Photosynthetic Characteristics of Winter-flowering Helianthus on the Gulf Coast

Helianthus annuus is the most common species of sunflower in North America and is commonly found growing and flowering year-round on the Gulf Coast of Texas in what appears to be at least two cohorts: winter and summer-flowering. Our hypothesis is that there is no difference in photosynthetic rates between these cohorts.  With that, these cohorts could do equally well in their respective seasons.  As climate changes, this could lead to the establishment of winter-flowering cohorts in more northern areas.  We used a LI-COR 6400 to measure photosynthetic characteristics of 51 H. annuus winter-flowering plants.  The mean photosynthetic rate of winter-flowering plants measured in January and February is 31.5 µmol m^-2sec^-1, which is lower than summer-flowering plants measured in another study.  To determine the effect of temperature, we measured winter and summer-flowering plants on the same day in March.  The winter-flowering mean is 38.8 µmol m^-2sec^-1 and the summer-flowering mean is 41.7 µmol m^-2sec^-1. We will use ANOVA to better understand differences that may exist at different measurement times.  Even if the rate for winter-flowering plants is lower than summer-flowering plants, the rates for winter-flowering plants are high for most plant species.  This would allow winter-flowering plants to be almost as successful as summer-flowering plants.  Ultimately, we would like to determine if progeny from winter and summer-flowering plants do equally well in both growing seasons and if H. annuus could extend its growing season in regions farther north of our area if winters become warmer in these areas.