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Thyme seedlings and thyme essential oil enhance nodulation and shoot weight of white clover.
Stewart, Jeneen1, McKenna, Mary 1, 1
ABSTRACT- This study examined a three way ecological interaction between a terpene-producing plant(Thymus serpyllum, wild thyme), a legume (Trifolium repens, white clover), and the symbiotic nitrogen-fixing bacteria, Rhizobium leguminosarum bv. trifolii. Both plant species inhabit similar field and meadow habitats in the Eastern U.S. Thyme essential oil contains the terpene, thymol, as a principal component. Many terpenes have phytotoxic and antimicrobial properties, but few studies have explored their ecological effects. We examined the effects of thymol, thyme essential oil, and wild thyme seedlings on root nodulation and growth of white clover. Sterilized, inoculated clover seeds were sown in a 5:1 mixture of sand and vermiculite (pH 5.3)fertilized with Hoagland solution, or McDermott and Graham solution, a formulation designed to enhance nodulation in white clover. There were four treatments:(1) clover (control); (2)clover with 6 ng thymol; (3) clover with 6 ng thymol in thyme essential oil; and (4) clover with 6 thyme seedlings. Ecologically realistic concentrations of thymol and thyme essential oil were used that approximate natural levels produced by 6 thyme seedlings. Each treatment contained 64 pots (256 pots total);plants were grown for 4 weeks in an environmental chamber (10L:14D; 25 degrees C (day):18 degrees C (night)). Data were analyzed with ANOVA and Mann Whitney tests (Systat ver. 6.1). There was a greater number of nodules per plant in all treatments compared to the control. Nodule weight also increased in pots with thyme essential oil or thyme seedlings. Clover shoot dry weight was greater in pots with thymol or thyme seedlings. Thyme essential oil may enhance nodulation by direct stimulation or by inhibiting microorganisms that interfere with Rhizobium infection. Enhancement may involve other components of thyme essential oil as well as thymol. This study shows that Thymus serpyllum can have a significant effect on growth of a neighboring legume species, mediated through indirect plant-microbial interactions.
KEY WORDS: thymus serpyllum, trifolium repens, rhizobium leguminosarum, nodulation