Micrometeorological Comparison of Canopy Temperature between Two Wheat Cultivars Grown under Irrigation in a Hot Environment in Sudan

: A thorough exploration of the micrometeorological aspects inﬂuencing canopy temperature in contrasting wheat cultivars can unveil the speciﬁc mechanisms of adaptation to heat stress. However, information on wheat microclimates in hot environments for crop improvement is lacking. Here, we used a micrometeorological method to investigate wheat’s response to high temperatures. Field experiments were conducted in the Gezira Scheme, Sudan, to compare two high-yielding heat-tolerant cultivars, Imam and Bohaine, in terms of canopy temperature depression (CTD), air temperature gradient (ATG), and vapor pressure gradient (VPG) from a 2 m height to canopy level. The maximum air temperature at 2 m during the main growing season was 37 ◦ C. Air temperature at canopy level was mostly lower in the Imam ﬁeld than in the Bohaine ﬁeld, and it was positively correlated with and higher than radiometric canopy surface temperature. The maximum CTD during the reproductive stage was 4.7–6.5 ◦ C in the Bohaine ﬁeld and 5.0–7.2 ◦ C in the Imam ﬁeld. ATG was also larger in the Imam ﬁeld, attributed to the greater leaf area of the Imam canopy, as presumed from the NDVI difference between ﬁelds. ATG was negatively correlated with VPG in both ﬁelds, and the relationship was stronger at lower nighttime wind speeds and weaker at higher daytime wind speeds. These results indicate that the micrometeorological approach can be used to compare cultivars in high-temperature environments.


Introduction
Wheat (Triticum aestivum L.) is strongly affected by climate change, with yields projected to decrease by 6% for every 1 • C increase in global temperature [1,2].The most obvious effect of high temperatures on wheat is accelerated phenological development, resulting in shorter growing seasons and reduced yields [3,4].Among the world's wheatproducing regions, the effects of climate change are expected to be particularly severe in irrigated areas in arid regions such as India and Sudan [5][6][7].In these regions, canopy temperature depression (CTD)-the difference between air temperature (Ta) and radiometric canopy surface temperature (Ts) at standard (2 m) height, viz., CTD = Ta − Ts-is used as an indicator to screen wheat genotypes for heat stress tolerance, as wheat cultivars with a cooler canopy grow and yield better in hot environments [8][9][10][11].However, many meteorological factors, such as humidity and wind and morphological characteristics, such as plant height and peduncle length, affect CTD [12,13].
Accurately measuring crop CTD is essential for identifying heat and drought-resistant wheat varieties.A range of techniques are employed for measuring CTD, including handheld infrared thermometers [8,11,[14][15][16], thermal cameras [17,18], and increasingly, thermal imaging [19].These methods are typically employed in clear weather conditions during the afternoon, and they directly assess the radiative heat emitted by the canopy [20], providing insights into drought stress [15,21,22].Conversely, air temperature at canopy level (Tc) is derived from standard sensors in the surrounding air, offering an overview of the air temperature around the canopy.This information aids researchers and agronomists in understanding microclimates within the canopy, which influence plant physiology, stress responses, and overall crop performance [23].Analyzing these temperatures facilitates the identification of heat stress patterns and guides the implementation of beneficial agronomic practices, such as optimizing irrigation [24] and heat stress mitigation techniques.This enhances crop management for improved yield in hot environments.
Since wheat is adapted to cool environments, high temperatures are detrimental.At the individual plant scale, they decrease photosynthesis, thus reducing transpiration, which is positively associated with stomatal conductance, which in turn is highly affected by the vapor pressure deficit (VPD) of the atmosphere [10,[25][26][27].At the field scale, the transpiration-VPD relationship is complicated by air movement.Unless water vapor moves from the evaporating canopy surface into the atmosphere, it accumulates near the canopy surface, and the low VPD suppresses transpiration [27].When the air is dry just above a well-watered canopy, the high VPD increases stomatal conductance, letting more water vapor be released from the canopy surface into the atmosphere.This behavior of water movement in the soil-plant-atmosphere continuum occurs along with the evaporative cooling of the canopy.In hot environments with high evaporative demand, the surface temperature of a well-developed canopy with adequate soil moisture is almost entirely dependent on transpiration cooling [28].Decreasing transpiration under elevated VPD has been proposed as a crucial trait for drought tolerance.This approach aims to mitigate excessive water loss, prevent hydraulic failure, and enhance overall water use efficiency [29,30].
At high temperatures, especially during grain filling, wheat yields are higher at higher VPD [31] as a result of the crop's response to atmospheric water vapor pressure (VP) throughout the growing season.The crop's sensitivity to VP can be characterized by the above-canopy vapor pressure gradient (VPG)-the difference between VP at standard height (VPa) and that at canopy level (VPc), viz., VPG = VPa − VPc.According to the Monin-Obukhov similarity theory of heat, water vapor, and mass fluxes [32], the relationship between air temperature and VP can be interpreted as the association of the above-canopy air temperature gradient (ATG)-the difference between Ta and Tc, viz., ATG = Ta − Tc-with VPG.This relationship with respect to the canopy surface has been studied in specific wheat cultivars [20,33] but not compared between cultivars.Moreover, these micrometeorological parameters have been reported mainly for daytime and specific growth stages.To fully understand the ATG-VPG relationship for cultivar comparison, diurnal changes in Tc and VPc need to be measured throughout the growing season.In addition, as Tc and Ts differ in theory, their relationship needs to be investigated.
The Gezira Scheme in Sudan is one of the hottest wheat-growing areas in the world, and its irrigated wheat yields are projected to decrease under climate warming [7,34].Adaptation to climate change requires accelerated crop improvement for heat tolerance in controlled environments [35].However, there is a lack of information on microclimateirrigated wheat in hot environments to explain crop responses to high temperatures.Therefore, the main objective of this study was to investigate cultivar differences in the canopy temperature of irrigated wheat using a micrometeorological method.The specific objectives were (i) to compare the CTD and ATG of two cultivars contrasting in growth habit and (ii) to determine the relationship between ATG and VPG.The effect of wind on this relationship is discussed in relation to the temperature-humidity-wind relationship above the canopy.The micrometeorological approach has seldom been used to identify cultivar differences, and this study is the first to use it to compare canopy temperature between cultivars in a very hot wheat-growing area.

Field Experiments
Field experiments were conducted during the dry season of 2020-2021 at the Gezira Research Station of the Agricultural Research Corporation, Sudan (14.38 • N, 33.50 • E), within the Gezira Scheme in the arid climate zone (Figure 1).The climate is characterized by low annual precipitation (about 300 mm) from July to September and almost nil during the wheat-growing season (November-March) [36].The temperatures are lowest from November to February, and the monthly average ranges between 23.6 • C in the coldest months and 33.1 • C in the warmest months [36].The soil is classified as Vertisol, with a clay content of 50-60% [37].

Field Experiments
Field experiments were conducted during the dry season of 2020-2021 at the Gezira Research Station of the Agricultural Research Corporation, Sudan (14.38°N, 33.50° E), within the Gezira Scheme in the arid climate zone (Figure 1).The climate is characterized by low annual precipitation (about 300 mm) from July to September and almost nil during the wheat-growing season (November-March) [36].The temperatures are lowest from November to February, and the monthly average ranges between 23.6 °C in the coldest months and 33.1 °C in the warmest months [36].The soil is classified as Vertisol, with a clay content of 50-60% [37].
The experiments in two adjacent fields were designed to compare two high-yielding heat-tolerant wheat cultivars, i.e., Bohaine and Imam, which are the most common commercial cultivars grown in dry environments in Sudan.Bohaine is a fast-maturing cultivar with a fully erect growth habit, whereas Imam is a later-maturing cultivar with a semiprostrate growth habit [38].Each field measured 180 m long by 70 m wide (1.26 ha), aligned north-south; Bohaine was sown on 26 November and Imam on 6 December 2020 at 120 kg ha −1 in north-south rows 0.2 m apart.The fields were flood-irrigated immediately after sowing and every 8 to 13 days until 1 March 2021-Imam 10 times and Bohaine 9 times.The fields received the same type and amount of fertilizers: 25 kg N ha −1 and 28 kg P ha −1 in the form of diammonium phosphate before sowing and 56 kg N ha −1 of urea split-applied at the second and fourth irrigations.Each field had an automatic micrometeorological station installed in the southern part on 13 January 2021.The experiments in two adjacent fields were designed to compare two high-yielding heat-tolerant wheat cultivars, i.e., Bohaine and Imam, which are the most common commercial cultivars grown in dry environments in Sudan.Bohaine is a fast-maturing cultivar with a fully erect growth habit, whereas Imam is a later-maturing cultivar with a semi-prostrate growth habit [38].Each field measured 180 m long by 70 m wide (1.26 ha), aligned northsouth; Bohaine was sown on 26 November and Imam on 6 December 2020 at 120 kg ha −1 in north-south rows 0.2 m apart.The fields were flood-irrigated immediately after sowing and every 8 to 13 days until 1 March 2021-Imam 10 times and Bohaine 9 times.The fields received the same type and amount of fertilizers: 25 kg N ha −1 and 28 kg P ha −1 in the form of diammonium phosphate before sowing and 56 kg N ha −1 of urea split-applied at the second and fourth irrigations.Each field had an automatic micrometeorological station installed in the southern part on 13 January 2021.

Data Collection and Calculation
Plant height was measured four times (vegetative, heading, post-anthesis, and grainfilling stages) on 21 January, 31 January, 14 February, and 28 February 2021 for Imam and 21 January, 1 February, 15 February, and 28 February 2021 for Bohaine.At each measurement, three quadrats consisting of three 1.11-meter-long rows (ca. 1 m 2 ) were selected on the south side of the micrometeorological station in each field.Ten plants were randomly sampled in each quadrat, and values were averaged for statistical analysis.The Normalized Difference Vegetation Index (NDVI) of each field was calculated as a proxy for the leaf area index (LAI) and aboveground biomass [39,40] from near-infrared and red radiance reflectance data from the European Space Agency Sentinel-2 satellite at a spatial resolution of 10 m × 10 m (https://sentinels.copernicus.eu/web/sentinel/home,accessed on 10 July 2023).Gridded NDVI data within each field were averaged over the area of fields for each satellite image for comparison.The heading date was recorded as when 50% of the plants had the base of the spikes emerging from the flag leaf.
Temperature, humidity, and wind data were collected in the center of the field with more than a 130 m green fetch towards the north direction, at two heights using HMP155 probes (Vaisala Oyj, Vantaa, Finland) protected by a forced-ventilation shelter and 03002-47A anemometers (R. M. Young Co., Traverse, MI, USA).The upper sensors were fixed at 2 m above the ground, and the lower sensors were periodically adjusted to 0.05-0.1 m above the canopy surface.To measure canopy surface temperatures (Ts), an SI-411 infrared radiometer (Apogee Instruments, Inc., Logan, UT, USA) capable of monitoring temperatures over a canopy surface area of 8.4 m 2 was fixed 2 m above the ground, tilted 45 • down to the vertical mounting pole of the station and perpendicular to the western crop row.Measurement intervals were 1 min for Ta, Tc, Ts, and relative humidity (RH) and 1 s for wind speed and direction (WS and WD).Data averaged every 10 min were recorded on a CR1000X datalogger (Campbell Scientific, Inc., Logan, UT, USA).
Hourly Ta, Tc, Ts, and WS at a 2 m height (WSa) and canopy level (WSc) were calculated by averaging 10 min data for each hour of the 24-h day.We calculated 10 min averaged VPa and VPc from RH and air temperature according to [41] and averaged them for each hour.WD at a 2 m height (WDa) and canopy level (WDc) were used to determine the fetch requirement for the micrometeorological measurements.We calculated the percentages of the WD data with WS > 0.5 m s −1 (the threshold speed at which the vane began to move) between north-northwest and north-northeast to ensure a sufficient distance from the northern end of the field to the station with respect to the wind direction.

Statistical Analysis
Plant height was compared between cultivars using an unpaired t-test to ensure the same conditions necessary for micrometeorological comparisons, i.e., the same height of canopy surface between fields.CTD and ATG were analyzed for each of three growth periods, namely 21 to 30 January (I), 31 January to 14 February (II), and 15 to 28 February (III), which were determined from the dates of plant height measurements.In each hour of the 24-h day in each growth period, CTD and ATG were compared between fields by using a paired t-test.To clarify the relationship between ATG and VPG in each growth period, we used linear regression analysis, dividing the data into daytime (from hours 7 to 18) and nighttime (from hours 19 to 6) on the basis of latitude.Pearson's correlation analysis was performed to determine the relationship between Ts and Tc during all periods.All statistical analyses were performed using R language version 4.3.1,Vienna, Austria [42].

Crop Data
There were no significant differences in plant height between the Bohaine and Imam fields on any measurement date (p > 0.05) (Figure 2).However, NDVI was significantly higher in Imam on all dates.Periods I, II, and III corresponded, respectively, to the late vegetative stage, heading to early grain filling, and mid-to late grain filling in Imam, and to the late vegetative stage to heading, early to mid-grain filling, and late grain filling in Bohaine.
Agronomy 2023, 13, x FOR PEER REVIEW 5 higher in Imam on all dates.Periods I, II, and III corresponded, respectively, to the vegetative stage, heading to early grain filling, and mid-to late grain filling in Imam, to the late vegetative stage to heading, early to mid-grain filling, and late grain fillin Bohaine.

Micrometeorological Data
During period I, Ta was lowest around sunrise and highest around early aftern (Figure 3).During periods I-III, the maximum Ta was 37.2 °C in the Bohaine field and °C in the Imam field, and the maximum Tc was 36.1 °C and 34.5 °C, respectively.Tc positively correlated with Ts and was higher than Ts, and the relationship did not d between daytime and nighttime (Figure 4).Both VPa and VPc increased from early m ing to around noon, and then decreased until late afternoon and remained low until e the next morning (Figure 3).The maximum VPa during periods I-III was 1.9 kPa in fields, which was lower than the maximum VPc (2.3 kPa in the Bohaine field and 2.6 in the Imam field).
Wind speeds were higher during the daytime than during nighttime (Figure 3), a period I-III average WSa of 3.2 m s −1 in the daytime and 1.7 m s −1 at nighttime in fields.The average WSc was 1.3 m s −1 in the daytime and 0.4 m s −1 at nighttime in the Im field and 1.6 and 0.5 m s −1 , respectively, in the Bohaine field.The period I-III night wind direction at 2.0 m (WDa) was northerly 70.8% of the time in the Bohaine field 65.7% of the time in the Imam field, and that at canopy height (WDc) 71.0% of the tim the Bohaine field and 73.7% in the Imam field.In contrast, the daytime WDa was north only 31.7% and 43.6% of the time, and the daytime WDc only 44.3% and 59.4% of the t respectively.

Micrometeorological Data
During period I, Ta was lowest around sunrise and highest around early afternoon (Figure 3).During periods I-III, the maximum Ta was 37.2 • C in the Bohaine field and 37.3 • C in the Imam field, and the maximum Tc was 36.1 • C and 34.5 • C, respectively.Tc was positively correlated with Ts and was higher than Ts, and the relationship did not differ between daytime and nighttime (Figure 4).Both VPa and VPc increased from early morning to around noon, and then decreased until late afternoon and remained low until early the next morning (Figure 3).The maximum VPa during periods I-III was 1.9 kPa in both fields, which was lower than the maximum VPc (2.3 kPa in the Bohaine field and 2.6 kPa in the Imam field).
Wind speeds were higher during the daytime than during nighttime (Figure 3), with a period I-III average WSa of 3.2 m s −1 in the daytime and 1.7 m s −1 at nighttime in both fields.The average WSc was 1.3 m s −1 in the daytime and 0.4 m s −1 at nighttime in the Imam field and 1.6 and 0.5 m s −1 , respectively, in the Bohaine field.The period I-III nighttime wind direction at 2.0 m (WDa) was northerly 70.8% of the time in the Bohaine field and 65.7% of the time in the Imam field, and that at canopy height (WDc) 71.0% of the time in the Bohaine field and 73.7% in the Imam field.In contrast, the daytime WDa was northerly only 31.7% and 43.6% of the time, and the daytime WDc only 44.3% and 59.4% of the time, respectively.

Comparison of CTD and ATG between Cultivars
The hourly average CTD in period I was significantly larger in the Imam field than in the Bohaine field during nighttime (from hours 19 to 3) but not during most hours of the daytime (Figure 5).That in period II differed between the fields from hours 14 to 8.That in period III differed during most hours of the day.In the Bohaine field, the maximum CTD was 6.

Comparison of CTD and ATG between Cultivars
The hourly average CTD in period I was significantly larger in the Imam field than in the Bohaine field during nighttime (from hours 19 to 3) but not during most hours of the daytime (Figure 5).That in period II differed between the fields from hours 14 to 8.That in period III differed during most hours of the day.In the Bohaine field, the maximum CTD was 6.5 °C in hour 17 in period I, 6.5 °C in hour 19 in period II, and 4.7 °C in hour 19 in period III.In the Imam field, it was 6.4 °C, 7.2 °C, and 5.0 °C, respectively, during the same hours.
The hourly average ATG was larger in the Imam field than in the Bohaine field during all hours in period I and during most hours in periods II and III (Figure 5).The maximum ATG was 2.

Relationship between ATG and VPG
Daytime ATG was correlated negatively with VPG in both fields during all periods (R 2 = 0.02 to 0.30), except in the Imam field in period III (Figure 6).The relationship was moderate in period I and weak in period II.The nighttime relationship between ATG and VPG was stronger in both fields during all periods (R 2 = 0.73 to 0.90; Figure 7).The hourly average ATG was larger in the Imam field than in the Bohaine field during all hours in period I and during most hours in periods II and III (Figure 5).The maximum ATG was 2.

Relationship between ATG and VPG
Daytime ATG was correlated negatively with VPG in both fields during all periods (R 2 = 0.02 to 0.30), except in the Imam field in period III (Figure 6).The relationship was moderate in period I and weak in period II.The nighttime relationship between ATG and VPG was stronger in both fields during all periods (R 2 = 0.73 to 0.90; Figure 7).

Discussion
Canopy temperature is an important trait for breeding wheat with heat stress tolerance.Genotypes have been evaluated mainly in small experimental plots (a few meters); since Ts can be measured over a small area of the canopy surface, a radiometric approach, i.e., CTD, has been used for cultivar comparison [8,10,11,43].Our study, which was conducted under very high temperatures of up to 37 • C (Figure 2), showed that the maximum CTD was 6.  [8].Thus, CTD under irrigated conditions in arid environments is large because dry air promotes transpiration, resulting in more evaporative cooling of the canopy.This indicates that the ambient temperature of the canopy in such microclimates can be modified to be considerably lower than the critical high temperature for growth and development, e.g., 31.0 • C around anthesis and 35.4 • C during grain-filling [44].On the other hand, CTD is negative in some cases; that is, Ts > Ta, owing to water stress in the field [10].
Although canopy temperature is rarely measured at night, it can be used to compare cultivars in high-temperature environments.For example, Balota et al. [44] showed differences in CTD among wheat cultivars during predawn hours.Similarly, our study showed that Ts at night was lower than Ta, and thus revealed cultivar differences in CTD (Figure 5).This lowered canopy surface at night could be due to radiative cooling [45] or to cooling by nocturnal transpiration [46] in calm air.This implies that nighttime cooling is favorable for crop growth, since leaf respiration is reduced when temperatures drop [47,48].
There are only a few previous comparisons of Ts and Tc in wheat.Our study showed that Ts < Tc (Figure 4).Similarly, previous studies in various climatic zones showed that Ts < aerodynamic surface temperature at high sensible heat flux in temperate central England [20] and arid Phoenix, Arizona, USA [49].The difference between the two temperatures is due to the fact that Ts, which reflects leaf moisture status, is based on radiative emission from the canopy [22,[50][51][52][53], whereas Tc is due to the energy balance of the canopy, especially sensible heat loss from the canopy to the boundary layer above the canopy surface [20,49].In addition, the relationship between aerodynamic surface temperature and Ts can be discriminated by wind speed [20], but the effect of WS on the ATG-CTD relationship was not clear in our results.
Clarifying the micrometeorological relationship between canopy temperature and atmospheric water vapor may provide a better understanding of the mechanisms that lead to cultivar differences in crop response to high-temperature environments.Wind speed has a significant effect on the loss of sensible heat from the canopy surface and thus on the aerodynamic estimation of canopy surface temperature from the vertical profiles (gradients) of temperature and humidity [20,54].ATG was negatively correlated with VPG, but the weaker daytime ATG-VPG relationship, at higher WS (Figure 6), could be due to the turbulent flux of heat and water vapor.The stronger relationship at nighttime (Figure 7), at lower WS, can be interpreted as the retention of cold, moist air below the boundary where warmer, dry air has little flow; that is, the horizontal flux of heat and water vapor over the canopy could be slowly stable.
Our study further showed a cultivar difference in ATG (Figure 5), as the canopy of Imam was cooler than that of Bohaine.This difference could be attributed to the large difference in NDVI (as a proxy for LAI and aboveground biomass) between cultivars (Figure 2), as in the positive relationship of CTD with LAI and aboveground biomass in a similar environment in northwest Mexico reported by Ayeneh et al. [55].As the two fields received full irrigation, Imam, which has a higher LAI and a denser canopy and is recommended for the dry irrigated environments of Sudan [38], could perform better than Bohaine under non-limiting water conditions in terms of the ability to meet atmospheric evaporative demand during the daytime and rehydrate overnight to recover from diurnal wilt, implying that cultivars with a prostrate dense canopy are better adapted to hot and dry environments than those with an erect canopy.This is supported by the recently reported finding [56] that the cultivar difference in CTD can result from their morphological difference.But when we consider the phenological characteristics of these two cultivars, fast-maturing Bohaine with a lower yield potential than long-maturing Imam is still needed when the optimum sowing period for Imam cannot be met.
Based on this study, it has been discovered through micrometeorological observations that wheat displays distinct reactions to high-temperature stress.This indicates that different cultivars possess varying mechanisms of adaptation to heat stress, with those that are more accustomed to high temperatures, demonstrating a greater ability to modify the microclimate.In addition, significant information provided in this study can fill the gap in understanding microclimate modification, which can be used in enhancing crop yield through various techniques available, such as shading, windbreak, antitranspirant application, optimized sowing parameters, intercropping, irrigation, and nutrient management [57].

Conclusions
This study demonstrates the potential of the micrometeorological approach to explain cultivar differences in the response of irrigated wheat to high temperatures.There were large differences in ATG and VPG between cultivars.ATG was negatively correlated with VPG, and the relationship was stronger at nighttime, when wind speeds are lower.The cultivar difference was clear during both daytime and nighttime in all three critical

Figure 1 .
Figure 1.Location of experimental fields in the study area, the Gezira Scheme (Sudan).The fields are indicated on a map of Sentinel-2 satellite-based NDVI on 1 February 2021.

Figure 1 .
Figure 1.Location of experimental fields in the study area, the Gezira Scheme (Sudan).The fields are indicated on a map of Sentinel-2 satellite-based NDVI on 1 February 2021.

Figure 2 .
Figure 2. Seasonal changes in plant height (bars, mean ± SD) and NDVI (points) of wheat culti Imam and Bohaine.

Figure 3 .
Figure 3. Seasonal changes in wind speed (WS), air temperature (T), and atmospheric water vapor pressure (VP) at 2 m height ("a") and canopy level ("c") in irrigated fields of wheat cultivars, Imam and Bohaine.

Figure 3 .
Figure 3. Seasonal changes in wind speed (WS), air temperature (T), and atmospheric water vapor pressure (VP) at 2 m height ("a") and canopy level ("c") in irrigated fields of wheat cultivars, Imam and Bohaine.
5 • C in hour 17 in period I, 6.5 • C in hour 19 in period II, and 4.7 • C in hour 19 in period III.In the Imam field, it was 6.4 • C, 7.2 • C, and 5.0 • C, respectively, during the same hours.

Figure 4 .
Figure 4. Relationships between canopy surface temperature (Ts) and air temperature at canopy level (Tc) during daytime (hours 7 to 18 of the 24-h day) and nighttime (hours 19 to 6) in irrigated fields of wheat cultivars, Imam and Bohaine, from the late vegetative to late reproductive stages (21 January to 28 February 2021).
4 °C in hour 21 in period I, 2.6 °C in hour 19 in period II, and 1.9 °C in hour 20 in period III in the Bohaine field, and 3.4 °C in hour 16 in period I, 3.4 °C in hour 20 in period II, and 1.9 °C in hour 20 in period III in the Imam field.

Figure 4 . 14 Figure 5 .
Figure 4. Relationships between canopy surface temperature (Ts) and air temperature at canopy level (Tc) during daytime (hours 7 to 18 of the 24-h day) and nighttime (hours 19 to 6) in irrigated fields of wheat cultivars, Imam and Bohaine, from the late vegetative to late reproductive stages (21 January to 28 February 2021).Agronomy 2023, 13, x FOR PEER REVIEW 8 of 14
4 • C in hour 21 in period I, 2.6 • C in hour 19 in period II, and 1.9 • C in hour 20 in period III in the Bohaine field, and 3.4 • C in hour 16 in period I, 3.4 • C in hour 20 in period II, and 1.9 • C in hour 20 in period III in the Imam field.

5 •
C in the Bohaine field and 6.4 • C in the Imam field during the late vegetative stage, 6.5 • C and 7.2 • C during the early reproductive stage, and 4.7 • C and 5.0 • C during the late reproductive stage.These values are comparable to those of previous studies in irrigated and similar environments; for example, CTD values for wheat varietal differences reported from south-central Mexico ranged widely from 5.0 to 7.9 • C during pre-heading, from 4.8 to 9.1 • C during heading/anthesis, and from 4.1 to 7.3 • C during grain filling