Abstract: Predicting how plants will respond and potentially adapt to warmer air temperatures resulting from anthropogenic climate change is dependent on understanding how plants have already adapted to different environments. Studies of local adaptation of populations of wide-spread plant species provide clues about vulnerabilities to increased temperatures as well as adaptive responses. However, we know little about multiple life-stage specific effects of heat on plants, despite the fact that selection can operate during either the diploid sporophytic stage or the haploid gametophytic stage. We studied plant responses to heat for populations of horsenettle (Solanum carolinense) from two different latitudes to quantify vulnerabilities and potential evolutionary divergence between the two regions. Our objectives were to: 1) test for effect of long-term heat on pre- and post-pollination reproductive traits; 2) examine differences in responses to temperature treatments between plants from northern versus southern latitudes; and 3) measure correlations between sporophytic and gametophytic temperature tolerances.
We grew clonal replicates of horsenettle plants from rhizomes collected in multiple populations in Texas and Minnesota. The mature plants were used to test for differential responses to heat in sporophytic traits (cell membrane stability, chlorophyll content, photosynthetic rate, pistil length, proportion of viable seeds , unfertilized ovules and aborted seeds) and gametophytic traits (pollen germination percentage, pollen tube growth rate, ovule number, pollen grain size). We found that northern plants had multiple traits showing greater heat tolerance than southern plants, including pollen germination, leaf chlorophyll content and cell membrane stability. There were also significant interactions between region and heat response in multiple traits, indicating that northern plants reduce pistil size more dramatically in response to heat and increase the proportion of seeds that are aborted more dramatically in response to heat compared to southern plants. No correlations between sporophytic and gametophytic traits were statistically significant. Thus, there is evidence of divergence between the two regions studied in how the differing populations of plants respond to heat. These results suggest that different life-stage specific traits will be affected differentially to climate change with implications for adaptation in a warmer world.
