Seed Production of Summer and Winter-flowering Helianthus on the Gulf Coast

Helianthus annuus is the most common species of sunflowers in North America.  This species is commonly found growing and flowering throughout the year on the Gulf Coast of Texas in what appears to be at least two cohorts: winter-flowering and summer-flowering. Another common species in this area, H argophyllus, has far fewer plants flowering in the winter. Because these species are annuals, seed production is extremely important to their persistence. To determine the reproductive success of these cohorts and to see if winter-flowering significantly contributes to their seed banks, we examined seed production of summer and winter-flowering plants by counting seeds per inflorescence.  We will also test viability of seeds using the tetrazolium test. We hypothesize that seed production will not differ between these cohorts within a species, but that winter-flowering plants will be much more important to the persistence of H. annuus than to H. argophyllus.  The mean number of seeds per inflorescence for summer-flowering H. annuus is 129.6 and for H. argophyllus is 113.8.  We will complete the rest of the study by early summer.  Our data thus far indicates that both species produced good numbers of seeds, even during a historic drought year (2011) in South Texas. Our ultimate goal is to determine if winter-flowering plants come from seeds produced by winter-flowering plants or from seeds produced at different times of the year.  Whether or not the cohorts are separate has implications for the persistence of annual plants, such as sunflowers, under climate change.