Silverleaf nightshade (Solanum elaeagnifolium) or SLN is a Weed of National Significance in Australia, mainly infesting the southern cereal cropping zone and with the potential to infest 400 million hectares. SLN management relies on herbicides, but efficacy is influenced by factors such as plant morphology, genetic background, growth stage and environmental conditions. This study investigated the extent and cause of morphological and genetic variation, the distribution of different phenotypes and genotypes, and how the modes of reproduction contribute to the adaptability of SLN in Australia. SLN is a herbaceous, perennial weed that reproduces both sexually and asexually. In Australia, plants germinate from the soil seedbank and rootbank from September (spring) to April (autumn), fruits normally form in January, and the aerial growth senesces in May. High morphological variation was found between 642 SLN individuals from south-eastern Australia. Leaf length, width and area ranged from 1.44 to 10.6 cm, 0.39 to 4.09 cm, and 0.41 to 25.8 cm2 , respectively. High trichome densities were found on both leaf surfaces (67 and 132 trichomes/mm2 on the adaxial and abaxial surface, respectively). Larger leaves usually have lower adaxial trichome densities than smaller leaves. On average, there were 603 and 814 stomata/mm2 on the adaxial (ranged from 284 to 942 stomata/mm2 ) and abaxial (ranged from 455 to 1519 stomata/mm2) surfaces, respectively. These morphological variations may influence foliar herbicide coverage, retention and uptake. Nineteen SSR primer-pairs and four AFLP primer combinations detected a high level of genetic diversity of 94 SLN populations, with average Jaccard’s coefficient for SSR and AFLP analysis at 0.43 and 0.26, respectively. High intra- and inter-population genetic diversity was found, suggesting SLN has capacity to adapt and persist under a range of management systems and environments. This study also improved the differentiation of SLN from the morphologically similar native perennial Solanum species S. esuriale (quena). Misidentification can result in delay of management as S. esuriale is not a problem unless high densities occur. Compared to S. esuriale, SLN has a significantly longer trichome intrusive base structure on the adaxial leaf surface. In addition, five and seven species-specific SSR bands (alleles) were identified for SLN and S. esuriale, respectively. Distribution of different SLN phenotypes and genotypes were correlated with abiotic factors. Individuals that were taller and had a larger leaf area were distributed in areas where higher rainfall was received during the growing season (2009-2010), indicating the likely impact of rainfall on SLN morphology. In addition, the Bayesian model-based genetic structure analysis assigned Australian SLN into two gene pools. The spatial distribution of these two gene pools correlated well with the early records (1901 and 1918) of SLN in Australia, which may suggest the possible sites of first establishment. The reproductive strategies of SLN can help to explain why it covers such a broad area in Australia. Plants that emerged in spring produced significantly more seeds and root biomass than plants that emerged in the late summer or early autumn, especially for root-generated individuals, thereby resulting in the replenishment of the soil seedbank and rootbank, and contributing to establishment. In addition, accidental transportation of seed in contaminated agricultural products will promote expansion of the species, lead to gene flow within and between populations and contribute to the genetic diversity and adaptability. By contrast, those plants that emerged later (plants from seeds later than January and plants from root later than March) only produced very limited root biomass (dry weight < 0.1 g per plant) before winter. It is not clear whether such a small amount of root biomass is sufficient for the plant to survive over winter and reproduce in subsequent years. The results of this PhD study provide an insight into the morphological and genetic diversity of SLN in Australia. The research also improves SLN identification and enhances an understanding on the adaptability of SLN. These findings will assist in designing effective management strategies.
|Qualification||Doctor of Philosophy|
|Award date||08 Feb 2013|
|Place of Publication||Australia|
|Publication status||Published - 2013|