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 the distribution of these two species, we examined photosynthetic characteristics.  In a 2008 study, we found a greater response in stomatal conductance in H. annuus after a rain event. We planned to better understand response to rain events over the season.  However, our area experienced a historic drought during the summer of 2009 which resulted in the premature death of one of our populations.  In late July, we determined photosynthetic rates and stomatal conductance for two populations of H. annuus and two populations of H. argophyllus.  We analyzed data for dry conditions but were unable to characterize response to a rain event. 

Results/Conclusions An ANVOA indicates there are significant differences for photosynthetic rate and stomatal conductance among our four sites (P < 0.0001).  A post hoc Tukey’s test indicates stomatal conductance for the H. annuus population that died by mid August is significantly below the other three populations.  This population’s photosynthetic rate is significantly lower than the other H. annuus population but only significantly lower than one of the H. argophyllus populations.  Photosynthetic rate for the surviving H. annuus population is significantly higher than all other populations.  Results are consistent with previous work indicating that H. annuus displays a larger range of values for photosynthetic rate and stomatal conductance than does H. argophyllus.  Presumably, because the stressed population of H. annuus was still photosynthesizing at rates that were lower but similar to other populations, this population could have recovered had there been a rain event.  The ability of H. annuus to maintain photosynthetic rates with low stomatal conductance under extremely dry conditions may be important in determining fidelity to soil type observed in the field.