EDMONTON – Inoculating pulse crops with rhizobium bacteria is one of farming’s great bargains, but stressed plants won’t be able to take advantage of the full savings.
Fran Walley, a soils professor at the University of Saskatchewan, told the recent Farm Tech conference in Edmonton that herbicides are among the biggest culprits.
Root hair curling is a good example, she said. It is critical to proper rhizobium infection because the bacteria are trapped as the root hair curls over and causes a “shepherd’s hook.” The bacteria then break down the cell wall and enter the root hair. That curl is the point of infection.
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However, Walley said some herbicides can cause root hair deformations that make the root hairs inaccessible to the rhizobium.
“Anything that affects the movement of carbohydrates and energy down into that nodule is going to affect nitrogen fixation,” she said.
“If you have a herbicide treatment that compromises your plant, weakens the plant even temporarily, I can guarantee the nodules are temporarily depressed in terms of activity. They don’t have as much energy available to them.”
Walley said plants that are recovering from damage will have less energy to send to the nodules.
“If it affects the plant, be it a herbicide application, insect infestation, drought or whatever, it’s going to affect nitrogen fixation.”
Plant stress can affect rhizobial growth and survival, host recognition and attachment and nodule formation. A plant badly affected by herbicides will not fix as much nitrogen.
Recently, Walley reviewed a number of scientific studies dealing with how herbicides affect nodulation and nitrogen fixation.
One study found that all the herbicides it reviewed had a negative impact on nodulation, total nitrogen fixation, photosynthate supply and leaf area. But when the researchers looked at nitrogenase activity, which is the enzyme responsible for fixing nitrogen, there was no difference in its activity.
“That was a good sign. It’s not impairing the mechanics of nitrogen fixing once it gets set up. But it does have some interaction with getting everything up and running.”
In this study, researchers examined rhizobium growth in a lab in the presence of various herbicides and found the rhizobium grew fine in the flasks, whether or not the particular herbicides were present.
Walley said the study concluded that the herbicides affected nitrogen fixation because they affected plant growth.
However, they did not affect the mechanisms of the rhizobium or the nitrogen fixing process. She said this is typical of the research done in various studies.
“If it’s damaging the plant, that’s the effect on fixation. It’s just affecting the photosynthate supply down to the root.”
Walley said herbicides can also affect how well the rhizobium stick to the plant root surface. She said one study found that the herbicide Paraquat sticks to attachment sites on plant root hairs. Researchers felt some herbicides might compete with the rhizobia for attachment sites on the root surface, she added.
Rhizobium bacteria that were exposed to chlorsulfuron in one study would grow, but lost the ability to nodulate, while rhizobium exposed to the same chemical in another study nodulated quite well.
“That’s the nature of dealing with these biological systems,” she said. “They can be touchy to deal with.”
Some herbicides can induce root hair deformations that will stop the rhizobium from infecting the root hair. If it’s going to affect the plant root, it could have an impact on whether infection can take place.
Walley said Australian researchers who applied post-emergent herbicide on six-week-old peas that had already nodulated found that the herbicide caused a senescence of the nodules that were already on the plant.
“After four or five weeks, new nodules formed on the plant, but the important message is, if you’ve got a stressed plant, it can result in a substantial loss of nodules from the roots, due to stress on the plant rhizobium symbiosis. You don’t want to be stressing your plant too much, in case that it fails to continue to support that nitrogen fixing system.”
Waterlogged soil and extreme drought can also cause lost nodules. Walley said the Alberta Pulse Growers recently supported research looking at impacts of in-crop and residual herbicides used in Western Canada.
Trials initiated in 2004 that will continue through 2006 at Beaverlodge, Alta., are studying nitrogen-fixing activity of peas with peat-based powder and granular inoculant, and a variety of herbicide combinations. So far, there hasn’t been a big difference in yield.
“In 2004, data suggested that while (herbicide applications) might have had an effect on nitrogen fixation, the benefits of better weed control outstripped any problems in terms of nitrogen fixation,” Walley said.
Residual herbicides included Curtail M, Everest and Sundance. They were applied in 2004 and the peas grown in 2005. Preliminary data after one year suggested the residual treatments seemed to have better yields compared to the check.
“If there were problems affecting nitrogen fixation, they were minor compared to benefits associated with weed control,” she said.
“Same thing when we were using a granular herbicide. They’re certainly as good as our checks. There was no indication at (one) site that there was any negative impact. I suspect that’s where we’ll end up, with this research. It’s balancing concerns regarding nitrogen fixation with getting good weed control, to achieve good yields.”
A number of herbicides, including Sencor and sulfentrazone, are being tested in a similar manner near Saskatoon. Walley said results indicate plant injury from Sencor, definitely at the double rate but even at the regular rate.
“Whether more nitrogen was being fixed didn’t matter. The best yield was achieved where there was good weed control.”
In cases where Sencor was applied at the double rate, leaves were burned off compared to control plants, which had a better developed root system and far better nodulation. The upper portions of the control plants were also healthier.
“(The treated) plant has spent the majority of its energy trying to regenerate its leaves, so very little photosynthate would be allocated down to the root,” Walley said.
“It’s recovering from stress and not able to support the nitrogen fixing system. With chickpea, the untreated control seemed to be fixing more nitrogen, but the yield wasn’t as good as sulfentrazone. Sulfentrazone might have caused a temporary decline in the amount of nitrogen fixed, but at the end of the day it was a better treatment than the untreated check because of weed control.”
Walley said studies that looked at the initial gene activation found that the chemical structure of the flavenoids is close to a number of herbicides.
“(The researchers) hypothesized that some organochlorine pesticides can mimic or interfere with biochemical signalling and when they looked at it, they found some herbicides could inhibit that initial signaling.”
She said extremely high rates of herbicides would have to be used to get that effect and she felt it would not be at rates normally used in a commercial field operation.
To highlight the benefits of proper inoculation to plant health, Walley reviewed the results from a growth chamber herbicide residue experiment, with Everest residue in the soil.
“It’s obvious that Everest residue is inducing significant problems in a pea plant,” she said.
“The residue is equivalent to a field rate application, so it would be as if the Everest hadn’t broken down at all over the past 12 months. Where we inoculated the plant versus an uninoculated treatment, so they both have the same amount of herbicide residue, the plant is in a much better situation to deal with some of that stress.
“Making sure you have a healthy plant that’s fixing nitrogen can also help the plant deal with stress.”
She said any chemical, whether it is a fertilizer, herbicide or seed treatment, can affect rhizobium.
“We need to protect that nitrogen fixing system, because it’s worth a lot of money with the price of nitrogen fertilizer. It’s reasonable to be cautious of herbicide residues, so you can avoid imposing unintended chemical stresses on that system.”
Walley said the good news from her review of this research is that rhizobium seems to be good at dealing with stresses, particularly when they’re in moist soil.
“When inoculating your crop, you want to make sure you get good inoculation and get the seed into moisture, not just to get the seed germinating, but so the bacteria are in the most favourable environment to handle any of those chemical stresses,” she said. “We need to mostly worry about what’s happening to the plant. If something bad is happening to the plant, we can rest assured it’s happening to the rhizobium.”
