Annual Pasture Legumes Research Articles

Developmental patterns of flowers and pods and the effect on seed number in French serradella (Ornithopus sativus) and yellow serradella (Ornithopus compressus) cultivars

Context Reliable seed production is a key requirement for successful year-on-year regeneration of annual pasture legumes. Aims The study aims were to investigate the developmental patterns of flowers and pods and the effect on seed number among cultivars of French (Ornithopus sativus Brot.) and yellow serradella (O. compressus L.); and to assess the effects of early flower loss. Methods Four cultivars of each species were grown in a glasshouse under non-limiting growth conditions. Date of flowering and numbers of flowers, pods and seeds were assessed for up to 20 reproductive nodes on two stem axes per plant (n = 5 plants). A flower removal treatment was imposed to assess whether early flower loss affected flower and/or pod production. Key results Flowering in the serradellas was indeterminate, but for all cultivars there was a peak period of flower and pod production, with the timing and duration of the peak period differing among cultivars. Peak flowering occurred primarily because the proportion of plants flowering began to decline, but the number of flowers per reproductive node and the number of pods formed per node also declined with time. Compensation for early flower loss was observed for most cultivars because of a longer duration of pod formation and/or greater numbers of pods developed on higher reproductive nodes. Conclusions This study demonstrated that there is diversity in the patterns of flowering and podding and number of seeds initiated among serradellas. Implications Diversity in flowering and podding patterns combined with a capacity to compensate for early flower loss may be used to develop serradellas better able to cope with environmental stressors (frost, drought, heat) experienced during the flowering window.

Development of methods to overcome physiological seed dormancy of temperate annual pasture legumes to assist speed breeding

Context and aims Freshly harvested seeds of temperate annual pasture legume species have physiological dormancy (PD), which delays the growth of the next generation and is thus an impediment to speed breeding. The aim of this study was to develop a method or methods of PD breaking that can achieve high germination (≥90%) within 5 days of picking freshly harvested seed. Methods In a series of experiments, PD breaking methods by themselves and in combination were tested on burr medic (Medicago polymorpha). The most promising methods were then tested on other annual medic species (Medicago spp.), subterranean clover (Trifolium subterraneum), and alternative species from six genera. Key results The successful PD breaking method involved drying pods overnight at 45°C, extracting and scarifying seed, placing seed on filter paper in a Petri dish along with non-dormant seed, imbibing seed with 4 μM benzylaminopurine, sealing the Petri dish, and incubating in the dark at 15°C. Highly dormant genotypes required the additional treatment of removing the testa. Conclusions We have developed an effective way of breaking seed PD in the 11 pasture legumes species from the seven genera tested. The rapid breaking of seed PD will assist speed breeding of annual pasture legumes, making it less laborious and time-bound than currently used methods. Implications The PD breaking methods are expected to assist basic science studies of annual pasture legume species and may be applicable to other species and genera.

Survival analysis of germination data in response to temperature for Ornithopus species and other temperate pasture legumes.

Cool temperatures can limit productivity of temperate grazing systems as poor pasture growth rates in winter create feed shortages for livestock. Ornithopus spp. (serradella) are broadly adapted annual pasture legumes that produce high-quality forage in soil types considered marginal for other temperate legume species. However, serradella establishment is perceived to be difficult in cool-season environments. We used survival analysis to compare germination rate and seedling emergence for two serradella species (yellow serradella and French serradella) against three reference species (Medicago sativa, M. polymorpha and Trifolium subterraneum ) in four temperature treatments (10/5, 15/10, 20/15 and 25/20°C; max/min). We also compared shoot relative growth rate and photosynthetic rate at 15/10°C (cool) and 23/18°C (warm). Cool temperatures (10/5, 15/10°C) did not slow germination rates for serradella relative to the reference species, but warm temperatures (20/15, 25/20°C) delayed emergence and reduced post-emergent shoot growth rates. Once established, Ornithopus spp. had similar mean photosynthetic rates and stomatal conductance at cool temperatures to the reference species. We conclude that, contrary to common perception, cool temperatures did not adversely influence germination, emergence, or early growth of Ornithopus spp. relative to the reference species.

Can arbuscular mycorrhizal fungi mitigate drought stress in annual pasture legumes?

Can arbuscular mycorrhizal fungi mitigate drought stress in annual pasture legumes?

Selecting higher nutritive value annual pasture legumes increases the profitability of sheep production

CONTEXTThe nutritive value of annual pasture legumes changes with phenology during each season, with a progressive decrease in feeding value as plants accumulate structural carbohydrates and nutrients are diluted. However, these changes differ widely among annual legume species, affecting the pattern of nutrient availability and production in a grazing enterprise. OBJECTIVEThis paper set out to examine the implications of differences in the nutritive value of annual pasture legumes in relation to their economic value for mixed farming systems of southern Australia. METHODSCalibrated GrassGro™ plant models with data generated from field experiments to represent Dry Matter Digestibility (DMD) profiles of six annual legume species; Medicago truncatula Gaertn. (barrel medic), B. pelecinus L. (biserrula), Trifolium spumosum L. (bladder clover), Ornithopus sativus Brot. (French serradella), Medicago littoralis Rhode ex Loisel. (strand medic) and Trifolium subterraneum L. (subterranean clover). The plant models were used in a modelling study to support a self-replacing Merino sheep enterprise, which was simulated at seven locations across southern Australia, where annual pasture species are heavily relied on for livestock production. RESULTS AND CONCLUSIONSModelled values of DMD of green material, averaged across all phenological stages, ranged from approximately 62% (French serradella) to 73% (bladder clover), with little variation due to location. Pastures with higher nutritive value had a reduced requirement for supplementary feeding of sheep. Variation in nutritive value had a greater effect on supplementary feeding costs and gross margins than differences in biomass production among the annual legume species. Differences in the levels of supplementary feeding required in systems with the different legumes was most pronounced during the period from May to July (late autumn to winter). For sheep enterprises compared at the same stocking rate, the total supplementary feed required was 18% lower with bladder clover pastures, and 9% lower in subterranean clover pastures, compared with an average of the remaining four pasture species. SIGNIFICANCEThis is the first study to compare the effects of variation in nutritive value among the annual pasture legumes developed for the soils and climatic conditions of southern Australia. Our results demonstrated that the magnitude of differences in nutritive value among annual pasture legumes are economically important and that the establishment of higher nutritive value legumes will reduce supplementary feeding costs in extensive grazing systems, in some cases even where their biomass production is lower.

An Improved Protocol for Agrobacterium-Mediated Transformation in Subterranean Clover (Trifolium subterraneum L.).

Subterranean clover (Trifolium subterraneum) is the most widely grown annual pasture legume in southern Australia. With the advent of advanced sequencing and genome editing technologies, a simple and efficient gene transfer protocol mediated by Agrobacterium tumefaciens was developed to overcome the hurdle of genetic manipulation in subterranean clover. In vitro tissue culture and Agrobacterium transformation play a central role in testing the link between specific genes and agronomic traits. In this paper, we investigate a variety of factors affecting the transformation in subterranean clover to increase the transformation efficiency. In vitro culture was optimised by including cefotaxime during seed sterilisation and testing the best antibiotic concentration to select recombinant explants. The concentrations for the combination of antibiotics obtained were as follows: 40 mg L−1 hygromycin, 100 mg L−1 kanamycin and 200 mg L−1 cefotaxime. Additionally, 200 mg L−1 cefotaxime increased shoot regeneration by two-fold. Different plant hormone combinations were tested to analyse the best rooting media. Roots were obtained in a medium supplemented with 1.2 µM IAA. Plasmid pH35 containing a hygromycin-resistant gene and GUS gene was inoculated into the explants with Agrobacterium tumefaciens strain AGL0 for transformation. Overall, the transformation efficiency was improved from the 1% previously reported to 5.2%, tested at explant level with Cefotaxime showing a positive effect on shooting regeneration. Other variables in addition to antibiotic and hormone combinations such as bacterial OD, time of infection and incubation temperature may be further tested to enhance the transformation even more. This improved transformation study presents an opportunity to increase the feeding value, persistence, and nutritive value of the key Australian pasture.

Salinity tolerance and avoidance mechanisms at germination among messina (

Messina (Melilotus siculus) cv. Neptune, an annual pasture legume native to the Mediterranean Basin, has recently been released for saltland pastures in southern Australia following demonstration of biomass production and persistence superior to other commercial pasture legumes in saline environments prone to winter waterlogging. Self-regenerating annual pasture legumes also require seed adaptations for both tolerating and avoiding salinity at germination in these environments. This study examined diversity within Neptune and 20 other messina accessions for salt tolerance at germination, recovery of germinability from temporary salt stress, and timing and extent of hardseed softening, compared with balansa clover (Trifolium michelianum) cv. Frontier, burr medic (Medicago polymorpha) cv. Scimitar and white melilot (Melilotus albus) cv. Jota. Germination rates after 14 days at 300 mM NaCl relative to 0 mM NaCl were ≥99% for Neptune and 18 other messina accessions, 66% for Scimitar, 21% for Jota and 11% for Frontier. No genotype germinated at 600 mM NaCl; however, when transferred to 0 mM NaCl after 14 days at 600 mM, all genotypes except Scimitar and Jota recovered partial germination, ranging from 13% to 93% of controls (0 mM NaCl for 28 days). The softening rate of hard (impermeable) seeds in the field varied among genotypes, with deferral of hardseed softening until late autumn–early winter, when rainfall is more likely, indicating greater persistence. The months in which the hardseed level first became significantly lower than the initial level (in freshly harvested seeds) after placement on the soil surface in December were: March for Frontier; April for Scimitar, Jota and Neptune; and March–July for all messina accessions. This study confirmed that messina has high salt tolerance and several avoidance mechanisms at germination that contribute to its adaptation to saline soils in southern Australia. Several messina accessions were superior to Neptune for individual traits which could be exploited for plant breeding. These results also have implications for saltland pastures in other regions of the world with Mediterranean-type climates.

Sensitivity of the messina (

Messina (Melilotus siculus) is a new annual pasture legume with better combined waterlogging and salt tolerance than other annual legumes. Messina cv. Neptune and a new salt-tolerant rhizobial symbiont (Sinorhizobium medicae SRDI-554) were made available to Australian growers in 2017. Messina is related to the annual medics (Medicago spp.) that are nodulated by the same genus of rhizobia and regarded as sensitive to soil acidity. Because some saltland soils are acidic, it is important to understand the sensitivity of messina to soil acidity in order to avoid failures during early adoption. Acidity tolerance of the messina–Sinorhizobium symbiosis was investigated in a hydroponic experiment (inoculation with SRDI-554, or the salt-intolerant strain WSM-1115 recommended for medics), and in three acidic soils (pHCa 4.3–5.5) (inoculation with SRDI-554 ± lime pelleting of seed), in the greenhouse. In the hydroponic experiment, the percentage of messina plants (with SRDI-554) that formed nodules declined at pH levels between 5.7 (43%) and 5.5 (4%). Strain SRDI-554 was slightly more sensitive to acidity than strain WSM-1115. In the acidic soils, more plants formed nodules than in the hydroponic experiment at similar pH levels; however, without lime pelleting, nodule number was inadequate at soil pHCa <5.5. Addition of lime to seed was beneficial to messina nodulation. Nodule number per plant increased from 4.0 to 9.6 with the addition of lime. The messina–Sinorhizobium symbiosis was confirmed as sensitive to low pH. At pHCa 5.5, which is the level recommended as the lower limit for growing messina, nodule number was constrained in both hydroponics and soil. The risk of suboptimal nodulation would be reduced if the recommended lower soil pH limit for growing messina is increased to pHCa 5.8, in line with most annual medics. Efforts to improve the acidity tolerance of the messina symbiosis would be best focused on the rhizobial symbiont, rather than the plant.

Harvesting subterranean clover seed – current practices, technology and issues

Subterranean clover (Trifolium subterraneum L.) is Australia’s most widely sown annual pasture legume. Its widespread use as a pasture plant requires a well-functioning seed production industry, and Australia is the only significant producer of subterranean clover seed globally. However, the sustainability of this industry is under threat due to its reliance on ageing harvest equipment and the resultant environmental impacts. In order to evaluate seed harvesting practices, technology, and issues, we report on case studies, workshops, and a survey of seed producers across southern Australia. The Horwood Bagshaw Clover Harvester, designed in the 1950s, remains the most popular subterranean clover seed harvester. We discuss its use and modifications, and document several contemporary issues facing the seed production industry. Issues are primarily soil erosion and degradation; the expensive, slow and labour-intensive harvest process; and poor reliability and maintainability of harvesters that are now at least 30 years old. We conclude the root cause of these issues is the suction harvest technology utilised by the Horwood Bagshaw Clover Harvester. Analysis of the current harvest system is provided to support the development of new approaches to harvest subterranean clover seeds.

Genetic analysis of boron tolerance in burr medic (

Soils with toxic levels of boron (B) are widespread in the cereal-livestock zone of southern Australia. The annual pasture legume burr medic (Medicago polymorpha L.) is widely grown in rotation with grain crops in this zone, but current cultivars are susceptible to high levels of B. We tested the boron tolerance of several putative B tolerant burr medic accessions and developed four F2 populations by crossing two tolerant accessions with two susceptible cultivars. We tested a B tolerance SSR marker developed for barrel medic (M. truncatula Gaernt) on tolerant burr medic accessions but found a new marker was required. We identified several B tolerant burr medic accessions. In our four F2 populations tolerance was inherited in a 3 : 1 (tolerant : susceptible) ratio and we identified a molecular marker that accounts for 0.84 of the variation. The B tolerant accessions, along with the B tolerance marker, will allow for the efficient introgression of B tolerance into widely adapted genetic backgrounds and will allow breeders to efficiently develop cultivar(s) that are tolerant of a widespread subsoil constraint.

Intrinsic root morphology determines the phosphorus acquisition efficiency of five annual pasture legumes irrespective of mycorrhizal colonisation.

Mycorrhizal fungi are ubiquitous in agroecosystems and form symbiotic associations that contribute to the phosphorus (P) acquisition of many plants. The impact of mycorrhizas is most pronounced in P-deficient soil and commonly involves modifications to the root morphology of colonised plants. However, the consequences of mycorrhizal colonisation on root acclimation responses to P stress are not well described. Five annual pasture legumes, with differing root morphologies, were grown to determine the effect of mycorrhizal colonisation on shoot yield, root morphology and P uptake. Micro-swards of each legume were established in pots filled with a topsoil layer that had been amended with five rates of P fertiliser. The topsoil overlaid a low-P subsoil that mimicked the stratification of P that occurs under pasture. Mycorrhizal colonisation improved P acquisition and shoot yield in the low-P soil treatments, but did not reduce the critical external P requirement of the legumes for near-maximum yield. The yield responses of the mycorrhizal plants were associated with reduced dry matter allocation to topsoil roots, which meant that the P acquisition benefit associated with mycorrhizal colonisation was not additive in the P-deficient soil. The contribution of the mycorrhizal association to P acquisition was consistent among the legumes when they were compared at an equivalent level of plant P stress, and was most pronounced below a P stress index of ~0.5. The intrinsic root morphology of the legumes determined their differences in P-acquisition efficiency irrespective of mycorrhizal colonisation.

Metabolic Profiling Provides Unique Insights to Accumulation and Biosynthesis of Key Secondary Metabolites in Annual Pasture Legumes of Mediterranean Origin.

Annual legumes from the Mediterranean region are receiving attention in Australia as alternatives to traditional pasture species. The current study employed novel metabolic profiling approaches to quantify key secondary metabolites including phytoestrogens to better understand their biosynthetic regulation in a range of field-grown annual pasture legumes. In addition, total polyphenol and proanthocyanidins were quantified using Folin–Ciocalteu and vanillin assays, respectively. Metabolic profiling coupled with biochemical assay results demonstrated marked differences in the abundance of coumestans, flavonoids, polyphenols, and proanthocyanidins in annual pasture legume species. Genetically related pasture legumes segregated similarly from a chemotaxonomic perspective. A strong and positive association was observed between the concentration of phytoestrogens and upregulation of the flavonoid biosynthetic pathway in annual pasture legumes. Our findings suggest that evolutionary differences in metabolic dynamics and biosynthetic regulation of secondary metabolites have logically occurred over time in various species of annual pasture legumes resulting in enhanced plant defense.

Morphological diversity within a core collection of subterranean clover (Trifolium subterraneum L.): Lessons in pasture adaptation from the wild.

Subterranean clover (Trifolium subterraneum L.) is a diploid self-pollinated annual pasture legume native to the Mediterranean region and widely sown in southern Australia and other countries with Mediterranean-type climates. This study utilised a core collection of 97 lines, representing around 80% of the genetic diversity of the species, to examine morphological diversity within subterranean clover. A total of 23 quantitative agro-morphological and 13 semi-quantitative morphological marker traits were assayed on the core collection and 28 diverse Australian cultivars as spaced plants in a replicated common garden experiment. Relationships between these traits and 24 climatic and edaphic parameters at their sites of origin were also examined within the core collection. Significant diversity was present for all traits. The Australian cultivars had similar levels of diversity to the core collection for several traits. Among the agro-morphological traits, time to flowering, leaf size and petiole diameter in mid-winter, plant area in late winter, maximum stem length, content of the oestogenic isoflavone biochanin A and total isoflavone content, were correlated with seven or more environmental variables. These can be considered highly adaptive, being the result of strong environmental selection pressure over time. For the first time in a clover species, morphological markers, including leaf mark, anthocyanin pigmentation and pubescence traits, have been associated with rainfall and soil parameters. This suggests they either have an adaptive role or the genes controlling them may be linked to other genes controlling adaptive traits. This study demonstrated the value of core collections to examine diversity within much larger global collections. It also identified adaptive traits from wild plants that can be utilised to develop more productive and persistent subterranean clover cultivars. The high heritability of these traits indicates that selection gains can be readily made.

Metabolomic approaches for the identification of flavonoids associated with weed suppression in selected Hardseeded annual pasture legumes

Weed suppressive potential of annual pasture legumes has been previously described, the mechanism of interference with weeds has not been clearly elucidated. We, therefore, aimed to delineate the role of secondary metabolites synthesized and released by pasture legumes through a series of studies to: 1) characterize key metabolites present in plant tissues, residue and the rhizosphere and 2) correlate their presence with weed suppressive properties. Field and in vitro experimentations coupled with targeted and non-targeted metabolic profiling was performed to evaluate the abundance of key phytotoxic metabolites using UHPLC QTOF-MS. Further, those metabolites strongly correlated with weed suppression and phytotoxicity were predicted by chemometric analyses and their concentration evaluated in field soils collected from the same legume site. Field experimentation, methanolic extracts and dried residues of Biserrula pelecinus L. and Ornithopus compressus L. exhibited marked phytotoxicity in a series of laboratory experiments. Metabolic profiling revealed that both foliar tissues and rhizosphere soils of pasture legumes possessed a high abundance of flavonoids and their precursors. Chemometric analyses suggested the strong association of quercetin, isoquercetin, kaempferol, and kaempferol-7-O-glucoside with weed suppression under field conditions. Specifically, the abundance of quercetin and kaempferol was significantly higher in soils collected from established stands of biserrula and yellow serradella in contrast to arrowleaf, gland and subterranean clover. Current study provided an evidence for the role of certain annual legume-produced flavonoids in weed suppression in southern Australia and further insight into their localization and release in the soil rhizosphere.

Anti-methanogenic advantage of biserrula (Biserrula pelecinus) over subterranean clover (Trifolium subterraneum) from in vitro fermentation is maintained across growth stages and cutting treatments

The annual forage legume biserrula (Biserrula pelecinus L.) offers a promising opportunity as a low-methanogenic, bioactive pasture for southern Australian grazing systems where subterranean clover (Trifolium subterraneum L.) is the dominant annual pasture legume. This in vitro study to assess methanogenic potential examined how growth stage and cutting of biserrula affect biomass, nutritive value and fermentative parameters including methanogenic potential compared with subterranean clover. Both species were grown in a glasshouse, where three growth-stage treatments comprised plant collection at vegetative, reproductive or maturity growth stages. Three cutting (simulated grazing) treatments included cutting at the vegetative or reproductive stage and an uncut control, with herbage collection at maturity. Methane production in biserrula was unaffected by the stage of growth. Other nutritive value and fermentative parameters varied significantly with growth stage, the highest fibre content and lowest crude protein (CP) being found at maturity. Regrowth from herbage cut at the reproductive stage had higher CP and lower biomass than other cut treatments. In biserrula, this regrowth also showed the highest fermentability of the three cutting treatments including elevated methane yield. Notwithstanding these treatment effects on fermentability, biserrula maintained its strong anti-methanogenic advantage over subterranean clover across all treatments, confirming its potential as an anti-methanogenic bioactive pasture.

Field benchmarking of the critical external phosphorus requirements of pasture legumes for southern Australia

In recent decades several pasture legumes have been available in southern Australia as potential alternatives to the most widely used annual pasture legume Trifolium subterraneum. Little is known about their soil phosphorus (P) requirements, but controlled environment experiments indicate that at least some may differ in their P fertiliser requirements. In this study, pasture legume varieties, including T. subterraneum as the reference species, were grown at up to four sites in any one year over a 3-year period (in total, seven site × year experiments) to measure herbage growth responses in spring to increased soil P availability. A critical soil test P concentration (corresponding to 95% maximum yield) was estimated for 15 legumes and two pasture grasses. The critical soil P requirements of most of the legumes did not differ consistently from that of T. subterraneum, indicating their soil fertility management should follow the current soil test P guidelines for temperate Australian pastures. However, the critical P requirement of Medicago sativa was higher than that of T. subterraneum, but remains ill-defined because extractable soil P concentrations in these experiments were often not high enough to permit a critical P estimate. Three forage crop legumes (Trifolium incarnatum, Trifolium purpureum, Trifolium vesiculosum) and two pasture legumes (Ornithopus compressus, Ornithopus sativus) had lower critical soil test P concentrations. It may be feasible to manage pastures based on these species to a lower soil test P benchmark without compromising yield.

Performance and weed-suppressive potential of selected pasture legumes against annual weeds in south-eastern Australia

Mixed farming systems have traditionally incorporated subterranean clover (Trifolium subterraneum L.) and lucerne (Medicago sativa L.) as key components of the pasture phase across south-eastern Australia. However, poor adaptation of subterranean clover to acidic soils, insufficient and inconsistent rainfall, high input costs, soil acidification and the emergence of herbicide-resistant weeds have reduced efficacy of some traditional clover species in recent years. To overcome these challenges, numerous novel pasture species have been selectively improved and released for establishment in Australia. Despite their suitability to Australian climate and soils, limited knowledge exists regarding their weed-suppressive ability in relation to establishment and regeneration. Field trials were therefore conducted over 3 years in New South Wales to evaluate the suppressive potential of selected pasture legume species and cultivars as monocultures and in mixed stands against dominant annual pasture weeds. Pasture and weed biomass varied significantly between pasture species when sown as monocultures, but mixtures of several species did not differ with regard to establishment and subsequent weed infestation. Arrowleaf clover (T. vesiculosum Savi.) and biserrula (Biserrula pelecinus L.) cv. Casbah showed improved stand establishment, with higher biomass and reduced weed infestation compared with other pasture species. Generally, weed suppression was positively correlated with pasture biomass; however, yellow serradella (Ornithopus compressus L.) cv. Santorini exhibited greater weed suppression than other pasture legumes while producing lower biomass, thereby suggesting a mechanism other than competition for resources affecting weed-suppressive ability. Over the period 2015–17, arrowleaf clover and biserrula cv. Casbah were generally the most consistent annual pasture legumes with respect to yearly regeneration and suppression of annual pasture weed species.

Contrasting communities of arbuscule-forming root symbionts change external critical phosphorus requirements of some annual pasture legumes

Annual pasture legumes with a superior ability to acquire soil phosphorus (P) and a low external critical P requirement could reduce the need for P fertiliser. Roots of pasture legumes grown in field soil will commonly be colonised by arbuscular mycorrhizal fungi (AMF). However, recent research suggests a second community of arbuscule-forming root-colonising fungi may be present: fine root endophytes (FRE). AMF are known to enhance P acquisition by plants under certain conditions, whereas very little is known about FRE and their impact on plant P status. We, therefore, grew plants at seven levels of P supply to determine and compare the external critical P requirements of two species of Ornithopus and two cultivars of Trifolium subterraneum when colonised predominantly by FRE (FRE-dominant treatment) or both FRE and AMF (mixed treatment). As expected from previous studies, the Ornithopus species had a lower critical P requirement than the T. subterraneum cultivars. However, for the Ornithopus species only, the FRE-dominant community had a significantly lower external critical P requirement (24–31 mg P kg−1 soil) than the mixed community (50–58 mg P kg−1 soil). Moreover, at low-P supply (≤15 mg applied P kg−1 soil), Ornithopus species in the FRE-dominant treatment had longer specific root length, smaller average root diameter, 45–128% greater total root length, and lower shoot and root P concentration than in the mixed treatment. We concluded that comparisons among experiments of root morphological traits and external critical P requirements can be affected by the presence of different communities of arbuscule-forming root-colonising fungi and that these effects may vary among plant species.

Differences in nutrient foraging among Trifolium subterraneum cultivars deliver improved P-acquisition efficiency

The growth and root morphology responses to soil phosphorus (P) fertility by five cultivars of Trifolium subterraneum (a temperate annual pasture legume) were examined to assess whether differences in root morphology and/or root acclimation to P stress influenced P-acquisition by the clover varieties, or their critical P requirements (i.e. the rate of P supply needed for maximum shoot yield). The clovers were grown as microswards in soil with P stratified in a topsoil layer to mimic growth conditions and soil P availability in a pasture. Yield and P content of shoots, and roots from the topsoil and subsoil layers was determined after 5 weeks growth in a controlled-environment cabinet. The lengths, diameters, and root hair lengths of nutrient foraging roots from the topsoil layer were quantified. The shoot yield of the cultivars was similar when grown with high soil P fertility. However, the cultivars varied up to 1.5-fold in their ability to yield at low levels of soil P supply, and by 1.6-fold in their critical P requirements. All cultivars acclimated to low P soil by increasing root length density in the topsoil but those that yielded relatively well did so by maintaining a large root hair cylinder volume (i.e. they explored more soil) under low soil P conditions. This was associated with maintenance of dry matter allocations to topsoil roots and higher specific root lengths. Both factors assisted development of high root length density for nutrient foraging. Root acclimation responses to P were compared among the cultivars at equivalent relative shoot yields to assess the influence of plant P stress as a trigger for nutrient foraging. The least P-efficient cultivars slowed their allocation of dry matter to foraging roots at lower levels of P stress. The results suggest variation within T. subterraneum for root proliferation and specific root length could be targeted in breeding for improved P-acquisition efficiency.

Root morphology acclimation to phosphorus supply by six cultivars of Trifolium subterraneum L

Aims Trifolium subterraneum L. is the predominant annual pasture legume in southern Australia. Cultivars with improved phosphorus (P) foraging ability would improve the P-use efficiency of agricultural systems. We therefore investigated variation in root traits related to P-uptake among six cultivars.