Transcript Allelopathic relations in the rhizosphere between

European Weed Research Society
2nd International Conference, Santorini, Greece
September 7-10, 2009
Allelopathic relations in the rhizosphere
between broomrapes and ordinary weeds
Ana Höniges1, Franz Hadacek2, Aurel Ardelean1
& Klaus Wegmann1
1Vasile-Goldiş-University,
Arad, Romania
2University of Vienna, Austria
Allelopathy among plants
The German botanist Hans Molisch coined the term
allelopathy, derived from the Greek „allelon“ (the
other) and „pathein“ (suffer), in order to describe
negative effects on germination, growth and
development of a different plant species. This
definition is unbalanced, because also positive
effects have become known.
Rüdiger Knapp in 1950 created his research field
„Experimental Sociology of Higher Plants“. Knapp
never analyzed and identified chemical compounds.
However, allelopathic effects are brought about by
chemical compounds, which we call allelochemicals.
Allelopathic effects by neighbour plants
Allelopathic effects caused by plants in the
neighborhood are well known. Germination
stimulation of parasitic plants by strigolactones in
root exudates of host or non-host plants belong
to them. Germination inhibition by metabolites,
exuded by roots or washout from leaves are
observed. Among other, rarer metabolites, the
cinnamic acid family (cinnamic, p-coumaric,
ferulic acids) are well known as germination
inhibitors.
Phelipanche/Orobanche suppression
by weeds?
The observation that a weedy part of a heavily
infested tobacco field in Germany was free of
Phelipanche ramosa led us to the suspicion
that allelopathic effects might be involved.
Therefore we carried out analyses of root
exudates.
Methods
Plants were cultivated in Vermiculite, then washed out
and transferred into hydroculture in half-concentrated
Murashige/Skoog medium. The water from the
rhizosphere was collected during several days,
concentrated in a rotation evaporator (50 °C), then preseparated into a hydrophilic phase (containing mainly
sugars, polyols and organic acids) and a hydrophobic
phase by an Amberlite XAD1180 column.
This hydrophobic phase was analyzed by HPLC
separation on a Dionex HPLC instrument equipped
with a UVD340U photodiode array detector and data
analysis. The absorption spectrum for each peak
between 220-590 nm was recorded and compared with
the data bank. The data bank proposes compounds
for spectra with 95% or more identity with known
compounds.
Rising plants in Vermiculite
Teucrium chamaedrys
Galium verum
Centaurea scabiosa
Cirsium vulgare
Hedera helix root exudate
Benzoic acid
Berberis vulgaris root exudate
Benzoic acid
Artemisia campestris root exudate
Benzoic acid
Pelargonium zonale root exudate
Benzoic acid
Root exudate of associated flora
Benzoic acid
Results of the analyses
A metabolite is identified, if the elution time in
the HPLC separation and the absorption
spectrum are matching with that compound.
Members of the cinnamic acid family have not
been identified.
Surprisingly, benzoic acid was a major
component in many root exudates (see list).
Benzoic acid in root exudates
Artemisia campestris
Berberis vulgaris
Carduus personata
Centaurea scabiosa
Cirsium vulgare
Galium verum
Hedera helix
Pelargonium zonale
Teucrium chamaedrys
Thymus pannonicus
Examples with no or little benzoic acid in root
exudates were Lycopersicon esculentum,
Medicago sativa, Salvia aethiopis and Urtica
dioica.
Germination experiments
Germination experiments have been carried out
under standard conditions with seeds of
Phelipanche ramosa and Orobanche cumana,
two important weedy broomrape forms.
Germination stimulant was the synthetic
strigolactone analogue GR 24 (10-6 M).
The following potential germination inhibitors were
COOH
COOH
added (10-3 M).
COOH
O
syringic acid
O
COOH
H3CO
O
OCH 3
OH
O
O
GR 24
CH3
t-cinnamic acid
OCH 3
OH
ferulic acid
benoic acid
O. cumana stimulated by GR 24
O. ramosa stimulated by GR 24
Results of the germination experiments
Stimulant
Dist. water (blank)
GR 24
GR 24 + cinnamic acid
GR 24 + ferulic acid
GR 24 + syringic acid
GR 24 + benzoic
Phelipanche Orobanche
ramosa
cumana
0
0
96
85
5
23
3
22
5
7
20
40
Conclusions
In previous studies the role of ordinary weeds for
seed bank setup and spreading of Phelipanche
ramosa has been studied, in particular their role as
associated hosts in colza (Gibot-Leclerc et al. 2003).
Suicide germination by germination stimulants in
weed root exudates would lead to parasitic seed
bank reduction, if the weeds are not parasitized.
We report here about a potential for reducing
parasitic weeds by germination inhibition by
benzoic acid exuded by common weeds (or crop
plants).