Waterlogging and Salinity Tolerance in Lucerne (medicago sativa)

Lucerne (Medicago sativa) is a perennial crop with a deep taproot system penetrating several metres down the soil profile and thus can be integrated into crop rotations to reduce watertable recharge to ameliorate saline and waterlogged sites and to redress the environmental threat of rising saline watertables. Despite this fact, not much is known about specific mechanisms mediating adaptive responses of luceme to waterlogging and salinity stresses. One of the aims of this project was to provide such information. Also, for luceme's ameliorative properties to be fully exploited, tolerant genotypes need to be identified. Keeping in mind a high degree of genotypic variation within luceme in terms of its tolerance to waterlogging and salinity, another aim of this study was to identify physiological indicators, which might become useful screening tools for tolerance to the stresses of waterlogging and salinity. Physiological responses to these abiotic stresses were characterized under a variety of conditions using photosystem (PS) II photochemistry, leaf gas exchange, pigment concentrations, biomass, nutrient dynamics and mesophyll cell anatomy, to find the most appropriate method for screening for stress tolerance in luceme. The practical application of this work is its use as suggested screening tools for the ACIAR funded project Lucerne for Animal Production and the Environment, (collaboration between Australia and China). Significant effects of waterlogging were measured across all genotypes. Chlorophyll fluorescence variation became apparent on day 7 of stress and therefore appeared to be a useful, early non-destructive indicator of waterlogging stress tolerance. Recovery dynamics following waterlogging were investigated in four genotypes by studying PS II photochemistry and relating these to nutrient concentration changes. Nutrient concentrations at the end of recovery regained pre-stress values and PSII photochemistry also largely recovered. Preliminary studies on the concurrent stresses of waterlogging and salinity measuring chlorophyll fluorescence on excised, waterlogged leaves exposed to varying concentrations of salinity indicated a significantly greater stress response than salinity on its own. Germination trials on ten genotypes and varying salinity levels pointed to a genotypic response to salinity, but correlations with parameters measured at later stages of plant ontogeny were not evident. Salinity effects on plant growth characteristics, pigment and nutrient composition, leaf sap osmolality, changes in anatomical and electrophysiological characteristics of leaf mesophyll, and net ion fluxes in roots of six luceme genotypes were investigated. Waterlogging caused a marked reduction in photosynthetic capacity as measured with `CO_2` assimilation rate and chlorophyll fluorescence. A wide range of Fv/Fm values (maximal photosynthetic capacity) was recorded within stressed cultivars, suggesting some scope in discovering more tolerant individuals. However, only minor genotypic differences were detected. This might be due to luceme being a highly cross-pollinating species, as individual luceme populations consist largely of a heterogeneous mixture of genetically heterozygous individuals. This fact seems to overshadow most of the differences between cultivars. Results of the salinity experiment suggest that different luceme genotypes use contrasting strategies to avoid toxic effects of sodium on cell metabolism. Sodium exclusion seemed to be used by at least one of the cultivars under investigation, whereas sodium inclusion and subsequent sequestering into vacuoles appeared to be used by other tolerant genotypes. When selecting genotypes for salt tolerance, it is important to consider the possibility of these different adaptation mechanisms being at play. Overall, this study suggests that multiple traits are involved in determining salt and waterlogging tolerance in lucerne. The problem is additionally exacerbated by a high degree of genetic variability within he!erozygous luceme populations; therefore screening for luceme improvement should rely on several, not just not one selection criteria. Ideally these criteria should be attributable to several physiological mechanisms involved. In addition, it appears that such screening should also be aimed at searching for outstanding individuals within a population, not only at comparing genotypes from a genetically diverse and promising pool of potentially tolerant luceme germplasm.