Host preference of the federally endangered hemiparasite Schwalbea americana L. (Scrophulariaceae)

We examined the host-preference and optimal hosts of the federally endanger ed hemiparasite, Schwalbea americana L. We also determined the effects of f our fertilizer treatments (NO3-140ppmN, NO3-280ppmN, NH4-140ppmN, and NH4-2 80ppmN) on the formation of haustoria of S. americana to two of these host species, Panicum tenue and Pityopsis graminifolia. Results of the host pref erence study show that Schwalbea americana seedlings are more likely to for m haustorial connections with Pityopsis graminifolia and Ilex glabra than w ith other common associates. We also found that size of Schwalbea americana seedlings played an important role in year to year survivorship independen t of host attachment. Regardless of nitrogen form, we observed a greater ra te of attachment of Schwalbea americana to Pityopsis graminifolia than to P anicum tenue. Nitrogen treatment had no effect on parasitism of Panicum ten ue, although more seedlings of Schwalbea americana survived without haustor ia with the NH4-280ppm treatment. Growth (leaf area and stem height) of Sch walbea americana increased with NH4-280 ppm treatment with Pityopsis gramin ifolia as a host. Panicum tenue had much higher nitrate reductase activity levels in the leaf tissue than Pityopsis graminifolia, and both hosts showe d low levels in the root tissue. Low levels of nitrate reductase activity w ere found in the leaves of Schwalbea americana. Host preference of Schwalbe a americana may be related to the host's ability to utilize NH4 as opposed to its ability to reduce NO3. Growth of Schwalbea americana appears to be i nfluenced by the amount of NH4 available to the host.