Negative cross resistance in atrazine resistant junglerice (Echinochloa colona (L.) Link) populations in sugarcane (Saccharum officinarum L.) fields

Introduction Negative and positive cross-resistance to other herbicides have been found in triazine-resistant biotypes. Thus, negative cross-resistance has been reported to bentazone for Brassica napus L. and A. retroflexus L., and to pyridate for B. napus and Epilobium ciliatum Raf.. In contrast, positive cross-resistance to bentazone has been found in Chenopodium album L. and Solanum nigrum L.. Negative cross-resistance, i.e., herbicide-resistant plants being more sensitive to herbicides than susceptible plants, has been documented in several triazine-resistant weed biotypes. The goal of this study was to search for herbicides that exert negative cross-resistance at the whole-plant level for control of atrazine-resistant populations of E. colona.
Materials and Methods Suspected resistant and susceptible seeds of junglerice to atrazine were collected from sugarcane fields and adjacent areas of Karun Agro-Industry Inc., Shushtar, southwestern of Iran in 2014 - 2015 growing season. These populations were named according to their population status and abbreviated as follows: R1, R2, R3 and S (susceptible population). All the collected seeds (R- and S-populations) were stored at room temperature (25 oC). Herbicides were tebuthiuron, linuron, diuron and diuron hexazinone. For the PRE application, 10 seeds were planted in 500-ml plastic pots containing clay:sand mix, and herbicides were applied using an flood nozzle and back sprayer calibrated to deliver 350 L ha-1 to the suspected resistant and susceptible junglerice biotypes 1 d after sowing. Plants were thinned to 4 plants per pot. The aboveground biomass was harvested 28 DAT, dried at 75 °C for 48 h, and weighed. The aboveground biomass data were expressed as a percentage of the untreated control. The experiment was a completely randomized block design with four replications and was repeated. The data were analyzed using a nonlinear regression model and R software (drc add on packeges), the three and four parameters log-logistic equation was fitted to the data to describe the responses of the populations to herbicides: susceptible population was estimated as an index in order to compare the resistance levels of the tested populations.
Results and Discussion The result of screening test showed that 3 populations were resistant to atrazine. The results of dose-response assay using atrazine revealed that resistance factor of R1, R2 and R3 populations were 12.26, 6.59 and 3.75 based on wet weight (% of control) and 5.37, 4.22 and 4.71 based on number of survival plants (% of control), respectively. The ED50 values of the R1, R2 and R3 populations were 36.85, 73.01 and 41.71 g ai ha-1 compared with 44.42 g ai ha-1 of the S-populaton of tebuthiuron. The Rf of the R1, R2 and R3 populations were 0.80, 1.53 and 0.72 of tebuthiuron, respectively. The ED50 values of the R1, R2 and R3 populations were 92.76, 107.73 and 106.84 g ai ha-1 compared with 152.51 g ai ha-1 of the S-populaton of linuron. The Rf of the R1, R2 and R3 populations were 1, 1.89 and 3.26 of tebuthiuron, respectively. The ED50 values of the R1, R2 and R3 populations were 74.21, 95.25 and 69.80 g ai ha-1 compared with 79.03 g ai ha-1 of the S-populaton of diuron. The Rf of the R1, R2 and R3 populations were 0.97, 0.85 and 1.18 of diuron, respectively. The ED50 values of the R1, R2 and R3 populations were 62.11, 49.48 and 54.30 g ai ha-1 compared with 88.72 g ai ha-1 of the S-populaton of diuron藬嫎⧨. The Rf of the R1, R2 and R3 populations were 0.62, 0.68 and 0.52 of diuron藬嫎⧨, respectively. The results showed that negative cross resistance to tebuthiuron, linuron, diuron and diuron hexazinone. Resistant biotypes, also, showed the highest negative cross resistance to diuron藬嫎⧨.
Conclusions Some herbicides that inhibit photosystem II bind more efficiently to the mutant triazine binding domain than to the wild (susceptible) type. Triazine-resistant weeds frequently show negative cross-resistance to other photosystem-II inhibitors, such as bentazon and pyridate; triazine-resistant weeds can also exhibit negative cross-resistance to herbicides that do not affect photosystem II. Negative cross resistance may be the major reason that atrazine resistance did not evolve where herbicide mixtures were used, when the mixed herbicide (usually a non-PS II inhibiting acetanilide) also controlled triazine-sensitive weeds. The value of negative cross-resistance linked with the general lack of fitness of almost all triazine-resistant weeds may be greater than we measured herein. The competition exerted by a crop in the field may further accentuate and exacerbate the lack of fitness and further lower the RI.