A cytogenetic and pollen study of annual Medicago species from Soummam Valley (Northeastern of Algeria)

This paper reports a cytogenetic study of eight Medicago L. species sampled from the Soummam Valley (northeastern Algeria). Chromosome numbers and meiosis irregularities during microsporogenesis were explored. Pollen viability rate and pollen size were also examined. The studied taxa are diploid and display biva-lent pairing and regular chromosome segregation during meiosis. Although meiosis appears regular, some anomalies were detected in relatively high cumulated rates (14.66%–26.14%). The most common meiotic abnormality examined here is related to cytomixis (from 14.66% in M. littoralis to 25.83% in M. laciniata). Other anomalies were also detected, including chromatic bridges, asynchronous divisions, micronuclei and multipolar cells. Consequently, the species exhibited varying percentages of pollen viability (from 70.11% in M. laciniata to 99.14% in M. littoralis). Pollen viability was negatively correlated with meiotic abnormalities (Pearson correlation coefficient R = −0.72, p = 0.043). The pollen grains were also heterogeneous in size. Medicago truncatula Gaertn. and M. laciniata (L.) Miller presented the most variable pollen size (relative standard deviation exceeding 19%). Medicago littoralis is distinguished from other species by possessing homogeneous and large sized pollen (relative standard deviation RSD = 6.73 %). The cytogenetic and pollen data provided by this study are discussed in the context of species systematics and in the perspective of genetic improvement.

Enhancing composition and persistence of mixed pasture swards in southern New South Wales through alternative spatial configurations and improved legume performance

This study examined whether the productivity and persistence of mixed pastures that included subterranean clover (Trifolium subterraneum L.), lucerne (Medicago sativa L.), and/or phalaris (Phalaris aquatica L.) were improved if species were spatially separated rather than being sown together in each drill row. The study also compared the productivity and persistence of subterranean clover with alternative annual legume species biserrula (Biserrula pelecinus L.) and strand medic (Medicago littoralis Rhode ex Loisel). Twelve pasture treatments were sown in replicated field experiments at five locations across the medium-rainfall region of south-western New South Wales in 2012 and monitored for 3 years. Pastures that included lucerne, phalaris and subterranean clover were generally more productive than pastures with only one or two of those species, regardless of sowing configuration. Averaged across sites, subterranean clover regeneration in year 3 was 29% higher and total cumulative biomass 13% higher where subterranean clover was sown in a 1 : 1 configuration with lucerne than where the species were mixed together in every drill row. There were fewer consistent benefits of alternative spatial configurations on swards containing phalaris with subterranean clover or with lucerne. Results of the present study appeared to be highly site-specific, or season-dependent, and therefore alternative spatial configurations cannot be recommended as a universal strategy at this time. More research is required to understand the factors driving the responses to alternative spatial configurations observed in the present study. Neither biserrula nor strand medic was superior to subterranean clover in a 3-year pasture phase with lucerne, despite abundant rhizobia compatible with all species at all sites. The density of subterranean clover in year 3 was 29% and 41% higher than of biserrula and strand medic, respectively. A narrow choice of adapted cultivars as well as excessive levels of hard seed for the legumes used in phased pastures with lucerne are suggested as contributing to the inferior performance of the alternative legumes species tested in the study.

Host range and saprophytic competence of Sinorhizobium meliloti — a comparison of strains for the inoculation of lucerne, strand and disc medics

Strains of Sinorhizobium meliloti were compared for their saprophytic competence (ability to survive and colonise) in mildly acidic (pHCa 4.8–5.4) soils, using a ‘cross-row’ technique at 3 field locations in the south-east of South Australia. Some strains of rhizobia had greater saprophytic competence than others. Strain WSM879 performed consistently well, nodulating 36% of lucerne seedlings (mean of 3 sites and 4 sampling regions) compared with former inoculant strain WSM826 which nodulated 27% of lucerne seedlings. At one site, strain WSM879 was compared with the former and current Australian inoculant strains (WSM826 and RRI128, respectively). Here, all 3 strains nodulated a similar percentage of lucerne seedlings. However, the addition of 5 t/ha of lime to the soil at this site increased the percentage of lucerne plants nodulated from 23 to 43%. This increase was due to a combination of better strain survival and colonisation and indicates there remains some potential to further improve these aspects of strain performance. The growth of 4 of the rhizobial strains from the field trials was measured on acidified agar media (between pH 4.0 and 7.5). There was virtually no colony growth (<10% of growth at pH 7.0) by strains WSM826, RRI128 and WSM879, at or below pH 6.0. Although strain MSUR52a was still able to grow (40% of potential) at pH 6.0 (in the absence of aluminium) this was not always reflected in better nodulation of lucerne seedlings by this strain in the field. Inclusion of aluminium in the media increased the sensitivity of the strains to acidity. The ability of 6 selected S. meliloti strains to form effective symbioses with 15 plant hosts (from Medicago sativa, Medicago littoralis and Medicago tornata) was compared. All S. meliloti strains formed effective symbioses with all plant hosts. Overall, strain RRI128 was the most effective strain with both the lucernes and the annual medics, resulting in shoot weights similar to those of plants supplied with mineral nitrogen.

Differential tolerance of annual medics, Nungarin subterranean clover and hedge mustard to broadleaf herbicides

The tolerance of 5 species of annual medics (Medicago spp.), Trifolium subterraneum cv. Nungarin, and hedge mustard (Sisymbrium orientale) to a range of post-emergence broadleaf herbicides was tested over 2 years. The least damaging to M. truncatula was 2,4-DB, which provided more consistent and effective control of hedge mustard than the other chemicals tested, including the less expensive tank mix of 2,4-DB + diuron. MCPA amine or sodium salt (300 g a.i./ha), bromoxynil (420 g a.i./ha), and tank mixes containing MCPA amine (150-175 g a.i./ha) severely damaged annual medics, particularly M. truncatula, with flowering delayed by up to 21 days, and dry matter and seed yields often significantly (P<0.05) reduced. Seed yields of most test plants indicated a degree of recovery from herbicide damage assessed visually after 10 days. Hedge mustard was not always as severely damaged by MCPA amine as was M. truncatula. Nungarin subterranean clover and M. aculeata SAD 2356 were more tolerant than the M. truncatula cultivars of MCPA amine, MCPA sodium salt, MCPA tank mixes, and bromoxynil, and less tolerant of 2,4-DB. Medicago littoralis, M. polymorpha and M. laciniata were severely damaged by bromoxynil but were more tolerant of MCPA than M. truncatula.

Host range and physiologic specialisation of Macrophomina phaseolina isolated from field peas in South Australia

Macrophomina phaseolina is reported as a pathogen of field peas, causing leaf, stem, and petiole lesions characteristic of ascochyta infection. Alternative hosts were found to include Vigna mungo, Medicago littoralis, Medicago scutellata, and Lens culinaris. Twenty-one isolates of M. phaseolina were differentiated into 15 pathotypes. Adequate sources of resistance were identified against all pathotypes. Breeding for specific resistance to M. phaseolina in field peas is discussed.