Is (−)-Catechin a Novel Weapon of Spotted Knapweed (Centaurea stoebe)?

Abstract

The novel weapons hypothesis states that some invasive weed species owe part of their success as invaders to allelopathy mediated by allelochemicals that are new to the native species. Presumably, no resistance has evolved among the native species to this new allelochemical (i.e., the novel weapon). In their native habitat, however, the plants that co-evolved with these invasive species have theoretically evolved defenses that obviate the allelochemical advantage. Previous studies have claimed that catechin is such a novel weapon of spotted knapweed (Centaurea stoebe = C. maculosa), an invasive species in the non-native habitat of North America. These studies indicated that (-)-catechin is more phytotoxic than (+)-catechin. Other studies have not found sufficient catechin in field soils to support this theory. We report that (-)-catechin and (+)-catechin are essentially equal, but poorly phytotoxic to a variety of plant species in bioassays without soil. In a dose/response experiment with Montana soils, we found the lowest dose for a growth reduction of two native Montana grasses (Koeleria macrantha and Festuca idahoensis) by a racemic mixture of (+/-)-catechin that ranged from about 25 to 50 mM, concentrations, orders of magnitude higher than expected in nature. Autoclaving the soil before adding the catechin did not affect the activity of catechin. We found (-)-catechin to be a potent antioxidant, in contrast to a previous claim that it acts as an allelochemical by causing oxidative stress. Our findings suggest that catechin is not a novel weapon of spotted knapweed and that other allelochemical(s) or alternative mechanisms must be found to explain the success of this species as an invader in North America.

The case against (–)-catechin involvement in allelopathy ofCentaurea stoebe(spotted knapweed)

“…Other papers provide rigorous evidence that the claims of strong phytotoxicity of (-)-catechin in either an aqueous medium or soil are unfounded. 19,20 Our most recent paper 20 strongly supports the view that neither enantiomer of catechin is likely to be involved in allelopathy of any species and questions whether (-)-catechin could act as a toxin through generation of oxidative stress, as claimed by Bais et al 12 We found (-) and (+)-catechin to have similar, weak phytotoxicities, with little or no phytotoxicity in soils at concentrations far exceeding those reported in any of the above papers. Furthermore, we confirmed that it is a strong antioxidant, virtually ruling out that it could harm plants by causing oxidative stress.…”

“…One could hypothesize that catechin is only substantially active when in the presence of some other component of C. stoebe. However, dramatic synergism between any allelochemicals has thus far not been rigorously proven (discussed by Duke et al 20 ). The half-life of catechin in soil is increased by the presence of other phenolic compounds, but it is still short and essentially inactive.…”

“…[12][13][14][15] In the latter example, once again, papers have appeared which seriously question the results. [16][17][18][19][20] Our addendum is written to expand the commentary from one of those dissenting papers. 20 …”

“…[16][17][18][19][20] Our addendum is written to expand the commentary from one of those dissenting papers. 20 …”

Tabanone, a New Phytotoxic Constituent of Cogongrass (Imperata cylindrica)

“…Some compounds that are quite active in soil-free bioassays such as those discussed above are virtually or completely inactive in soil (e.g., Duke et al 2009;Hiradate et al 2010). Activity of a compound can also vary considerably between different soil types (Hiradate et al 2010).…”