Effects of herbicide applications in wheat fields

Abstract

The present review encompasses the physiological and yield constraints of herbicide applications with special reference to wheat productivity. Post-independence lagging of Indian agriculture to feed its population led to haphazard use of chemical pesticides and weedicides which deteriorated the productivity pay-off particularly of wheat and rice. Past some decades witnessed the potential use of certain phytohormones in augmenting abiotic stress to get rid of yield gap and productivity constraints. We summed up with reviewing the potential role of these natural regulators in overcoming above mentioned drawbacks to substitute or to integrate these chemicals with the use of plant hormones.

Keywords: :

• Phalaris minor
• Triticum aestivum
• brassinosteroids
• herbicides
• isoproturon toxicity
• phytohormones
• safeners

Introduction

Wheat is of prime importance in the realms of food crops in the world. The total area of the world under wheat is around 212.99 million ha with grain yield of 596.20 million tons.¹ The major wheat producers that include China, India, USA, France, Russia, Canada, Australia, Pakistan, Turkey, Argentina, Iran and Italy contribute 76% of global production. Area wise India shares 12.40% of the global cropped area while on the basis of wheat production; India occupies 11.63% of the global production. In India, it is the second important food crop being next to rice. The major wheat producing states in India are Uttar Pradesh, Punjab and Haryana contributing 34%, 21% and 13% respectively.² The most important species of wheat is Triticum aestivum occupying 85% of total area under wheat cultivation and the next important species is Triticum durum occupying 14% of the wheat area.³ Wheat is rabi crop of temperate zone with cool winters and hot summers being very conducive for its good growth. Indo-Gangetic plain is the most important area where wheat is grown. India is broadly divided into five wheat zones based on agro-climatic conditions viz. North-West, North-East, Central, Peninsular and Northern hill region. The planting to harvesting time ranges from late Oct to May–June. Well drained loams and clayey loams are considered good for wheat. Good crops of wheat are also raised in sandy loams and black soils. Optimum date of sowing depends on the type of variety, weather, soil, irrigation facility etc. Most appropriate time of sowing is when the daily ambient temperature drops to 20–22°C. Therefore, second fortnight of November is optimum time of sowing in northern plains. The crop is harvested when the grains harden and the straw become dry and brittle. Almost 75% of wheat cultivation depends on rain for irrigation. The annual rainfall in wheat zones varies from 12.5 to 100 cm and most of it is received during summer or monsoon season.

Productivity Constraints and Weed Problem

Wheat grown area in India is about 27.99 million ha with a production of 75.81 million tons. After the independence (1950–51) the production of wheat in India was only 6.46 million tons and productivity was mere 663 kg/ha, insufficient to feed the Indian population (almost 1214.3 million). The initiation of green revolution in mid sixties, expansion of irrigation and adoption of high yielding varieties helped a lot in increasing wheat productivity. The advent of dwarf wheat and establishment of All India Coordinated Wheat Improvement Project (AICWIP) proved an important milestone for systematic wheat research and getting real breakthrough in its productivity but still many constraints affected its yield. The main factors which influence crop production are radiation, soil moisture, nutrient availability and length of growing season. Yield reducing factors also encompass disease, insects and weed infestation.

The presence of weeds within the crop may adversely affect production in a number of ways. Weeds compete with crop species for water, nutrients and light and ultimately reduce crop yield.⁴ Weeds are unwanted plant species growing in the domesticated crops. The competition of weeds for nutrients may results in such obvious responses as dwarfing in plant size, nutrient starved conditions, wilting and actual dying out of plants.⁵ Weeds are notorious yield reducers that are, in many situations, economically more important than insects, fungi or other pest organisms.^6,7 Weeds have inhibitory effect on crops.^8,9 The growth of most of the crops involves a constant battle with the weeds in addition to insect pests and diseases. Weeds not only reduce the crop yield, but also deteriorate the quality of the produce thereby, reducing its market value. Weeds reduce yield by affecting the sunlight reaching the plants. In some more serious cases it may lead to complete failure of crop.¹⁰ Therefore, the eradication of weeds from the crop fields is essential for obtaining maximum returns. The various methods for eradication of weeds are hoeing, weeding, dabbing, tillage, bar harrowing, crop rotation biological and chemical controls.

Indo-Gangetic or northern plains of India are mostly comprised of wheat-rice cropping system. The major weeds prevalent in wheat fields are dicot and monocot, grown in Rabi season viz. Bathua (Chenopodium album), Gazari (Fumaria parviflora), Katili (Cersium arvensis), Krishnneel (Anagallis arvensis), Akari (Vicia hirsuta), Sengi (Melilotus alva/Meliotus indica), Chatari matari (Lathyrus aphaca), Satyanashi (Argemone maxicana) etc. Likewise, monocot weeds viz., Gehusa/Gullidanda / Gehun ka mama (Phalaris minor), wild oats (Avena fatua), Piazi (Asphodelus tenuifolius) etc. that impose serious problems in wheat fields. In addition to these, doob (Cynodon dactylon) is a major perennial weed. The most noxious weed in wheat field is Phalaris minor Retz. (Littleseed canary grass). Surveys of wheat crops in the states of Punjab^11,12 and Haryana^13-15 established P. minor as the most dominant weed of wheat in northwest India. It is very difficult for the farmers to identify due to their resemblance with the wheat plants in early stages of growth. Its infestation has been a longstanding management problem for farmers. Its morphological similarity and competitive fast growth with wheat are important problem. The weed problem dates back to green revolution of dwarf wheat varieties. Untreated weed infestation can result in dramatic reduction in wheat yield by 57%,¹⁶ therefore farmers are being forced to harvest immature crops. Complete failure of crop can occur in extreme cases.

Weedicides Application in Wheat Fields: Efficacy and Drawbacks

Traditional methods of weed control such as crop rotation, manual hoeing or tractor drawn cultivator and costly labor have made the use of herbicides popular among Indian farmers. Keeping the importance of these circumstances in view, it is necessary to select the suitable chemicals capable of controlling effectively and economically all the type of weeds present in wheat crop. There are many kinds of chemicals (herbicides) which are used for controlling the weeds. The herbicides are most effective in controlling annual as well as perennial weeds. However, it is essential to select an appropriate kind of chemical and to use it at a specified rate; otherwise they may damage the crop. It should be kept in mind that the chemicals not only destroy the weeds but also get mixed with air, water and soil. Crops take the chemicals from the soil due to which their effect remains in crops. However, the eradication of weeds through chemicals is considered suitable for more area during short period of time. Herbicide is a chemical used to kill or inhibit the growth of weeds and other unwanted plant pests. Herbicides can be classified in several ways, including the weed control spectrum, labeled crop usage, chemical families, mode of action, application timing/method, and others. Contact herbicides kill only the plant parts in contact with the chemical, whereas systemic herbicides are absorbed by the roots or foliage and translocated throughout the plant. Herbicide activity can be either selective or nonselective. Selective herbicides are used to kill weeds without significant damage to desirable plants. Nonselective herbicides kill or injure all plants present if applied at an adequate rate. To be effective, herbicides must adequately be in contact with plants, be absorbed by plants, move within the plants to the site of action without being deactivated, and reach to toxic levels at the site of their action. The term “mode of action” refers to the sequence of events from absorption into plants to plant death, or, in other words, how an herbicide works to injure or kill the plant. A number of weed species that were once susceptible to and easily managed by certain herbicides have developed resistance with time. These weeds are no longer controlled by applications of previously effective herbicides. As a result the repeated use of a specific type of herbicide on the same land has developed resistance in some type of weeds to these chemicals. Their application can therefore result in visible crop injuries i.e., leaf chlorosis, necrosis, plant deformations, decolorization, leaves withering, growth retardation.^17,18

Effect of Herbicides on Wheat Plants

Growth and development

The bioaccumulation and effects of herbicide isoproturon on two freshwater rooted macrophytes Elodea densa and Ludwigia natans were studied.¹⁹ The results showed a significant growth inhibition of E. densa cuttings to isoproturon concentration close to 10 μg L⁻¹. Oxygen concentrations in the medium were weakly but significantly reduced after 24 h exposure to 2 μg L⁻¹ isoproturon. Khan and co- workers²⁰ conducted an experiment and reported that isoproturon 75 WP at 30 DAS application had a phytotoxic effect on the wheat crop.

Ali and coworkers²¹ showed that herbicidal treatments of metribuzin and isoproturon + diflufenican produced smaller plants which can be due to their phytotoxic effect on wheat crop. Metribuzin and isoproturon+ carfentrazone treated plots resulted in lowest plant height as compared with other herbicides. Oxidative stress was also induced. In an experiment²² conducted it was shown that the tested dose of atrazine, isoproturon and metribuzin significantly reduced the nodulation (nodule number and dry mass) in green gram. Minimum grain protein was obtained at 400 μg kg⁻¹ of isoproturon (124 mg g⁻¹). An experiment²³ was conducted to know the response of wheat to chlorotoluron, a phenyl urea herbicide. The experiment showed that it induced oxidative stress in wheat (Triticum aestivum). Treated plants showed accumulation of O₂- and H₂O₂ in leaves and resulted in peroxidation of plasma membrane lipids in plants. Yin and coworkers²⁴ reported that isoproturon induced oxidative stress in Triticum aestivum. Significant decrease in chlorophyll content at low concentration of isoproturon (2mg/kg) was observed. Treatment with isoproturon at 2, 3.5, 5, 10, and 20 mg/kg progressively inhibited the shoot growth, as expressed by dry weight. However, the significant inhibition occurred at 10–20 mg/kg. Treatment with 20 mg/kg isoproturon decreased the root length to 44% of the control. Isoproturon²⁵ reduced the fresh weight, dry weight, chlorophyll and carotenoid content of 10 d old maize seedlings in the conducted experiment. Kieloch and Rola²⁶ in an experiment showed that the plots treated with the mixture pendimethalin + isoproturon were thinned markedly and leaf withering was observed. Cultivar clever proved to react negatively to the mixture of pendimethalin + isoproturon, which led to significant wheat thinning as compared with the untreated plots.

Photosynthesis

The effect of isoproturon was investigated²⁷ in two wheat cultivars (Triticum sativum L. cvs. castan and esquilache) and a weed (Lolium rigidum Gaud.) and reported that root growth inhibition was considerably significant in esqilache. Further,²⁸ the use of isoproturon affected the ultrastructure of photosynthetic apparatus of wheat cv esqilache and decreased the activity of ribulose bisphosphate carboxylase. A decrease in protein and chlorophyll content was also observed. The grain yield was poor. The content of chlorophyll significantly decreased even after the exposure to 2 mg/kg of isoproturon, the chlorophyll content decreased by 11% as compared with the control. Exposure of wheat plants to isoproturon led to lipid peroxidation in roots and leaves. Sharma and Bandana²⁹ conducted an experiment to study the effect of isoproturon on chlorophyll and sugar content in wheat leaves at different stages of growth. The treatments included control, handweeding (twice) and three concentrations of isoproturon (35 DAS) viz. 1.0 kg/ha, 1.0 kg/ha + 0.1% surfactant and 1.5 kg/ha. All the three isoproturon treatments resulted in a decrease in chlorophyll and sugar content in wheat leaves at 30 d after herbicide application.

Chlorophyll fluorescence

Chlorophyll fluorescence is an important technique in basic and applied plant physiology research. It is used to look at the photosynthetic activity³⁰ expressing the plant health. Its measurement is rapid and non- invasive. The level of PSII was inhibited inside the leaves followed by changes in chlorophyll fluorescence intensity after droplet deposition of isoproturon on leaf fragments in Triticum aestivum.³¹ The effects of 5 μg L⁻¹ isoproturon³² on photosynthetic activity of leaf cells of the freshwater macrophyte Elodea densa was studied and a noticeable effect was observed in chlorophyll ‘a’ in vivo fluorescence, F_I/F_P ratios determined on the basis of induction curves. The effects on chlorophyll fluorescence in wheat, KR-1 (R) and H-2 (S) biotypes of Phalaris minor were measured.¹⁷ A 4-h treatment of excised leaves incubated in isoproturon solution (0.025 and 0.05mM concentration) resulted in a decreased fluorescence coefficient [F_v/F_m ratio, in which F_v = variable fluorescence (F_m - F_o); F_m = the maximum fluorescence and F_o = initial fluorescence] in wheat (T. aestivum L.) and both biotypes of P. minor. Dewez and co- workers³³ investigated photosynthetic-fluorescence parameters to be used as valid biomarkers of toxicity when alga Scenedesmus obliquus was exposed to isoproturon [3-(4-isopropylphenyl)-1, 1-dimethylurea] effect. Chlorophyll fluorescence induction of the treated algal cells showed inactivation of photosystem II (PSII) reaction center and strong inhibition of PSII electron transport. Chlorophyll fluorescence measures a number of parameters linked to physiology like quantum yield (Fv/Fm), which is a measure of maximum photochemical efficiency of photosystem II. An alternate measurement is fluorescence decline ratio (R_fd) which Horgan and Zabkiewicz³⁴ investigated in wheat and two other plant species with herbicides dalapon and diuron and noticed visible damage in leaves of wheat after 3 d.

Enzyme activity and metabolism

An experiment³⁵ conducted on maize by treating it with isoproturon and concluded that isoproturon resulted in the accumulation of H₂O₂ in leaves of 10 d old maize seedling. SOD activity was significantly advanced whereas ascorbate peroxidase activity was significantly reduced. It also reduced Glutathione S-transferase isoform activities. The activities of antioxidant enzymes in wheat plants²⁴ showed a substantial change compared with the control, when subjected to isoproturon exposure. Application of isoproturon up to 20 mg/kg led to the decreased activity of SOD. Moreover, the activity of GST, one of typical detoxifying enzymes, was elevated in response to isoproturon.

Herbicides retard wheat growth and grain yield

Chhokar and Malik³⁶ showed that under field conditions, at post-emergence isoproturon at 1000 g/ha 32 d after sowing failed to control littleseed canarygrass and as a result wheat grain yield decreased by 65% compared with the weed-free control. Singh³⁷ reported that dry weight of S biotype was significantly reduced at 0.25 kg/ha of isoproturon. Toxicity of isoproturon to wheat and the R biotype increased several fold when P-450 inhibitors were added to herbicide solution. Chokkar and coworkers³⁸ showed that infestation of isoproturon resistant population caused > 65% wheat grain yield reduction with the recommended rate of isoproturon (1000 g ha⁻¹) application. Similarly, an experiment²⁵ on isoproturon treated maize showed reduced fresh and dry weight of shoots and roots as well as chlorophyll and carotenoid contents of 10 d old maize seedlings during the following 20 d.

Isoproturon and associated problems in wheat: Grown in fields

Among different herbicides, isoproturon was extensively used due to its broad-spectrum weed control and flexibility in application timing and method. Isoproturon was recommended for the control of P. minor in wheat, and was largely accepted by the Indian farmers³⁹ due to its broad-spectrum weed control and wide application window along with its selectivity under wheat. Extensive use of isoproturon over many years has led to the evolution of resistance in Phalaris minor in northwest India.^36,40 Therefore, continued reliance on isoproturon after the development of resistance resulted in a heavy build-up of Phalaris minor populations, as competition from other weeds was removed. This caused heavy yield losses in wheat. Evolution of resistance in Phalaris minor to isoproturon is due to a number of factors. (1) The farmers include continuous use of isoproturon for many years, under dosing and inappropriate application methods.^40-42 (2) The recommended dose of isoproturon (1000 g a.i./ha) was not effective against P. minor due to resistance development and a double dose (2000  g a.i./ha) failed to provide effective control and resulted in large scale yield reduction in wheat.^36,40 Alternative herbicides used for control of isoproturon resistant Phalaris minor were cloninafop, fenoxaprop, trialkoxydim and sulphosulfuron. Sulphosulfuron³⁶ at 25 g /ha was found to be best for control of Phalaris minor and many broad leaf weeds. Metribuzin and Flufenacet have also been investigated in winter wheat and other cereals.^43-48

There were instances when wheat growers were forced to harvest their immature crop as fodder in the absence of effective alternative herbicides.⁴⁰ These herbicides are effective primarily on annual broadleaves, while some provide control of grasses as well.

The Toxic Effects of Isoproturon

The use of isoproturon [3-(4-isopropylphenyl)-1,1-dimethyl urea], a phenyl urea herbicide induces oxidative stress and decreased chlorophyll content in T. aestivum at even very low concentrations.²⁴ Isoproturon blocks the flow of electrons through PS I of photosynthesis by binding to D₁ protein in the thylakoid membrane. This herbicide inhibits the Hill reaction in photosynthetic electron transport, with subsequent inhibition of ATP and NADPH₂ formation; irreversible damage to the photosynthesis process leads to a permanent lack of food production in the susceptible plant. Photosynthesis-inhibiting herbicides block the light reactions of photosynthesis thus inhibiting sugar production. In trapping the light energy, the electrons are borrowed from chlorophyll (the green material in leaves) which are then replaced by electrons split from water. Under the circumstances in which chlorophyll electrons are not replaced, the chlorophyll is destroyed and the plant’s food manufacturing system breaks down. The plant slowly starves to death due to lack of energy. Adoption of fenoxaprop-P, clodinafop, and sulfosulfuron in isoproturon-resistant areas since 1997 initially led to high yields, but resulted in a weed flora shift which eventually reduced yields and increased the cost of weed management. Although isoproturon recommendation has been withdrawn from rice–wheat cropping zones, resistance in littleseed canarygrass is spreading in other areas where isoproturon has been used for several years because it is inexpensive and has broad-spectum weed control. In some cases herbicides are not completely selective to a particular crop. Their application can result in visible crop injuries i.e., leaf chlorosis, necrosis, plant deformations, decolorization, leaves withering, growth retardation.^17,18 The main reason for winter wheat cultivars varied tolerance to herbicide is because of diverse viability to plant metabolic and morphologic properties that govern herbicide uptake and translocation.^49,50

Potential of Phytohormones to Counter Herbicidal Activities

Herbicides are used to manage unwanted vegetation or weeds but their inappropriate use causes damage to non-target plants as well. Plant hormones have been cited to play an important role in regulating plant responses to a wide range of biotic and abiotic stresses such as herbicidal phytotoxicity. It has now been found that the compound 2,4-Dichlorophenoxyacetic acid is suitable for protecting cultivated plants against the phytotoxic action of clodinafop-propargyl.⁵¹ Two field experiments⁵² were performed to study the reversal effect of glyphosate induced phytotoxicity on growth and yield and its components of fava bean by the application of growth factors, i.e., growth regulators, amino acids and nutrient elements at different concentrations. GA₃ alone or in a mixture with cytokinin reversed the phytotoxic effect of the glyphosate herbicide on decreasing the plant height. Cytokinin at 4/1000, as well as GA₃ at 50 ppm, reversed the phytotoxic effect of glyphosate herbicide on decreasing the dry weight/plant, the number and dry weight of pods, seed yield per plant and per ha. Brassinosteroids enhance resistance of plants to various stresses such as cold, fungal infection, herbicide injury and salt.^53-56 BRs reduce the damaging effects of simazine, butachlor or pretilachlor in rice.⁵⁷ Reduction in the residue levels of various pesticides in cucumber (Cucumis sativus L.) were also reported.⁵⁸ BRs also increased the ability of resistance in plants against a wide variety of environmental and other stresses like herbicide safening under field conditions.^59,60 Analyses of chlorophyll fluorescence together with the measures of photosynthetic CO₂ assimilation and plant growth indicate that the harmful effects caused by s-triazine herbicides can be alleviated by brassinosteroids. A group of chemically diverse compounds called safeners having unique ability to protect grass crops from herbicide injury^61,62 by increasing the expression of genes encoding herbicide-metabolizing enzymes, such as the glutathione S-transferases (GSTs), cytochrome P450 monooxygenases (P450s), and several others.^63-65 Safeners stimulate herbicidal metabolism by increasing activity of Glutathione S-transferase (GST) enzymes which detoxify endogenous toxins or xenobiotics.⁶⁵ They act as bioregulators by intensifying the detoxification process of some herbicides.^61,64,66-69 A group of enzymes catalyzing various oxidative, hydrolytic and conjugation reactions are able to metabolize most of the herbicides.^70-75 Grossmann⁷⁶ also used 2,4-D (auxin) as weed killer for better crop production. Cobb and Reade⁷⁷ proposed that for more than 60 y, auxins are being used as herbicides which have low persistence and are unlikely to create environment related problems in future also.

Future Prospects

To feed the growing population there is a need to increase the wheat production without much dependence on chemicals like fertilizers and herbicides which have unpredictable harmful effects on environment and human health. Herbicide resistant weeds and herbicide toxicity to non-target crops are the major obstructions in the way of high production of cultivated crops. There is a need to improve weed management techniques for better crop production. So, we must adopt environment friendly products like plant hormones which are good weed-killers or alternatively select herbicide resistant crops. Herbicide safeners can also be a good option. These are chemical compounds used in combination with herbicides to make them safer, by reducing the toxic effect of herbicides on crop plants and improve selectivity between crop plants and weed species that are the major target by herbicide. These safeners interact with the receptor proteins of herbicides to downregulate the impact of herbicide at the target. Herbicide safeners improve crop tolerance to herbicides by regulating the expression of genes involved in herbicide metabolism. They interact with those biochemical processes or target proteins whose activity would normally be inhibited by the herbicide. BRs can be considered as safeners, which induce the activity of numerous plant P450s and enhance glutathione conjugation involved in the biodegradation of herbicides. Various safeners which can be used for wheat are Cloquintocet-mexyl, Fenchlorazole-ethyl, Mefenpyr- diethyl and Furilazole, but their continuous use can cause their persistence in ground water and indirectly in humans. The use of safeners also results in change of phenolic metabolism in wheat seedlings but its significance is yet to be determined.⁷⁸ BRs are natural plant steroids with well-known stimulation of cytosolic antioxidant pool along with quenching enzymes of ROS. However, their role in pesticide metabolism is yet to be established. Therefore, potential use of BRs as natural safeners could prove to be a better choice for protecting cereal crops from herbicide injury. Improved herbicide formulations with low or no toxicity except for the target weeds seems to be the demand of near future in agricultural sector so that it may not pose any further complications in yield improvement and the security of health productivity or food security threat.

Abbreviations:
BRs	=	brassinosteroids
DAS	=	days after sowing
GST	=	glutathione S-transferase
R	=	resistant
ROS	=	reactive oxygen species
S	=	sensitive
SOD	=	superoxide dismutase
WP	=	wettable powder

Acknowledgment

The authors are thankful to Chairman, Department of Botany, Aligarh Muslim University, India for providing the necessary facilities. The authors greatly thank the King Saud University, Deanship of Scientific Research, College of Science Research Centre for their financial support.