Mycorrhizal Colonization Increases Herbicide Toxicity in Apple

Abstract

<jats:p>Herbicides are increasingly used in orchards. Since apple trees strongly depend on mycorrhizae, the effects of three commonly used herbicides on the host plant and endophyte were examined. Symbiosis between tissue-cultured P16 apple rootstocks and <jats:italic>Glomus versiforme</jats:italic> (Karsten) Berch was established under greenhouse conditions. Simazine (1, 2, 10, and 20 μg a.i./g), dichlobenil (1, 5, 10, and 25 μg a.i./g), paraquat (0.5, 1, 10, and 100 μg a.i./g), or water was applied to mycorrhizal and nonmycorrhizal plants as a soil drench. The response of mycorrhizal plants to herbicide was greater, and the relative elongation rate was more sharply reduced in mycorrhizal (76%) than in nonmycorrhizal plants (33%). Six weeks after herbicide application, dry mass reduction due to herbicides was similar (39% and 36%) for mycorrhizal and nonmycorrhizal plant shoots, respectively, while root dry mass reduction was larger for mycorrhizal (63%) than nonmycorrhizal plants (46%). None of the herbicide treatments affected root colonization. However, an in vitro hyphal elongation test with <jats:italic>G. intraradices</jats:italic> Schenck &amp; Smith and herbicide-amended (0, 1, 10, 100, and 1000 μg a.i./g) gellan gum solidified water showed that either dichlobenil or paraquat, even at the lowest concentrations, could significantly reduce hyphal elongation. Simazine did not affect hyphal elongation in vitro, a result suggesting that improved absorption capacity of mycorrhizae explains, at least in part, the increased phytotoxicity of some herbicides. It was found that plant mortality was higher among mycorrhizal than nonmycorrhizal apple trees for all herbicide treatments. The increased CO<jats:sub>2</jats:sub> assimilation rates of dichlobenil-treated mycorrhizal plants contrasted with the decreased rates of control plants measured 1 week after dichlobenil treatment. This indicates a physiological interaction between mycorrhizal colonization and dichlobenil in the toxic response of apple plants. Chemical names used: 2-chloro-4,6-bis-ethylamino-s-triazine (simazine), 2,6-dichlorobenzonitrile (dichlobenil), 1,1'-dimethyl-4,4'bipyridinium (paraquat).</jats:p>

Strong indirect herbicide effects on mycorrhizal associations through plant community shifts and secondary invasions

“…) but not unprecedented (Hamel et al. ). More likely are indirect effects mediated by a shift in overall plant abundance and/or community composition.…”

“…Because picloram mimics a hormone not known to occur in fungi, we did not expect any direct responses although direct responses have been observed with other herbicides (Hamel et al. ).…”

“…Herbicides target plant-specific pathways, such as photosynthesis, so direct effects on soil biota are therefore unlikely (Dayan et al 2010) but not unprecedented (Hamel et al 1994). More likely are indirect effects mediated by a shift in overall plant abundance and/or community composition.…”

“…Arbuscular mycorrhizal fungi can also influence plant tolerance to herbicides. For example, if AMF take up and translocate herbicides to plants, herbicide tolerance may decline and toxicities increase (Hamel et al 1994). On the contrary, AMF may increase herbicide tolerance if fungi help plants cope with the reduced root branching and modified root morphology caused by some herbicides (Ronco et al 2008).…”

Potential roles for microbial endophytes in herbicide tolerance in plants

“…This review has highlighted some of the potential ways in which microbes can directly influence crop and weed responses to herbicides, and this would seem to be a fertile area for future research. Attention should be drawn to cases where unsuitable endophytic organisms are susceptible to and can increase herbicide toxicity to host plants, as in the case of mycorrhiza‐infected apple trees, possibly owing to increased herbicide uptake . Based on their significant impact on crop growth and health, it is not surprising that beneficial endophytes are already being developed for new applications in crop improvement, with a focus on screening for new strains and methods of selective inoculation .…”