As a consequence, plants often develop Fe deficiency symptoms, which include, for instance, the interveinal chlorosis of young leaves, a decrease in the photosynthesis rate and a reduced growth 5. To overcome Fe shortage, plants induce both morphological and molecular adaptations, as for instance by modifying their root system architecture and by triggering the expression of genes involved in Fe solubilization and acquisition from rhizosphere compartment 6,7. Additionally, several pieces of evidence have demonstrated that also the contribution of rhizosphere microorganisms can be fundamental to help plant coping with suboptimal Fe concentrations in the growth medium 8–10.
Among different strategies to remediate Fe deficiency in crop plants, the use of Fe-based fertilizers still represent the most frequent and economically sustainable approach adopted in agriculture, applied either to the canopy, as foliar spray, or to the soil 11. Iron is generally supplied in a chelated form with aminocarboxylate synthetic ligands, like HEDTA, EDTA, DTPA and EDDHA, which are aimed at increasing the Fe availability for plant uptake. Based on this feature, synthetic chelates also allow reducing the amount of fertilizer applied to crops 11, thereby having an economic benefit. However, when considering fertilization at soil level, the stability of Fe complexes is strongly influenced by soil pH; only the most stable chelate (i.e., o,o-EDDHA/Fe3+), and yet the most expensive, is able to guarantee Fe bioavailability in highly calcareous soils 11. However, the use of aminocarboxylate synthetic ligands for Fe fertilization also displays several drawbacks, as for instance i) the persistence in the plant system (ligands can be accumulated in plant tissues), ii) the speciation of the complexes can be influenced by other cations in the soil (e.g. ligand exchange reactions with Zn and Cu), iii) the undesired mobilization of heavy metals (e.g. Pb) that can enter the food chain, thereby affecting both the quality and the safety of agricultural products 12.
Plants biostimulants (PBs) are defined as a class of substances able to improve crop productivity and quality, increasing the availability of nutrients in the soil, ameliorating nutrient use efficiency of plants, and promoting the degradation and humification of organic substances in soils 13. Overall, PBs feature a variegate nature thereby including a broad spectrum of substances, all of these exerting the above-mentioned beneficial effects on plants, albeit their precise mode of action is still elusive 14. Among PBs, in the last years, beneficial microorganisms, humic substances and protein hydrolysates (PHs) have been attracting great attention as a possible greener alternative respect to traditional fertilizers for managing Fe nutrition in crop plants 8,15,16. In particular, PHs are a mixture of bioactive compounds as amino acids and peptides obtained from animal or vegetal protein sources through a process of enzymatic and/or thermal-chemical hydrolysis 17. PHs may also contain carbohydrates, phenols, mineral elements, phytohormones and other organic compounds contributing to their biostimulant activity. One of most significant bioactive effects of PHs is the increase of plant nutrient acquisition, achieved by enhancing the bioavailability of nutrients in soil solution and promoting more root growth and active uptake process 17. PH-mediated improvement of nutrient bioavailability in soil solution has been associated to the conversion of mineral nutrient as inorganic ions to complexes that have higher solubility. The formation of mineral-organic complexes between mineral nutrients and various PH-derived ligands such as peptides and amino acids has been reported for mineral cations including iron.
In this context, plant nutrition can be improved by combining biostimulant effects and nutrient supply in advanced fertilizers like biochelates. Biochelate can be defined as an organic compound consisting of a central metal atom attached to one or more naturally occurring organic molecules, called ligands (e.g., peptides). Metal biochelates can be obtained with plant nutrients existing in cationic form (e.g. calcium, iron, manganese, zinc, copper). Metal biochelates with micronutrients are more bioavailable in the soil solution than the corresponding inorganic salts or oxides for plant uptake especially under alkaline conditions. Moreover, biochelates contain environmentally friendly chelating agents that are fully biodegradable and nontoxic for humans and animals, and therefore they can be used in agriculture without the health and environmental concerns raised for synthetic chelates, as discussed above. Despite the great potential of biochelates as fertilizers, this technology is still poorly used in agriculture. Biochelates, such as peptides, are currently largely adopted in food and feed industry to enhance mineral bioavailability for human or animal nutrition 18,19. A variety of metal-chelating peptides has been generated and identified from different food sources, such as milk, egg, soybean and sea cucumber 20. The mineral-chelating properties of peptides are attributed to the structural diversity of their backbone, which contains both the terminal carboxyl and amino groups, and the side chains of amino acid residues 18.
Recently, innovative fertilizers containing metal-biochelates were introduced in EU and US markets. These fertilizers containing a range of micronutrients (e.g., Fe, Zn, Mn) and calcium chelated with peptides resulting from vegetal-derived PHs used as plant biostimulant 21,22 were developed. Preliminary studies showed these metal-biochelates have a good stability in the pH range 6-8. These observations have been further confirmed by three agronomic trielas, carried out on three agriculture-relevant crops, namely cucumber, tomato and strawberry. The aim was to compare the agronomic performances of a Fe-biochelate containing vegetal-derived peptides and the widely used syntetic Fe chelate, adopting either an optimal and alkaline substrate pH, 6.0 and 8.0, respectively12. Different parameters generally affected by an inadequate Fe nutrition of plants (e.g. leaf chlorophyll content, photosynthetic efficiency, Fe uptake machinery, biomass accumulation and crop yield) have been assessed and, independently from the crop considered, Fe-biochelate showed the very same efficiency of syntetic Fe chelate in supplying plants with adequate amounts of the micronutrient12. Considering the potential negative impact of synthetic chelates on the environment and the long persistence of these compounds in the plant tissues, the above research findings are of great interest to enhance the sustainability of crop production. In the context of the transition towards sustainability promoted by EU, the development, and the subsequent application, of new generation fertilizers based on biochelates represents a virtuous example of novel agricultural practice, ensuring profitability, environmental health, and social and economic equity.
