The invasive potential of parthenium weed: A role for allelopathy

Abstract

Parthenium weed (Parthenium hysterophorus L.) is an herbaceous plant that has now invaded over 44 countries worldwide having an impact upon crop and pasture production, natural community biodiversity and human and animal health. Allelopathy is a welldefined physiological trait that is possessed by some crops and native plant species, but especially by invasive weeds. It is a biological phenomenon involving the release of chemicals that may cause a stimulatory, but more often an inhibitory effect upon another plant?s growth, reproduction and survival. Numerous studies have associated the invasive success of parthenium weed with a proficient allelopathic attribute, able to cause an inhibition of either germination, seedling or plant growth, phenological development and/or flowering in a range of agriculturally important plant species, including weeds and some native species. The presumed allelochemicals are produced by a number of plant parts including the stem, leaves, roots, pollen and other floral parts. The two major allelochemical groups that are thought to be involved are the sesquiterpene lactones and the phenolic acids, and both have been found to be released by living plants and decomposing plant residues as well as being present in rhizosphere soil under a parthenium weed population. Different bioassay approaches have been used to help illucidate the allelopathic nature of this weed. These include laboratory assays using aqueous extracts applied directly to germinating seeds, or those evaluating the impact of parthenium weed seedlings on the growth of neighbouring seedlings, both using a liquid medium (relay seeding bioassay) or in an agar medium (plant box bioassay). Other laboratory bioassays have looked at the impact of parthenium weed litter upon germination and seedling growth (sandwich bioassay) while some glasshouse pot tests carrying sand or soil have also been undertaken looking at the impact of aqueous extracts, leaf litter or rhizosphere soil. Each approach has its own advantages and disadvantages. This chapter will evaluate the evidence and decide if allelopathy makes a major or a minor contribution to the invasiveness of this weed. © 2015 Nova Science Publishers, Inc.