Associate Professor North Dakota State University fargo, Minnesota, United States
Background/Question/Methods
Climate change is altering temperature regimes throughout the world, threatening plant species sensitive to environmental conditions. To persist, species must have the phenotypic plasticity to acclimate to novel conditions, adapt through natural selection, or shift ranges. Examining intraspecific variation in populations across varying environmental conditions will provide clues to how species will respond to a warming globe. In this study, we compared temperature tolerance in populations of Solanum carolinense from Texas and Minnesota in the sporophytic and gametophytic life stages. Selection can occur in both stages, acting as independent pathways by which traits evolve. Evidence of overlap in gene expression between the sporophyte and gametophyte suggests that selection in both stages could allow rapid evolution. Because monthly temperatures in Texas and Minnesota differ, we hypothesized that local adaptation of traits related to thermotolerance has resulted in a divergence between northern and southern populations. We also predict that temperature tolerance in the sporophyte and gametophyte are positively correlated. To explore sporophytic temperature tolerance, we measured photosynthetic rate, cell membrane stability, and chlorophyll content in leaves exposed to extreme heat and cold. For gametophytic tolerance, we measured pollen germination and pollen tube growth rate at five temperatures ranging from 10°C to 40°C.
Results/Conclusions
We found mixed results for the sporophytic variables, suggesting that the mechanisms involved in maintaining photosynthesis, chlorophyll content, and the integrity of cell membranes have divergent responses to thermal stress. Cell membrane stability, a typical measure of thermotolerance, did not differ between north and south when exposed to extreme heat, but was significantly different when exposed to extreme cold, in which southern plants incurred less damage. Chlorophyll content was significantly different between the two regions for both treatments. Plants from Minnesota had less chlorophyll alteration in both the hot and cold treatments. There was no significant difference between plants from TX and MN for photosynthetic rate. We found a significant difference in pollen germination between plants from Minnesota and Texas at 40°C. Plants from Minnesota had a higher percent germination than plants from Texas, which is not what we expected. However, pollen tube length was not significantly different between plants from the two regions. These results suggest that plasticity in thermotolerance is extensive and that any process of divergence between northern and southern populations is incomplete. There was no significant correlation between the sporophytic and gametophytic stages, suggesting that pathways involved in thermal tolerance in pollen and leaves may differ.
