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 and that the germination phase of development does not appear to play a role in this distribution.  To better understand possible differences in photosynthetic characteristics between these species and if these differences may play a role in their distribution, we determined photosynthetic characteristics in June 2008 at the end of a period six weeks without rain (dry conditions) and again in July 2008 after a significant rain event caused by the passage of Hurricane Dolly (wet conditions).  

Results/Conclusions

Stomatal conductance increased for both species between dry and wet conditions (P=0.043).  However, the significant interaction term (P=0.0015) indicates the increase was greater for H. annuus than for H. argophyllus.  Helianthus annuus has higher photosynthetic rates than does H. argophyllus at both dry and wet conditions (P<0.0001).  The rates increased for both species between dry and wet conditions in a similar manner (interaction term P=0.96).  These results indicate that a more complete study of photosynthetic characteristics of these species grown on both soil types may help us to understand fidelity to soil type observed in the field.