Photosynthetic characteristics of 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.  In previous studies, we found H. argophyllus was restricted to sandy soil sites, while H. annuus was nearly restricted to clay soil sites.   While almost all H. argophyllus populations senesce in October, it is not uncommon to see H. annuus growing and flowering during every month of the year.  Data from a related study indicates that there are different cohorts of H. annuus throughout the year and winter-active H. argophyllus are plants that did not senesce in October.  We measured photosynthetic characteristics of 25 H. annuus and 15 H. argophyllus winter-active plants in January and February 2012.  Height, leaf area and specific leaf area were determined for 10 winter-active H. annuus plants.  Ultimately, we would like to determine if progeny from winter-active and summer-active 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. 

Results/Conclusions

An ANOVA indicates photosynthetic rates for summer-active plants are significantly higher than are photosynthetic rates for winter-active plants (P < 0.0001) and that photosynthetic rates for winter-active H. annuus and H. argophyllus are not significantly different.  However, the rates for winter-active plants are not low for most plant species.  Temperature, and to a lesser extent light level, may be partly responsible for these differences.  Perhaps, due to lower winter temperatures, stomatal conductance is generally lower (P < 0.0001) for winter-active plants.  We hope to determine if differences in photosynthetic rates between plants growing during different seasons are due to differences in photosynthetic characteristics between these populations or are due to different environmental conditions during these seasons.