, which, solubilizing Fe from an insoluble Fe phosphate, favored indirectly the mobilization and the availability of phosphate (Shaw et al 2006;Tomasi et al 2008). With respect to Fe, it has been demonstrated that the Fe-mobilization capacity of these organic ligands (Cesco et al 2000(Cesco et al , 2010 and the contribution of their Fe complexes in the acquisition process of the micronutrient by plants (Cesco et al 2002(Cesco et al , 2006Pinton et al 1999;Tomasi et al 2009a, b) could be quite different among the complexes. In the mobilization process, MS s can act alone or in combination with simple carboxylic acids and organic reducing agents of different origin; in this combined form, an additive effect with an enhanced extent of Fe dissolution from minerals has been observed (Dhungana et al 2007;Dehner et al 2010).…”

“…A link between organic acid excretion and proton extrusion by roots of white lupin has been clearly demonstrated (Tomasi et al 2009a, b). Plasma membrane H + -ATPase activity of roots, mainly involved in the proton extrusion coupled with the release of citrate previously described, is known also for being stimulated by soil HS (Pinton et al 1997a). In soils with low Fe availability, the occurrence of organic acid (citric, malic, and oxalic) also contributes to pH decrease when their exudation is coupled with proton efflux; the concentration in the rizodeposition varies among plant species and depends on soil pH (Hinsinger et al 2003).…”

“…Differently, HS fractions with low molecular mass, like fulvic acids, can contribute to the available pool of Fe present in soil solution due to their solubility and capability in forming Fe complexes. With respect to these low-molecular-weight humic fractions, it has been clearly demonstrated that they can act efficiently as natural substrates for Fe acquisition by roots of monocots and dicots (Pinton et al 1997b(Pinton et al , 1999Cesco et al 2002). HS increase Fe bioavailability through their Fechelating properties and have redox-reactive properties (Kögel-Knabner et al 2008).…”

Mechanisms of plant growth stimulation by humic substances: The role of organo-iron complexes

“…The corrective effect was attributed to complexation of Fe by HS in the organic materials. The ability of Fe-deficient plants to use Fe bound by water extractable (from peat) HS fraction was investigated by Pinton et al (1999). Studies conducted by our group have shown (Amichai 2001) that various fractions of DaM extracted, isolated and purified from compost bind Fe, maintain it in NS and provide this essential element to plants.…”

The Physiological Mechanisms of Desiccation Tolerance in Rhizobia

“…Previously, an aqueous extract of sphagnum peat was found to be an iron-rich substrate due to its content of water-soluble humic substances, which may explain the downregulation of proteins involved in iron transport in cells grown in the peat extract (Pinton et al, 1998). Cytryn et al (2007) reported on the genome-wide transcriptional analysis of B. japonicum response to desiccation stress.…”