Effect of Mycorrhizal Fungi and Trifluralin Herbicide on Emergence, Growth and Root Colonization of Clover (Trifolium repens L.)

Herbicides, despite of their control of weeds, have the potential to affect sensitive crops in rotation and also beneficial non-targeted soil microbes including vesicular arbuscular mycorrhiza (VAM) fungi (6). AM fungi can increase the growth of crops through increasing uptake of phosphorus and insoluble micronutrients, and indirectly by improving soil quality parameters (30). However, several authors have reported different effects of herbicides on VAM symbiosis, which ranges from no adverse effects to slightly or highly toxic effects (6). Pesticides have also been reported to stimulate colonization of plant roots by AM fungi (27). Therefore, the objective of this study was to investigate the interaction effects of mycorrhizal fungi and Trifluralin herbicide on the growth and root colonization of clover.
A factorial experiment was arranged in randomized complete block design with three replicates at the College of Agricultural, University of Shahrood during 2012. Treatments were included three levels of mycorrhiza inoculation, M1: non mycorrhiza (control), M2: Glommus mosseae and M3: Glommus intraradices and herbicide treatments were included four levels of Trifluralin(T1: 0, T2: 1000, T3: 1500 and T4: 2000 ml ha-1). In mycorrhizal treatments, 20 g inoculums were thoroughly mixed with soil. Seeds of clover (Trifolium repens L.) were sown in the pots maintained near the field in order to provide normal environmental conditions. Seedlings were thinned to two plants per pot at three leaf stages. At the time of harvesting, the emergence and growth characteristics of clover and root colonization was also registered. Statistical analyses of data were performed with statistical software MSTATC. Significant differences between means refer to the probability level of 0.05 calculated by LSD test.
The results showed that emergence, uniformity (EU) values decreased and time to 10% (D10) and 90% (D90) of emergence increased significantly with increasing herbicide dose. The maximum emergence (Emax) value and clover shoot dry matter was obtained with inoculation by mycorrhiza species in all levels of herbicide doses than non-inoculation treatments (figures 1 and 3). Inoculation of clover with G. intraradices increased root length by 34 and 57% at the herbicide doses of 1000 and 1500 ml ha-1 than non-inoculation treatments, respectively (figure 2). Results showed that, the root colonization percentage improved in low rate of Treflan by inoculation of mycorrhizal than non-inoculation treatment significantly (figure 4). These results are in agreement with the results reported by Garcia-Romeria et al. (14). They reported that shoot dry weights of AM fungi infected pea plants, were higher when the cyanazine was applied at the rate of 0.05mg ml -1 than applied at the rate of 0.1 ml per pot (14). Busse et al. (6), reported that triclorpyr, imazapyr and sulfometuron methyl herbicides did not alter the capability of mycorrhizal fungi to infect roots, even at concentrations detrimental to seedling growth. Makarian et al., (21) reported that herbicide application significantly decreased the growth (dry matter, chlorophyll content and height) of maize and barley plants but mycorrhizal colonization increased plant growth at low levels of herbicide concentration. It seems that, one of the main reasons that enable the mycorrhizal plant to partially or completely overcome the stress of herbicides as compared with non-mycorrhizal plant is the enhanced nutrient uptake (6). Enhanced nutrients uptake is due, in part, to the hyphal extension into the soil and subsequent transfer of them back to the root (30). Recent researches have established that AM fungi are able to absorb organic compounds, macro and micro nutrients from the soil and transfer them to plants (29). Pesticides have also been reported to stimulate colonization of plant roots by AM fungi. This may be due to a reduction or elimination of competing microorganisms or to a pesticide mediated change in plant metabolism resulting in increased production of materials stimulatory to AM fungi in the rhizosphere (27).
In conclusion, the results of this study indicated that with increasing Treflan doses the clover seed emergence, root colonization, dry matter production and root length of clover significantly decreased than non-herbicide application treatment. But inoculation of clover seeds with mycorrhizal fungi increased seed emergence and shoot dry matter at the all doses and root length and colonization percentage of the low rates of herbicide. Based on our results, mycorrhiza fungi can alleviate crop stress due to low doses of Treflan injury through increasing of plant growth.