Annual Medicago Research Articles

Growth Analysis of Spring and Summer Seeded Annual Medicago spp

Understanding patterns of summer and fall growth and dry matter partitioning of annual medics (Medicago species) in the north central USA should be useful in developing harvest management recommendations for these legumes. Our objective was to describe changes in biomass yield and biomass partitioning to leaf, stem, and floral fractions during development of annual medics. Five Australian annual medic species and ‘Nitro’ alfalfa (M. sativa L.) were planted in spring and summer at St. Paul and Rosemount, MN. Leaf to stem dry weight ratio (L:S) of medics declined with maturation of both spring and summer seedings. Whole plant maximum DM yield averaged 5395 kg ha−1 for spring seeding and 3786 kg ha−1 for summer seeding, and these yields occurred from 10 to 14 wk after planting depending on species and environment. ‘Sapo’ (M. rugosa Desr.) and Sephi (M. truncatula Gaertn) were consistently among the highest in leaf and stem yields for spring seeding, and ‘Sava’ [M. scutellata (L.) Mill] was consistently among the highest in leaf and stem yields for the summer seeding. ‘Harbinger’ (M. littoralis Rhode), ‘Santiago’ (M. polymorpha L.), and Sava were consistently the earliest legumes to flower and the earliest to achieve maximum flower and pod yields during summer growth. They also had the highest flower and pod yields. Flowering of annual medics was not fully expressed during fall growth because of the temperature and daylength changing pattern. Varied yield responses of annual Medicago species will necessitate species‐specific harvest management practices.

Forage Yield and Quality of Six Annual Medicago Species in the North‐Central USA

AbstractAnnual medics (Medicago spp.) have been extensively used as winter annual forage and green manure crops in Mediterranean regions, but their potential use in the north‐central USA has not been evaluated. Our objective was to determine the summer and fall forage yield and forage quality of spring‐ and summer‐seeded annual medics. Twelve commercial annual medic cultivars from Australia (representing five species) and ‘George’ black medic (M. lupulina L.), ‘Nitro’ alfalfa (M. sativa L.), and ‘Arlington’ red clover (Trifolium pratense L.) were seeded in spring and summer at Rosemount and Morris, MN, in 1991 and at St. Paul and Becker, MN, in 1992. Forage yields were taken at a single harvest in summer (when most medics were reproductive) from spring‐seeded stands and in the fall first frost (0°C) from summer‐seeded stands. Forage yields of spring and summer‐seeded annual medics ranged from 1.2 to 5.7 and 0.5 to 5.5 Mg ha−1, respectively, depending on location and species. Barrel medic (M. truncatula Gaertn.), snail medic [M. scutellata (L.) Mill.], and burr medic (M. polymorpha L.) cultivars were among the highest yielding, while Nitro alfalfa, George black medic, and Arlington red clover were consistently among the lowest yielding forage species. George black medic, Harbinger strand medic (M. littoralis Rohde ex Loisel.), and Kelson snail medic were among the highest in forage crude protein (CP) concentration for both summer and fall harvests. Forage fiber concentrations of all annual medics except Sava snail medic were equal to or lower than those in Nitro alfalfa. Our results indicate that annual medics have the potential for producing high yields of quality forage in the north‐central USA and might be used as short‐season annual crops for harvest in fall and summer when traditional forage supplies are inadequate.

Sinorhizobium medicae sp. nov., isolated from annual Medicago spp.

The taxonomic position of isolates of a new genomic species (designated genomic species 2) obtained from several annual Medicago species and originating from different geographical locations was established through the results of phenotypic tests (including the results of auxanographic and biochemical tests and symbiotic properties) and 16S rRNA phylogenetic inferences. A comparison of the complete 16S rRNA sequence of a representative of genomic species 2 (strain A 321T [T = type strain]) with the 16S rRNA sequences of other members of the Rhizobiaceae and closely related taxa showed that genomic species 2 was phylogenetically related to Sinorhizobium meliloti, Sinorhizobium fredii, Sinorhizobium saheli, and Sinorhizobium teranga. The levels of sequence similarity and observed numbers of nucleotide substitutions in Sinorhizobium strains indicated that A 321T and S. meliloti exhibited the highest level of sequence similarity (99.7%), with four nucleotide substitutions and one deletion. The results of a numerical analysis based on data from 63 auxanographic and biochemical tests clearly separated genomic species 2 isolates from S. meliloti. Genomic species 2 isolates nodulated and fixed nitrogen with Medicago polymorpha, whereas S. meliloti isolates were ineffective and formed rudimentary nodules on this host plant. On the basis of phenotypic and 16S sequence analysis data, genomic species 2 isolates cannot be assigned to a previously described species. We propose that these isolates belong to a new species, Sinorhizobium medicae.

Comparison of nucleotide diversity and symbiotic properties of Rhizobium meliloti populations from annual Medicago species

Forty-three isolates of Rhizobium meliloti were trapped from soil with five annual species of Medicago (M. polymorpha, M. truncatula, M. rigidula, M. orbicularis and M. minima) and one perennial species of Medicago (M. sativa). The annual species were growing naturally near the soil sampling site, and the commonly studied perennial species was used for comparison. Each R. meliloti was characterized by PCR-RFLP methods applied to two DNA regions nested between 16S rRNA and 23S rRNA genes and between nifD and nifK genes. They fell into two highly divergent groups (groups I and II), separated at a genetic distance of 0.024 by rDNA-amplified pattern analysis (profiles R1 and R2) and at 0.029 by nif-amplified pattern analysis (profiles N1-N2 and N3). These two groups were consistent with some cross-nodulation and -fixation results: rhizobia with the R1 genetic background elicited rudimentary nodules and could not fix nitrogen on M. polymorpha, while they were able to nodulate the five other species of Medicago. In contrast, rhizobia with an R2 profile were highly effective on M. polymorpha and poorly nodulated M. rigidula species, but were able to nodulate efficiently the other species. The striking phenotypic traits on M. polymorpha were also shared by reference strains: strains genetically closed to R2 type triggered typical and efficient nodules on M. polymorpha while those close to R1 type elicited rudimentary and non-efficient ones. Our results suggest that the presence of R. meliloti with R2 genetic backgrounds could be favoured by the distribution of M. polymorpha species.

Comparison of nucleotide diversity and symbiotic properties of Rhizobium meliloti populations from annual Medicago species

Forty-three isolates of Rhizobium meliloti were trapped from soil with five annual species of Medicago (M. polymorpha, M. truncatula, M. rigidula, M. orbicularis and M. minima) and one perennial species of Medicago (M. sativa). The annual species were growing naturally near the soil sampling site, and the commonly studied perennial species was used for comparison. Each R. meliloti was characterized by PCR-RFLP methods applied to two DNA regions nested between 16S rRNA and 23S rRNA genes and between nifD and nifK genes. They fell into two highly divergent groups (groups I and II), separated at a genetic distance of 0.024 by rDNA-amplified pattern analysis (profiles R1 and R2) and at 0.029 by nif-amplified pattern analysis (profiles N1-N2 and N3). These two groups were consistent with some cross-nodulation and -fixation results: rhizobia with the R1 genetic background elicited rudimentary nodules and could not fix nitrogen on M. polymorpha, while they were able to nodulate the five other species of Medicago. In contrast, rhizobia with an R2 profile were highly effective on M. polymorpha and poorly nodulated M. rigidula species, but were able to nodulate efficiently the other species. The striking phenotypic traits on M. polymorpha were also shared by reference strains: strains genetically closed to R2 type triggered typical and efficient nodules on M. polymorpha while those close to R1 type elicited rudimentary and non-efficient ones. Our results suggest that the presence of R. meliloti with R2 genetic backgrounds could be favoured by the distribution of M. polymorpha species.

Purification and Characterization of Vegetative Storage Proteins from Alfalfa (Medicago sativa L.) Taproots

Summary Alfalfa ( Medicago sativa L.) accumulates C and N reserves in taproots and utilizes these reserves for shoot growth in spring and for shoot regrowth after defoliation. Three proteins are very abundant in taproots and undergo a cyclic pattern of utilization during early shoot growth followed by reaccumulation during late shoot development. Our objectives were to purify and characterize these putative vegetative storage proteins from alfalfa taproots. The proteins were purified using organic-solvent and ionic-precipitation techniques, gel filtration, and affinity chromatography. Polyclonal antibodies were raised against the purified proteins, and electrophoresis and immunoblotting were utilized to determine protein distribution and relative abundance. These proteins are present in high concentrations in alfalfa taproots, but were not found in seeds, nodules, leaves, or stems of alfalfa. Taproots of all perennial Medicago species examined contained these proteins, whereas roots of annual Medicago species had very low to undetectable amounts of these proteins. Taproots of other forage legume species ( Lotus, Melilotus, and Trifolium ) did not contain proteins that cross-reacted with antibodies raised against the three alfalfa taproot proteins. The three proteins have molecular masses of 15, 19, and 32 ku, are glycosylated, and have epitopes in common. The amino acids asparagine and aspartate make up 15 mole percent of the three alfalfa taproot proteins. These proteins possess features consistent with their role being vegetative storage proteins.

Competition between Medicago truncatula and wheat for 15N labeled soil nitrogen and influence of phosphorus

To determine competition for nitrogen uptake, a greenhouse experiment was conducted using annual Medicago truncatula cf. Jemalong (medic) and Triticum turgidum spp. durum cv. Karim (wheat) grown alone or in mixture, over five successive crop cycles. Pots were supplied with P equivalent to 0, 50, 100 and 150 kg P 2O 5 ha −1 at the beginning of each crop cycle. The soil was labeled by growing wheat plants on soil fertilized with N enriched with 10.43 atom % 15N excess and by incorporating the labeled wheat as organic matter (2.08 atom % 15N excess) back into the soil. This method allowed estimation of N 2 fixed by medic and N transfer to wheat. Compared to the sole medic crop, dry herbage yield and total N of medic grown in mixture, were reduced. Medic plants alone (unmixed) assimilated 80% more soil N that wheat alone at the higher rate of P. When in mixture, wheat plants took up more soil N than medic plants. The percentage N derived from synbiosis (%Ndfa), by the sole medic, averaged 60%. In mixture, %Ndfa of the medic was 80%. The total N difference method gave higher values for %Ndfa (6–22%) than the isotope dilution technique, depending on the P rate. This was because P rates affected the N absorption of medic and wheat differently. Dry herbage yield and total N of medic and wheat grown alone and in mixture were improved by P supply. Competition between medic and wheat was affected by P fertilization, favoring wheat which became more competitive for uptake of soil N at the higher P rates. When no P was added to the soil, mixed medic took up 60% of the soil N absorbed by sole medic; at the higher P rate, this was reduced to 34%. Transfer of N fixed by medic to wheat was detected only after the second crop cycle.

Effect of water stress on competition betweenMedicago truncatulaand wheat in15N labeled soil

Crop mixtures that include legumes increase yield and restore soil fertility. These mixtures are limited by drought, which is frequent in Morocco due to lack of and poor distribution of rain. However, annual Medicago species are very important to forage production, and they have a tolerance to water stress. The effect of water stress on N2 fixation and N transfer by Medicago truncatula cv. Jemalong (medic), when mixed with wheat (Triticum durum cv. Karim), was investigated in three greenhouse experiments. Annual medic and wheat were seeded in 20‐L pots in pure and mixed stands. Three watering treatments were used: ‐0.9, ‐1.5, and ‐2.5 MPa in the first experiment; and ‐0.9, ‐2.5, and ‐3.5 MPa in the second and third experiments. Competition between medic and wheat was affected by hxdric treatments; the medic was more competitive for soil water than the wheat, especially under severe water stress. Shoot dry weight of wheat grown with medic was higher than that of wheat grown in pure stands. Medic alone took...

Methods of developing a core collection of annual Medicago species

A core collection is a subset of a large germplasm collection that contains accessions chosen to represent the genetic variability of the germplasm collection. The purpose of the core collection is to improve management and use of a germplasm collection. Core collections are usually assembled by grouping accessions and selecting from within these groups. The objective of this study was to compare 11 methods of assembling a core collection of the U.S. National collection of annual Medicago species. These methods differed in their use of passport and evaluation data as well as their selection strategy. Another objective was to compare core collections with sample sizes of 5%, 10% and 17% of the germplasm collection. Core collections assembled with evaluation data and cluster analysis better represented the germplasm collection than core collections assembled based solely on passport data and random selection of accessions, The Relative Diversity and the logarithm methods generated better core collections than the proportional method. The 5% and 10% sample size core collection were judged insufficient to represent the germplasm collection.

Regeneration of Medicago truncatula from protoplasts isolated from kanamycin-sensitive and kanamycin-resistant plants

Medicago truncatula (barrel medic) is an annual legume of agricultural and biological interest. In this report regeneration from isolated mesophyll protoplasts is described. A specifically developed, highly regenerable seed line is essential for regeneration. Other critical requirements for regeneration are the starting plant material, the use of agarose droplets incubated in a shallow layer of liquid medium, and protoplast density. Plants are grown in controlled environment conditions. Protoplasts are purified using a Percoll-based flotation procedure, then embedded in 100 μl agarose droplets containing a basal medium plus 25 μM NAA and 4 μM BAP (the same medium as in the surrounding shallow liquid layer) to induce protoplast division. A protoplast density of 6-8×10(5) ml(-1) is required for maximum colony formation. M. truncatula plants previously transformed for kanamycin resistance yielded embryogenic callus and also regenerated plants. Protoplasts from other annual Medicago (M.intertexta and M.scutellata) species readily form calli by the procedure we have described.

Resistance in annual Medicago species to Phoma medicaginis and Leptosphaerulina trifolii under field conditions

Advanced breeding lines and potential new cultivars of annual Medicago species were screened for resistance to phoma leaf and stem disease (caused by Phoma medicaginis) and pepper spot (caused by Leptosphaerulina trifolii) in the field, and their performances were compared with appropriate control cultivars. Medicago murex had the best overall disease resistance of the Medicago species tested. There were large differences between varieties in the incidence of both diseases. Based on end-of-season scores, the most resistant to phoma leaf disease included M. murex cv. Zodiac and SAR 3490, M. tornata SA10012, and M. truncatula cv. Parabinga, with scores 52.8 on a 0-10 scale (0, nil disease; 10, >90% leaves affected). On the basis of area under disease progress curve (AUDPC), the most resistant to phoma leaf disease, with AUDPC values of <157, included all M. murex varieties except 87FB2.30, and M. truncatula cv. Caliph. Based on end-of-season scores, the most resistant to phoma stem disease, all with scores c4, included M. murex cv. Zodiac, SAR 3490, and 87F01.48; M. sphaerocarpos cv. Orion (SEP 29.1), SEP 29.2, and SEP 30.3; and M. tornata cv. Tornafield and SA10012. Resistance to phoma stem disease in these lines was confirmed by AUDPC values, especially for M. murex cv. Zodiac, SAR3490, and 87FO1.48, and for M. tornata SA10012, with values <154. Based on end-of-season scores, the most resistant to pepper spot leaf disease included M. polymorpha cvv. Serena and Circle Valley, with scores 52.8. On the basis of AUDPC, all M. murex varieties except SAR3490 and 87FO1.5 could be included among those most resistant to pepper spot leaf disease, with AUDPC values <275. Coumestrol levels in stems ranged from 117 to 1420 mg/kg DW, with lowest levels (5330 mg/kg) in M. murex cv. Zodiac, 87FO 1.48, and GRC 87; M. sphaerocarpos cv. Orion (SEP 29.1), SEP 29.2, and SEP 26.2.1; M. tornata cv. Tornafield; and M. truncatula Z601. There was positive correlation between the severity of phoma disease on stems and end-of-season (September-November) coumestrol concentrations.

Relative resistance, associated yield losses and phyto-oestrogen production from fungal foliar diseases in new and old annual Medicago cultivars

The relative resistance, associated herbage and seed yield losses and phyto-oestrogen production from fungal foliar diseases in two new annual Medicago cultivars, viz. M. sphaerocarpos cv. Orion and M. truncatula cv. Caliph, were compared with those of four old cultivars, viz. M. murex cv. Zodiac, M. polymorpha var. brevispina cw. Santiago and Circle Valley, and M. truncatula cv. Cyprus. Orion was much more resistant to Phoma stem disease than Circle Valley and Santiago, but was more susceptible than Zodiac. However, Orion was the most susceptible cultivar to Phoma leaf disease. Orion and Caliph were susceptible to Leptosphaerulina stem disease, resembling Santiago, but much more susceptible than Circle Valley and Zodiac. Caliph was comparable with Cyprus with moderate susceptibility to both Phoma and Leptosphaerulina on stems. Both Caliph and Cyprus were resistant to Pseudopeziza. In mid-October and early November, foliar diseases reduced overall herbage yield of the six varieties by 16.2 and 20.3% respectively, compared with the fungicide-treated plots of the same cultivars. Foliar diseases reduced seed yield by 37.3%, and mean seed weight by 13- 7% compared with fungicide treated plots. Foliar disease in the disease block increased the content of the phyto-oestrogen coumestrol in stems from 230 to 500 ppm and in pods from 30 to 130 ppm, compared with the fungicide block. There was a positive correlation between the severity of Phoma on stems and leaves and the level of coumestrol in stems and pods. Orion produced large amounts of coumestrol in stems and pods in response to foliar diseases (e.g. 470 ppm in stems) but less than Zodiac. Caliph also produced large amounts of coumestrol in stems and pods in response to foliar diseases (e.g. 230 ppm in stems), and was similar to Cyprus but had less coumestrol than all other cultivars tested (stems) or Zodiac, Circle Valley and Santiago (pods). Orion carried the highest level of P. medicaginis seed contamination (28%), followed by Santiago (15%), Cyprus (11%), Circle Valley (l0%), Zodiac (10%) and Caliph (6%).

Studies on seed and pollen transmission of Alfalfa Mosaic, Cucumber Mosaic and Bean Yellow Mosaic Viruses in cultivars and accesions of annual Medicago species

Seed and pollen transmission of alfalfa mosaic (AMV), cucumber mosaic (CMV) and bean yellow mosaic (BYMV) viruses was investigated in annual medic species (Medicago spp.). For seed transmission studies with AMV, graft inoculation was used to establish early infection and maximize possible transmission rates to seedlings via seed, but with CMV and BYMV aphid and/or graft inoculation was used. For pollen transmission studies, pollen taken from virus-infected plants was used to pollinate healthy plants, the seed collected and seedlings tested. The rates of AMV isolate OUI-2 transmission to seedlings through seed produced on infected plants ranged from 6 to 53% for commercial cultivars and from 7 to 65% for accessions. Accession DZA 3181.1.1 of M. sphaerocarpos had the highest overall AMV transmission rate. Only two cultivars, cvv. Borung and Hannaford of M. truncatula, and accession SA 4268 of M. orbicularis, had transmission rates of less than 10%. The rates of CMV transmission to seedlings via seed produced on infected plants of the cultivars and accessions tested were 0.3 to 13%, the greatest being found in M. polymorpha cv. Serena, but 6 out of 11 had no detectable transmission. The rates of BYMV transmission to seedlings via seed of the cultivars and accessions tested were 0.3 to 1%, but in 12 out of 15 none was detected. AMV isolate OUI-2 was transmitted to 52% of seedlings via seed produced on healthy M. polymorpha cv. Circle Valley plants pollinated from infected plants. In contrast, no transmission to seedlings by either graft-inoculation or pollination of M. polymorpha plants was detected with a second AMV isolate, OUI-1, which appeared to have lost its ablilty to be seed transmitted. No CMV or BYMV transmission to seedlings via pollination of healthy plants with pollen from infected plants was detected in M. polymorpha cvv. Circle Valley or Santiago. When empty immature pods, and dissected seed coats and embryos from immature seeds produced on AMV-infected plants of M. polymorpha were tested, AMV isolates OUI-I and OUI-2 were detected in all pods and seed coats, but only in 59% of embryos with isolate OUI-2 and in none with isolate OUI-1. CMV was detected in 12% of embryos tested from immature seeds produced on CMV-infected M. polymorpha cv. Serena plants. Transmission of all three viruses through seed, and of AMV through pollen, is cause for concern in annual medic breeding and evaluation programs. Moreover, carry-over outside the growing season in medic pastures is possible through seed with all three viruses.

Delayed inoculation and competition of Rhizobium meliloti in annual Medicago species

Seed inoculation is frequently essential for annual Medicago establishment in Mediterranean dryland farming systems. As post-planting soil inoculation is often practiced when seed inoculation fails, the effect of delayed inoculation was investigated. Roots of Medicago rigidula (L.). All selection 716, and Medicago rotata Boiss. selection 2132, were initially pre-exposed to Rhizobium meliloti strains. Subsequently, roots were exposed to a secondary inoculum after 6, 48 and 168 h to simulate delayed inoculation. and subsequent establishment of others strains in the nodules were investigated. Combinations of highly effective and host compatible ICARDA strains (M15 and M53) and effective-ineffective strains (M15 and M28) were used to evaluate proportional nodulation responses. Plants were harvested after 6 weeks of growth. Nodules were assessed for distribution in the root system and for occupancy based on their differential resistance to kanamycin and streptomycin, and, in the case of ICARDA M28, on nodule characteristics. The strain ICARDA M15 was a better competitor than ICARDA M53 when applied in equal density in M. rotata, at zero time. While forming nodules with M. rigidula, ICARDA M53 was equally competitive under the same conditions. With ICARDA M15 as the primary inoculum, and ICARDA M53 delayed for 6, 48 and 168 h, the incidence of ICARDA M15 nodules increased on M. rigidula from 43% (at zero time) to 86, 94, and 97% and, on M. rotata, from 78% (at zero time) to 88%, 95%, and 98% for the three time delays, respectively. Pre-exposure of 2-day old M. rigidula seedlings to the ineffective strain ICARDA M28 as the primary inoculant was followed by significant nodule number increases ( P ⩽ 0.01) as compared with ICARDA M15. Nonetheless, when ICARDA M15 was the primary inoculum, M28 was able to produce about 35% of the nodules when applied at the 6 and 48 h time delays. However, with ICARDA M28 as the primary inoculum, followed by ICARDA M15, nodule occupancy of ICARDA M28 in M. rigidula increased from 40 (at zero time) to 75%, 80%, and 95% for the three time delays, respectively. The percentage of total nodulation by M28, applied at the three delay times, was markedly lower (10%, 5% and 1%, respectively) when M. rotata was pre-exposed to ICARDA M15. This suggested a host preference for ICARDA M15, even if applied as a late inoculum. Results indicate that the early events in the nodulation process of annual medics coupled with host-specificity factors are perhaps the most critical for competition among R. meliloti strains for nodule formation.

Genome size and base composition in Medicago sativa and M. truncatula species

The genome size (1C value) and base composition of 14 ecotypes of two species of tetraploid and diploid Medicago have been assessed by flow cytometry. These parameters vary both between and within species. The diploid annual Medicago truncatula Gaertn. had the smallest genome of the group studied (which also covered M. sativa L. subsp. sativa, M. sativa L. subsp. caerulea (Less. ex Ledeb.) Schmalh., M. sativa L. subsp. quasifalcata Sinsk., M. sativa L. subsp. x varia (Martyn) Arcangeli; however, its ecotypes revealed substantial intraspecific variation. The smallest M. truncatula genome observed was ecotype 108-1 with 1C = 0.49 pg and 38.1% GC and the largest was Jemalong with 1C = 0.57 pg and 38.6% GC. The degree of polysomaty in these Medicago was low, although in some tissues the frequency of cells with 4C nuclei reached 50%.

Morphologische und agronomische Merkmale autochthoner annueller Medicago‐Arten in Marokko

AbstractMorphological and agronomic characteristics of autochthonous annual Medkago species in MoroccoAutochthonous annual Medicago species were collected on non‐alkaline soils and evaluated as to their potential agricultural use. 411 Medicago ecotypes were found at 108 sites. With increasing rainfall and altitude of collecting sites, a decrease was recorded in the length of sprouts, in sward height, in leaf size, and in dry matter yield. High rainfall regions were found to select late flowering ecotypes. All Medicago ecotypes tested showed broad intraspecific variation in seed yield, leaf size and seed weight. Coumestrol content of leaves at flowering was generally low; on average 115 ppm of dry matter. No relationship was found between collection sites and coumestrol content. In a field trial comparing 112 ecotypes with nine introduced cultivars at two contrasting sites, thirty ecotypes proved to be better adapted to local conditions than the cultivars.

Infection of alternative hosts associated with annual medics (Medicago spp.) by alfalfa mosaic virus and its persistence between growing seasons

Under conditions of natural alfalfa mosaic virus (AMV) spread, five plant species found associated with annual Medicago spp. (medics) were infected commonly and another seven sporadically. Ten of these were new records. Because seed of herbaceous plant hosts provides a possible route for virus persistence through dry summer conditions, AMV seed transmission was tested for in alternative hosts. Of ten species systemically infected by sap inoculation with AMV, seed transmission was detected in Melilotus indica (l0%), Ornithopus compressus (0 1%) and Stachys arvensis (2%). Seed of seven naturally infected potential alternative host species was tested, and seed transmission found in Crassula decumbens (0. 1%), M. indica (3%), 0. compressus (0.2%), S. arvensis (0.4%) and Trifolium subterraneum (2%). Carry-over of AMV through seed transmission was detected in seedlings of Hypochaeris glabra (0.2%) and M. indica (0.6-0.8%) that germinated naturally in the field. In grazed, self-regenerated Medicago murex and M. polymorpha swards sown in 1987, the virus persisted for at least seven growing seasons. Levels of infection in M. polymorpha seed produced each year declined, but levels in the general seed bank remained higher due to presence of older seed. It is concluded that under the conditions of broadacre agriculture in the Mediterranean-type climate of Western Australia, seed transmission in C. decumbens, H. glabra and volunteer 0. compressus is an alternative means by which AMV can persist over summer to act as sources of AMV for spread within annual medic pastures. However, persistence through seed of the annual medic cultivars sown and of naturalized annual Medicago species is the principal means of persistence. AMV persists readily from year to year following sowing of infected seed and is likely to cause a recurrent disease problem in annual medic pastures.

An improved method for artificial hybridization in annual Medicago species

An improved technique for successful artificial hybridization in annual medic (Medicago spp.) is described. Using a previously reported method, only four out of seven species were successfully crossed, with the percentage of success ranging from 3 to 22%. Initial modifications to this technique gave a 7-8 fold increase in the successful crossing rate in M. murex and M. polymorpha medic, from 9 to 64% with M. murex and from 10 to 82% with M. polymorpha. Further modifications to the technique resulted in a success rate of 100% in both species. The numbers of seeds per pod obtained from crosses in both species were also increased by using the modified techniques compared to the established method. Selection of larger, more mature flowers, differences in flower cutting position, as well as post pollination position were the main modifications which accounted for the greatly improved success rate. The modified technique was subsequently applied successfully to obtain for the first time inter-specific crosses involving M. polymorphax M. murex, M. polymorphax M. sphaerocarpos, M.murexx M. sphaerocarpos, M. solerolii x M. littoralis/M.truncatula hybrid, M. solerolii x M, tornata, and M. littoralis/M.truncatula hybrid x M. sphaerocarpos.

A Core Collection for the United States Annual Medicago Germplasm Collection

The United States National Plant Germplasm System contains 3159 accessions from 36 species of annual Medicago. Although there is increasing interest in the annual medics for use in sustainable agriculture, the U.S. collection is under utilized because of lack of agronomic information. Development of a core collection could facilitate easier access to the germplasm collection and enhance its use. The core collection should consist of a sample of accessions that represents the range of variability within the germplasm collection with minimum redundancies. To select a core collection of annual Medicago species, a subset of 1240 accessions was evaluated during the summer of 1990 for 16 agronomic and morphological traits. Accessions within species were grouped by cluster analysis utilizing an unweighted pair group method with arithmetic averages. Intraspecific phenotypic diversity determined the number of accessions for that species selected for the core collection. One accession per cluster was selected for each species for the core collection. Accessions were chosen within a species to represent the greatest diversity in geographical regions. The selected core collection of 211 accessions was reevaluated during the summer of 1991. The core collection was found to represent the variability of the germplasm collection and to remain stable between the two evaluation years. This study can be used as a model method for selecting a core collection for multispecies germplasm collections.

Germination, Recruitment and Survival in the Weedy Annual Medicago polymorpha in Successive Wet and Dry Years

-Germination, establishment and survival in Medicago polymorpha, a weedy winter annual, were studied during 2 successive seasons in California. Seeds were dormant when produced in spring and remained largely dormant following exposure to summer conditions. Approximately 40% of viable seeds in the seed bank were carried over from year to year. Consequently, autumn recruitment of seedlings is due in large part to germination of seeds produced in previous years. Seedling emergence was mostly simultaneous following the first substantial autumn rains. Survival to reproduction differed between years; few individuals produced seeds prior to senescence in the dry year, whereas in the wet year most overwintering individuals successfully produced fruits.