PS 57-121
A comparison of photosynthesis and secondary chemistry between Helianthus winteri and Helianthus annuus
Winter’s sunflower (Helianthus winteri, Hw), a newly described sunflower species, is generally found on rocky, un-grazed, south facing slopes of the Southern Sierra Nevada foothills between the valley edge and several hundred feet upslope. We were interested in characterizing leaf and gas exchange differences between Hw and the more common valley floor Helianthus annuus (Ha) to identify potential mechanisms that contribute to their habitat preferences. Seed mass (n=10) was determined and plants were pot-grown for five months. Stem caliper diameter, stomatal characteristics (using microscopy, n=6) and photosynthetic measurements (using a Li-Cor 6400 gas exchange system, n=5) of both species were measured in four -month-old plants. Field collected leaf and flower samples (n=6) were used for determination of phenolic and monoterpene content. Samples were ground in liquid nitrogen and extracted with methanol (phenolics) or hexane (terpenoids). Phenolic compounds were separated and quantified on a Shimadzu high performance liquid chromatograph equipped with a photodiode array detector and identified by matching retention times to known standards. Terpenoid compounds were separated and identified using a Shimadzu gas chromatograph equipped with a mass spectrometer. Values are presented as mean±SE (n) and differences between means were analyzed using a t-test.
Results/Conclusions
Seed mass of Hw (5.7±0.14 mg) was greater (P=0.048) than Ha (5.2±0.18 mg). Neither stem caliper diameter nor stomatal characteristics (density or length) on the adaxial or abaxial leaf surfaces differed between the species. However, Hw had lower (P=0.007) specific leaf area than Ha, 157±3.3 (n=10) and 180±6.8 cm2/gDW (n=10), respectively. Photosynthetic characteristics of young fully expanded leaves (respiration rate, quantum efficiency, maximum photosynthetic rate, light compensation point and light saturation point) did not differ between the species. The total phenolic content of flowers and leaves was marginally greater in Hw than Ha (P=0.056 and P=0.084, respectively). Hw and Ha possessed similar total floral monoterpene content and profile, but Hw leaf monoterpene content exceeded (P=0.017) that of Ha (3896±533, n=7 vs. 1984±380 mg/g, n=6). There were significant differences in the concentrations of specific phenolic and terpenoid species. Differences in habitat initially suggested that Hw might exhibit more drought-tolerant characteristics relative to Ha. This interpretation is supported by SLA differences, but is not corroborated by other measurements. As this study used a common garden structure with abundant water, it is probable that physiological differences may be amplified in the field where environment-by-genotype interactions are more profound.