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Sodicity is a major soil constraint in many arid and semi-arid regions worldwide, including Australia, which adversely affects the ability of crops to take up water and nutrients from the soil, reducing yield. Reliable methods and tools are required for appropriate selection of traits, may provide a better understanding of crop responses to multiple stresses, especially in sodic soil. A novel strategy was developed using unmanned aerial vehicle (UAV)-thermal imaging and agglomerative hierarchical clustering-based techniques to evaluate and rank the physiological performance of 18 contrasting wheat genotypes grown on a moderately sodic and a highly sodic soil in north-eastern Australia. We obtained UAV-thermal imaging data at different times of the day (9:30, 12:00, and 15:00 hrs) close to flowering stage. Crop biophysical parameters (Leaf potassium concentration, normalized difference vegetation index, crop water uptake, stomatal conductance, plant moisture content, and aboveground biomass) were measured at close to flowering by destructive plant sampling and ground-based proximal sensing and yield was machine harvested at maturity. Canopy temperatures derived from thermal imagery between 28.9 and 35.4 °C were observed at the moderately sodic site, and between 36.2 and 41.0 °C at the highly sodic site from 9:30 to 15:00 hrs. Canopy temperature was consistently higher than corresponding ambient air temperatures indicating plant water stress at both sites. While the air temperature was not significantly different (p > 0.05) between the two sites, canopy temperature was significantly higher (p < 0.01) on highly sodic soil compared to moderately sodic soil, indicating greater water stress at the highly sodic site. This difference was most likely due to the adverse impacts of sodic soil constraints and not primarily due to environmental variations. Hence, our study revealed that sodic soil constraints can intensify plant water stress. Statistical analysis between canopy temperature (9:30, 12:00, and 15:00 hrs) and crop biophysical parameters showed close negative correlations at both moderately sodic (R2 = 0.54 to 0.83) and highly sodic (R2 = 0.30 to 0.89) sites. A closer correlation was observed at 15:00 hrs for both sites. Thus, high-resolution UAV-thermal imaging has potential to detect water-stressed plants on sodic soil. Agglomerative hierarchical clustering was used as an unsupervised machine learning tool for ranking of physiological performance of wheat genotypes. Results suggest that UAV-thermal imaging and AHC techniques can discriminate cultivars tolerant to sodicity. The study improves our understanding of crop physiological behaviour and can assist farmers in selection of water stress tolerant genotypes to sustain food security in sodic soil under water-limited environments. [ABSTRACT FROM AUTHOR]

Despite widespread application in studying climate change impacts, most crop models ignore complex interactions among air temperature, crop and soil water status, CO 2 concentration and atmospheric conditions that influence crop canopy temperature. The current study extended previous studies by evaluating T c simulations from nine crop models at six locations across environmental and production conditions. Each crop model implemented one of an empirical (EMP), an energy balance assuming neutral stability (EBN) or an energy balance correcting for atmospheric stability conditions (EBSC) approach to simulate T c . Model performance in predicting T c was evaluated for two experiments in continental North America with various water, nitrogen and CO2 treatments. An empirical model fit to one dataset had the best performance, followed by the EBSC models. Stability conditions explained much of the differences between modeling approaches. More accurate simulation of heat stress will likely require use of energy balance approaches that consider atmospheric stability conditions. [ABSTRACT FROM AUTHOR]

Even brief periods of high temperatures occurring around flowering and during grain filling can severely reduce grain yield in cereals. Recently, ecophysiological and crop models have begun to represent such phenomena. Most models use air temperature ( T air ) in their heat stress responses despite evidence that crop canopy temperature ( T c ) better explains grain yield losses. T c can deviate significantly from T air based on climatic factors and the crop water status. The broad objective of this study was to evaluate whether simulation of T c improves the ability of crop models to simulate heat stress impacts on wheat under irrigated conditions. Nine process-based models, each using one of three broad approaches (empirical, EMP; energy balance assuming neutral atmospheric stability, EBN; and energy balance correcting for the atmospheric stability conditions, EBSC) to simulate T c , simulated grain yield under a range of temperature conditions. The models varied widely in their ability to reproduce the measured T c with the commonly used EBN models performing much worse than either EMP or EBSC. Use of T c to account for heat stress effects did improve simulations compared to using only T air to a relatively minor extent, but the models that additionally use T c on various other processes as well did not have better yield simulations. Models that simulated yield well under heat stress had varying skill in simulating T c . For example, the EBN models had very poor simulations of T c but performed very well in simulating grain yield. These results highlight the need to more systematically understand and model heat stress events in wheat. [ABSTRACT FROM AUTHOR]

The changing climatic condition of the world significantly affecting the average grain yield of wheat crop. In the present study, fifty-six wheat genotypes including checks (Akbar-2019 and Barani-2017) were tested under drought stress. The crop was grown under rainfed conditions (only pre-sowing irrigation was applied at the time of land preparation) following alpha lattice design with two replications at Wheat Research Institute, Faisalabad, Punjab, Pakistan during 2020–21. Several yield and yield contributing traits were examined under drought stress. The analysis of variance (ANOVA) depicted significant differences among all traits which showed existence of great variation among them. Pearson's correlation studies showed significant positive correlation of yield (kg ha−1) with spike length (0.98**), flag leaf area (0.97**), normalized difference vegetation index (NDVI) at booting (0.87**) and grain filling stage (0.97**). Similarly, bi-plot analysis also revealed association of yield (kg ha−1) with spike length (cm), flag leaf area (cm2), NDVI at booting and NDVI at grain filling stage under drought stress condition. Estimation of canopy temperature at grain filling stage expressed negative relationship with yield (−0.11) under drought stress. Genotypes V-19274 (4536.2 kg ha−1) and V-19177 (4450.5 kg ha−1) were found best regarding yield and its contributing traits (spike length and flag leaf area) under drought stress. Cluster analysis separated the wheat genotypes in to four groups. Among all clusters the group four expressed high magnitude of yield (5163.4 kg ha−1). The genotypes in this cluster expressed high magnitudes of spike length (15.18 cm), flag leaf area (64.03 cm2), grain filling period (40.5 days), NDVI at booting (0.81) and NDVI at grain filling stage (0.72), while showed lower magnitude regarding plant height (97 cm), days to 50% maturity (141.5), days to 50% heading (101) and canopy temperature at grain filling stage (21.55) as compared to other three clusters. The identified wheat genotypes can be further used in different wheat breeding programs for water deficient conditions in changing climate scenario of the region to develop drought tolerant wheat cultivars. • Forty-six wheat genotypes were tested under drought stress for yield and its related traits. • The yield showed significant positive correlation with spike length, flag leaf area, and NDVI at booting and grain filling stage, and the bio-plot analysis also confirmed this association. • Canopy temperature at grain filling showed negative relationship with yield under drought stress. • Genotype V-19274 and V-19177 were found best regarding yield and its contributing traits under drought stress. [ABSTRACT FROM AUTHOR]

Vegetation indices and canopy temperature are the most usual remote-sensing approaches to assess cereal performance under Mediterranean conditions. However wide differences exist in the costs of the different equipment deployed to measure vegetation indices (e.g. spectroradiometers versus conventional red/green/blue cameras) or canopy temperature (infrared thermometers versus thermal cameras). In this study we compared different methodological approaches measuring vegetation indices and canopy temperature in durum wheat ( Triticum turgidum L. ssp . durum (Desf.) Husn.) and bread wheat ( Triticum aestivum L.) under different water conditions. These two categories of indices were correlated against grain yield during two consecutive years and with total biomass as well as water status (evaluated as carbon isotope composition, δ 13 C, and stomatal conductance) and nitrogen status (nitrogen and chlorophyll content) of the flag leaf during the first year. For the two crop species, and regardless of the device used, vegetation indices (the green area and the greener area) obtained with conventional cameras and the normalized difference vegetation index (NDVI) measured with a spectroradiometer were significantly correlated with grain yield, δ 13 C and stomatal conductance. Moreover, while canopy temperature measured with a thermal camera was better related to grain yield and δ 13 C than when measured with a low cost device such as an infrared thermometer, the latter approach still performs reasonably well when assessing yield and water status. This study highlights the usefulness of low-cost approaches to assess crop growth and water status in wheat under Mediterranean conditions. [ABSTRACT FROM AUTHOR]

In the extensive rice–wheat system of north-west (NW) India, harvesting is by large combines and the rice residues are normally burnt after harvest, followed by irrigation and intensive tillage prior to sowing wheat. While in-field retention of crop residues can play an important role in replenishing soil quality and reducing environmental pollution from stubble burning, until recently, there has been no suitable technology for seeding wheat in rice residues. To address this need, a series of machines (‘Happy Seeders’) was developed over the past 10 years, culminating in the development of version 2 of the 9-row Turbo Happy Seeder (v.2). The 9-row Turbo Happy Seeder(v.2) has a weight of 506 kg and can be operated by a 33.6 kW tractor at a work rate of 0.3 ha h −1 . Numerous on-farm trials show that yield of wheat sown into rice residues with the 9-row Turbo Happy Seeder is similar to or higher than yield with straw burning and conventional tillage prior to sowing, while providing many benefits to the farmer. These include greatly reduced fuel consumption and cost of crop establishment, and the ability to sow as soon as desired after harvest, ensuring the possibility of sowing at the optimum time and reducing the need for irrigation. However, adoption has been low to date, despite a 50% price subsidy. Constraints to adoption include the low window of operation of the machine (25 days per year), the low machine capacity compared with conventional seed drills, the inability to operate in wet straw, and the lack of straw spreaders on combine harvesters. Removal of subsidies for diesel and electricity (for pumping groundwater) and implementation of the policy banning in-field straw burning would help to accelerate adoption of technology for direct drilling wheat into rice residues. [ABSTRACT FROM AUTHOR]

Genotype-environment interaction (GxE) has a great impact on wheat physiology, morphology and grain yield (GY). We evaluated an association mapping panel of spring wheat advanced lines for chlorophyll content, canopy temperature (CT), and yield-related traits under three different watering regimes in two consecutive growing seasons. Genome-wide association mapping identified 457 SNPs, with significant effects that varied with the watering regimes and growing seasons, of which 199 and 69 SNPs showed pleiotropic and conditionally neutral effects, respectively, on the measured traits. We mapped 61 SNPs with effects higher than 10% on all traits, showing antagonistic pleiotropic effects on CT, corresponding to 46 genes; some of these genes represent good candidates to control wheat response to water availability. Surprisingly, no significant SNPs were mapped in the semi-dwarfing genes, Rht-B1b or Rht-D1b. However, haplotype analysis of the SNPs located at the positions of both genes revealed significant interactions of GY with the watering regimes for Rht-B1b and with the growing season for Rht-D1b. We selected genotypes that outperformed two local check cultivars; some of them overlapped across the three watering regimes and could be used to create a multi-parent population to further unravel the genetic factors underlying yield component traits across drought stress. Our results demonstrate the importance of incorporating GxE in mapping models to better understand wheat response to different watering regimes and to select stable markers for selection. • Three genotypes outperformed two local check cultivars under all treatments, whereas the remaining genotypes showed GxE. • Out of the 457 significant SNPs, 199 and 69 SNPs showed pleiotropic and conditionally neutral effects, respectively. • 61 main-effect SNPs can assist significantly in marker-assisted breeding programs. • Two favorable QTLs on 5D and 7D increased GY, and as an adaptive strategy to cope with drought, lowered CT. [ABSTRACT FROM AUTHOR]

Abstract: Plant water status is a key factor impacting crop growth and agricultural water management. Crop water stress may alter canopy temperature, the energy balance, transpiration, photosynthesis, canopy water use efficiency, and crop yield. The objective of this study was to calculate the Crop Water Stress Index (CWSI) from canopy temperature and energy balance measurements and evaluate the utility of CWSI to quantify water stress by comparing CWSI to latent heat and carbon dioxide (CO2) flux measurements over canopies of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). The experiment was conducted at the Yucheng Integrated Agricultural Experimental Station of the Chinese Academy of Sciences from 2003 to 2005. Latent heat and CO2 fluxes (by eddy covariance), canopy and air temperature, relative humidity, net radiation, wind speed, and soil heat flux were averaged at half-hour intervals. Leaf area index and crop height were measured every 7 days. CWSI was calculated from measured canopy-air temperature differences using the Jackson method. Under high net radiation conditions (greater than 500Wm−2), calculated values of minimum canopy-air temperature differences were similar to previously published empirically determined non-water-stressed baselines. Valid measures of CWSI were only obtained when canopy closure minimized the influence of viewed soil on infrared canopy temperature measurements (leaf area index was greater than 2.5m2 m−2). Wheat and maize latent heat flux and canopy CO2 flux generally decreased linearly with increases in CWSI when net radiation levels were greater than 300Wm−2. The responses of latent heat flux and CO2 flux to CWSI did not demonstrate a consistent relationship in wheat that would recommend it as a reliable water stress quantification tool. The responses of latent heat flux and CO2 flux to CWSI were more consistent in maize, suggesting that CWSI could be useful in identifying and quantifying water stress conditions when net radiation was greater than 300Wm−2. The results suggest that CWSI calculated by the Jackson method under varying solar radiation and wind speed conditions may be used for irrigation scheduling and agricultural water management of maize in irrigated agricultural regions, such as the North China Plain. [Copyright &y& Elsevier]

Abstract: Over the last century, most of the genetic gain in the grain yield of Italian durum has been achieved by an increase in the harvest index and promotion of flowering time. We report the indirect effects of this long-term selection on a number of leaf and canopy traits associated with photosynthesis. In a 2-year experiment, using different sowing dates and nitrogen regimes, these characters were assessed for a set of 20 cultivars released between 1900 and 2000. Leaf area, specific leaf area, specific leaf nitrogen content and stomatal resistance were all negatively correlated with year of release, whereas canopy temperature showed a positive correlation. The substantial reduction in plant height, by which the rise in harvest index was achieved, was partly responsible for the rise in canopy temperature, acting via its effect on aerodynamic resistance. Senescence, when expressed in thermal time after anthesis, was delayed for the modern cultivars, and this delay could be associated with a greater quantity of nitrogen allocated to the grain. The association of senescence onset with earliness suggests a likely role of increased earliness in delayed senescence. The yield increase over time was correlated with a decrease in stomatal resistance and in specific leaf nitrogen content. [Copyright &y& Elsevier]

Canopy temperature measured with infrared thermometers or other remote infrared sensors is an important tool for detecting crop water stress. The crop water stress index (CWSI) is the most often used index which is based on canopy temperature to detect crop water stress. This study evaluates the application of three different forms of CWSI for winter wheat water stress monitoring in the North China Plain (NCP): the Idso empirical model, the Jackson theoretical model, and the new Alves model, which replaces the radiometric surface temperature with a surface “wet bulb” temperature, thereby avoiding the evaluation of the surface resistance of the crop. The results show that the CWSI based on Jackson’s definition and Alves’ definition are better than the empirical CWSI for monitoring winter wheat water stress in NCP. Both definitions are useful tools to evaluate winter wheat water stress in NCP, but the CWSI based on Alves’ definition is more practical for monitoring winter wheat water stress in NCP, mainly due to not having to estimate the crop surface resistance, while the CWSI based on Jackson’s definition is more reasonable for quantifying winter wheat water stress in NCP. [Copyright &y& Elsevier]

Canopy water use efficiency (above-ground biomass over lifetime water loss, WUE canopy) can influence yield in wheat and other crops. Breeding for WUE canopy is difficult because it is influenced by many component traits. For example, intrinsic water use efficiency (WUE i), the ratio of net carbon assimilation (A net) over stomatal conductance, contributes to WUE canopy and can be estimated from carbon isotope discrimination (Δ). However, Δ is not sensitive to differences in the water vapor pressure deficit between the air and leaf (VPD leaf). Alternatively, measurements of instantaneous leaf water use efficiency (WUE leaf) are defined as A net over transpiration and can be determined with gas exchange, but the dynamic nature of field conditions are not represented. Specifically, fluctuations in canopy temperature lead to changes in VPD leaf that impact transpiration but not A net. This alters WUE leaf and in turn affects WUE canopy. To test this relationship, WUE canopy was measured in conjunction with WUE i , WUE canopy , and canopy temperature under well-watered and water-limited conditions in two drought-tolerant wheat cultivars that differ in canopy architecture. In this experiment, boundary layer conductance was low and significant changes in leaf temperature occurred between cultivars and treatments that correlated with WUE canopy likely because of the effect of canopy temperature on VPD leaf driving T. However, deviations between WUE i , WUE leaf , and WUE canopy were present because measurements made at the leaf level do not account for variations in leaf temperature. This uncoupled the relationship of measured WUE leaf and WUE i from WUE canopy and emphasizes the importance of canopy temperature on carbon uptake and transpired water loss. [ABSTRACT FROM AUTHOR]

• Mixtures of 4 winter wheat cultivars with different ripening patterns were evaluated. • Under water stress, heterogeneity in the ripening pattern and grain yield declined. • No mixture produced higher grain yield than the maximum monoculture. • Cultivar mixtures could not mitigate the post-anthesis water stress significantly. • Despite the general expectations, using cultivar mixtures may have adverse results. Wheat cultivar mixtures have a well-explored potential for improving crop diversity, yield stability, and agronomic features. However, it is unclear whether heterogeneity of phenological traits in such mixtures may help to cope with increasing environmental stresses and uncertainties. To investigate the option of using wheat cultivar mixtures with different ripening patterns for mitigating the adverse effects of post-anthesis water stress, a two-year field experiment was conducted during 2014−15 and 2015−16 growing seasons at the research field of School of Agriculture, Shiraz University, Iran. The factorial experiment was a Randomized Complete Block Design with 3 replicates, in which 15 mixture treatments including monocultures and every 11 possible mixtures of four early- to middle-ripening wheat cultivars were grown under two normal and post-anthesis deficit-irrigation conditions. The results indicated that under the stressful conditions, heterogeneity in the ripening pattern of mixtures was diminished. Consequently, dissimilarities in grain yields as well as various agronomic characteristics of mixture treatments were also lessened. Although cultivar mixtures showed some casual beneficial effects, such advantages were not consistent across all conditions. Besides, cultivar mixtures could not mitigate the post-anthesis water stress significantly. However, only when the total averaged values of Susceptibility Index across the two seasons were assessed, a tendency for decreasing SI (up to 5.9 %) was observed in response to the extended phenology, which may suggest further studies for investigating the cultivar mixtures in long-term cropping systems. Despite the general expectation for beneficial ecological services from cultivar mixtures, in many cases there were disadvantageous blends, which led to a considerable reduction in grain yield and water productivity in single growing seasons. Therefore, it is suggested that unless the performance, and preferably the involved mechanisms of cultivar mixtures are fully understood, use of blends as an alternative for conventional high-input wheat cropping systems may lead to adverse results. [ABSTRACT FROM AUTHOR]

• Thermal imaging and multivariate techniques were used to characterise water stress. • A Novel stress index - Normalized Water Stress Tolerance Index (NWSTI) was developed. • NWSTI was validated against CWSI, IG and CSI3 for water stress characterization. • NWSTI was used to screen wheat genotypes for water stress tolerance. The present study was undertaken to understand the combined impact of thermal imaging and multivariate techniques to characterize and screen different wheat genotypes under water stress condition. Ten wheat genotypes namely HD-2987, HDR-77, PBW-343, PBW-175, HD-2967, HD-2781, HD-2985, HD-3043, C-306 and PBW-502 were grown under two irrigation conditions (I and Io corresponding to IW/CPE ratio of 1.0 and 0.4, respectively). The water stress conditions in wheat genotypes were characterized thermo-graphically using canopy temperature-based stress indices, namely crop water stress index (CWSI), stomatal conductance index (IG) and stomatal resistance index (CSI3). Simultaneously, plant biophysical parameters like normalised difference vegetation index (NDVI), leaf area index (LAI), transpiration, stomatal conductance, photosynthesis, were measured at peak vegetative stage along with yield and its attributes (panicle number, tiller number, 1000 seed weight, crop yield, total dry matter, harvest index) were observed. These parameters were analyzed using multivariate techniques namely principal component analysis (PCA), hierarchical cluster analysis (HCA) and discriminant analysis (DA) to develop a new index called normalized water stress tolerance index (NWSTI), to group wheat genotypes depending on their ability to tolerate water stress. Based on these multivariate analysis, wheat genotypes were classified as tolerant (C-306, HD-3043, HD-3987 HD-3985 and HD-2781), moderately tolerant (HDR-77, PBW-175 and PBW-502) and sensitive (HD-2967, PBW-343) to water stress with NWSTI score of >0.66, 0.33–0.66 and <0.33, respectively. Structural matrix from DA analysis showed that biophysical parameters like stomatal conductance, harvest index, photosynthesis, transpiration, NDVI and panicle number have significant (p < 0.001) role in discriminating wheat genotypes into three different groups of water stress tolerance. Distinct location of group centroid in the canonical distribution function further signifies (p < 0.001) the truthfulness of the HCA based dendrogram prepared using the normalized values of crop biophysical parameters. A significantly higher correlation (R2 , p < 0.001) between NWSTI and the thermal image-based stress indices (that characterize the instantaneous crop condition for the ten wheat genotypes), prove the usefulness of NWSTI as a stress index. In future, the newly developed water stress index NWSTI, may play a potential role in the screening and selection of drought tolerant wheat genotypes. [ABSTRACT FROM AUTHOR]