Helianthus paradoxus Heiser (Asteraceae, puzzle sunflower), is a federally threatened hybrid species found in salt marshes of west Texas and New Mexico. Helianthus annuus L. (Asteraceae, common sunflower) is one of the parent species and is found throughout North America, but it is not present in the inland salt marshes where H. paradoxus is found. Helianthus paradoxus has previously been described as a halophyte, but its tolerance to Na2SO4, one of the major salts found in its habitat, has not been investigated. However, salinity has been identified as a major abiotic factor influencing the limited distribution of H. paradoxus populations. In this greenhouse study, the effects of elevated concentrations of Na2SO4 and NaCl, at equal ionic strengths (0.00, 0.09, 0.17, 0.34, and 0.51), on the survival and dry mass of both H. paradoxus and H. annuus were examined. In the three-way factorial experiment, the effects on dry mass observed were dependent on the species, the type of salt and the ionic strength of the salt. Helianthus paradoxus produced more dry mass than H. annuus in both salt treatments; however, NaCl was more inhibitory of dry mass production for both species with plants unable to survive the highest salt treatments. While dry mass of H. annuus decreased with increasing ionic strengths of both salts, dry mass of H. paradoxus increased by 38 to 72% in low to moderate ionic strengths of Na2SO4 relative to the nonsaline treatment. Both species were less tolerant of NaCl than Na2SO4 with H. paradoxus seeming to have moderate and high tolerance to elevated Cl− and SO42− ionic strength, respectively, while H. annuus had low to moderate tolerance. Greater dry mass production in Na2SO4, along with tolerance to both salts, suggests that low to moderate sulfate soil salinity will enhance the dry mass production of H. paradoxus.
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Vol. 58 • No. 3