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Seminar

Analytical Strategies for Ecologically Active Phytochemicals

Time:
Date:
Venue:
Presenter: 		3.30pm - 4.30pm
Thursday 14 July, 2011
Conference Room, Wagga Wagga Agricultural Institute
Jeffrey D. Weidenhamer
Department of Chemistry,
Ashland University, Ashland, OH 44805 USA 
E: jweiden@ashland.edu

Plants produce a wide variety of highly phytotoxic chemicals, some of which have activities comparable to synthetic herbicides. The possibility that plants exert direct chemical, or allelopathic, effects on neighboring plants as well as on soil microflora has attracted considerable research interest. The success of certain invasive plants has been attributed to phytotoxic root exudates. Other work suggests that in conifer forests, monoterpenes released from pine roots and litter inhibit nitrogen mineralization and nitrification.  But there is a general lack of information about the spatial and temporal dynamics of allelochemicals in the soil that is a barrier to testing hypotheses of allelopathic effects.

When soils beneath suspected allelopathic plants are analyzed, concentrations are typically low, and this has been cited as evidence that these compounds do not play a significant role in plant-plant interactions. However, static concentrations in the environment reflect the current balance of input versus output rates for a compound. Because plant roots compete with microorganisms and other processes that remove allelochemicals from soil solution, flux rates are likely to be a key component of toxicity.

Polydimethylsiloxane (PDMS) sorbents, which are widely used in analytical techniques such as solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE), are being applied to measure the dynamics of thiophenes produced by marigold roots in soil. Both PDMS-coated wires and coils of PDMS microtubing (silicone tube microextraction, STME) placed directly into soil have been successfully used to measure thiophenes in soil. STME allows repeated sampling of soil without disturbance of plant roots. The two ends of the tubing remain out of the soil so that compounds can be recovered by methanol elution for HPLC analysis. Results show the distribution of marigold allelochemicals in the root zone is spatially heterogeneous and dynamic over time. Given the high potency of these thiophenes in bioassays, the amounts recovered can readily be conceived to be biologically active. STME should be broadly applicable to the analysis of lipophilic root exudates, and thus provides a new means to test hypotheses about the role of root exudates in plant-plant interactions. 

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