Herbicide Residues in Soil Affect Hormone Levels in Crops
A new study funded by the Academy of Finland has found that glyphosate residues in soil affect phytohormones in aboveground plant parts.
Glyphosate-based herbicides are commonly used to kill weeds before crops are sown on agricultural fields. It was assumed that glyphosate degrades quickly in the soil, with none to negligible effects on the crop plants. However, herbicide residues are increasingly found in soils with agricultural history while, at the same time, soil health and plant resilience decreases.
The researchers conducted an experiment where they tested the effects of soilborne glyphosate-based herbicides on plant hormone — phytohormone — levels of three important crop species: oats, potatoes and strawberries. Plant hormones are small molecules with essential signaling functions in the plant, such as regulating plant growth, flowering, senescence and responding to stressors such as drought, damage or pathogen infection. Furthermore, plant hormones are involved in fine-tuning the plant responses to feeding by herbivores — in particular, the production of compounds that help plants repel herbivores in order to minimize damage.
Glyphosate inhibits a specific enzyme in the shikimate pathway, which is needed for the biosynthesis of essential aromatic amino acids in plants. The analyses of plant samples for a variety of phytohormones revealed that oat plants growing in soil that contained minimal concentrations of glyphosate residues showed decreased levels of phytohormones deriving from either one of those aromatic amino acids targeted by glyphosate. Surprisingly, this co-occurred with lower plant damage by herbivores, indicating an increase in plant-resistance traits.
These results, published in Frontiers in Plant Science, demonstrate the hidden impact of ubiquitous agrochemical residues on phytohormones and plant-herbivore interactions. At the scale of agricultural fields, these effects may affect insect biodiversity patterns and might affect insect biodiversity in agricultural environments.
In contrast to oats, potato plants responded to herbicide residues in soil by elevating stress-related phytohormones and an increased plant growth, while strawberry plants were largely not responding to herbicide residues in soil; this shows how species-specific the responses to glyphosate residues in soil can be.
What Does This Mean for Growers?
From John Kempf:
All plants respond dramatically to changes in phytohormone levels. The use of plant hormone products usually corresponds to a crop’s value. High-value crop growers quickly recognize they cannot afford to not use these products because of the exceptional crop responses they can produce when applied in a timely fashion. We often observe instances where cultural management practices produce a profound negative effect on a crop, because the impact on phytohormone levels is not appreciated.
It seems logical to expect that low-level glyphosate residues in the soil profile result in changed phytohormone profiles in crops, given glyphosate’s disruption of the shikimate pathway, which is the basis for several plant metabolites. The central role of phytohormones in regulating plant growth and responses to abiotic and biotic environments has been ignored in studies examining the effects of glyphosate residues on plant performance and trophic interactions.
Herbicide residues are ubiquitous, and it is necessary to unravel their consequences for ecological interactions and their involvement in shaping evolutionary processes. While this research is relevant for all soils and crops that have had glyphosate applied, it is of particular relevance to tree crops, where large amounts of glyphosate have been applied in the tree row.
Soil Health and Nutrient Density: A Preliminary Comparison of Regenerative and Conventional Farming
A newly published study in the journal PeerJ uses several independent comparisons to indicate that regenerative farming practices enhance the nutritional profiles of crops and livestock.
Measurements from paired farms across the United States show differences in soil health and crop nutrient density between fields worked with conventional (synthetically fertilized and herbicide-treated) or regenerative practices for five to ten years.
Regenerative farms that combined no-till, cover crops and diverse rotations produced crops with higher soil organic matter levels, soil health scores, and levels of certain vitamins, minerals and phytochemicals. In addition, crops from two regenerative no-till vegetable farms — one in California and the other in Connecticut — had higher levels of phytochemicals than values reported previously from New York supermarkets.
Moreover, a comparison of wheat from adjacent regenerative and conventional no-till fields in northern Oregon found a higher density of mineral micronutrients in the regenerative crop.
Finally, a comparison of the unsaturated fatty acid profile of beef and pork raised on one of the regenerative farms to a regional health-promoting brand and conventional meat from local supermarkets found higher levels of omega-3 fats and a more health-beneficial ratio of omega-6 to omega-3 fats.
Despite small sample sizes, all three crop comparisons show differences in micronutrient and phytochemical concentrations that suggest soil health is an under-appreciated influence on nutrient density, particularly for phytochemicals not conventionally considered nutrients but nonetheless relevant to chronic disease prevention. Likewise, regenerative grazing practices produced meat with a better fatty acid profile than conventional and regional health-promoting brands.
These comparisons offer preliminary support for the conclusion that regenerative soil-building farming practices can enhance the nutritional profile of conventionally grown plant and animal foods.
Smart Soil Bugs for Controlling Crop Diseases
An innovative method of controlling a range of damaging crop diseases using native, beneficial soil bacteria has been discovered by a team of researchers at the John Innes Centre in Norwich, England. The innovation hopes to give farmers a way to reduce the cost and environmental damage caused by the chemical treatments currently in use to control crop diseases.
The team isolated and tested hundreds of strains of Pseudomonas bacteria from the soil of a commercial potato field and then sequenced the genomes of 69 of these strains. By comparing the genomes of those strains shown to suppress pathogen activity with those that did not, the team was able to identify a key mechanism in some of the strains that protected the potato crop from harmful disease-causing bacteria.
Then, using a combination of chemistry, genetics and plant infection experiments, they showed that the production of small molecules called cyclic lipopeptides is important to the control of potato scab, a bacterial disease that causes major losses to potato harvests. These small molecules have an antibacterial effect on the pathogenic bacteria that cause potato scab, and they help the protective Pseudomonas move around and colonize the plant roots.
The experiments also showed that irrigation causes substantial changes to the genetically diverse Pseudomonas population in the soil.
First author of the study Dr. Alba Pacheco-Moreno said, “By identifying and validating mechanisms of potato pathogen suppression, we hope that our study will accelerate the development of biological control agents to reduce the application of chemical treatments which are ecologically damaging.
“The approach we describe should be applicable to a wide range of plant diseases because it is based on understanding the mechanisms of action that are important for biological control agents,” she added.
The study, which appears in eLife, proposes a method by which researchers can screen the microbiome of virtually any crop site and take into account varying soil, agronomic and environmental conditions. By exploiting advances in high-speed genetic sequencing, the method can screen the soil microbiome for therapeutic bacteria and work out which molecules are being produced to suppress pathogenic bacteria.
The next step for the new approach is to put the beneficial bugs back into the same field in greater numbers, or in cocktails of mixed strains, as a soil-microbiome-boosting treatment. Potential methods to apply the microbiome boosters include applying the bacterial cocktails as seed coatings, as a spray or via drip irrigation.
A corresponding author of the study adds, “In the future it’s not the molecule produced by the bacteria that we would use — it would be the Pseudomonas strain itself. It offers a more sustainable route — we know these bacteria colonize the soil where potatoes grow, and they provide protection to the crop. Using a bacterium, you can easily grow and formulate it in an appropriate way and apply it to the field, and it is much greener than using a synthetic chemical.”
Previous studies on the suppression of potato scab have indicated a potential biocontrol role for Pseudomonas. However, progress was hampered by a lack of mechanistic knowledge. It was also widely known that irrigation can suppress Streptomyces scabies infection; this study suggests that this is because of the effect that water has on microbial populations.
How Will This Knowledge Help Growers?
From Dr. Robert Kremer:
The Pseudomonas bacteria are cosmopolitan inhabitants of many environments, including soils and root zones (rhizospheres) in which many species are known antagonists of competing microorganisms — including plant pathogens — and are considered beneficial plant-growth-promoting rhizobacteria. This group of bacteria typically suppresses pathogens through the synthesis of antibiotics or chelating agents (siderophores) that block uptake of micronutrients to pathogens.
However, the authors of this study, using genomics methods, discovered new growth-suppressive compounds (cyclic lipopeptides — CLP) that function by disrupting cellular integrity and reproduction of the potato scab pathogen, resulting in greatly reduced infection of potato. Their innovative approach of matching the gene coding for the CLPs with those Pseudomonas species capable of producing CLPs will greatly aid in selecting effective microorganisms from soils for future development as disease biocontrol agents available for agroecological farmers.
The authors further indicate that a biological product would be based on mass-produced cultures of selected Pseudomonas species that could establish in soil after application as a probiotic and deliver the active pathogen-suppressive compounds directly to the disease sites in the rhizosphere and on the potato tubers. It is suggested that CLPs then would not serve as templates for industry to develop chemical pesticides and thereby avoid introduction of new synthetic chemicals into the field.
Before a probiotic based on this research becomes commercially available, considerably more work will be required on formulation to assure survival of the living bacteria applied to soil and on the soil conditions that may affect inoculant performance. The original article reports that the Pseudomonas bacteria were selected from a “commercial potato field,” however no details regarding cropping system or any soil properties were presented. Such factors influence abundance and functioning of the soil microbiome, including the pathogen component.
It is important to know if the field was continuous potato or rotated with other crops. Ecologically based farmers realize that rotation with other row crops or with cover crops will deter pathogen establishment and disease development. Rotation will also strengthen soil microbiome diversity, which likely includes a significant proportion of the beneficial Pseudomonas species. An indication of typical soil properties such as texture, soil organic matter, pH, and nutrient values is necessary to interpret microbial results.
For example, soil pH and manganese content directly affect potato scab disease development as well as beneficial microbial activity. This information needs to be reported to understand the environmental conditions under which the potato disease-suppressive bacteria developed and to know if potential probiotic biological control products will be effective under a range of soil and crop conditions.
Considering current agroecological farming practices, the potato diseases discussed in the article are likely held in check under stringent crop rotation and soil nutrient management. Availability of future biological probiotic products based on the reported research may be useful for enhancing the soil microbiome in potato-producing areas as indicated by soil health biological assays and in “emergency” situations of potato disease outbreaks occurring during transition from conventional production systems.
Fickle Sunshine Slows Down Rubisco and Limits Photosynthetic Productivity of Crops
All of the carbon in our bodies, in food and in the entire biosphere results from the assimilation of carbon dioxide in photosynthesis by a single enzyme, known to biologists as Rubisco. Not surprisingly, given its importance, this protein is the most abundant in the world.
Researchers from Lancaster University working to improve the sustainable productivity of key crops in sub-Saharan Africa have discovered a new imperfection in the way Rubisco functions in cowpea and believe this imperfection is likely shared with other crops.
“Rubisco plays a central role in photosynthesis and frequently limits carbon assimilation in crop plants,” said Elizabete Carmo-Silva, professor of crop physiology at Lancaster. “Leaves adjust the activity of Rubisco to the abundance of solar energy. However, we found that this adjustment is imperfect, and frequently there is a mismatch between how active Rubisco is and how much solar energy is available for photosynthesis.”
Cowpea is grown throughout Africa because of its high protein content, but it is particularly important in West Africa, where it is the most significant source of vegetable protein. In a recent study, published in Nature Plants, Carmo-Silva and Senior Research Associate Sam Taylor found that as cowpea leaves go into the shade, the activity of the enzyme Rubisco drops more rapidly than was previously appreciated.
This is important because every day, as the sun inevitably tracks across the sky above crops in farmers’ fields, leaves cast neighboring plants from sunlight into the shade and back again. When a shaded leaf comes back into the sun, Rubisco activity takes several minutes to gear up to the new abundance of solar energy, resulting in missed opportunities to convert that energy into sugars. Adding up the effect of those lost minutes of productivity across a day has been estimated to cost at least 20 percent of potential carbon dioxide uptake.
“Photosynthetic responses are not immediate. Leaves take quite a few minutes to adjust when going from shade to high light, and during those minutes the leaf is not assimilating as much CO2 as it has the light energy for, so there is a substantial loss,” said Carmo-Silva, who is leading this research for the Realizing Increased Photosynthetic Efficiency (RIPE) project. “We set out to identify differences among cowpea varieties that affect the speed of activation, to try and identify which ones are faster.”
The amount of carbon lost during the Rubisco process depends not only on the speed with which Rubisco can be re-activated but also on the starting point: the Rubisco activity at the moment when sunlight returns. This factor is determined by the speed of natural de-activation of Rubisco that happens in the shade. Faster de-activation means a bigger hit on carbon assimilation in farmers’ crops.
The researchers used a high-throughput biochemical method to show that cowpea leaves only need to be in shade for as little as five minutes for Rubisco activity to bottom out, so even brief shading of leaves will lower the plant’s photosynthetic productivity.
Despite these challenges, there are reasons to be optimistic. Only four different types of cowpea were measured, from the thousands of variants that exist, and the researchers did find differences in the speed at which Rubisco de-activated. This holds out hope that within the wider gene pool of cowpea, varieties with much slower rates of Rubisco de-activation can be found. That would allow targeted breeding for cowpea, and perhaps other crops — improving productivity by minimizing the impact of this newly identified imperfection in Rubisco function.
Why Does This Matter for Growers?
From Dr. Robert Kremer:
Information in this report that suggests shading limits the efficiency of the carbon-fixing ability of the enzyme Rubisco that can lead to reduced carbohydrate production and overall lower productivity in cowpea is useful to farmers because it can help in management decisions for specific crop production systems. For example, with intercropped cowpea in corn for soil health improvement or livestock grazing, seed yield may not be as important as a high protein biomass to supplement the carbonaceous corn residue remaining after harvest.
However, if fresh pea or dry grain yield is the primary food-production goal, the farmer may wish to grow cowpea as a monoculture to reduce shading effects. The researchers admit that results are based on only four of thousands cowpea varieties, suggesting that farmers might consider testing several varieties in their particular system (intercropping or monoculture) to find those that perform optimally under these conditions. The prospects for selection of cowpea varieties with improved carbon-fixing and overall photosynthetic activity are encouraging.
Rubisco can fix more carbon when exposed to elevated levels of carbon dioxide, which may offset some of the effects of shading. Ecologically minded farmers typically maintain higher soil organic matter content relative to conventional farmers and use cover crop residues, organic amendments and surface mulches in their production system, all of which provide additional carbon dioxide as decomposition proceeds during the season and can be fixed by the growing crop.
The reported research failed to indicate that adequate nutrients, especially magnesium and iron, are necessary for Rubisco activity. The soil microbiome associated with plant roots also influences overall plant growth, and ultimately photosynthesis, whereby some rhizobacteria aid the plant to tolerate environmental stresses such as shading. Healthy soils on agroecologically managed farms are nutrient sufficient and support the diverse microbiome that enables plant growth — including all physiological processes, such as photosynthesis — to function as optimally as possible.
Powerful Sensors on Planes Detect Crop Nitrogen with High Accuracy
Synthetic nitrogen fertilizers transformed agriculture as we know it during the Green Revolution. Yet despite improvements in crop nitrogen-use efficiency, fears of underperformance spur fertilizer overapplication to this day. Excess nitrogen then ends up in waterways, including groundwater, and in the atmosphere in the form of potent greenhouse gases.
Predicting the amount of nitrogen needed by a particular crop in a particular year is tricky. The first step is understanding crop nitrogen status in real time, but it’s neither realistic nor scalable to measure leaf nitrogen by hand throughout the course of a season.
In a first-of-its-kind study, a University of Illinois research team put hyperspectral sensors on planes to quickly and accurately detect nitrogen status and photosynthetic capacity in corn.
“Field nitrogen measurements are very time- and labor-consuming, but the airplane hyperspectral sensing technique allows us to scan the fields very fast, at a few seconds per acre. It also provides much higher spectral and spatial resolution than similar studies using satellite imagery,” says Sheng Wang, research assistant professor at U of I.
“Our approach fills a gap between field measurements and satellites and provides a cost-effective and highly accurate approach to crop nitrogen management in sustainable precision agriculture,” he adds.
The plane, fitted with a top-of-the-line sensor capable of detecting wavelengths in the visible and near-infrared spectrum (400-2,400 nanometers), flew over an experimental field in Illinois three times during the 2019 growing season. The researchers also took in-field leaf and canopy measurements as ground-truth data for comparison with sensor data.
The flights detected leaf and canopy nitrogen characteristics, including several related to photosynthetic capacity and grain yield, with up to 85 percent accuracy.
“That’s close to ground-truth quality,” says Kaiyu Guan, co-author on the study. “We can even rely on the airborne hyperspectral sensors to replace ground-truth collection without sacrificing much accuracy. Meanwhile, airborne sensors allow us to cover much larger areas at low cost.”
Remote sensing picks up energy reflected from surfaces. The chemical composition of leaves, including their nitrogen and chlorophyll content, subtly changes how much energy is reflected. Hyperspectral sensors detect differences of just 3 to 5 nanometers across their entire range — a sensitivity unmatched by other remote sensing technologies.
“Other airborne remote sensing technologies pick up the visible spectrum and possibly near-infrared — just four spectral bands. That’s not even close to what we can do with this hyperspectral sensor. It’s really powerful,” Guan says.
The researchers see a use for their findings in the popular Maximum Return to Nitrogen (MRTN) corn nitrogen rate calculator.
Wang explains, “Under our approach, we can detect the nitrogen status of the crop and make some real-time adjustments for the agricultural stakeholders. MRTN provides recommended nitrogen fertilization rates based on the economic tradeoff between soil nitrogen fertilizer rates and end-of-season yield. Our remote-sensing approach can feed plant nutrient status into the MRTN system, enabling real-time crop nitrogen management. It can potentially shift the current recommendations based on pre-growing season, soil-centric fertilization to a diagnosis based on real-time plant nutrition, improving agroecosystem nitrogen use efficiency.”
Importantly, the research team worked out the best mathematical algorithm to detect nitrogen reflectance data from the hyperspectral sensor. They expect it will be put to use as newer technologies come on board.
“NASA is planning a new satellite hyperspectral mission, as are other commercial satellite companies. Our study can potentially provide the algorithm for those missions because we already demonstrated its accuracy in the aircraft hyperspectral data,” Wang says.
Guan says bringing this technology to satellites is the end goal, enabling a view of every field’s nitrogen status early in the growing season. The advancement will allow farmers to make more informed decisions about nitrogen side-dressing.
Ultimately, of course, the goal is to improve the environmental sustainability of nitrogen fertilizers in agronomic systems. And Guan says precision is the way to get there.
“Essentially, you can’t manage what you can’t measure. That is why we put so much effort into this technology.”
In the Presence of Nature
Remembering Teena Kreymar-Tainio
Athena (Teena) Kreymer-Tainio passed away on January 23, 2022, after battling neurological disease. Teena was the president of Tainio Biologicals and was the guardian of the legacy of her husband, Bruce, the founder of the company and one of the early leaders of the ecological agriculture movement.
Teena spent her childhood in Puyallup, Washington, happily camping, fishing, skiing and developing her enduring love of nature. With grit and determination, Teena put herself through nursing school while raising her two daughters as a single mother.
Soon after realizing her dream of becoming an RN, Teena met and married Bruce Tainio and eventually joined in the running of his agricultural business, Tainio Technology (later Tainio Biologicals). Building on all that she learned from Bruce and others in the field, Teena eventually realized another life-long dream of becoming a published author with her book Farming in the Presence of Nature, published by Acres U.S.A. in 2017.
The following is an excerpt from the book that highlights Teena’s deep understanding of nature and its importance in agriculture.
In the rolling hills of the Inland Northwest Palouse country, a field of barley senses the change of light as a cloud races across the blue summer sky, casting a moving shadow over the verdant land.
The plants are on full alert as a doe and her spotted fawn amble serenely through the field, nibbling on tender shoots and leaves. They can feel the animals’ legs brushing against their leaves, and detect the signals of the injured plants that have been grazed or stepped on.
A warm summer wind sighs and ripples through the field, sending waves of green across the hills and valleys; and riding upon those waves comes a band of dreaded plant predators, aphids.
Aphids multiply rapidly, and within a few days, in the average chemically grown monoculture crop, the vulnerable plants are often infested with disease-carrying, sap-sucking herbivores. But in this particular biologically managed field, these healthy plants work together to protect themselves. For diversity, the smart farmer plants a cultivar mix of barley varieties, and doesn’t worry much about an occasional patch of weeds here and there. A healthy stand of barley is a good weed suppressor in and of itself, and the thistle stem gall fly, released by the county Extension, helps keep the Canada thistle from becoming too aggressive.
As soon as the first plant falls victim to aphid attack, it goes into defense mode, producing a volatile scent that makes it less palatable to the aphids. The neighboring plants “smell” the volatiles produced by the injured plant, and in turn produce their own insect-repelling chemicals.
In nature there is strength in diversity. The varied cultivars of barley and the weeds have influenced each other to release their defensive volatiles, giving the whole community a stronger mix of pheromones to repel the aphids, fending off carnage and sending out distress signals to an army of beneficial insects.
Aphids are a favorite food of the ladybug, and the same volatiles that are repelling to the herbivores are the carnivorous insect’s dinner bell, signaling to the voracious predators that food is nearby (Ninkovic, Glinwood and Pettersson). Soon the ladybugs arrive en masse, and begin munching aphids and laying eggs. Within a few days, hordes of hungry larvae hatch and devour many times their weight in aphids, saving the barley plants and their wild companions from annihilation.
But the community’s line of defense doesn’t stop there. In the rhizosphere is a network of mycelial strands of mycorrhizal fungi connecting the roots of the plants to one another. Not only do the fungi transport nutrients and soil water to the plant roots in exchange for carbon, they also serve as an underground communication system that enables plants to alert each other of impending danger. Scientists don’t know conclusively yet, but the current assumption is that messages sent by plants via this underground “Internet” system are chemical-based (Babikova, Bruce and Birkette).
This diverse community of plants and insects, along with the invisible world of macro- and microbiota on and beneath the soil surface, are a well-balanced system, working together as Nature intended. The deep-rooted weeds and the windbreak of trees at the field’s border break up compacted soil and bring minerals and water to the surface from depths most annual crop plants can’t reach, and provide shelter for beneficial insects and wildlife.
Conversely, chemical farming creates a downward spiral of disease and addiction. Synthetic chemicals may control weeds and specific diseases, and cheap synthetic fertilizers may facilitate a decent crop yield, but not without great cost to the soil and plant community. Wild plants, beneficial insects, worms, soil microbes — all vital components of Nature’s immune system — are suppressed or killed, leaving lifeless dirt and the ensuing side effects of erosion, compaction, salt accumulation, contaminated groundwater, super weeds that an arsenal of herbicides can no longer control, and vulnerable, chemically dependent crops.
