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

Abstract Stimulatory effects of humic substances (HS) on plant growth have been observed and widely documented. Studies have often shown positive effects on seed germination, root initiation and total plant biomass. The consistency of these observations has been uncertain, predominantly due to the lack of understanding of the plant growth promotion mechanism. Often these effects have been attributed to a direct effect of plant growth hormones; whereas in other instances the term “hormone-like activity” has been used to describe the plant growth stimulation (Chen and Aviad, Humic Substances in Soil and Crop Sciences: Selected Readings, American Society of Agronomy, Soil Science Society of America, 1990; Nardi et al., Humic Substances in Terrestrial Ecosystems, Elsevier Science B.V., 1996). Yet, investigators have been unable to prove that plant growth regulators are present in HS preparations, or the evidence provided remains unconvincing. An alternative hypothesis suggesting that growth enhancement of plants grown in nutrient solution (NS) containing HS is the result of improved micronutrient availability, Fe in particular, has been postulated and tested in the present study. Nutrient solutions containing N, P, K, Ca, Mg, S, B, Mo, Cu, Mn, Zn, and Fe at concentrations considered to be optimal for plant growth were tested for solubility of the Fe, Zn, and Mn, 7 days after preparation. In addition to control solutions at pH values of 5, 6, 7, and 7.5, 0 to 200 mg L−1 of leonardite humic acid (HA) were added to the solutions and they were tested for Fe and Zn solubility. The HA greatly enhanced the maintenance in solution of Fe, in all the tested solutions, and Zn at pH 7.5. Mn mostly remained in solution in its inorganic forms. Plant growth experiments were carried out on both dicotyledonous plants (melons and soybean) and monocotytedonous Poaceae plants (ryegrass), due to the major difference in their Fe uptake mechanism. Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts. The addition of HA or fulvic acid (FA) without addition of Fe, and Zn resulted in partial growth enhancement and correction of Fe deficiency, or none of the two, in the various experiments. This suggests that the growth enhancement effect observed in solutions containing Fe, Zn, and HS was related to the micronutrients rather than to phytohormones. However, the addition of Fe, Zn and either EDTA, HA or FA resulted in healthy, chlorophyll rich plants and enhanced growth, thereby providing evidence that improved Fe, and possibly Zn nutrition is a major mechanism of plant growth stimulation by HS. The use of the term hormone-like activity could be the result of the similarity of the physiological effects obtained in plants enjoying sufficient supply of Fe and Zn.

[1]  M. Schnitzer,et al.  Effects of a soil fulvic acid on the growth and nutrient content of cucumber (Cucumis sativus) plants , 1981, Plant and Soil.

[2]  V. Römheld,et al.  Uptake of 59Fe from soluble 59Fe-humate complexes by cucumber and barley plants , 2002, Plant and Soil.

[3]  R. Pinton,et al.  Water-extractable humic substances enhance iron deficiency responses by Fe-deficient cucumber plants , 1999, Plant and Soil.

[4]  V. Römheld,et al.  Strategies of plants for acquisition of iron , 1994, Plant and Soil.

[5]  Y. Chen,et al.  Manure and peat based iron-organo complexes , 2004, Plant and Soil.

[6]  Y. Chen,et al.  Manure and peat based iron-organo complexes , 2004, Plant and Soil.

[7]  V. Römheld,et al.  Solubilization of iron by water‐extractable humic substances , 2000 .

[8]  F. Reniero,et al.  Chemical and biochemical properties of humic substances isolated from forest soils and plant growth , 2000 .

[9]  S. Cutrupi,et al.  IAA detection in humic substances , 1998 .

[10]  M. Shenker,et al.  The Role of Ligand Exchange in the Uptake of Iron from Microbial Siderophores by Gramineous Plants , 1996, Plant physiology.

[11]  Yona Chen.  Organic Matter Reactions Involving Micronutrients in Soils and Their Effect on Plants , 1996 .

[12]  G. Concheri,et al.  Biological Activity of Humus , 1996 .

[13]  David E. Crowley,et al.  Evidence for direct utilization of a siderophore, ferrioxamine B, in axenically grown cucumber , 1993 .

[14]  Y. Hadar,et al.  Utilization by tomatoes of iron mediated by a siderophore produced by Rhizopus arrhizus , 1992 .

[15]  Yona Chen,et al.  Effect of humic substances on plant growth , 1988 .

[16]  V. Römheld,et al.  Different strategies in higher plants in mobilization and uptake of iron , 1986 .

[17]  Walter J. Horst,et al.  Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants , 1986 .

[18]  F. J. Stevenson,et al.  Soil organic matter interactions with trace elements , 1986 .

[19]  B. Steinitz,et al.  The effect of various iron-containing fertilizers on growth and propagation of Gladiolus grandiflorus , 1982 .

[20]  Phillip Barak,et al.  The evaluation of iron deficiency using bioassay-type test , 1982 .

[21]  Yona Chen,et al.  Remedy of lime‐induced chlorosis with iron‐enriched muck , 1982 .

[22]  V. Römheld,et al.  Iron deficiency stress induced morphological and physiological changes in root tips of sunflower , 1981 .

[23]  D. Vaughan,et al.  Some effects of humic acid on cation uptake by parenchyma tissue , 1976 .