Allelopathy in Lucerne (Medicago sativa L)

Allelopathy, the chemical interaction between plants, can influence plant community structure. Allelochemical compounds can be distributed to the environment in different ways, with root exudation very important among them. This thesis reports research to examine the allelopathic potential of lucerne (Medicago sativa L.) on other pasture species growing in association.

The Equal Compartment Agar Method (ECAM) was used to evaluate lucerne
seedling allelopathy on annual ryegrass ( Lolium rigidum) from a collection of 40
lucerne genotypes. Significant differences were found among lucerne genotypes as measured by an inhibition index which ranged from 3.5% to 45.5% inhibition of annual ryegrass root growth relative to the control. Lucerne genotypes were classified from strong (eg. cv. Sardi 5) to weak (eg. cv. SARDI 7 Series 2) in respect of their impact on annual ryegrass root growth. Annual ryegrass seed germination was also reduced in the presence of strong allelopathic lucerne cultivars. A preliminary analysis of the chemical basis of lucerne allelopathy was undertaken using a metabolomics approach. The sensitive QTOF-MS technique was employed to identify root allelochemicals produced by the extreme and moderate allelopathic lucerne genotypes identified in the laboratory experiments. Chemical analysis of root extracts and root exudates of each genotype was conducted on 14 day-old-seedlings. A total of 1879 and
1895 mass signals were recorded in the root tissue and exudates of the lucerne
genotypes respectively. Distributions of total metabolites differed between the
genotypes. 2-Oxoadipate and 2-hydroxyphenylacetic acid and 2'-deoxyuridine 5'- mono-phosphate were found only in the root exudates of weakly allelopathic
genotypes Titan9 and SARDI 7 Series 2 respectively. These may play a role as
buffering agents in weakly allelopathic lucerne genotypes against annual ryegrass. These findings suggest a genetic basis for allelopathic capability between lucerne genotypes.

The effects of annual ryegrass (ARG) root exudates on lucerne growth and
nodulation in laboratory and glasshouse experiments were also investigated. Lucerne genotypes showed a range of sensitivities to annual ryegrass root exudates. In the laboratory, at the highest density of annual ryegrass, 30 seedlings per beaker, lucerne root inhibition was lower for Sardi 5 and L70 , and higher for Cropper 9.5, Haymaster7, Force 10, Genesis, WL 925HQ, Titan9 and Stamina GT6. At this density, annual ryegrass reduced root growth of Sardi 5 and L70 to 31%and 30% of their respective controls; whereas Cropper 9.5, Haymaster7, Force 10, Genesis, WL 925HQ, Titan9 and Stamina GT6 root length was restricted to 14 - 18% of their control. The presence of ryegrass also reduced nodule numbers of lucerne genotypes in both laboratory and
glasshouse conditions.

Allelopathic interference between lucerne genotypes and annual pasture legumes [subterranean clover (Trifolium subterraneum) Leura and Riverina, biserrula (Biserrula pelecinus) and French serradella (Ornithopus sativus)] was assessed, the effects varying depending on the donor and receiver plant species. Among the 10 lucerne genotypes chosen for the experiment, two genotypes, L70 and L56 stimulated Riverina and Leura subterranean clover root length; L70 increased Riverina and Leura root lengths by 40% and 30% respectively. The other lucerne genotypes inhibited root lengths of all four annual legumes. The genotype Cropper 9.5 produced the stronger inhibition to the root length of annual legumes compared with Haymaster7 and sardi 5, with Riverina
subterranean clover and Margurita showing the largest reduction of 24% and Leura the least with 14% reduction compared with their control. Root exudates of all four annual legumes inhibited root length of five lucerne genotypes by at least 49%. L70 was the most tolerant genotype to the allelopathic effects of root exudates of annual legumes. Other growth parameters of annual legumes and lucerne genotypes were also affected depending on the combination of the donor and the receiver species.

The effects of ARG and legume (lucerne and subterranean clover) mixed
combinations were examined. At the density of 20 seedlings /beaker, lucerne genotypes, L56 and L70 increased subterranean clover (cv. Riverina) root length by 59% and 48% respectively; whereas ryegrass root length was decreased by 44% and 39% by SARDI Five and Titan9 respectively under laboratory conditions. In glasshouse conditions, the performance of the middle species in mixed culture of ARG and legume was compared with its respective control grown alone for above ground parameters. Of the six combinations, L70-ARG-Riverina and SAR5-ARG-Riverina produced the largest annual ryegrass leaf area, leaf dry weight and shoot dry weight compared with annual ryegrass monoculture. The legume in the mixed pastures developed fewer nodules than when grown alone.

It is concluded that substantial genetic variability exists in lucerne genotypes in
respect of allelopathic capability. The extent to which allelopathy influences total
sward production will also depend on the associated pasture species and their
allelopathic capabilities. More research is needed to understand the chemical basis and genetic control of these allelopathic capabilities.