Does the cutting of lucerne (Medicago sativa) encourage the movement of arthropod pests and predators into the adjacent crop?

Lucerne (Medicago sativa) has been suggested as an ideal refuge habitat as part of an integrated pest management (IPM) program because it harbours high numbers of beneficial arthropods. Whether or not cutting of lucerne encourages the movement of these beneficials into adjacent target crops is unknown. Vacuum samples were used to determine the effects of cutting lucerne on arthropod abundance (pests and predators) within lucerne and adjacent soybean (Glycine max) crops. Vacuum-sample collections of arthropods were conducted before and after lucerne cutting on seven occasions in four fields over two seasons. In the lucerne, 10 m by 1 m strips parallel to the crop interface were sampled at 5, 10, 15, 20 and 30 m from the interface. In the soybean, 10 m of row were sampled at the same distances from the crop interface. The abundance of predators in lucerne was reduced immediately after cutting at all distances from the interface. Predator abundance in soybean did not show any change. The cutting of lucerne significantly reduced pest numbers within the lucerne but had little effect on pest abundance in the adjacent soybean. The temporal pattern in pest and predator abundance was very different for each field sampled. Generally, arthropods decreased in abundance after cutting and gradually increased as the lucerne grew back. In soybeans, arthropod numbers fluctuated regardless of the cutting of the lucerne. Cutting of lucerne alone does not guarantee movement of predators into the adjacent target crop. The presence of lucerne fields within a cropping area may have some impact on regional predator populations, and so still be useful for IPM programs, but this has yet to be tested critically.

Potential to increase yield in lucerne (Medicago sativa subsp sativa) through introgression of Medicago sativa subsp falcata into Australian adapted material

The effect of interspecific heterosis in crosses between Medicago sativa subsp. sativa and M. sativa subsp. falcata was assessed. Three sativa and 3 falcata plants were crossed in a diallel design. Progeny dry matter yield and natural plant height were assessed in a replicated field experiment at Gatton, Queensland. Yield data were analysed using the method of residual maximum likelihood (REML) and Griffing's model 1. There were significant differences between the reciprocal, general combining ability (GCA), and specific combining ability (SCA) effects. As expected, S-1 populations were lower yielding than their respective intraspecific cross and falcata x falcata crosses were significantly lower yielding than sativa x sativa crosses. Some of the interspecific crosses indicated substantial SCA effects, yielding at least as well as the best sativa x sativa crosses. We have demonstrated the potential usefulness of unselected M. sativa subsp. falcata as a heterotic group in the improvement of yield in northern Australian adapted lucerne material, and discuss how it could be incorporated into future breeding to overcome the yield stagnation currently being experienced in Australian programs.

DNA markers linked to yield, yield components, and morphological traits in autotetraploid lucerne (Medicago sativa L.)

We have mapped and identifed DNA markers linked to morphology, yield, and yield components of lucerne, using a backcross population derived from winter-active parents. The high-yielding and recurrent parent, D, produced individual markers that accounted for up to 18% of total yield over 6 harvests, at Gatton, south-eastern Queensland. The same marker, AC/TT8, was consistently identified at each individual harvest, and in individual harvests accounted for up to 26% of the phenotypic variation for yield. This marker was located in linkage group 2 of the D map, and several other markers positively associated with yield were consistently identified in this linkage group. Similarly, markers negatively associated with yield were consistently identified in the W116 map, W116 being the low-yielding parent. Highly significant positive correlations were observed between total yield and yield for harvests 1-6, and between total yield and stem length, tiller number, leaf yield/plant, leaf yield/5 stems, stem yield/plant, and stem yield/5 stems. Highly significant QTL were located for all these characters as well as for leaf shape and pubescence.

Strategies to obtain stable transgenic plants from non-embryogenic lines: complementation of the nn(1) mutation of the NORK gene in Medicago sativa MN1008

Strategies to introduce genes into non-embryogenic plants for complementation of a mutation are described and tested on tetraploid alfalfa (Medicago sativa). Genes conditioning embryogenic potential, a mutant phenotype, and a gene to complement the mutation can be combined using several different crossing and selection steps. In the successful strategy used here, the M. sativa genotype MnNC-1008(NN) carrying the recessive non-nodulating mutant allele nn(1) was crossed with the highly embryogenic alfalfa line Regen S and embryogenic hybrid individuals were identified from the F1 progeny. After transformation of these hybrids with the wild-type gene (NORK), an F2 generation segregating for the mutation and transgene were produced. Plants homozygous for the mutant allele and carrying the wild-type NORK transgene could form root nodules after inoculation with Sinorhizobium meliloti demonstrating successful complementation of the nn(1) mutation.

Utility of cotyledon and detached leaf assays for assessing root reactions of lucerne to Phytophthora root rot caused by Phytophthora medicaginis

Phytophthora root rot, caused by Phytophthora medicaginis, is a major limitation to lucerne production but it can be managed through the use of resistant cultivars. Current resistance screening methods, using mature plants or post-emergence seedling assays, are costly and time consuming. The use of zoospore inoculum on detached leaves and intact cotyledons as an assay for plant resistance was assessed using genetically defined segregating populations. The detached leaf assay was a reproducible test, but this test could not be used for accurately predicting root ratings. The cotyledon tests using zoospores gave results at the population level that were indicative of the root responses of 19 cultivars and lines tested. The cotyledon reaction of individual plants also showed a strong association with root response. The cotyledon test, while not completely predictive of mature root responses, allowed the selection of Phytophthora resistant plants at a higher frequency than could be achieved by random selection.

Characterisation of Rhizoctonia solani isolates causing root canker of lucerne in Australia

The fungus causing Rhizoctonia root canker of lucerne in western Queensland has been characterised as a new subgroup within anastomosis group (AG 6) of Rhizoctonia solani. Isolates from two sites showed identical rDNA ITS sequence homology but could be differentiated based on DNA fingerprints. The lucerne isolates did not cause disease on wheat, indicating they are genetically different from the AG 6 subgroup that causes crater disease on wheat in South Africa. Root canker symptoms were produced on all commercial Australian cultivars of lucerne tested.

Incidence of Stagonospora meliloti and Acrocalymma medicaginis in Lucerne crowns and roots in eastern Australia, and their comparative aggressiveness to Lucerne and inheritance of reaction to S. meliloti in lucerne

The research presented indicates that lucerne crown and root rot caused by Stagonospora meliloti is prevalent in southern New South Wales, whereas Acrocalymma medicaginis is the more commonly observed pathogen in Queensland. Although both pathogens cause reddening of internal root and crown tissue of lucerne, they can be distinguished by symptomatology. S. meliloti causes a diffuse red blotching of the internal tissue accompanied by the presence of an external lesion, whereas A. medicaginis causes red streaking at the extremity of wedge-shaped, dry-rotted tissue. Inoculation of propagules of a susceptible lucerne clone indicated that S. meliloti was the more aggressive pathogen. Although A. medicaginis does not cause leaf disease, there was a strong relationship between the leaf and root reaction of clones to S. meliloti. Inheritance of resistance to S. meliloti in lucerne appeared to be conditioned by a single dominant gene, based on segregations observed in S-1 and F-1 populations, but not in a backcross population from the same family where an excess of susceptible individuals (74% v. expected of 50%) was obtained in a cross of a resistant F-1 individual to the susceptible parent. Resistance appears to be highly heritable, however, and amenable to population improvement by breeding. A conclusion of the research is that breeding for resistance to S. meliloti for lucernes to be grown in southern Australia would appear to be a worthwhile objective. Presently, no highly resistant cultivars exist anywhere in the world.

A genetic linkage map in autotetraploid lucerne adapted to northern Australia, and use of the map to identify DNA markers linked to resistance to Phytophthora medicaginis

Phytophthora root rot, caused by Phytophthora medicaginis, is a major limitation to lucerne ( Medicago sativa L.) production in Australia and North America. Quantitative trait loci (QTLs) involved in resistance to P. medicaginis were identified in a lucerne backcross population of 120 individuals. A genetic linkage map was constructed for tetraploid lucerne using 50 RAPD ( randomly amplified polymorphic DNA), 104 AFLP (amplified fragment length polymorphism) markers, and one SSR ( simple sequence repeat or microsatellite) marker, which originated from the resistant parent (W116); 13 markers remain unlinked. The linkage map contains 18 linkage groups covering 2136.5 cM, with an average distance of 15.0 cM between markers. Four of the linkage groups contained only either 2 or 3 markers. Using duplex markers and repulsion phase linkages the map condensed to 7 homology groups and 2 unassigned linkage groups. Three regions located on linkage groups 2, 14, and 18, were identified as associated with root reaction and the QTLs explained 6 - 15% of the phenotypic variation. The research also indicates that different resistance QTLs are involved in conferring resistance in different organs. Two QTLs were identified as associated with disease resistance expressed after inoculation of detached leaves. The marker, W11-2 on group 18, identified as associated with root reaction, contributed 7% of the phenotypic variation in leaf response in our population. This marker appears to be linked to a QTL encoding a resistance factor contributing to both root and leaf reaction. One other QTL, not identified as associated with root reaction, was positioned on group 1 and contributed to 6% of the variation. This genetic linkage map provides an entry point for future molecular-based improvement of lucerne in Australia, and markers linked to the QTLs we have reported should be useful for marker-assisted selection for partial resistance to P. medicaginis in lucerne.

Identifying and discriminating among lucerne cultivars using plant morphological characters

The use of morphological data obtained from field (plot test) and glasshouse trials to identify and discriminate among four Iranian and two New Zealand lucerne (Medicago sativa L.) cultivars was investigated, following guidelines established by the International Union for the Protection of New Varieties of Plants (UPOV) for cultivar registration and the Organisation for Economic Co-operation and Development (OECD) for seed certification. Data were collected for terminal leaflet length, width and ratio, angle of stem growth, date of first flowering, stem height at first flowering, flower colour, cutting recovery height, and disease scores. None of these characters were sufficient to identify or discriminate among the six cultivars. The results indicate a need to find cost-effective and efficient laboratory techniques to enhance the assessment of distinctness of lucerne cultivars (UPOV) and for determining cultivar purity for lucerne seed certification (OECD).

Effects of litter and fine root composition on their decomposition in a Rhodic Paleustalf under different land uses

Plant litter and fine roots are important in maintaining soil organic carbon (C) levels as well as for nutrient cycling. The decomposition of surface-placed litter and fine roots of wheat ( Triticum aestivum ), lucerne ( Medicago sativa ), buffel grass ( Cenchrus ciliaris ), and mulga ( Acacia aneura ), placed at 10-cm and 30-cm depths, was studied in the field in a Rhodic Paleustalf. After 2 years, = 60% of mulga roots and twigs remained undecomposed. The rate of decomposition varied from 4.2 year -1 for wheat roots to 0.22 year -1 for mulga twigs, which was significantly correlated with the lignin concentration of both tops and roots. Aryl+O-aryl C concentration, as measured by 13 C nuclear magnetic resonance spectroscopy, was also significantly correlated with the decomposition parameters, although with a lower R 2 value than the lignin concentration. Thus, lignin concentration provides a good predictor of litter and fine root decomposition in the field.

Genetic improvement of lucerne for anthracnose (Colletotrichum trifolii) resistance

Anthracnose, caused by Colletotrichum trifolii, is one of the most serious diseases influencing lucerne persistence and productivity in eastern Australia. The disease is largely controlled by plant resistance; however, new pathotypes of C. trifolii have developed in Australia, seriously limiting the productive life of susceptible cultivars. This paper describes an incompletely recessive and quantitatively inherited resistance to C. trifolii identified in a clone (W116) from cv. Sequel. S-1, F-1, F-2 and backcross populations of W116 and D (highly susceptible clone) were studied for their reaction to C. trifolii race 1. Resistance was found to be quantitatively inherited, and quantitative trait loci associated with resistance and susceptibility were identified in a backcross population (D x W116) x D using random amplified polymorphic DNA and amplified fragment length polymorphic markers. A multi-locus region on linkage group 4 was found to contribute significantly to the resistance phenotype. The application of DNA markers to allow exploitation of this quantitatively inherited resistance in lucerne breeding is discussed.

Do predators aggregate in response to pest density in agroecosystems? Assessing within-field spatial patterns

1. The spatial heterogeneity of predator populations is an important component of ecological theories pertaining to predator-prey dynamics. Most studies within agricultural fields show spatial correlation (positive or negative) between mean predator numbers and prey abundance across a whole field over time but generally ignore the within-field spatial dimension. We used explicit spatial mapping to determine if generalist predators aggregated within a soybean field, the size of these aggregations and if predator aggregation was associated with pest aggregation, plant damage and predation rate. 2. The study was conducted at Gatton in the Lockyer Valley, 90 km west of Brisbane, Australia. Intensive sampling grids were used to investigate within-field spatial patterns. The first row of each grid was located in a lucerne field (10 m from interface) and the remaining rows were in an adjacent soybean field. At each point on the grid the abundance of foliage-dwelling and ground-dwelling pests and predators was measured, predation rates [using sentinel Helicoverpa armigera (Hubner) egg cards] and plant damage were estimated. Eight grids were sampled across two summer cropping seasons (2000/01, 2001/02). 3. Predators exhibited strong spatial patterning with regions of high and low abundance and activity within what are considered to be uniform soybean fields. Ground-dwelling and foliage-dwelling predators were often aggregated in patches approximately 40 m across. 4. Lycosidae (wolf spiders) displayed aggregation and were consistently more abundant within the lucerne, with a decreasing trap catch with distance from the lucrene/soybean interface. This trend was consistent between subsequent grids in a single field and between fields. 5. The large amount of spatial variability in within-field arthropod abundance (pests and predators) and activity (egg predation and plant damage) indicates that whole field averages were misleading. This result has serious implications for sampling of arthropod abundance and pest management decision-making based on scouting data. 6. There was a great deal of temporal change in the significant spatial patterns observed within a field at each sampling time point during a single season. Predator and pest aggregations observed in these fields were generally not stable for the entire season. 7. Predator aggregation did not correlate consistently with pest aggregation, plant damage or predation rate. Spatial patterns in predator abundance were not associated consistently with any single parameter measured. The most consistent positive association was between foliage-dwelling predators and pests (significant in four of seven grids). Inferring associations between predators and prey based on an intensive one-off sampling grid is difficult, due to the temporal variability in the abundance of each group. 8. Synthesis and applications. This study demonstrated that generalist predator populations are rarely distributed randomly and field edges and adjacent crops can have an influence on within-field predator abundance. This must be considered when estimating arthropod (pest and predator) abundance from a set of samples taken at random locations within a field.

Yield response of rice (Oryza sativa L.) genotypes to drought under rainfed lowlands 4. Vegetative stage screening in the dry season

Screening for drought resistance of rainfed lowland rice using drought score (leaf death) as a selection index has a long history of use in breeding programs. Genotypic variation for drought score during the vegetative stage in two dry season screens was examined among 128 recombinant inbred lines from four biparental crosses. The genotypic variation detected for drought score in the dry season was used to examine the reliability of the dry season screening method to estimate relative grain yield of genotypes under different types of drought stress in the wet season. Large genotypic variation for drought score existed in two experiments (A and B). However, there was no relationship between the drought scores of genotypes determined in these two experiments. Different patterns of development and severity of drought stress in these two experiments, i.e. slow development and mild plant water deficit in experiment A and fast development and severe plant water deficit in experiment B, were identified as the major factors contributing to the genotypes responding differently. Larger drought score in the dry season experiments was associated with lower grain yield under specific drought stress conditions in the wet season, but the association was weak to moderate and significant only in particular drought conditions. In most cases, a significant phenotypic and moderate genetic correlation between drought score in the dry season and grain yield in the wet season existed only when both drought score and grain yield of genotypes were affected by similar patterns and severity of drought stress in their respective experimental environments. The dry season environments used to measure genotypic variation for drought score should be managed to correspond to relevant types of drought environment that are frequent in the wet season. The efficiency of using the drought score as an indirect selection criterion for improving grain yield for drought conditions was lower than the direct selection for grain yield, and hence wet season screening with grain yield as a selection criterion would be more efficient. However, using drought score as a selection index, a larger number of genotypes can be evaluated than for wet season grain yield. Therefore, it is possible to apply higher selection intensities using the drought score system, and the selected lines can be further tested for grain yield in the wet season. (C) 2004 Elsevier B.V. All rights reserved.

Genotypic consistency in the expression of leaf water potential in rice (Oryza sativa L.)

Early work has shown variation in the grain yield of rice cultivars grown under water stress conditions to be associated with the plant water status, mainly with the maintenance of high leaf water potential (LWP) at flowering and grain filling stage. Considerable variation for LWP among rice varieties has been recorded. The present work was designed to investigate genotypic consistency in water potential within the plant and under canopy manipulation to vary plant water requirement. In a glasshouse experiment, with six rice genotypes, a consistent water potential gradient from stem base to leaf tip has been observed. Leaf tip water potential has been found as the minimum LWP that can be recorded at any time of stress. Genotypes with similar canopy size could maintain different levels of LWP under stress conditions. In a field experiment, with four selected lines, four canopy sizes and two canopy mixture treatments were introduced prior to the imposition of control, mild and severe water stress conditions. It was found that the line differences in LWP and relative water content (RWC) were expressed under both mild and severe stress conditions, regardless of canopy size, tiller number and whether they were mixed with another line with different capacity to maintain LWP. Although there were some differences among canopy size treatments for radiation interception in three water conditions, canopy manipulation (plant size) within a line did not affect the expression of LWP and hence genotypic variation in LWP was maintained. Under both glasshouse and field conditions, lines that maintained high LWP had larger xylem diameter and stem areas than those that had low LWP. The results indicated that the size of the vascular bundles could influence the maintenance of plant water relations under water deficit. (c) 2005 Elsevier B.V. All rights reserved.