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Wheat, a major food crop, faces significant yield constraints due to losses caused by various diseases, especially rusts and powdery mildew. Since the causal organisms are always evolving, there is a never-ending hunt for new genes/quantitative trait loci (QTLs) for resistance to control the damage. For this purpose, Triticum durum–Aegilops speltoides backcross introgression lines (DS-BILs) developed in our wide hybridization programme were screened against stripe rust and powdery mildew at both seedling and adult plant stages. DS-BILs showed complete to moderate resistance at the adult plant stage while varying resistance and susceptibility at the seedling stage. A total of 1095 single-nucleotide polymorphisms (SNPs) identified on 14 chromosomes of T. durum, using genotyping by sequencing, were used for QTL mapping. Eleven unique QTLs, across six chromosomes (chr1B, chr2A, chr2B, chr3B, chr6B and chr7B) were identified for resistance, four QTLs for field mixture of stripe rust pathotypes, two QTLs for stripe rust pathotype 78S84 and five QTLs for field mixture of powdery mildew pathotypes using stepwise regression-based likelihood ratio test for additive effect of markers and single-marker analysis. Eleven DS-BILs carrying multiple QTLs were identified which will serve as a useful resource to transfer the respective resistance to susceptible cultivars to develop all stage resistant elite cultivars where QTL for stripe rust resistance QYrAs.pau-2A.1 (LOD 3.8, PVE 24.51 linked to SNP S2A_16016633) and QTL for powdery mildew resistance QPmAs.pau-6B (logarithm of the odds (LOD) 3.2, phenotypic variation explained (PVE) 17.75 linked to SNP S6B_26793381) are major targets of the transfer.
The genetics of the root system is still not dissected for wheat and lack of knowledge prohibits the use of marker-assisted selection in breeding. To understand the genetic mechanism of root development, Synthetic W7984 × Opata M85 doubled-haploid (SynOpDH) mapping population was evaluated for root and shoot characteristics in PVC tubes until maturity. Two major quantitative trait loci (QTLs) for total root biomass were detected on homoeologous chromosomes 2A and 2D with logarithm of the odds scores between 6.25–10.9 and 11.8–20.86, and total phenotypic effects between 12.7–17.7 and 26.6–40.04% in 2013 and 2014, respectively. There was a strong correlation between days to anthesis and root and shoot biomass accumulation (0.50–0.81). The QTL for biomass traits on chromosome 2D co-locates with QTL for days to anthesis. The effect of extended vegetative growth, caused by photoperiod sensitivity (Ppd) genes, on biomass accumulation was always hypothesized, this is the first study to genetically support this theory.
The subject of this chapter is the introduction and success of varieties of wheat from the steppes in the Great Plains. After considering the types of crops grown in the two regions, it analyzes the various ways in which they made the journey: with immigrants, especially Mennonites; introduction by government “plant explorers”; exchanges between Russian/Soviet and American crop scientists; and the international grain trade. The persistence of stories about the role of Mennonite immigrants is subjected to particular scrutiny.
Improving fertilizer use efficiency has remained a challenge, particularly for small-scale farming in undulating ‘abnormal’ landscapes of East Africa. Milne's 1930s concept on ‘Catena’ was considered as a breakthrough in understanding soil variability and its implication on productivity in East African highlands. However, there is limited information on how the ‘Catena’ features could be used for fine tuning fertilizer recommendations. We initiated multiple on-farm replicated experiments in three wheat-growing districts (Endamohoni, Lemo and Worreilu) in the Ethiopian highlands in 2014, 2015 and 2016 to assess landscape positions affecting crop-nutrient responses, identify yield limiting nutrients across the ‘Catena’ (N, P, K, S and Zn) and quantify effects of landscape positions on resources use efficiency. We clustered farmlands across the ‘Catena’ (Hillslopes, Midslopes and Footslopes) based on land scape positions in the respective locations. Wheat yield was more strongly and significantly affected by landscape positions (P < 0.001) than by nutrient sources or rates. The crop response to fertilizers was 50 to 300% higher in foot slopes than in hillslopes, depending on locations and inputs levels. With increasing slope, there was a decrease in a crop fertilizer response due to a significant decrease in soil organic carbon, clay content and soil water content, with r2 of 0.95, 0.86 and 0.96, respectively. The difference in the crop response between landscape positions was significantly higher (P < 0.05) with higher rates of nutrient applications (>N92 P46) while differences between landscape positions diminish at lower rates. Yield benefits due to application of K was significant only in the dry years (P < 0.05), while there was hardly any yield benefit from the application of zinc and sulfur. The crop nitrogen recovery fraction and crop water productivity decreased with an increasing slope regardless of nutrient combinations. The results indicated that the landscape position could be considered as a proxy indicator for targeted fertilizer application, particularly in farms with undulating topographic features. Hillslopes are better served by the application of organic fertilizers along with conservation measures as applying higher rates of mineral fertilizer in hillslopes would rather increase the risk of downstream nutrient movement.
Soil salinity is one of the major abiotic stresses affecting seed germination, crop growth and productivity. In this study, seeds of three wheat (Triticum aestiveum L.) and three barley (Hordeum vulgare L.) cultivars were treated with different concentrations of NaCl to investigate the effect of salt on seed germination physiology and metabolism through the characterization of seed germination pattern, gamma-aminobutyric acid (GABA) shunt metabolite accumulation [GABA, glutamate (Glu) and alanine (Ala)] and glutamate decarboxylase (GAD) expression using RT-PCR. A trend of decreasing germination percentage with increasing NaCl concentrations was observed. Under all salt stress treatments, data showed significant increase with positive correlation (r = 0.50–0.99) between abundance of GABA shunt metabolites and salt concentration in all wheat and barley cultivars for 5 days. Increased GABA content was associated with a small but significant increase in Ala and Glu content in all cultivars. In all NaCl treatments, the transcription of GAD in terms of RNA abundance showed a significant increase in all cultivars with positive correlation (r = 0.50–0.98). Data showed significant association between GAD RNA transcription and the response of germinating seeds to salt stress in terms of GABA shunt metabolite accumulation. The elevated expression of GAD under salinity suggests the need for elevated activity of the GAD-mediated conversion of Glu to GABA during seed germination, which provides alternative metabolic routes to the respiratory machinery, balancing carbon and nitrogen metabolism and osmolyte synthesis in germinating seeds of wheat and barley under salt stress.
Two field experiments were conducted during 2018 at Paskeville and Arthurton, South Australia, to identify effective herbicide options for the control of thiocarbamate-resistant rigid ryegrass in wheat. Dose–response experiments confirmed resistance in both field populations (T1 and A18) of rigid ryegrass to triallate, prosulfocarb, trifluralin, and pyroxasulfone. T1 and A18 were 17.9- and 20-fold more resistant to triallate than susceptible SLR4. The level of resistance detected in T1 to prosulfocarb (5.9-fold) and pyroxasulfone (4-fold) was lower compared to A18, which displayed 12.1- and 7.8-fold resistance to both herbicides, respectively. Despite resistance, the mixture of two different preplant-incorporated (PPI) site-of-action herbicides improved rigid ryegrass control and wheat yield compared to a single PPI herbicide only. Prosulfocarb + triallate and prosulfocarb + S-metolachlor + triallate did not reduce rigid ryegrass seed set when compared to prosulfocarb applied alone at the higher rate (2,400 g ai ha–1). Pyroxasulfone + triallate PPI followed by glyphosate (1,880 g ai ha-1) as a weed seed set control treatment reduced rigid ryegrass seed production by 93% and 95% at both sites, respectively. These herbicides also significantly improved grain yield of wheat at Paskeville (22%) and Arthurton (38%) compared to the untreated.
Isolated gametes can be used to investigate fertilization mechanisms, and probe distant hybridization between different species. Pollen grains of wheat and Setaria viridis are tricellular, containing sperm cells at anthesis. Sperm from these plants were isolated by breaking open pollen grains in a osmotic solution. Wheat ovules were digested in an enzyme solution for 20 min, and then transferred to an isolation solution without enzymes to separate egg cells from ovules. The fusion of wheat egg cells with wheat and S. viridis sperm was conducted using an electro-fusion apparatus. Under suitable osmotic pressure (10% mannitol), calcium concentration of 0.001% (CaCl2·2H2O), and a 30–35 V alternating electric field for 15 s, egg cells and sperm adhered to each other and became arranged in a line. Electroporation of the plasma membrane of egg cells and sperm using a 300–500 V direct-current electric field (45 µs amplitude pulse) caused them to fuse.
New farming systems and management options are needed in South Asia as the intensive rice–wheat production system is set to become increasingly unsustainable under climate change. In the current study, six cropping systems options/treatments varying in tillage, crop establishment method, residue management, crop sequence and fertilizer and water management were evaluated using a cropping systems model under current (1980–2009) and future (2030 and 2050) climate scenarios in the state of Bihar, India. The treatments were current farmers' practice (CP), best fertilizer and water management practices, zero tillage (ZT) with no crop residue retention, ZT with partial crop residue retention (ZTPR), future conservation agriculture-based rice–wheat intensive cropping system (FCS-1) and future conservation agriculture-based maize–wheat intensive cropping system (FCS-2). The results indicate that climate change is likely to reduce rice–wheat system productivity under CP by 4% across Bihar. All the crop management options studied increased yield, water productivity and net returns over that of the CP under the current and future climate scenarios. However, the ZTPR treatment gave significantly higher relative yield, lower annual yield variability and a higher benefit-cost-ratio than the other treatments across cropping system components and climate periods. Although all the new cropping system treatments had a positive yield implication under the current climate (compared to CP), they did not contribute to adaptation under the future climate except FCS-2 in wheat. It is concluded that adaptation to future climate must integrate both cropping system innovations, and genetic improvements in stress tolerance.
We analyze the effects of Price Loss Coverage (PLC), Agriculture Risk Coverage (ARC), individual revenue protection insurance (RP), and Supplemental Coverage Option (SCO) on the RP coverage level, certainty equivalent, and program payments. The model is calibrated to a representative wheat farm in Mitchell County in Kansas to analyze the effects of various policies. The result highlights that when insurance is framed as an investment, cumulative prospect theory predicts farmers’ coverage decisions accurately at 70%. ARC or PLC program increases the RP coverage level to 75%, but PLC and SCO jointly decrease the RP coverage level to 70%.
The aim of this article is to analyse the dynamics of international trade in cereals, primarily wheat, in the first third of the twentieth century, with a special focus on the causes of the fall in exchanges and prices that took place in the 1930s. Developments over this period are compared with the general trade in food and agricultural products. An examination of the structure of the trade in wheat, maize, and rice shows the operation of their respective markets, giving special attention to the import and export flows between consumers and producers. To understand the functioning of the market for these products, the article examines the changes in supply, demand, and prices, and the emergence and development of intermediary companies in this business. The argument draws from a new database, based on the statistics published by the International Institute of Agriculture.
The wheat bZip transcription factor TaABF1 mediates both abscisic acid (ABA)-induced and ABA-suppressed gene expression. As levels of TaABF1 protein do not change in response to ABA, and TaABF1 is in a phosphorylated state in vivo, we investigated whether TaABF1 could be regulated at the post-translational level. In bombarded aleurone cells, a TaABF1 protein carrying phosphomimetic mutations (serine to aspartate) at four sites (S36D, S37D, S113D, S115D) was three to five times more potent than wild-type TaABF1 in activating HVA1, an ABA-responsive gene. The phosphomimetic mutations also increased the ability of TaABF1 to downregulate the ABA-suppressed gene Amy32b. These findings strongly suggest that phosphorylation at these sites increases the transcriptional regulatory activity of TaABF1. In contrast to the activation observed by the quadruple serine to aspartate mutation, a single S113D mutation completely eliminated the ability of TaABF1 to upregulate HVA1 or downregulate Amy32b. Thus phosphorylation of TaABF1 can either stimulate or inhibit the activity of TaABF1 in regulating downstream genes, depending on the site and pattern of phosphorylation. Mutation of S318 and S322 (in the bZIP domain) eliminated the ability of TaABF1 to activate HVA1, but had no effect on the ability of TaABF1 to downregulate Amy32b, suggesting that TaABF1 represses Amy32b expression through a mechanism other than direct DNA binding. An important step towards understanding how ABA and gibberellin (GA) signals are integrated through TaABF1 phosphorylation to regulate downstream gene expression is to clarify the effects of those hormones on the expression of specific genes. In contrast to some other ABA-induced genes, we found that HVA1 induction by ABA or TaABF1 is not inhibited by GA.
In 2015, winter wheat growers in Virginia reported commercial failures of thifensulfuron to control mouse-ear cress. This was the first reported case of field-evolved acetolactate synthase (ALS) resistance in mouse-ear cress, so research was conducted to evaluate alternative herbicide options as well as to document potential yield loss in winter wheat from mouse-ear cress. Efficacy studies were conducted at three site-years in 2015 to 2016 and 2016 to 2017 as well as a POST greenhouse trial. In the PRE study, flumioxazin, pyroxasulfone, saflufenacil, and metribuzin resulted in more than 80% mouse-ear cress control 15 wk after planting across all sites with no observable wheat injury. No differences were observed in wheat yield in two of three sites in the PRE herbicide study; yield differences were attributed to common chickweed and not to mouse-ear cress. In the POST herbicide study, 2,4-D, dicamba, and metribuzin resulted in greater than 75% control in the field and greenhouse. Metribuzin, dicamba, and pyroxsulam resulted in crop injury 3 wk after treatment at some sites, but injury was transient. Yield from all POST treatments was similar to the nontreated plots. No yield loss was observed by mouse-ear cress densities greater than 300 plants m–2, indicating that mouse-ear cress is not very competitive with winter wheat. Growers should make herbicide decisions based on other weeds in the field and can incorporate the aforementioned herbicides for mouse-ear cress control.
Humankind has existed for 2·5 million years but only in the past 10 000 years have we been exposed to wheat. Therefore, it could be considered that wheat (gluten) is a novel introduction to humankind's diet! Prior to 1939, the rationing system had already been devised. This led to an imperative to try to increase agricultural production. Thus, it was agreed in 1941 that there was a need to establish a Nutrition Society. The very roots of the Society were geared towards necessarily increasing the production of wheat. This goal was achieved and by the end of the 20th century, global wheat output had expanded by 5-fold. Perhaps, as a result, the epidemiology of coeliac disease (CD) or gluten sensitive enteropathy has changed. CD now affects 1 % or more of all adults. Despite this, delays in diagnosis are common, for every adult patient diagnosed approximately three–four cases are undetected. This review explores humankind's relationship with gluten, wheat chemistry, the rising prevalence of modern CD and the new entity of non-coeliac gluten or wheat sensitivity. The nutritional interventions of a low fermentable oligo-, di- and mono-saccharides and polyols diet and gluten-free diet (GFD) for irritable bowel syndrome and the evidence to support this approach (including our own published work) are also reviewed. There appears to be a rising interest in the GFD as a ‘lifestyler’, ‘free from’ or ‘clean eater’ choice, causing concern. Restrictive diets may lead to potential nutritional implications, with long-term effects requiring further exploration.
The effectiveness of adaptation strategies is crucial for reducing the costs of climate change. Using plot-level data from a specifically designed survey conducted in Pakistan, we investigate the productive benefits for farmers who adapt to climate change. The impact of implementing on-farm adaptation strategies is estimated separately for two staple crops: wheat and rice. We employ propensity score matching and endogenous switching regressions to account for the possibility that farmers self-select into adaptation. Estimated productivity gains are positive and significant for rice farmers who adapted, but negligible for wheat. Counterfactual gains for non-adapters were significantly positive, which is potentially a sign of transactions costs to adaptation. Other factors associated with adaptation were formal credit and extension, underscoring the importance of addressing institutional and informational constraints that inhibit farmers from improving their farming practices. The findings provide evidence for the Pakistani Planning and Development Department's ongoing assessment of climate-related agricultural losses.
Drought stress ‘particularly at seedling stage’ causes morpho-physiological differences in wheat which are crucial for its survival and adaptability. In the present study, 209 recombinant inbred lines (RILs) from synthetic wheat (W7984)× ‘Opata’ (also known as SynOpRIL) population were investigated under well-watered and water-limited conditions to identify quantitative trait loci (QTL) for morphological traits at seedling stage. Analysis of variance revealed significant differences (P < 0.01) among RILs, and water treatments for all traits with moderate to high broad sense heritability. Pearson's coefficient of correlation revealed positive correlation among all traits except dry root weight that showed poor correlation with fresh shoot weight (FSW) under water-limited conditions. A high-density linkage map was constructed with 2639 genotyping-by-sequencing markers and covering 5047 cM with an average marker density of 2 markers/cM. Composite interval mapping identified 16 QTL distributed over nine chromosomes, of which six were identified under well-watered and 10 in water-limited conditions. These QTL explained from 4 to 59% of the phenotypic variance. Six QTL were identified on chromosome 7B; three for shoot length under water-limited conditions (QSL.nust-7B) at 64, 104 and 221 cM, two for fresh root weight (QFRW.nust-7B) at 124 and 128 cM, and one for root length (QRL.nust-7B) at 122 cM positions. QFSW.nust-7B appeared to be the most significant QTL explaining 59% of the phenotypic variance and also associated with FSW at well-watered conditions. These QTL could serve as target regions for candidate gene discovery and marker-assisted selection in wheat breeding.
Unavailability of irrigation water for early sowing has remained a constant problem in cold arid deserts of Ladakh. In order to get a solution to this problem, a 2-yr farmers’ participatory research trial with best bet agronomic management on artificial glacier water harvesting technology was conducted. The technology involves collecting water from natural glaciers that melt during late December. The water is diverted toward a shed constructed with stone embankments set up at regular intervals. The area is chosen where there is minimum interference of solar radiation, generally between two mountain slopes or ridge that is on the leeward side. The melted water is that melts from the natural glacier impeded by the embankments and get frozen here. This frozen water starts melting in late March and is used for both pre sowing and initial crop water requirement. It also ensures early sowing of wheat by creating additional 45-day window which leads to introduction of long- and medium-duration wheat varieties to replace decades old locally grown short-duration varieties. The work was initiated with a benchmark survey of 100 farmers to get an understanding of present irrigation scenario, crop management practices and date of sowing. Data from 99 farmer participating trial of wheat conducted after or from bench mark survey clearly indicated that the effect of water shortage can be seen on yield and yield attributing characters due to unavailability of pre sowing irrigation and water requirement at imperative growth stages and may also lead to terminal heat stress in wheat crop. Out of total number of irrigations applied, initial two irrigations can be compensated by artificial glacier water harvesting technique, leading to a revolution in the agriculture scenario of the tribal population by introduction of long- and medium-duration wheat varieties in cold arid desert of Ladakh for the very first time. It was observed that wheat seeding done in first fortnight of April gave better yields in comparison to late seeded wheat. Moreover, the long-duration varieties (LDVs) or medium-duration varieties (MDVs) sown under late condition gave better yield in comparison to locally grown short-duration varieties sown at same time. Yield potential of LDVs and MDVs of wheat under late sowing was found quite low in comparison to early-sown wheat, still when compared with the performance of locally grown wheat the yields were more even if the local varieties were sown early. The outcome of this study will help the farmers of tribal, cold arid community in harvesting better wheat yields by timely sowing of the wheat crop accompanied with better bet agronomic management practices. Government initiative is further required to ensure better outreach of complete crop management strategies to the tribal farming community of the region in order to ensure food security and improve their socioeconomic status.
Intercropping of legumes in cereals and manuring are important measures to sustain soil fertility and enhance crop productivity in general and under organic and limited moisture conditions in particular. This study evaluated different wheat (Triticum aestivum L.) + chickpea (Cicer arietinum L.) row ratios as intercrops under organic and limited moisture conditions in Northwest India with an aim to find out the suitable row ratio to get higher system productivity under these conditions. Chickpea, being a legume, produces nitrogen compounds that help the plant to grow itself and also makes them available to the companion wheat plants and thus helps in wheat nutrition, which otherwise becomes limited due to less mineralization of nitrogen under organic and cold weather conditions. The primary aim of the study was to get better productivity of wheat crop as this is an assured crop of this region. The intercropping system was evaluated in a randomized complete block design with four replications at Ludhiana, Punjab in Northwest India during 2013–2014 and 2014–2015. Effective tillers m−1 row length, number of grains spike−1 and 1000-grain weight of wheat were higher in wheat + chickpea (2:1) intercropping system as compared with sole wheat. This intercropping system produced significantly higher wheat grain yield, wheat equivalent yield and land equivalent ratio than sole wheat. Wheat + chickpea (2:1) and wheat + chickpea (3:1) intercropping systems gave higher water-use efficiency than sole wheat. However, chickpea gave higher yield attributes and seed yield as sole crop than that in different intercropping systems. Wheat + chickpea (2:1) and wheat + chickpea (3:1) intercropping systems produced mean wheat grain yields of 5.11 and 4.79 Mg ha−1, respectively, along with additional mean chickpea seed yields of 0.28 and 0.24 Mg ha−1, respectively.
Obesity is strongly associated with insulin resistance (IR), along with mitochondrial dysfunction to metabolically active tissues and increased production of reactive O2 species (ROS). Foods rich in antioxidants such as wheat germ (WG), protect tissues from damage due to ROS and modulate some negative effects of obesity. This study examined the effects of WG supplementation on markers of IR, mitochondrial substrate metabolism and innate antioxidant markers in two metabolically active tissues (i.e. liver and heart) of C57BL/6 mice fed a high-fat–high-sucrose (HFS) diet. Male C57BL/6 mice, 6-week-old, were randomised into four dietary treatment groups (n 12 mice/group): control (C, 10 % fat kcal), C+10 % WG, HFS (60 % fat kcal) or HFS+10 % WG (HFS+WG). After 12 weeks of treatment, HFS+WG mice had significantly less visceral fat (−16 %, P=0·006) compared with the HFS group. WG significantly reduced serum insulin (P=0·009), the insulinotropic hormone, gastric inhibitory peptide (P=0·0003), and the surrogate measure of IR, homoeostatic model assessment of IR (P=0·006). HFS diet significantly elevated (45 %, P=0·02) cardiac complex 2 mitochondrial VO2, suggesting increased metabolic stress, whereas WG stabilised this effect to the level of control. Consequently, genes which mediate antioxidant defense and mitochondrial biogenesis (superoxide dismutase 2 (Sod2) and PPARγ coactivator 1-α (Pgc1a), respectively) were significantly reduced (P<0·05) in the heart of the HFS group, whereas WG supplementation tended to up-regulate both genes. WG significantly increased hepatic gene expression of Sod2 (P=0·048) but not Pgc1a. Together, these results showed that WG supplementation in HFS diet, reduced IR and improved cardiac mitochondrial metabolic functions.
This research paper describes the effect of partially replacing wheat with maize grain and canola meal on milk production and body condition changes in early lactation Holstein-Friesian dairy cows consuming a grass silage-based diet over an 83-d period. Two groups of 39 cows were stratified for age, parity, historical milk yield and days in milk (DIM), and offered one of two treatment diets. The first treatment (CON) reflected a typical diet used by Western Australian dairy producers in summer and comprised (kg DM/cow per d); 8 kg of annual ryegrass silage, 6 kg of crushed wheat (provided once daily in a mixed ration), 3·6 kg of crushed lupins (provided in the milking parlour in two daily portions) and ad libitum lucerne haylage. The second treatment diet (COMP) was identical except the 6 kg of crushed wheat was replaced by 6 kg of a more complex concentrate mix (27% crushed wheat, 34% maize grain and 37% canola meal). Lucerne haylage was provided independently in the paddock to all cows, and no pasture was available throughout the experiment. The COMP group had a greater mean overall daily intake (22·5 vs 20·4 kg DM/cow) and a higher energy corrected milk (ECM) yield (29·2 vs 27·1 kg/cow; P = 0·047) than the CON cows. The difference in overall intake was caused by a higher daily intake of lucerne haylage in COMP cows (4·5 vs 2·3 kg DM/cow). The CON group had a higher concentration of milk fat (42·1 vs 39·3 g/kg; P = 0·029) than COMP cows. Milk protein yield was greater in COMP cows (P < 0·021); however, milk fat yield was unaffected by treatment. It is concluded that partially replacing wheat with canola meal and maize grain in a grass silage-based diet increases voluntary DMI of conserved forage and consequently yields of ECM and milk protein.
The research concerns the changes of spectral characteristics of reflected radiation (360 to 1 000 nm) of spring wheat leaves under nitrogen deficiency and moderate soil drought. The efficiency of factorial influence (η2) on chlorophyll index was equal to 20% and 4% under nitrogen and water deficiency, respectively. Most significantly soil drought influenced the water index WRI (η2=55%) and the light diffusion index R800 (η2=28%), which was caused by changes in leaf structure. At low levels of nitrogen supply, these parameters did not change or changed only slightly (η2=2%). It may be deduced that the data base for crop monitoring in precision farming systems must contain a series of optical criteria for assessing specific and non-specific changes in optical characteristics of a crop canopy under the impact of various stress factors.