Seed production of summer and winter-active Helianthus on the Gulf Coast

Background/Question/Methods

Helianthus annuus and H. argophyllus are the most common species of sunflowers in Nueces County, Texas.  Almost all H. argophyllus populations senesce in October.  However, it is not uncommon to see H. annuus growing and flowering during every month of the year.  It appears that there are different cohorts of H. annuus throughout the year and that winter-active H. argophyllus are plants that did not senesce in October.  As annuals, seed production for these species is vital for their continued presence in our area.  In an effort to determine the relative contribution to future generations of different cohorts, we examined seed production and viability of summer-active and winter-active plants of both species.  We expect that the contribution of winter-active H. argophyllus is minor.  For winter-active H. annuus, we eventually hope to determine if winter-active plants come from seeds produced by winter-active plants or from seeds produced at different times of the year. 

Results/Conclusions

The mean number of seeds per inflorescence for summer-active H. annuus (129.6) is not significantly different than the mean number of seeds per inflorescence for summer-active H. argophyllus (113.8).  In a previous study, we found that H. argophyllus produces significantly more (P < 0.0001) inflorescences per plant than does H. annuus.  Using these data, we estimate that an average H. argophyllus plant produces 1055 seeds compared to 938 seeds for an average H. annuus plant.  Because density for these two species is similar, the seed bank for H. argophyllus has the potential to be slightly larger than the seed bank for H. annuus.  These data indicate that both species produced good numbers of seeds during a historic drought year in South Texas.