Soil salinity, a condition stressful for most land plants, is increasing globally. The goal of this study is to understand the ecological and genetic factors that contribute to the origin of extreme salt tolerance in Helianthus sunflowers. Specifically: (1) Does hybridization contribute novel genetic variation necessary for the evolution of salt tolerance? We tested this by comparing the survival of three artificial salt-selection lines (within H. annuus, within H. petiolaris & hybrid annuus x petiolaris) relative to hybrid halophyte H. paradoxus in a high salinity paradoxus field site. (2) Alternatively, can extreme salt tolerance evolve within H. annuus populations occurring in high salinity environments? We estimated phenotypic traits related to salt tolerance for H. annuus plants from naturally occurring high- versus low-salinity sites. Plants were grown under control, medium-saline, and high-saline conditions. Growth in plant height and leaf senescence were both recorded, which are predicted to reflect osmotic and ionic salt stress, respectively. Variation in leaf succulence, total aboveground and belowground biomass were estimated after plant harvest. We predicted that plants from high-salinity H. annuus source populations would exhibit a lesser negative effect on growth and leaf damage under high salinity relative to plants from low-saline H. annuus populations.
We predicted high salt tolerance would evolve in the hybrid salt-selection line due to novel genetic combinations, whereas salt tolerance would remain low in parental lines (annuus & petiolaris). As expected, individuals from wild annuus and petiolaris populations did not survive >6 days in saline field conditions, whereas 70% of paradoxus individuals survived the 42-day experiment. Unexpectedly, after four generations of salt-selection, individuals from the within-annuus and within-petiolaris lines survived at a moderate rate (38-52%), similar to the survival of individuals from the hybrid line (34%). This suggests that hybrid variation is not necessary for the evolution of salt tolerance. Rather, salt tolerance may evolve from standing genetic variation when populations are exposed to high salinity.
Comparing phenotypic traits from naturally occurring saline versus non-saline annuus sites, we detected the expected detrimental effect of salinity treatment on all estimates of plant performance. However, we did not detect significant differences between saline and non-saline sources in trait values or interaction between treatment and salinity-source-type for key salt tolerance traits. Overall this suggests that high-saline annuus populations might be maladapted, and restricted gene flow may play an important role in the evolution of extreme salt tolerance.