Abstract
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.
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Claims 10
Claim · model-written summary
Humic acid maintained iron in solution under all tested solution conditions.
Not reportedTaxon
Verbatim
e0“The HA greatly enhanced the maintenance in solution of Fe, in all the tested solutions, and Zn at pH 7.5.”
Claim · model-written summary
Humic acid maintained zinc in solution at pH 7.5.
Not reportedTaxon
Verbatim
e0“The HA greatly enhanced the maintenance in solution of Fe, in all the tested solutions, and Zn at pH 7.5.”
Claim · model-written summary
Manganese mostly remained dissolved in inorganic forms.
- Outcome
- Mne0
Not reportedTaxonDirection
Verbatim
e0“Mn mostly remained in solution in its inorganic forms.”
Claim · model-written summary
Plants without iron showed severe iron deficiency.
- Taxon
- PlantsViridiplantaee0
- Treatment
- absence of Fee0
- Outcome
- Fe deficiencye0
Not reportedComparatorDirection
Verbatim
e0“Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts.”
Claim · model-written summary
Mineral iron salts only partly corrected iron deficiency.
Verbatim
e0“Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts.”
Claim · model-written summary
Humic acid or fulvic acid without added iron and zinc gave variable partial growth enhancement.
- Taxon
- plantsViridiplantaee1
- Treatment
- HA or fulvic acid (FA) without addition of Fe, and Zne0
- Outcome
- growth enhancemente0
- Direction
- partiale0
- Qualifier
- or none of the two, in the various experimentse0
Not reportedComparator
Verbatim
e0“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.”
t0“plant growth”
e1“Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts.”
Claim · model-written summary
Humic acid or fulvic acid without added iron and zinc variably corrected iron deficiency.
- Taxon
- plantsViridiplantaee1
- Treatment
- HA or fulvic acid (FA) without addition of Fe, and Zne0
- Outcome
- Fe deficiencye0
- Direction
- partiale0
- Qualifier
- or none of the two, in the various experimentse0
Not reportedComparator
Verbatim
e0“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.”
t0“plant growth”
e1“Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts.”
Claim · model-written summary
Iron and zinc with EDTA, humic acid or fulvic acid yielded healthy chlorophyll-rich plants.
- Taxon
- plantsViridiplantaee0
- Treatment
- Fe, Zn and either EDTA, HA or FAe0
- Outcome
- healthy, chlorophyll rich plantse0
Not reportedComparatorDirection
Verbatim
e0“the addition of Fe, Zn and either EDTA, HA or FA resulted in healthy, chlorophyll rich plants and enhanced growth”
t0“plant growth”
Claim · model-written summary
Iron and zinc with EDTA, humic acid or fulvic acid enhanced plant growth.
- Taxon
- plantsViridiplantaee0
- Treatment
- Fe, Zn and either EDTA, HA or FAe0
- Outcome
- growthe0
- Direction
- enhancede0
Not reportedComparator
Verbatim
e0“the addition of Fe, Zn and either EDTA, HA or FA resulted in healthy, chlorophyll rich plants and enhanced growth”
t0“plant growth”
Claim · model-written summary
The authors identify improved iron and possibly zinc nutrition as a major growth stimulation mechanism.
- Taxon
- plantsViridiplantaee1
- Compound
- Fe, and possibly Zne0
- Outcome
- plant growth stimulatione0
- Qualifier
- major mechanisme0
Verbatim
e0“improved Fe, and possibly Zn nutrition is a major mechanism of plant growth stimulation by HS”
t0“plant growth”
e1“Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts.”
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