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Results and discussion
Evaluated wheat varieties showed significant genetic variation in
their allelopathic activities ranging between +10.9% (cv. Naveed) --
-3O.2% (cv. PI 4512) in terms of their effect on increase or decrease
in dry weight of Avenue, and +14.0% (cv. Naveed) -- -29.8% (cv. Inia)
on its height. Nine of the evaluated varieties exhibited more than
20% inhibition of the dry weight of the test weed. However, seven
genotypes/varieties e.g. Baiyat, Ghods, Inia, Kavkaz, Khazar-1, PI
4512 and Surkhu caused 25.8, 27.9, 28.3, 25.7, 28.3, 30.2 and 26. 1%
reduction in the dry weight, respectively, indicating the existence
of gene(s) for allelochemical(s) production which in turn inhibit the
growth of A. ludoviciana. Interestingly, Bezostaya-1,
Naveed and Niknejad varieties exerted a positive allelopathy, and
promoted the dry weight of Avenue by 6.6, 10.9 and 10.4%,
respectively (Table 1).
When the number of wheat seeds were increased in the assay boxes, the
extent of inhibition was further enhanced. However, varieties,
Bezostaya-1, Naveed and Niknejad that had positive effects on the
growth of the test weed at low seed density, showed some inhibitory
effect but there was no significant change in the height of wheat
plants when grown with increased density (Table
2). This indicates that the adverse or inhibitory effect of wheat
varieties on Avenue is mediated through the release of chemical
inhibitors (allelopathy) rather than simple competition, otherwise
growth of wheat plants would have also been affected. Furthermore,
none of the wheat varieties showed any autotoxicity.
It is generally presumed that plant species with high growth vigour
are better competitors of weeds and are allelopathically superior.
However, we could not get any correlation between the growth (height)
and allelopathic activity of different wheat varieties. Bezostaya-1,
a variety with maximum height (35.1 cm) caused a 6.6% promotion in
Avenue dry weight whereas Inia with a comparatively less height (26.3
cm) inhibited the test weed by 28.3%. Naveed and Niknejad varieties
had almost similar plant heights with those of Karaj-1, Karaj-3 and
Khazar-1 but the earlier ones had a 10% promotory effect while the
latter varieties inhibited the test weed by 13.0% to 28.3% (Fig.
1). These results indicate that in this case the allelopathic
activity is independent of the early growth vigour of wheat
varieties. In order to see any possible genetic variation in
allelopathic activity of wheat genotypes at different growth stages
(the age of plants) their effect on dry matter and. plant height of
Avenue was studied. All the allelopathic varieties showed a similar
trend of inhibition and the allelopathic activity increased with the
advancing age of wheat seedlings (Fig. 2 and
Fig. 3). Out of the tested wheat genotypes,
Inia, Jenab, Karaj-3, Kavkaz, Khazar-1, PI -4512 and Surkhu showed
more than 60% inhibition in the growth rate of Avenue during 22nd to
26th day (Table 1, Fig.
2 and Fig. 3). PI-4512 caused the maximum
inhibition (74.6%). Inhibition caused by different wheat genotypes
can he attributed to the presence of major gene(s) controlling the
production of allelochemicals like acetic, propionic, butyric,
vanillic, syringic, p-coumaric, p-hydroxybenzoic and hydroxamic
(2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) acids which are known
to cause reduction in plant growth, total biomass and act as
herbicide (Tang and Waiss 1978; Perez 1990; Alsaadawi et al.
1998).
These preliminary results indicating great genetic variability in
bread wheat genotypes for allelochemical production can be of
paramount importance in view of the success achieved with
allelopathic rice (Olofsdotter 1998), in the control of weeds and
reduction in the use of herbicides. These results have also revealed
that it is not the early growth vigour of some of the wheat
genotypes, helpful in the control or reduction of weed population in
wheat field, rather it is the ability of wheat genotypes to produce
certain types of allelochemicals which suppress, inhibit or kill the
competing weed. Possibility of identifying strongly allelopathic
wheat varieties possessing major gene(s) to control weeds under
field. condition lies in screening a large number of genetically
diverse genotypes and their wild relatives. Putnam and Duke (1978)
have suggested that wild types and progenitors of existing crops may
have possessed high allelopathic activity and this character was
reduced or lost as they were hybridised and selected for other useful
characteristics. The facts that allelopathic activity is genetically
inherited (Panchuk and Prutenskaya 1973), and is caused in
synergistic manner involving a complex of chemicals (Rizvi et al.
1992) which is produced under the action of more than one gene
(Alborn et al. 1992) make the wheat allelopathy research very
attractive. However, unfortunately in wheat which is one of the most
widely researched and grown crop throughout the world, the research
on its allelopathic trait is negligible. These studies clearly
indicate the possibilities of exploiting allelopathic trait through
genetic manipulations while breeding new wheat cultivars which will
help the farmers in reducing the cost of production and at the same
time the approach will be environment friendly.
Allelopathic crop varieties may not only lead to the reduction in the
use of herbicides, these could also be used as a 'source' of
allelopathic trait for breeding crops with strong 'weed resistance'.
However, evaluation of a large number of genetically diverse wheat
genotypes, identification of useable gene(s) controlling
allelochemicals production, inheritance, their mode of action and
non-target toxicity, are prerequisite before allelopathic wheat
genotypes can he used commercially. Experiments are in progress on
the above mentioned aspects.
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