Prasanta C. Bhowmik

Challenges and opportunities in implementing allelopathy for natural weed management

Abstract Plants may favorably or adversely affect other plants through allelochemicals, which may be released directly or indirectly from live or dead plants (including microorganisms). Due to increase in the number of herbicide-resistant weeds and environmental concerns in the use of synthetic herbicides, there have been considerable efforts in designing alternative weed management strategies. The conventional synthetic herbicides are becoming less and less effective against the resistant weed biotypes. The objective of this article is to examine the role of allelopathic cover crops/crop residues, natural compounds, and allelopathic crop cultivars in natural weed management. Numerous examples of employing crop residues, cover crops and allelopathic crop cultivars in weed management are provided. Although we cannot eliminate the use of herbicides, their use can be reduced by exploiting allelopathy as an alternate weed management tool for crop production against weeds and other pests.

Colloidal behavior of aluminum oxide nanoparticles as affected by pH and natural organic matter.

The colloidal behavior of aluminum oxide nanoparticles (NPs) was investigated as a function of pH and in the presence of two structurally different humic acids (HAs), Aldrich HA (AHA) and the seventh HA fraction extracted from Amherst peat soil (HA7). Dynamic light scattering (DLS) and atomic force microscopy (AFM) were employed to determine the colloidal behavior of the NPs. Influence of pH and HAs on the surface charges of the NPs was determined. zeta-Potential data clearly showed that the surface charge of the NPs decreased with increasing pH and reached the point of zero charge (ZPC) at pH 7.9. Surface charge of the NPs also decreased with the addition of HAs. The NPs tend to aggregate as the pH of the suspension approaches ZPC, where van der Waals attraction forces dominate over electrostatic repulsion. However, the NP colloidal suspension was stable in the pHs far from ZPC. Colloidal stability was strongly enhanced in the presence of HAs at the pH of ZPC or above it, but in acidic conditions NPs showed strong aggregation in the presence of HAs. AFM imaging revealed the presence of long-chain fractions in HA7, which entangled with the NPs to form large aggregates. The association of HA with the NP surface can be assumed to follow a two-step process, possibly the polar fractions of the HA7 sorbed on the NP surface followed by entanglement with the long-chain fractions. Thus, our study demonstrated that the hydrophobic nature of the HA molecules strongly influenced the aggregation of colloidal NPs, possibly through their conformational behavior in a particular solution condition. Therefore, various organic matter samples will result in different colloidal behavior of NPs, subsequently their environmental fate and transport.

Phenolic-linked variation in strawberry cultivars for potential dietary management of hyperglycemia and related complications of hypertension

Fruit extracts of different strawberry cultivars were evaluated for their potential to contribute to the dietary management of hyperglycemia-linked to type 2 diabetes and related hypertension. In vitro inhibition of alpha-amylase, alpha-glucosidase, and angiotensin-1-converting enzyme (ACE) activity was evaluated using fruit extracts and correlated to phenolic content and antioxidant activity. There were significant differences between cultivars in both phenolic-linked antioxidant activity and inhibitory activity for the targeted disease relevant enzymes. Honeoye, Idea, and Jewel cultivars exhibited moderate alpha-amylase inhibition. Strawberry cultivars, in general, exhibited uniform alpha-glucosidase inhibition with Ovation having the highest inhibitory activity. Water extracts of Jewel and Ovation cultivars had moderate ACE inhibition compared to low inhibition observed in other cultivars. Strawberry cultivars with combined inhibitory potential against alpha-glucosidase and ACE and with moderate or low alpha-amylase inhibitory potential could be targeted for potential management of hyperglycemia-linked type 2 diabetes and related complication of hypertension.

Colloidal Stability of Al2O3 Nanoparticles as Affected by Coating of Structurally Different Humic Acids

The colloidal stability of three structurally different humic acid (HA)-coated Al(2)O(3) nanoparticles (HAs-Al(2)O(3) NPs) was studied in the presence of Ca(2+). HAs were obtained after sequential extractions of Amherst Peat Soil. Highly polar HA1-coated Al(2)O(3) NPs exhibited strong aggregation in the presence of Ca(2+). HA3 and HA7-coated NPs showed weaker aggregation due to their increased aliphaticity and low polarity. HA7-Al(2)O(3) NPs displayed the weakest aggregation behavior even at relatively high Ca(2+) concentration. The inverse stability ratio (alpha = 1/W) was the lowest for HA7-Al(2)O(3) NPs, reflecting that strong steric stabilization enhanced colloidal stability. Atomic force microscopy (AFM) of pure Al(2)O(3) NPs on Ca(2+)-saturated mica clearly demonstrated significant aggregation following classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model for hard spheres. On the contrary, weakly polar HA fraction produced approximately 10 nm thick corona of adsorbed layer around each Al(2)O(3) NP, thus stabilizing coated NP suspension through steric effect. Under alkaline conditions and at low ionic strength, adsorbed HA chains swelled, increasing their osmotic potential, which in turn resulted in stabilization of the colloids. Inherent structural variations of natural organic matter (NOM) played a significant part in colloidal stability of the coated NPs. Thus, development of sterically stabilized NPs may have potential application for water remediation in marine and high salinity conditions.

Potential Use of Halophytes to Remediate Saline Soils

Salinity is one of the rising problems causing tremendous yield losses in many regions of the world especially in arid and semiarid regions. To maximize crop productivity, these areas should be brought under utilization where there are options for removing salinity or using the salt-tolerant crops. Use of salt-tolerant crops does not remove the salt and hence halophytes that have capacity to accumulate and exclude the salt can be an effective way. Methods for salt removal include agronomic practices or phytoremediation. The first is cost- and labor-intensive and needs some developmental strategies for implication; on the contrary, the phytoremediation by halophyte is more suitable as it can be executed very easily without those problems. Several halophyte species including grasses, shrubs, and trees can remove the salt from different kinds of salt-affected problematic soils through salt excluding, excreting, or accumulating by their morphological, anatomical, physiological adaptation in their organelle level and cellular level. Exploiting halophytes for reducing salinity can be good sources for meeting the basic needs of people in salt-affected areas as well. This review focuses on the special adaptive features of halophytic plants under saline condition and the possible ways to utilize these plants to remediate salinity.

The Biology of Invasive Alien Plants in Canada. 5. Polygonum cuspidatum Sieb. & Zucc. (= Fallopia japonica (Houtt.) Ronse Decr.)

Polygonum cuspidatum (Japanese knotweed) is an introduced perennial geophyte in the buckwheat family (Polygonaceae). The phytogeographic distribution of P. cuspidatum in North America suggests a large number of intentional introductions via ornamental plantings from 1870 to 2000, followed by secondary spread from these foci. This species is most pernicious along riparian corridors and road and railroad rights-of-way, reducing visibility, displacing native species, negatively affecting native wildlife, and causing alterations in natural hydrologic processes. Although non-hybrid seed recruitment has not been observed in Europe because of the presence of male-sterile clones only, dispersal of seeds and stem and rhizome fragments by flowing water does occur in North America and populations are readily established from these sources. The primary means of local and regional range expansion is human-mediated transport of rhizome-infested soil. Hybridization is common with the congener P. sachalinense in the intr...

Bioavailability of Allelochemicals As Affected by Companion Compounds in Soil Matrices

Multicompound allelochemical interactions were studied using Centaurea maculosa as a model source to understand how the bioavailability of complex allelochemical mixtures is modified in soil-microbial systems. Litter decomposition of C. maculosa in sandy loam soil yielded five phenolic acids, namely, hydroxybenzoic, vanillic, protocatechuic, p-coumaric, and ferulic acids. The degradation studies were conducted by exogenous application of catechin, the primary allelochemical exuded by C. maculosa, and the phenolic acid cosolutes in a sandy loam and silt loam soil. Compared to a single-solute system, in a multisolute system the persistence of individual allelochemicals was significantly increased in both soils. Oxidation and sorption were primarily involved in the disappearance of allelochemicals. Mass spectrometric data showed that catechin rapidly underwent polymerization to form procyanidin dimer both in soil and in bioassay medium, resulting in reduced persistence and phytotoxicity. Hence, catechin phytotoxicity could occur only under conditions that would inhibit these condensation reactions. This study clearly demonstrates that various soil mechanisms including competitive sorption and preferential degradation would increase the persistence of allelochemical mixtures in a soil matrix.

Sorption and Fractionation of a Peat Derived Humic Acid by Kaolinite, Montmorillonite, and Goethite

Sorption of humic acid (HA) on mineral surfaces has a profound interest regarding the fate of hydrophobic organic contaminants (HOCs) and carbon sequestration in soils. The objective of our study is to determine the fractionation behavior of HA upon sorption on mineral surfaces with varying surface properties. HA was coated sequentially on kaolinite (1:1 clay), montmorillonite (2:1 clay), and goethite (iron oxide) for four times. The unadsorbed HA fractions were characterized by elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and solid state 13C nuclear magnetic resonance spectroscopy (NMR). The mineral-HA complexes were characterized by DRIFT. Polarity index [(N+O)/C] revealed higher polarity of the unadsorbed HA fractions after coating on kaolinite, reflecting that relatively higher polarity fractions of HA remain unadsorbed. Sorption of aliphatic alcohol fraction along with carbohydrate was prominent on kaolinite surface. DRIFT results of the unadsorbed HA fractions indicated more sorption of aliphatic moieties on both kaolinite and montmorillonite. DRIFT results of the unadsorbed HA fractions after sorption on kaolinite and goethite showed the sorption of the proteinaceous fractions of HA. The HA fractions obtained after coating on goethite showed significant sorption of carboxylic moieties. The results mentioned above comply reasonably well with the DRIFT spectra of the mineral-HA complexes. 13C NMR results showed higher sorption of anomeric C on kaolinite surface. Higher sorption of paraffinic fraction was observed on montmorillonite. NMR data inferred the sorption of carboxylic moieties on goethite surface. Overall, this study showed that aliphatic moieties of HA preferentially sorbed on kaolinite and montmorillonite, while carboxylic functional groups play a significant role in sorption of HA on goethite. The sorbed fractions of HA may modify the mineral surface properties, and thus, the interaction with organic contaminants.

Dual purpose secondary compounds: phytotoxin of Centaurea diffusa also facilitates nutrient uptake

Traits that allow more efficient foraging for a deficient resource could increase the competitiveness of a species in resource-poor habitats. Considering the metal-nutrient mobilization ability of many allelochemicals, it is hypothesized that, along with the reported toxic effect on the neighbors, these compounds could be directly involved in resource acquisition by the allelopathic plant. Using nutrient manipulation treatments in hydroponic culture, this hypothesis was tested using Centaurea diffusa, an invasive species that produces the putative phytotoxin 8-hydroxyquinoline (8HQ). The exudation of 8HQ by C. diffusa was very limited and transient. It was further shown that: C. diffusa utilizes 8HQ for its own acquisition of iron, a nutrient deficient in many of its alkaline, invaded habitats; there possibly exists a unique mechanism for the uptake of the 8HQ-complexed iron (Fe) in C. diffusa, which is novel to the nongraminaceous species; although phytotoxic at very low concentrations, the toxic effect of 8HQ showed a conditional response in the presence of metals, and was significantly reduced when 8HQ was complexed with copper (Cu) and Fe. This study, in addition to elucidating one of the possible adaptive mechanisms conferring competitive advantage to C. diffusa, also outlines measures to negate the phytotoxicity of its putative allelochemical. The results indicate that the exudation of 8HQ by C. diffusa could be primarily for nutrient acquisition.

Sorption of isoxaflutole by five different soils varying in physical and chemical properties

Isoxaflutole is a new pre-emergence corn herbicide which controls both grass and broadleaf weeds. Experiments were performed in the laboratory to study the sorption of isoxaflutole in five different soils (Moorhead, MN; East Monroe, CO; Ellendale, MN; South Deerfield, MA; and Chelsea, MI) using the batch equilibration technique. Total initial isoxaflutole solution concentrations for each soil were 0.05, 0.15, 0.3. 0.8, 1.5, 2.0 and 4.0 mg litre−1. Analysis of [ring-14C] isoxaflutole was performed using liquid scintillation counting, and sorption data were fitted with the Freundlich model. Isotherms of isoxaflutole in all the soils were non-linear as depicted by the exponent (n   Moorhead, MN > East Monroe, CO > South Deerfield, MA > Ellendale, MN. The negative magnitude of Gibbs free energy of sorption (ΔG x) indicates the spontaneity of the given sorption process in the soils from Moorhead, MN; East Monroe, CO and Chelsea, MI. © 1999 Society of Chemical Industry