Interactions between plant-root exudates and soils in extracting humic-like substances

Plants have evolved with roots in close contact with the solid phase of the soil therefore organic acid root exudates, in plant–root–microbial interactions, have attracted much interest not so much in terms of carbon source but as ‘signals’ for recognition or as precursors of phytohormone production (Marschner, 1995). Recent studies have demonstrated that organic acids (fumaric and succinic acids) occurring in the root exudates can dissociate HS in low and high molecular weight structures. This interpretation may support the hypothesis that the conformational behavior of dissolved humus in the rhizosphere, and therefore also the interaction of humic components with plant-root cells, may be controlled by the presence of root-exuded or microbe-released organic acids in the soil solution (Piccolo et al., 2003). Therefore, root exudates may be a better medium for extracting low molecular size (LMS) organic fractions than currently used alkaline solutions. Our objective was to compare the chemical and biological activity of LMS extracts using maize (Zea mays L.), and two forest species (Picea abies and Pinus sylvestris) root exudates to humic substances extracted by KOH (Stevenson, 1994). The identification of some organic acids species present in extracts have been investigated by GC/MS technique and the biological activity of the LMS humic/organic substances extracted was evaluated by determining their hormone-like activity. Two forest species (Picea abies Karst. and Pinus sylvestris L.) and two commercial maize hybrids (Zea mays L.: cultivars Mytos and Samantha; Dekalb, Italy) were used for this study. The root exudates were collected from maize seedlings treated with sterile Hoagland’s solution (Hoagland and Arnon, 1950), and forest plants raised in sterile conditions as described in a previous work (Nardi et al., 1997). Root exudates were determined as reported in Nardi et al. 2002. The LMS humic/organic substances were obtained by treating 2 g of an Eutric cambisol (EC) and a Rendzic leptosol (RL) (A horizons) with 20 ml of water or 20 ml of root exudates and shaken them at room temperature for 16 h under a N2 atmosphere. The suspensions were centrifuged at 10 °C and 5000 g for 30 min and the supernatants (extracts) were analyzed for total carbon. We ascertained that agrarian and forest seedlings release different types of exudates in the rhizosphere, and they have higher extracting abilities towards carbon then water when in contact with poorer substrates. Maize exudates are endowed with fumaric and succinic acids, while the forest ones exhibited only large amounts of succinic acid. GC/MS spectra revealed that the LMS fractions had a greater variety of fatty acids than the HS. The behaviour of these species could be related to their different environmental conditions (Ingestad, 1960). All soil extracts by exudates exhibited a hormonal activity that was not present in either water extracts or in the original exudates. The extracts from agrarian soil exhibited a higher hormone-like activity with respect to the extracts from forest soil. Concerning phenol content in the humus extract obtained by using the exudates, benzoic acid and in minor extent phtalic acid resulted to be the more present. Analysis of the phenolic pool demonstrated specificity on the extracted molecules not directly correlated with phenols present in pure exudates. Studies have shown that phenolic compounds have been found to be highly effective in plant defence against pathogens, nematodes, phytophagous insects (Rich et al., 1977; Russel et al., 1978; Dakora, 1995; Dakora and Phillips, 1996). In particular, benzoic acid is one of the most active phenolic compound inhibitors of fungi (Ravin et al., 1989; Weidenborner et al., 1989 & 1990). This paper reconfirms the important role of exudates in breaking out humic associations and in releasing hormone-like activities and pest inhibitors, suggesting the appropriate combination of factors for the best use of the humic fraction as plant growth regulators as well as plant pest control.