Allelochemicals Biologica... - Name

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ANA LUISA ANAYA
to assist with molecular studies conducted in today’s expanding field. Such instrumental
techniques are herein recommended as economically efficient means of advancing
the rigor of allelopathy research and assisting the development of a better understanding
of the chemical basis for the allelopathy phenomenon (Haig, 2001).
4. THE MODE OF ACTION OF ALLELOCHEMICALS
Allelopathic chemicals alter plant growth and development by a multiplicity of actions
on physiological processes because there are hundreds of different structures and many
of the compounds have several phytotoxic effects. Whole plants bioassays and
physiological tests designed to use a small quantity of compound are keys to strategies
for elucidating mechanisms of action. Insight into the action of the responsible
chemicals is critical to a more complete explanation of allelopathy and to its application
for improving crop production. Unfortunately, we still find that linkages between
inhibition of growth and the corresponding physiological mechanism are elusive. A
major part of the problem is the array and diversity of allelochemicals. Several hundred
different compounds have been identified, and we speculate that many others will be
eventually being discovered. Most instances of allelopathic inhibition are the result of
the simultaneous action of several compounds, and often these include compounds
whose chemistry is divergent (Einhellig, 2002). The visible evidence in bioassays is
that many phenolics, quinones, sesquiterpene lactones, alkaloids, and others alter
root morphology. Although the cell membrane is an early interface with
allelochemicals, relatively little attention has been given to membrane-related effects
and their molecular targets.
Effects of a single allelochemical or their mixtures on physiological processes
include disruption of membrane permeability (Galindo, et al., 1999), ion uptake (Yu
and Matsui, 1997), inhibition of electron transport in both the photosynthesis and
respiratory chains (Calera et al., 1995a; Abrahim, et al., 2000), alteration of enzymatic
activity (Politycka, 1999, Romagni, et al., 2000), and inhibition of cell division (Cruz-
Ortega et al., 1988; Anaya and Pelayo-Benavides, 1997). In other examples, bioactivitydirected
fractionation of the methanol extract of the roots of Ratibida mexicana resulted
in the isolation of two bioactive sesquiterpene lactones, isoalloalantolactone and elema-
1,3,11-trien-8,12-olide. Both compounds caused a significant inhibition of the radicle
growth of Amaranthus hypochondriacus and Echinochloa crus-galli, exerted moderate
cytotoxicity activity against three different solid tumour cell lines and inhibited the
radial growth of three phytopathogenic fungi. Isoalloalantolactone also caused the
inhibition of ATP synthesis, proton uptake, and electron transport (basal,
phosphorylating and uncoupled) from water to methylviologen therefore acting as a
Hill’s reaction inhibitor.The lactone inhibited only photosystem II (Calera et al., 1995b).
Other compounds studied by Calera et al., (1995c) were the resin glycoside mixture
from Ipomoea tricolor. They tested its effect on seedling growth and plasma membrane
H + -ATPase activity in Echinochloa crus-galli. The resin glycoside mixture as well as
Tricolorin A, the main compound in the mixture, inhibited the activity of the plasma
membrane ATPase. In other studies, Cruz-Ortega et al. (1998) observed plasma