While I enjoy growing a variety of produce from tomatoes, basil and garlic to blueberries, one thing remains the bane of my existence: weeds. My least favorite: quackgrass. I even dug up an entire garden bed to rid myself of the weed and its long rhizomes. How well do you think that worked? I found that the quackgrass happily grew around the popcorn I planted in that same location. What is a gardener to do? Well, recent research by Veiga et al. demonstrates that fungus can lend a helping hand and suppress growth of some weed species.
In this case, the study focused on arbuscular mycorrhizal fungi (AMF). Like many fungi, AMF live in the soil and send out thin filaments in search of nutrients. These specific fungi are interesting because they are obligate symbiotes with plants. That is, the fungus infiltrates plant roots, bringing water, phosphorus and other nutrients collected from its far-ranging filaments, while the plant supplies the fungus with carbohydrates. The fungal exchange structures inside plant root cells are called arbuscules, giving rise to the name. The authors wanted to know if AMF in the presence or absence of crop plants was enough to affect weeds that could be found with these crop plants.
The weeds selected for the study included quackgrass and eight other weeds, the crops grown were corn, wheat and red clover, and the AMF tested were three different Glomus species. These weeds, crops and AMF species were used in various combinations in two different experiments.
The first experiment was done using soil inoculated with only Glomus intraradices. The nine weeds and three crops were germinated in sterile sand and transplanted into pots containing either the AMF soil inoculum or control soil that had been autoclaved, and the seedlings grown for eight weeks in a greenhouse environment before harvesting. Red clover was the positive control as it is known to grow better in the presence of AMF. Plant growth was assessed by separating the above-soil biomass and and below-ground biomass. The above-soil plant material was dried in an oven and weighed. A portion of the roots was dried to assess below-ground biomass, and the rest was stained to determine the level of AMF colonization.
As expected, red clover had the strongest growth response to G. intraradices despite <50% colonization of its roots. However, AMF colonized >50% of the roots of maize, wheat, quackgrass, creeping thistle and four other weeds. For these highly colonized weeds and crops, six of the eight species had a decrease in biomass that was greater than 20% compared to controls grown in autoclaved soil substrate. Unfortunately for me, growth reduction in quackgrass was not statistically significant.
For the second experiment, an equal mixture of G. intraradices, Glomus claroideum and Glomus mosseae was inoculated into the soil substrate for growing maize either alone, or with one of three different weed species that showed >20% biomass decrease in the first experiment (Apera crus-galli [loose silkybent], Setaria viridis [green foxtail] and Solanum nigrum [black nightshade]). Nylon mesh was used to divide pots in half to keep roots from touching but allowed AMF filaments to pass through. One half of the pot was sown with maize or one weed species or both sides were sown. After germination, seedlings were thinned to a single maize plant and three weed plants per half pot and grown for 12 weeks in a greenhouse before harvesting.
Interestingly, S. viridis and S. nigrum demonstrated no differences in AMF root colonization when grown alone or with maize, but maize did show a decrease in colonization when grown with either of these two weed species versus grown alone. However, A. crus-galli showed increased root colonization when grown with maize, while maize showed no statistically significant difference when grown alone or with A. crus-galli. The biomass changes in this experiment correlated with the results of the first experiment when all four species were grown alone, that is, AMF colonization reduced growth. For S. viridis and S. nigrum grown with maize, biomass was reduced but to similar levels seen in monoculture. However, the maize-A. crus-galli-AMF interaction demonstrated a measurable difference compared to the single plants grown alone. A. crus-galli had a greater negative effect (–16% alone versus –56% with maize) while maize grew slightly better in the same pot (–2% alone versus 10% with A. crus-galli).
This experiment demonstrated that for some weed species, AMF negatively affected growth and even amplified the growth reduction in the presence of maize while boosting growth for the crop. As the authors pointed out, AMF colonization is not a deterrent for weeds because they still grew and and produced seed. However, AMF is a possible tool that benefits crops, plants humans want to grow, by inhibiting growth of weeds, plants humans do not want contaminating their crops. How crops, weeds and AMF interact in soil outdoors with the chaotic mix of microorganisms and wild plants is also a consideration and could change the outcome of the experiment. Regardless, AMF is not a panacea for my quackgrass woes, but I better appreciate what fungi could do for me and the popcorn I grow.
Veiga, R.S., Jansa, J., Frossard, E., & van der Heijden, M.G. (2011). Can arbuscular mycorrhizal fungi reduce the growth of agricultural weeds? PLoS ONE, 6 (12) PMID: 22164216