European Journal of Sustainable Agroecosystems

Background: Root exudates, which are a mixture of organic acids, sugars, amino acids, flavonoids, mucilage, and enzymes released into the rhizosphere by plant roots, are currently recognized as essential factors in determining the efficiency of nutrient acquisition in legumes. By virtue of their particular exudation profiles, nutrient-efficient legume cultivars are able to mobilize, in a certain manner, phosphate, iron, and other nutrients that are available in small quantities from the substrate, and thus ensure symbiotic nitrogen fixation. However, both the mechanistic and comparative aspects of this phenomenon are yet to be adequately covered in scientific literature.

Objective: The aim of this review is to give a comprehensive overview of the scientific literature (2015-2025, with the inclusion of important discoveries made in the early 2000s) on the composition and role of root exudates in nutrient-efficient legume varieties in order to identify common patterns, inconsistencies, and persisting gaps in research.

Literature Search Sources: A structured search through Scopus, Web of Science, PubMed, and Google Scholar, alongside reports from FAO and CGIAR yielded a total of 611 records without duplicates. Of these, 35 studies met the inclusion criteria based on PRISMA screening. Key Findings: Phosphorus-efficient legume genotypes, such as common beans, soy beans, chickpeas, and faba beans exude larger quantities of carboxylates, particularly citrate, malate, and oxalate, than non-efficient ones. This way these legumes open up phosphates which are otherwise bound with calcium and aluminum. The exudation of flavonoids and isoflavonoids, especially daidzein and genistein helps to initiate the expression of rhizobial nod genes and improve nitrogen-fixation, although the significance of the flavonoids in activating nodD genes is diminishing. Environmental factors affecting legume exudation such as drought, iron deficiency, aluminum toxicity alter its composition towards gaining osmolytes. Despite the fact that there exists a great diversity between species of legumes, field trials in agriculture are limited.

Research shortcomings: Among the main challenges that remain is fossil-fuel detection almost exclusively based on hydroponic or pot-grown plants, irregularities in collection and equalization procedures for exudates, few connections between various omics genres, few instances of using orphan legumes in research (e.g. lentils, pigeon pea, cowpea), and lack of understanding regarding the effect of exudates on microbial communities living in the soil.

Conclusions: Exudate composition of roots can be considered a genetic component that can be manipulated for improving efficiency of nutrient usage. Researchers are advice to validate their laboratory results in the field, standardize methods of research, and implement results into practical work.