Aaron Dain MSc, Yakimowski Lab Phenotypic variation of a moderately salt tolerant sunflower (Helianthus annuus) and a halophytic sunflower (Helianthus paradoxus) under extreme environmental salinity Environmental salinity is a global phenomenon that is increasing with anthropogenic change. Salt is toxic to most plants, but less than 1% of plant species are considered highly salt tolerant. Helianthus annuus is considered a moderately salt tolerant glycophyte, although populations occur in the saline Great Salt Lake region of Utah. This raises the question: can populations of glycophytes in saline habitats locally adapt to more extreme salinity? Furthermore, is the range and upper limit of H. annuus within the range associated with its homoploid hybrid descendent, the halophyte H. paradoxus? To investigate variation in the relation between environmental salinity and phenotypic variation, we conducted a common-garden experiment using H. annuus seeds collected from 20 populations of the species’ range with varying concentrations of environmental salinity, and seeds collected from 5 populations of H. paradoxus. Seedlings were treated with control, moderate (field), and high (field x2) salinity treatment solutions mimicking the ionic composition of H. paradoxus habitat. Phenotypic traits related to salt tolerance (above- and below-ground biomass, plant height, and growth rate) of juvenile plants were measured and compared among populations and treatment groups. Quantitative soil salinity data was collected for ten of these populations, and used to investigate the relation between environmental salinity and salt tolerance. In the Great Salt Lake region soil salinity (% sodium) ranged from 1.3 to 37.9 (n = 26 populations); thus, high-saline sites exhibit salinity in the range associated with its halophytic relative H. paradoxus. Under non-saline conditions, H. annuus plants originating from non-saline habitats are larger than those originating from saline habitats. However, under field-like salinity, saline-originating H. annuus plants retain more biomass than non-saline originating plants. H. paradoxus plants are smaller than H. annuus under non-saline conditions but are larger under field-like salinity. This suggests biomass is shaped by environmental salinity. In comparison, growth rate and plant height do not significantly differ between saline- and non-saline H. annuus populations, and those from populations with the highest salinity grow more slowly than those from populations with lower salinity. Additionally, H. paradoxus plants grow more slowly than H. annuus plants. This suggests that plant height and growth rate are not shaped by environmental salinity in the same way, and that growth rate may not be selected for in saline habitats.
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