Host Preference of the Federally Endangered Hemiparasite Schwalbea americana L. (Scrophulariaceae)

Abstract

HELTON, R. C., L. K. KIRKMAN, AND L. J. MUSSELMAN. Host preference of the federally endangered hemiparasite Schwalbea americana L. (Scrophulariaceae) J. Torrey Bot. Soc. 127:300-306. 2000.-We examined the host-preference and optimal hosts of the federally endangered hemiparasite, Schwalbea americana L. We also determined the effects of four fertilizer treatments (NO3-140ppmN, N03-280ppmN, NH4-140ppmN, and NH4280ppmN) on the formation of haustoria of S. americana to two of these host species, Panicum tenue and Pityopsis graminifolia. Results of the host preference study show that Schwalbea americana seedlings are more likely to form haustorial connections with Pityopsis graminifolia and Ilex glabra than with other common associates. We also found that size of Schwalbea americana seedlings played an important role in year to year survivorship independent of host attachment. Regardless of nitrogen form, we observed a greater rate of attachment of Schwalbea americana to Pityopsis graminifolia than to Panicum tenue. Nitrogen treatment had no effect on parasitism of Panicum tenue, although more seedlings of Schwalbea americana survived without haustoria with the NH4-280ppm treatment. Growth (leaf area and stem height) of Schwalbea americana increased with NH4-280 ppm treatment with Pityopsis graminifolia as a host. Panicum tenue had much higher nitrate reductase activity levels in the leaf tissue than Pityopsis graminifolia, and both hosts showed low levels in the root tissue. Low levels of nitrate reductase activity were found in the leaves of Schwalbea americana. Host preference of Schwalbea 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 influenced by the amount of NH4 available to the host.