Germination of Helianthus species on two soil types and across a salt gradient

Helianthus annuus and Helianthus argophyllus are the main species of sunflowers in Nueces County, Texas.  In a previous study, Helianthus argophyllus was restricted to sandy sites, and H. annuus was nearly restricted to clay sites.  To determine if the germination phase plays a role in this distribution, we germinated locally collected seed of H. annuus and H. argophyllus on sand and clay soils for 30 days.  Although each species germinated on both soil types, percent germination on the preferred soil type was significantly higher.  Because the sandy soils on which H. argophyllus is found include dunes near saline environments, differences in their distribution may in part be due to salt tolerance.  Therefore, we compared germination of these two species in sand-filled Petri dishes at four salinity levels over a 15-day period.  Salinity levels were 0% (distilled water), 5%, 10%, and 15% salt.  ANOVA revealed a significant interaction between salt concentration and species.  Inspection of the data indicated that germination for H. argophyllus declined more steeply across the salinity gradient than it did for H. annuus.  This result is contrary to our expectations based on field distributions of these species.  Consequently, although germination may indeed play a role in distribution of these species, differences in salinity tolerance during germination do not appear to explain these differences.