The impact of plant competition cues on nitrate levels and their assimilation in major crop plants remains largely unknown. This study investigated how low red to far-red (R:FR) light, a signal of plant competition, affects nitrate levels and assimilation in maize and soybean seedlings. Maize and soybean seedlings were exposed to low R:FR light conditions (R:FR ≈ 0.5) that mimicked nearby weeds or artificial sources. Additional treatments included individual soybean seedlings subjected to a soybean canopy. Biochemical assays and RNA-sequencing were used to assess nitrate levels, assimilation-related enzymes, and gene expression. Low R:FR light led to increased leaf nitrate levels in maize by 95% and 52% compared to the weed-free control (R:FR ≈ 2.8), and in soybean by 50% and 63%, while ammonium levels remained unchanged. A 37% increase in leaf nitrate level was also observed in soybean exposed to canopy conditions. In maize, nitrate accumulation was linked to reduced activity of ferredoxin-dependent glutamine:2-oxoglutarate aminotransferase (fd-GOGAT), while activities of nitrate reductase, nitrite reductase, glutamine synthetase, and NADH-GOGAT were unaffected. RNA-sequencing of maize leaves did not show altered expression of tonoplast localized nitrate transporter genes. These findings suggest that low R:FR light, as a plant competition cue, promotes nitrate accumulation in maize and soybean, potentially by altering nitrate assimilation pathways rather than transport or storage. This response may influence crop growth and metabolism under competition stress.

Oxidative stress responses of Arabidopsis to reflected low red to far-red signals (R:FR ≈ 0.3) generated by neighboring weeds or an artificial source of FR light were compared with a weed-free control (R:FR ≈1.6). In the low R:FR treatments, induction of the shade avoidance responses (SAR) coincided with increased leaf production of singlet oxygen (1O2). This 1O2 increase was not due to protochlorophyllide accumulation and did not cause cell death. Chemical treatments, however, with 5-aminolevulinic acid (the precursor of tetrapyrrole biosynthesis) and glutathione (a quinone A reductant) enhanced cell death and growth inhibition. RNA sequencing revealed that transcriptome responses to the reflected low R:FR light treatments minimally resembled previously known Arabidopsis1O2 generating systems that rapidly generate 1O2 following a dark to light transfer. The upregulation of only a few early 1O2 responsive genes (6 out of 1931) in the reflected low R:FR treatments suggested specificity of the 1O2 signaling. Moreover, increased expression of two enzyme genes, the SULFOTRANSFERASE ST2A (ST2a) and the early 1O2-responsive IAA-LEUCINE RESISTANCE (ILR)-LIKE6 (ILL6), which negatively regulate jasmonate level, suggested that repression of bioactive JAs may promote the shade avoidance (versus defense) and 1O2 acclimation (versus cell death) responses to neighboring weeds.

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Nitrogen stress plays a critical role in corn yield reduction. Thermal remote sensing has many applications: as an assessment tool for urban heat island, as an ecological indicator of ecosystem development, and as a water-stress-detection tool. In this study, it was hypothesized that corn crops supplied with optimum or high rates of nitrogen would have lower surface temperatures compared to corn grown under nitrogen-stressed conditions. Two experiments were conducted in the greenhouse at the University of Guelph, Canada, from the period between 2015 and 2016, involving three rates of nitrogen (high, medium, and low rates) supplied to corn plants after seed emergence. Leaf and whorl temperatures were collected by using a high-resolution thermal camera, an infrared handheld point measurements gun, and a type T thermocouple, respectively. An approximate difference of 2 °C was observed in temperatures between plants receiving high and low rates of nitrogen. These results supported the hypothesis that nitrogen-stressed plants have higher temperatures compared to less stressed plants, at a 0.05 significance level. This study investigated the application of the exergy destruction principle through thermal remote sensing, to detect crop stress at early growth stages under greenhouse conditions, to increase the production and reduce the harmful environmental impact.

Recent studies indicate that thiamethoxam (TMX), a neonicotinoid insecticide, can affect plant responses to environmental stressors, such as neighboring weeds. The molecular mechanisms behind both stable and environmentally specific responses to TMX likely involve genes related to defense and stress responses. We investigated the effect of a TMX seed treatment on global gene expression in maize coleoptiles both under normal conditions and under low ratio red to far-red (R:FR) light stress induced by the presence of neighboring plants. The neighboring plant treatment upregulated genes involved in biotic and abiotic stress responses and affected specific photosynthesis and cell-growth related genes. Low R:FR light may enhance maize resistance to herbivores and pathogens. TMX appears to compromise resistance. The TMX treatment stably repressed many genes that encode proteins involved in biotic stress responses, as well as cell-growth genes. Notably, TMX effects on many genes' expression were conditional on the environment. In response to low R:FR, plants treated with TMX engage genes in the JA pathway, as well as other stress-related response pathways. Neighboring weeds may condition TMX-treated plants to become more stress tolerant.

Crop stress is a serious problem that affects crop production worldwide. Nitrogen stress affects a plant's ability to grow and develop. Many methods have been used to detect nitrogen stress, however, these methods have many limitations including sensitivity to soil and sunlight variations. This study proposes the inverse relationship between crop surface temperature and crop stress under greenhouse conditions as predicted by the exergy destruction principle (EDP). It is hypothesized that stressed crops have higher surface temperatures compared to less stressed crops. In this study, the main objectives are to confirm the decrease in crop surface temperature with nitrogen rate increase under greenhouse conditions, and to investigate the effect of nitrogen stress on leaf thermal emissivity.

AbstractMultiple herbicide-resistant populations of horseweed [Conyza canadensis (L.) Cronquist] continue to spread rapidly throughout Ontario, notably in areas where no-till soybean [Glycine max (L.) Merr.] is grown. The occurrence of multiple herbicide resistance within these populations suggests that the future role of herbicide tank mixtures as a means of control will be limited. An integrated weed management strategy utilizing complementary selection pressures is needed to reduce the selection intensity of relying solely on herbicides for control. Field studies were conducted in 2018 and 2019 to test the hypothesis: if fall-seeded cereal rye (Secale cereale L.) can reduce C. canadensis seedling density and suppress seedling growth, then the interaction(s) of complementary selection pressures of tillage, cereal rye, and herbicides would improve the level of C. canadensis control. Laboratory studies were conducted to determine whether the allelopathic compound 2-benzoxazolinone (BOA) affected the root development of C. canadensis seedlings. The interactions observed among multiple selection pressures of tillage, cereal rye, and herbicides were inconsistent between the 2 yr of study. A monoculture of cereal rye seeded in the fall, however, did reduce seedling height and biomass of C. canadensis consistently, but not density. This reduction in seedling height and biomass was likely caused by the allelopathic compound BOA, which reduced seedling root development. Control of C. canadensis seedlings in the spring required the higher registered rates of dicamba or saflufenacil. The addition of shallow fall tillage and the presence of cereal rye did not improve the variability in control observed notably with 2,4-D or the lower rates of saflufenacil or dicamba. With the implementation of complementary weed management strategies, environmental variables in any given year will likely have a direct influence on whether these interactions are additive or synergistic.

Weed thresholds are an integral component of an integrated weed management system (IWM). In this paper we review the 10literature on weed thresholds and examine the theory and applicability of thresholds within the context of a systems approach to IWM. The development of empirical models describing single and multi-species thresholds are reviewed and discussed in terms of the magnitude of weed threshold values in various crops and the importance and limitations of the parameters used to calculate these values. Mechanistic weed threshold crop competition models are suggested as a means of overcoming some of the limitations of empirically based threshold models. A mechanistic approach to the development of weed threshold models is desirable since relative crop and weed responses to environmental factors, cultural practices and the dynamic nature of competition are considered. Guidelines for the application of weed thresholds within a cropping system are outlined. [Article copies available for a fee from The Haworth Document Delivery Service: 1-800-342-9678. E-mail address: getinfo@ haworthpressinc.com]

Photosynthetic responses of common bean ( Phaseolus vulgaris L.) to increasing durations of weed-free and weedy environments were investigated using a critical period for weed control study under field conditions. The presence of weeds induced the shade avoidance response and was accompanied by a reduced red to far-red ratio (R/Fr) of reflected light supporting previous assertions it is an important signal regulating crop-weed interactions. Despite increases in stomatal conductance and leaf intercellular [CO 2 ] with increasing duration of weed presence, CO 2 assimilation and photosynthetic efficiency continually declined. This coincided with reduced Calvin cycle capacity suggesting induction of biochemical rather than stomatal limitations on photosynthesis. Weed removal prior to reproductive stages resulted in maintenance of high photosynthetic capacity. When weed presence extended to reproductive stages and beyond the critical period for weed control, however, CO 2 assimilation and photosynthetic efficiency never recovered. Yield was highly correlated with photosynthetic efficiency and in a similar manner, declined with increasing durations of weed presence through reduced seeds per plant. We conclude that the lasting consequence of weed competition is impairment of photosynthesis, which may provide an important mechanism to explain yield loss.

The number of fertilized florets around silking determines the number of kernels in maize (Zea mays L). In this study, we examined the relationship between plant dry matter accumulation, ear initial length and floret number. What does this relationship look like between the V7 to V9 growth stages and how does it change when the plants are subjected to stress events? We tested the hypothesis that if floret number per ear row, is related positively to plant dry matter accumulation during early stages of growth then the occurrence of stress during this period will result in a loss of floret number. Three modem hybrids were subjected to three different stresses, drought, plant population density and light quality. Light quality and drought stresses were applied for two seasons in our field hydroponics system, while the plant population density stress was conducted for two field seasons. The relationship between plant dry matter and floret number follows the classic relationship with a minimum dry matter level required and a plateau. Not all stresses impacted these relationships in the same manner. Drought stress was most severe resulting in a reduction in dry matter, ear length and floret number for the sampled period. Plant density stress was similar to drought, resulting in a reduction in plant dry matter and ear length, however, floret number per ear row was unchanged. Both drought and plant density stress resulted in reductions in the predicted minimum dry weight for floret number and the rate of increase of floret number per unit dry matter. Light quality defined as a reduced red-to-far red ratio caused reductions in dry matter up to and including the V7 stage of growth, however, the relationship between floret number and plant dry matter accumulation did not vary from the control. These results confirm that floret number in maize is established well in advance of flowering and also suggests that floret number is related to plant dry weight sampled between V7 to V9-10 stage of growth. This research presents new knowledge on the relationship between floret number, plant cumulative dry matter and early season stress.

The development of a linuron-free weed management strategy for carrot production is essential as a result of the herbicide reevaluation programs launched by the Pest Management Regulatory Agency in Canada for herbicides registered before 1995 and the discovery of linuron-resistant pigweed species in Ontario. Field trials were conducted in one of Ontario's main carrot-growing regions on high organic soils in 2016 and 2017. Pigweed species seedlings were effectively controlled with PRE treatments of prometryn, pendimethalin, S-metolachlor, or glufosinate. POST treatments of pyroxasulfone and metribuzin followed by predetermined biologically effective dose (>= 90% control of pigweed seedlings) of acifluorfen, oxyfluorfen, fluthiacet-methyl, and fomesafen achieved excellent crop selectivity and commercially acceptable pigweed species seedling control under field conditions. Carfentrazone-ethyl or fomesafen applied PRE severely reduced seedling emergence and yield in the wet growing season of 2017. This study demonstrated clearly that an alternative linuron-free strategy can be developed for carrots. The strategy of exploring the potential to use the biologically effective dose of selected herbicides to achieve crop selectivity and control of pigweed species seedlings was verified.

Nitrogen is one of the most important yield‐limiting nutrients for corn (Zea mays). The ability of thermal remote sensing to detect nitrogen deficiency in corn may enable precision agriculture to modify nitrogen rates according to field conditions. This study applies the exergy destruction principle as a theory to explain the inverse relationship between surface temperature and nitrogen rate. Two hypotheses were developed. First, it was hypothesized that agricultural crops experiencing greater growth and providing greater yield will have lower surface temperature. The second hypothesis was that corn grown under optimum levels of nitrogen will have lower surface temperatures compared to corn grown under nitrogen stressed conditions. Field studies were conducted during two summer seasons (2016 and 2017) on an established long‐term field trial of corn yield response to varying rates of nitrogen. It was found that corn surface temperature decreased as the rate of nitrogen increased. A shallow but statistically significant (P < 0.05) negative slope was observed consistently with increasing rates of nitrogen. Surface temperature measurements, however, were variable. This variability was the result of external and weather dependent variables that influenced leaf surface temperature. Despite this variability, the exergy destruction principle provides a theory from which thermal remote sensing can be applied through the use of surface temperature measurements to detect physiological stress in crop plants.Core Ideas Thermal remote sensing was proposed to detect nitrogen stress in corn plants. Nitrogen stressed plants had higher surface temperatures than less stressed plants. Temperature trends were consistent with the exergy destruction principle. Nitrogen‐temperature correlations were statistically significant at the 0.05 significance level. Corn yield increases with nitrogen rate increase and surface temperature decrease.

SummaryThe mechanisms by which weeds compete with crop plants are poorly understood. To gain insight into these mechanisms, we characterised early physiological responses of soyabean to neighbouring weeds using a biological weedy system that generated a consistent far‐red‐enriched light environment and excluded direct resource competition. Neighbouring weeds decreased superoxide dismutase activity in unifoliate leaves. This coincided with increased hydrogen peroxide (H2O2) and oxidized ascorbate levels, while the steady‐state level of superoxide, catalase activity and lipid peroxidation remained unchanged. These responses suggested increased leaf production of singlet oxygen (1O2), which was demonstrated by detection of increased Singlet Oxygen Sensor Green (SOSG) fluorescence within 3 h after staining of unifoliate leaves. This finding was further supported by increased ratios of the photosensitiser protochlorophyllide to both chlorophyllide a, and total chlorophyll in the dark as well as enhanced sensitivity to cell death by a 1O2‐generating compound in the light. These responses coincided with dramatic changes in photosynthesis, carbon partitioning and biomass allocation with a persistent decline in leaf sucrose level and biomass production at later growth stages. This study provides direct experimental evidence that under resource‐independent competition, far‐red‐enriched light reflected by neighbouring weeds can alter the balance between ROS production and detoxification and thereby generate an oxidative stress signal in soyabean leaves.

The discipline of weed science is at a critical juncture. Decades of efficient chemical weed control have led to a rise in the number of herbicide-resistant weed populations, with few new herbicides with unique modes of action to counter this trend and often no economical alternatives to herbicides in large-acreage crops. At the same time, the world population is swelling, necessitating increased food production to feed an anticipated 9 billion people by the year 2050. Here, we consider these challenges along with emerging trends in technology and innovation that offer hope of providing sustainable weed management into the future. The emergence of natural product leads in discovery of new herbicides and biopesticides suggests that new modes of action can be discovered, while genetic engineering provides additional options for manipulating herbicide selectivity and creating entirely novel approaches to weed management. Advances in understanding plant pathogen interactions will contribute to developing new biological control agents, and insights into plant-plant interactions suggest that crops can be improved by manipulating their response to competition. Revolutions in computing power and automation have led to a nascent industry built on using machine vision and global positioning system information to distinguish weeds from crops and deliver precision weed control. These technologies open multiple possibilities for efficient weed management, whether through chemical or mechanical mechanisms. Information is also needed by growers to make good decisions, and will be delivered with unprecedented efficiency and specificity, potentially revolutionizing aspects of extension work. We consider that meeting the weed management needs of agriculture by 2050 and beyond is a challenge that requires commitment by funding agencies, researchers, and students to translate new technologies into durable weed management solutions. Integrating old and new weed management technologies into more diverse weed management systems based on a better understanding of weed biology and ecology can provide integrated weed management and resistance management strategies that will be more sustainable than the technologies that are now failing.

Meeting future food demands for 9 billion people in the next 30 years will require either agricultural expansion or intensification to increase production. However, agriculture is already a major driver of biodiversity loss, as well as freshwater withdrawals, nutrient inputs, and greenhouse gasses, among other pressing environmental issues. In this paper, we look for solutions to this production-conservation challenge at the subfield scale. We use precision agriculture yield data from three farms in Southern Ontario and convert them into "profit maps" that show which regions of a field have management costs that exceed the market value of the commodities produced. We analyse the profit of three farms over time and identify areas that consistently show low or negative profit and thus constitute a compelling case for taking these areas out of production. We find, for example, that up to 14% of farmland can result in money loss and even more than 50% of the land might still not meet minimum revenue expectations. Further, we assess the economic feasibility of conservation strategies on these set-aside lands and find that investing in environmental benefits (even minimally) can often times be inexpensive when compared with economic losses due to failed harvests. We argue that profit mapping can serve as a management tool for farmers that will allow them to identify optimal crop areas, optimize nutrient inputs, plan for ecological intensification, and avoid economic loss all while providing ecosystem services at the local scale.

In order to intensify agricultural productivity while at the same time enhance ecosystem services, it is necessary to evaluate carefully how current weed-management technologies are deployed, including herbicides and herbicide-resistant crops. Herbicide chemistries and herbicide-resistant crops have provided excellent technologies that have resulted in significant changes to the way weeds can be controlled. This chapter highlights several key components that must form the basis for an effective integrated weed-management strategy, including tillage, the importance of understanding weed emergence relative to the crop, critical periods for weed control, crop morphology, row width, nutrient management and crop rotation.

Sustainably feeding the next generation is often described as one of the most pressing “grand challenges” facing the 21st century. Generally, scholars propose addressing this problem by increasing agricultural production, investing in technology to boost yields, changing diets, or reducing food waste. In this paper, we explore whether global food production is nutritionally balanced by comparing the diet that nutritionists recommend versus global agricultural production statistics. Results show that the global agricultural system currently overproduces grains, fats, and sugars while production of fruits and vegetables and protein is not sufficient to meet the nutritional needs of the current population. Correcting this imbalance could reduce the amount of arable land used by agriculture by 51 million ha globally but would increase total land used for agriculture by 407 million ha and increase greenhouse gas emissions. For a growing population, our calculations suggest that the only way to eat a nutritionally balanced diet, save land and reduce greenhouse gas emissions is to consume and produce more fruits and vegetables as well as transition to diets higher in plant-based protein. Such a move will help protect habitats and help meet the Sustainable Development Goals.

Six field trials were conducted over a 2-yr period (2014 and 2015) at two locations in southwestern Ontario to compare the level of weed control provided by dicamba applied alone and in combination with dimethenamid-P applied before planting (preplant, PP) in glyphosate-and dicamba-resistant soybean to current industry standards when used in a two-pass weed management program. Crop injury, weed control, soybean seed yield, environmental impact (EI), and profitability were evaluated in this study. No statistically significant injury was documented. Several PP herbicides provided excellent early-season grass and broadleaf weed control, although early-season weed control of those weed species was not acceptable with glyphosate applied alone or in combination with dicamba, dicamba + dimethenamid-P, 2,4-D, or saflufenacil. At 8 wk after application, the sequential application of a PP herbicide followed by glyphosate applied after emergence (POST) provided at least 86% control of the weed species evaluated in this study. Weed interference with no herbicide treatments caused a soybean seed yield loss of 64%. The sequential application of glyphosate had the lowest EI value. The addition of chlorimuron + metribuzin or chlorimuron + imazethapyr did not increase the EI substantially but did improve the level of weed control and reduced weed density and biomass. The inclusion of a PP herbicide in a weed management program has several stewardship benefits and may reduce the selection for herbicide resistant weeds.

Summary Intraspecific competition among crop plants is undesirable. Less competitive crops are predicted to increase yield and decrease the need for added resources. Wild plants demonstrate the ability to recognize kin and potentially help their relatives by reducing their competitive behaviours, a form of altruism. Altruism can also evolve through multilevel selection. Are these processes relevant to sustainable agriculture? Crops do grow predictably with kin. However, their evolution is more strongly dictated by artificial selection (crop breeding), which incorporates individual and group selection, making multilevel selection more relevant than kin selection in favouring altruism. While current crop breeding protocols attempt to target the reduction of competitive traits, early mass selection may have the opposite effect. We predict that kin recognition itself is not relevant to crops, because of the consistently high relatedness within crop stands. Nonetheless, crops have shown cultivar and kin recognition. We argue that these responses cannot be assumed to demonstrate altruism, as current breeding practices offer little opportunity for kin selection. Synthesis. There is the opportunity to favour altruism through artificial breeding. Here we suggest how crop breeding protocols could be changed to favour cooperation by increasing group selection during early breeding.