Chemical Characteristics and Biological Activity of Organic Substances Extracted from Soils by Root Exudates

using caustic alkaline solution (Stevenson, 1994). Some studies have suggested that HS have only an apparent Plants have evolved with roots in close contact with the solid phase high molecular size, which can be reversibly disrupted of the soil. Therefore, root exudates may be a better medium for by treating humic solutions with low concentrations of extracting low molecular size (LMS) organic fractions than currently mono-, di-, and tricarboxylic acids (Dell’Agnola and used alkaline solutions. Our objective was to compare the chemical and biological activity of LMS extracts using maize (Zea mays L.), Nardi, 1987; Nardi et al., 1996). More recently, numerPicea abies, and Pinus sylvestris root exudates to humic substances ous studies have shown that the amphiphilic properties (HS) extracted with alkaline solution. Gas chromatographic/mass of the organic acids in root exudates can dissociate HS spectrometric (GC/MS) spectra revealed that the LMS fractions had into low molecular size (LMS) and high molecular size a greater variety of fatty acids than the HS. Fourier transform infrared (HMS) structures (Nardi et al., 1997, 2000; Piccolo, 2002, (FT-IR) spectra of LMS fractions also indicated different amounts of 2003). This new interpretation may support the hypothfunctional groups by comparison with HS. The possible biological esis that the conformational behavior of dissolved hurole of LMS fractions with respect to HS was assessed by measuring mus in the rhizosphere, and therefore also the interhormone-like activity and nitrate uptake in P. sylvestris seedlings. action of humic components with plant-root cells, may The LMS fractions from agricultural soil stimulated nitrate uptake be controlled by the presence of root-exuded or miand nitrate reductase (NR) and glutamine synthetase (GS) activities, whereas those from a forest soil increased ammonium uptake, NR, crobe-released organic acids in the soil solution (Piccolo and glutamate dehydrogenase (GDH) activities. The stimulation of et al., 2003). nitrate and ammonium uptake via a NR-GS or NR-GDH metabolic Humic substances are also known to affect plant growth, pattern was consistent with the different chemical composition of the producing a wide spectrum of physiologic effects, alLMS fractions. This indicates LMS fractions in soil have consequential though the overall mechanism explaining how HS interaffects on the plant’s capacity to adapt to different environmental conact with the plant’s physiological system is not clear ditions. (Varanini and Pinton, 2001; Clapp et al., 2001). Nardi et al. (2002a) showed that the LMS humic fraction was small enough to cross the plasma membrane and actively I the last decade, a large amount of information participate in plant metabolism. Other LMS humic frachas accumulated on humic substances (HS), wartions obtained from different substrates, earthworm feranting the creation of an independent science of humic ces, and forest soils showed a hormone-like activity and compounds (Tan, 2003). Two different concepts have positive effects on some metabolic pathways (Pizzeemerged, one claiming humic compounds to be operaghello et al., 2001; Quaggiotti et al., 2004). tional or fake compounds produced by the analytical One of the most prominent physiologic effects of HS extraction procedures and the other considering them on plants concerns N uptake and assimilation (Quagto be natural compounds occurring in soils, rivers, lakes, giotti et al., 2004). For N to be assimilated by plants, oceans, and their sediments (Tan, 2003). The two opposafter NO3 uptake, the nitrate must first be reduced to ing opinions have apparently created some confusion ammonia. This is done by two enzymes: a cytoplasmic as to what HS are and their role in plant metabolism. nitrate reductase (NR), which converts the nitrate to Humic substances are hydrophilic, acidic, and high nitrite, and a chloroplastic nitrite reductase that catalyin molecular weight, ranging from several hundred to ses the reduction of nitrite to ammonium. Ammonium thousands of daltons, and are usually obtained from soils is toxic and does not accumulate anywhere in the plant. by extraction, fractionation, and isolation procedures In higher plants, ammonium is assimilated, at low or normal levels, more through glutamine synthetase (GS) activity than the reductive amination of 2-oxoglutarate, S. Nardi, M. Tosoni, and D. Pizzeghello, Dipartimento di Biotecnologie Agrarie, Università di Padova, Facoltà di Agraria,-Agripolis, Viale catalyzed by glutamate dehydrogenase (GDH). Glutadell’Università 16, 35020 Legnaro, Padova, Italy; M.R. Provenzano mate dehydrogenase serves the main physiologic purand A. Cilenti, Dipartimento di Biologia e Chimica Agroforestale ed pose of deaminating glutamate, thereby supplying suffiAmbientale, Università di Bari, Via Amendola 165/A, 70126 Bari, cient carbon skeletons for the tricarboxylate cycle to Italy; A. Sturaro and R. Rella, Consiglio Nazionale delle Ricerche, function (Robinson et al., 1991). This enzyme also has Corso Stati Uniti 4, I-35127 Padova, Italy. A. Vianello, Dipartimento di Biologia ed Economia Agro-industriale, Sezione di Biologia Vegean important role in plant N metabolism, providing a tale, Università di Udine, Via Cotonificio 108, 33100 Udine, Italy. link between the citrate cycle (via 2-oxoglutarate) and Received 23 Dec. 2004. *Corresponding author (serenella.nardi@ amino acid biosynthesis (via glutamate). unipd.it). Abbreviations: EC, eutric cambisol; FT-IR, Fourier transform infrared Published in Soil Sci. Soc. Am. J. 69:2012–2019 (2005). Soil Fertility & Plant Nutrition spectra; GC/MS, gas chromatographic/mass spectrometric; GDH, glutamate dehydrogenase; GS, glutamine synthetase; HMS, high molecudoi:10.2136/sssaj2004.0401 © Soil Science Society of America lar size; HS, humic substances; LMS, low molecular size; NR, nitrate reductase; RL, rendzic leptosol. 677 S. Segoe Rd., Madison, WI 53711 USA 2012 Published online October 27, 2005