Mechanisms of Cl(-) transport in plants are poorly understood, despite the importance of minimizing Cl(-) toxicity for salt tolerance. This review summarizes Cl(-) transport processes in plants that contribute to genotypic differences in salt tolerance, identifying key traits from the cellular to whole-plant level. Key aspects of Cl(-) transport that contribute to salt tolerance in some species include reduced net xylem loading, intracellular compartmentation and greater efflux of Cl(-) from roots. We also provide an update on the biophysics of anion transport in plant cells and address issues of charge balance, selectivity and energy expenditure relevant to Cl(-) transport mechanisms. Examples are given of anion transport systems where electrophysiology has revealed possible interactions with salinity. Finally, candidate genes for anion transporters are identified that may be contributing to Cl(-) movement within plants during salinity. This review integrates current knowledge of Cl(-) transport mechanisms to identify future pathways for improving salt tolerance.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2009.02060.x

Mechanisms of Cl- transport contributing to salt tolerance

Australian farmers and scientists have embraced the use of new pasture legume species more than those in any other country, with 36 annual and 11 perennial legumes having cultivars registered for use. Lucerne (Medicago sativa), white clover (Trifolium repens), and red clover (T. pratense) were introduced by the early European settlers and are still important species in Australia, but several other species, notably annual legumes, have been developed specifically for Australian environments, leading to the evolution of unique farming systems. Subterranean clover (T. subterraneum) and annual medics (Medicago spp.) have been the most successful species, while a suite of new annual legumes, including serradellas (Ornithopus compressus and O. sativus), biserrula (Biserrula pelecinus) and other Trifolium and Medicago species, has expanded the range of legume options. Strawberry clover (T. fragiferum) was the first non-traditional, perennial legume commercialised in Australia. Other new perennial legumes have recently been developed to overcome the soil acidity and waterlogging productivity constraints of lucerne and white clover and to reduce groundwater recharge and the spread of dryland salinity. These include birdsfoot trefoil (Lotus corniculatus), Talish clover (T. tumens), and hairy canary clover (Dorycnium hirsutum). Stoloniferous red clover cultivars and sulla (Hedysarum coronarium) cultivars adapted to southern Australia have also been released, along with a new cultivar of Caucasian clover (T. ambiguum) aimed at overcoming seed production issues of cultivars released in the 1970s. New species under development include the annual legume messina (Melilotus siculus) and the perennial legume narrowleaf lotus (L. tenuis) for saline, waterlogged soils, and the drought-tolerant perennial legume tedera (Bituminaria bituminosa var. albomarginata). Traits required in future pasture legumes include greater resilience to declining rainfall and more variable seasons, higher tolerance of soil acidity, higher phosphorous utilisation efficiency, lower potential to produce methane emissions in grazing ruminants, better integration into weed management strategies on mixed farms, and resistance to new pest and disease threats. Future opportunities include supplying new fodder markets and potential pharmaceutical and health uses for humans and livestock. New species could be considered in the future to overcome constraints of existing species, but their commercial success will depend upon perceived need, size of the seed market, ease of establishment, and management and safety of grazing animals and the environment. Molecular biology has a range of potential applications in pasture legume breeding, including marker-assisted and genomics-assisted selection and the identification of quantitative trait loci and candidate genes for important traits. Genetically modified pasture plants are unlikely to be commercialised until public concerns are allayed. Private seed companies are likely to play an increasingly important role in pasture legume development, particularly of mainstream species, but the higher risk and more innovative breakthroughs are likely to come from the public sector, provided the skills base for plant breeding and associated disciplines is maintained.

https://bioone.org/journals/Crop-and-Pasture-Science/volume-63/issue-9/CP12194/Temperate-pasture-legumes-in-Australiatheir-history-current-use-and-future/10.1071/CP12194.pdf

Temperate pasture legumes in Australia—their history, current use, and future prospects

Salinity and waterlogging as constraints to saltland pasture production: A review

Halophytes as forages in saline landscapes: Interactions between plant genotype and environment change their feeding value to ruminants

Increasing the proportion of the landscape planted to deep-rooted perennial pasture species is recognised as one of several remedial actions required for the control of dryland salinity in southern Australia. The widespread use of perennials in farming systems is limited at present by the lack of well-adapted perennials that can be grown to reduce recharge in a landscape where drought, soil acidity, temporary waterlogging, infertile soils and unrestricted grazing prohibit the use of many species. The range of plants adapted to salinity also needs to be expanded to stabilise and ameliorate soils already degraded by rising watertables and to increase the profitability of grazing discharge regions within the landscape. This paper describes the steps involved in a national forage screening and breeding program initiated by the Cooperative Research Centre (CRC) for Plant-based Management of Dryland Salinity1, seeking to expand the range of perennial and or salt-tolerant forage plants that can be incorporated into farming systems of southern Australia. It describes the target environments, soil constraints, farming systems and the criteria being considered when assessing the potential of new plants, including assessment of the weed risk posed by introducing new species. This paper forms an introduction to a special issue which presents the outcomes of the pasture species field evaluation and plant breeding program conducted by the CRC.

The search for new pasture plants to achieve more sustainable production systems in southern Australia

Herbage production and persistence of 42 annual pasture legumes from 33 species were measured at five sites across southern Australia. Medicago polymorpha L. was highly productive on soils, particularly those not prone to waterlogging, with soil surface (0–10 cm soil depth) salinity [estimated by electrical conductivity (ECe)] levels in summer of up to 36 dS/m, whereas M. truncatula Gaertn. was productive on ECe levels of at least 11 dS/m. Trifolium michelianum Savi and T. resupinatum L. were highly productive on soils subject to waterlogging, but only where 0–10 cm summer ECe levels were less than 8 dS/m. No commercial species were adapted to highly saline (0–10 cm summer ECe levels >8 dS/m), waterlogged sites. However, Melilotus siculus (Turra) Vitman ex B.D. Jacks. has the potential to fulfil this role, provided an appropriate Rhizobium strain can be selected. Mixtures of species and cultivars should be sown to account for high spatial variability for salinity and waterlogging. Traits for annual legume success in saline landscapes include salinity and waterlogging tolerance in germinating seedlings and mature plants, early flowering, hardseededness and delayed softening of hard seeds. Establishment of regenerating seedlings is associated with the timing of hardseed softening in relation to rainfall events capable of leaching topsoil salts. It is proposed that salinity measurements to determine annual legume suitability for winter-dominant rainfall areas are made in summer or early autumn, when at their highest levels. Transects along salinity and waterlogging gradients are suggested as an alternative method to traditional plots for genotype evaluation.

/pdf/production-and-persistence-of-annual-pasture-legumes-at-five-4vx8y13uqu.pdf

Production and persistence of annual pasture legumes at five saline sites in southern Australia

Herbage production and persistence of 24 perennial legumes from 20 species and 19 perennial grasses from 10 species were measured at five sites across southern Australia that differed in annual rainfall and extent of salinity and waterlogging. At Cranbrook, Western Australia, a site with occasional waterlogging and a summer salinity [estimated by electrical conductivity (ECe)] of 6.9 dS/m in the surface 10 cm of soil, strawberry clover (Trifolium fragiferum L.) and Lotus uligonosis Schkuhr produced 2.7 t/ha in the second year and had the highest legume plant frequencies in year 3, while herbage production of L. tenuis Waldst. & Kit. ex Willd. and L. corniculatus L. was not significantly lower. No grasses produced more than 0.3 t/ha in the second year, but tall wheatgrass (Thinopyrum ponticum (Podp.) Z.-W. Liu & R.-C. Wang) was the most persistent. At Girgarre, Victoria, another site with occasional waterlogging and a summer ECe of 8.0 dS/m, phalaris (Phalaris aquatica L.) produced 8.5 t/ha in the second year, while production of tall wheatgrass and perennial ryegrass (Lolium perenne L.) was not significantly lower. Sulla (Hedysarum coronarium L.) was the only legume that produced more than 1 t/ha. Phalaris, tall wheatgrass and sulla had the highest densities in the third year. At Duranillin (Western Australia) and Keith (South Australia), which both experienced extensive winter waterlogging and had summer ECe >30 dS/m, puccinellia (Puccinellia ciliata Bor) and tall wheatgrass were the only grass or legume species that persisted beyond the first summer. Puccinellia was the only species to produce useful quantities of herbage (1.0 t/ha) in year 3 at Duranillin. No perennial grasses or legumes produced more than 0.35 t/ha in either year 2 or year 3 at Tammin, Western Australia, the lowest rainfall site (330 mm mean annual rainfall) with summer ECe of 10.9 dS/m. Genotypic differences within sites were indicated for several species, suggesting opportunities to develop cultivars better adapted to saline soils

/pdf/production-and-persistence-of-temperate-perennial-grasses-1jey0kqnla.pdf

Production and persistence of temperate perennial grasses and legumes at five saline sites in southern Australia

The effects of salinity and hypoxia on growth, nutritive value, and ion relations were evaluated in 38 species of Trifolium and 3 check legume species (Trifolium fragiferum, Trifolium michelianum, and Medicago sativa) under glasshouse conditions, with the aim of identifying species that may be suitable for saline and/or waterlogged conditions. In the first set of experiments, plants were grown hydroponically at four NaCl concentrations (0, 40, 80, and 160 mm NaCl) and harvested after exposure to these treatments for 4 weeks. NaCl concentrations up to 160 mm reduced dry matter production in most species; however, there were differences in salt tolerance among species, with T. argutum, T. diffusum, T. hybridum, and T. ornithopodioides performing well under the saline conditions (dry matter production was reduced by less than 20%). Concentrations of Na+ and Cl− in the shoots increased with increasing salinity levels, and species again differed in their capacity to limit the uptake of these ions. Dry matter digestibility at 0 mm ranged from 49.8% (T. palaestinum) to 74.0% (T. vesiculosum) and decreased with increasing NaCl concentrations. A second set of experiments evaluated the tolerance of Trifolium species to hypoxic conditions in the glasshouse. Shoot growth, and to a lesser extent root growth, were reduced in all Trifolium species when plants were exposed to stagnant, non-aerated conditions for 28 days, but T. michelianum, T. resupinatum, T. squamosum, T. nigrescens, T. ornithopodioides, T. salmoneum, and T. fragiferum were the least affected species. All species acclimated to the oxygen-depleted conditions by increasing the gas-filled porosity in the roots. This study has provided information that will assist in the identification of forage species for saline and/or waterlogged areas.

M.E. Rogers

Papers

Production and persistence of annual pasture legumes at five saline sites in southern Australia

Production and persistence of temperate perennial grasses and legumes at five saline sites in southern Australia

The influence of NaCl salinity and hypoxia on aspects of growth in Trifolium species