Early root growth traits in Helianthus

We examined early rooting traits in the Helianthus hybrid system to explore how root growth and morphology have evolved in response to selection in a nutrient-poor habitat.  The active sand dune habitat of the hybrid species H. anomalus contains lower levels of soil nutrients than the habitats of the parental species H. annuus and H. petiolaris.  Therefore, we expected the hybrid species to have a lower specific root length (root length/mass), associated with greater root longevity, yielding a growth strategy that conserves resources that typically limit productivity.  Additionally, the presence of deep water stores in the H. anomalus habitat likely makes the early production of deep roots important for the species.  All three species were grown under optimum nutrient conditions in a glasshouse experiment.  Half of the plants were harvested one week after transplant, and the rest were harvested two weeks later.  Helianthus anomalus had a lower specific root length, higher average root diameter, and deeper rooting depth than its parental species at the first harvest.  By harvest two, the differences between parentals and hybrid were less distinct.  We conclude that early rooting traits differ as expected in H. anomalus, but the initial differences in root morphology are only evident in the very early stages of growth. This may be attributable to the slower relative growth rate of H. anomalus compared to its parental species.  Further comparisons of the species under nutrient-limiting conditions are planned to determine if species exhibit different rooting characteristics in response to nutrient stress.