Study on the Differences in the Responses of Different Winter Wheat Cultivars to Dry Hot Wind

Dry hot wind is one of the main prevailing agro-meteorological disasters during the grain lling stage of winter wheat in Northern China. In this study, three major winter wheat cultivars, including Henong 6119 (HN6119), Gaoyou 5218 (GY5218), and Jimai 325 (JM325) in Hebei Province were selected to analyze their responses to dry hot wind. Under the combined conditions of eld natural dry hot wind and articially simulated dry hot wind experiments, we characterized the three cultivars’ physiological parameters as affected on the day with dry hot wind, and on the day before and after hot wind conditions. Comparative analysis of different correlations among the three cultivars’ physiological parameters toward dry hot wind showed that, during eld nature dry hot wind conditions, HN6119 showed less water loss of leaves by reducing the stomatal conductance and transpiration rate, while GY5218 and JM325 showed more water loss of leaves by increasing the stomatal conductance and transpiration rate. The net photosynthetic rate, transpiration rate, and stomatal conductance of HN6119 were recovered during the recovery time after dry hot wind conditions, while these parameters of GY5218 and JM325 showed a continuous decreasing trend. During dry hot wind day, HN6119 showed signicant positive correlation between physiological parameters, while GY5218 and JM325 showed poor correlation. The stress of severe dry hot wind on thousand kernel weight (TKW) of HN6119, GY5218 and JM325 is 0.01%, 3.51%, 3.57%, respectively. The stress of mild dry hot wind on thousand kernel weight (TKW) of HN6119, GY5218 and JM325 is 0.36%, 8.12%, 8.84%, respectively. In summary, HN6119 showed strong resistance to dry hot wind, followed by GY5218, and JM325; JM325 had the weakest resistance to dry hot wind. hot wind measured on different dates. The results show that the net photosynthetic rate, transpiration rate and stomatal conductance of three cultivars all decreased on the rst day after the end of dry hot wind. Under the condition of articial simulated dry hot wind, the photosynthetic physiological parameters of different cultivars all decreased compared with the value without stress, it indicates that dry hot wind has retarding effect on photosynthetic physiological parameters. As shown in the previous literature (Zhao, et al. 2013), under the condition of articial simulated dry hot wind, the photosynthetic transpiration of wheat ag leaves at grain lling stage were signicantly retarded and caused partial closure of stomata, while a similar study (Zhang, et al. 2015) showed that the net photosynthetic rate, transpiration rate, and stomatal conductance of wheat ag leaves on the rst day after suffering from dry hot wind were lower than the control group. The conclusion is similar to our study. As shown in the previous literature (Zhao, et al. 2013), the observation of stomatal opening and closing and the determination of transpiration intensity under articial simulated dry hot wind experiment condition were performed, the results showed that the maximum value of stomatal opening of cultivars with no resistance to dry hot wind showed a trend of earlier appearance, the duration was signicantly prolonged and the transpiration intensity had no signicant change. The maximum value of stomatal opening of cultivars with strong resistance to dry hot wind showed no signicant change, and the transpiration intensity had a signicant trend of decreasing. There were

Province is one of the main wheats producing provinces in China (the planting area is more than 30 Van, et al. 2020). Therefore, study on the responses of different winter wheat cultivars to dry hot wind is of great practical signi cance for understanding the characteristics of different winter wheat cultivars and the safe production of winter wheat.
Researchers from various countries have paid much attention to the study on the responses of winter wheat to the dry hot wind. The former Soviet Union began to study the indicators, causes, geographical distribution, and defense measures of dry hot wind in the 1920s (Research Cooperation Group of Hot Dry Wind in Northern wheat area, 1983). American scholars simulate the effect of dry hot wind on wheat growth by means of wind tunnel test, and found that dry hot wind reduced the number of spikes, grain number per spike and grain weight of winter wheat (Smika et al. 1980). The study of dry hot wind in China began in the late 1950s. In early 1980s, the damage mechanism, meteorological index and defense technology of dry hot wind were studied by Research Cooperation Group of Hot Dry Wind which consisted by thirteen provinces in Northern China wheat area. Some researchers discussed the difference of dry hot wind resistance among different wheat cultivars (Gong 1981;Lu Zhengduo et al.1981).With the development of science and technology, based on the previous studies, the effects of dry hot wind on photosynthetic physiological parameters were studied by using the arti cially simulated dry hot wind experiments (Zhao et al. 2013, Zhang et al. 2015 ). However, there are few studies on the differences in the responses of different winter wheat cultivars to dry hot wind. Most of the existing studies focus on the comparison of photosynthetic physiological parameters before and after the affected by dry hot wind, while there are few studies on the variation characteristics of each parameter on the day affected by dry hot wind. And most of the studies were carried out by using a single variety. With the continuous updating of winter wheat cultivars, the previous research results cannot meet the needs of modern agricultural production. Therefore, in this research, three main winter wheat cultivars in Hebei Province were taken as examples, eld experiments under natural dry hot wind conditions were adopted to analyze the response differences among different cultivars to dry hot wind. The variation characteristics of physiological parameters such as net photosynthetic rate (P n ), transpiration rate (T r ), and stomatal conductance (G s ) of winter wheat during the days affected by dry hot wind, as well as the variation characteristics of physiological parameters before and after the days affected by dry hot wind were analyzed, and also the correlations among the parameters. Meantime, combined with arti cially simulated dry hot wind experiments, the stress of dry hot wind on each physiological parameter during and after the days affected by dry hot wind were analyzed. The results could reveal the resistance variation of different cultivars toward dry hot wind, which provide scienti c basis for winter wheat cultivars selection and production layout.

Experimental Materials and Design
The experimental area is located at Mazhuang (37°58′N, 115°13E′) experimental station of Hebei Agricultural University in Xinji City, which is the main wheat region of Hebei Province. This area belongs to warm temperate zone with semi-humid continental climate, the annual average temperature is 13.6°C, annual average precipitation is 466.4 mm, the annual average relative humidity is 63%, and annual average sunshine hours are 2610.1 hours.
The rst owering date was May 3rd, and there were no plant diseases, insect pests, and drought during the whole growth period. Dry hot wind with daily maximum temperature above or equal to 32℃, relative humidity below or equal to 30% at 14:00, wind speed above or equal to 3 m/s at 14:00 is de ned as mild dry hot wind; dry hot wind with daily maximum temperature above or equal to 35℃, relative humidity below or equal to 25% at 14:00, wind speed above or equal to 3 m/s at 14:00 is de ned as severe dry hot wind (Huo, et al. 2007).
Mild dry hot wind occurred on May 22nd, 2019, lasted from 13:00 to18:00; severe dry hot wind occurred on May 23rd, 2019, lasted from 13:00 to17:00 (Huo, et al. 2007). The measurements were carried out on May 21st to May 24th, May 28th, and May 31st when the weather condition is well. Table 1 shows the hourly meteorological element values on the day suffering from dry hot wind in 2019.

Measurement Items and Methods
Flag leaf is the primary photosynthetic organ for grain lling, it has important effects on grain lling and the yield (Evans, 1975). Five ag leaves with a similar growth pattern were randomly selected for each cultivar to carry out the physiological parameter measurement. As an input parameter of the portable photosynthesis instrument (LI-6400XT), the natural light intensity was measured rst on the measurement day. In order to avoid the in uence of pm, 16:00 pm and 18:00 pm on dry hot wind days)and in 2020 the measurement was carried out right after the dry hot wind stress experiment. Three repeated measurements were carried out for each winter wheat cultivars. After these measurements, cut ve leaves and weighed the fresh weight. Then weighed the dry weight after treatment at 120°C for 20 minutes and drying at 80°C for 10 hours in the oven. Then the relative water content (RWC) of the leaves is calculated by the following formula: The stress of dry hot wind on each parameter is calculated by the following formula: Where, n b is the value observed after the dry hot wind stress, n a is the value without stress; if n b >n a , it indicates that there is no signi cant negative impact on crops after the dry hot wind stress. The greater the SI value is, the stronger the stress is.
F-test and correlation method were used to analyze the response of different winter wheat cultivars to dry hot wind. Table 2 shows the signi cance test results of photosynthetic physiological parameters measured in 2019.
3 Results  Under the in uence of dry hot wind, the variation characteristics of P n showed differences among cultivars, which indicating that the responses of different cultivars to dry hot wind were different. For HN6119, the P n on May 21st

Differences in the Responses of Net Photosynthetic Rate (P n ) of Different Winter Wheat Cultivars
was in a high level; when happened, the crop responded to the dry hot wind by decreasing the P n . While it showed a signi cant increase trend during May 28th to May 31st during the recovery time after dry hot wind conditions, it indicates that dry hot wind had little effect on it. For GY5218, the P n on May 21st also was in a high level; when dry hot wind happened, it showed no signi cant change and remained in a high level; when dry hot wind ended, there were no signs of recovery and showed a signi cant decrease trend, it indicates that dry hot wind had effect on it.
For JM325, when dry hot wind happened, the crop responded to it by increasing the P n ; when dry hot wind ended, there were no signs of recovery and showed a continuous decrease trend, it indicates that dry hot wind had effect on it too. Figure 1b is the daily variation curve of P n on the day suffering from dry hot wind. For HN6119, the daily change curve of P n showed a bimodal pattern on the May 22nd, with peaks at 10:00 am and 16:00 pm respectively; which indicated that mild dry hot wind had no obvious effect on this cultivar. It is similar to the observation showed in the previous literature (Xu, 1984). On May 23rd, P n still reached a higher level at 10:00 am, but followed with a continuous decrease when the temperature increased as showed in Table 1, and no peak appeared at 16:00, which indicated that severe dry hot wind had signi cant effect on this cultivar. For GY5218 and JM325, peak of P n showed at 12:00 pm On May 22nd ; the peak time showed a signi cant difference from the previous literature (Xu, 1984). It indicates that the mild dry hot wind had a certain effect on these cultivars, but it was not severe. On May 23rd, P n also reached a higher level at 10:00 am, and the change afterwards is similar to HN6119, which indicated that severe dry hot wind had caused damage to these cultivars.
Therefore, the characteristics differences of P n among different cultivars showed that the resistance to dry hot wind of HN6119 is strong, while the resistance to dry hot wind of GY5218 and JM325 is weak. As shown in Fig. 2a and The increase trend of GY5218 and JM325 from May 21st to May 23rd is signi cantly different, but the decease trend is almost the same, the overall change trend from May 23rd to May 31st is decreasing.

Differences in the Responses of Transpiration Rate (T r ) of Different Winter Wheat Cultivars
Under the in uence of dry hot wind, the variation characteristics of T r showed differences among cultivars, which indicating that the responses of different cultivars to dry hot wind were different. For HN6119, the variation characteristics of T r is similar to P n , the crop responded to the dry hot wind by decreasing the T r , it indicates that dry hot wind had little effect on it. For GY5218 and JM325, the crop responded to the dry hot wind by increasing the T r ; when dry hot wind ended, it showed a signi cant decrease trend, it indicates that dry hot wind had effect on it. The value of T r was closely related to the amount of water loss in leaves. According to the difference of relative water content of leaves on the rst day (May 24th ) after dry hot wind and the day (May 21st ) before dry hot wind, the relative water content of HN6119, GY5218, and JM325 decreased by 0.24%, 1.94%, and 3.71% respectively. It indicated that the T r decrease of HN6119 resulted in less water reduction of leaves, while the T r increase of GY5218 and JM325 resulted in more water loss from leaves. Figure 2b is the daily variation curve of T r on the day suffering from dry hot wind. For HN6119, T r remained a low level from the beginning and showed a continuous decrease on the day suffering from severe dry hot wind. For GY5218 and JM325, T r remained a high level from the beginning, and showed the peak at 12:00 pm on May 23rd.
The results showed that when affected by dry hot wind the T r of HN6119 was decreased in order to reduce the water loss of leaves; while T r of GY5218 and JM325 was increased leading to the increased water loss of leaves.
Therefore, the characteristics differences of T r among different cultivars showed that the resistance to dry hot wind of HN6119 is strong, while the resistance to dry hot wind of GY5218 and JM325 is weak. For GY5218 and JM325, the crop responded to the dry hot wind by increasing the G s ; when dry hot wind ended, it showed a continuous decrease trend and no signs of recovery, it indicates that dry hot wind had effect on it.

Differences in the
From Fig. 3b, it can be seen that G s of HN6119 remained a low level from the beginning, which was consistent with the variation characteristic of P n and T r . HN6119 showed strong resistance to dry hot wind by reducing the stomatal conductance to reduce T r and P n , in order to reduce the damage caused by dry hot wind. For GY5218 and JM325, G s remained a high level when affected by dry hot wind, which was consistent with the variation characteristic of T r . However, the increase of G s and T r at 12:00 pm, 14:00 pm, and 16:00 pm on the day affected by severe dry hot wind was signi cant than the increase of P n . It indicates that the increase of T r did not cause the response of P n , only increased the water loss of leaves, so the resistance to dry hot wind of GY5218 and JM325 is weak.
Therefore, the characteristics differences of G s among different cultivars showed that the resistance to dry hot wind of HN6119 is strong, while the resistance to dry hot wind of GY5218 and JM325 is weak.

Differences in the Responses of Photosynthesis Physiological Parameters of Different Winter Wheat Cultivars
In order to discern the differences in the responses to dry hot wind of each cultivar, the intensive observation was carried out on the day before affected by dry hot wind, the rst day after affected by dry hot wind and the day affected dry hot wind. The data were used to analyze the correlation differences of the photosynthesis physiological parameters of three cultivars. As shown in Table 3 (intensive observations not  correlation of these parameters discerned that HN6119 showed better self-stability when affected by dry hot wind, the correlation coe cients of P n and T r showed signi cant correlation. The metabolism balance was not disturbed and still maintain normal regulation. For GY5218 and JM325, the correlation coe cients of P n , T r and G s showed poor correlation when affected by dry hot wind, which indicated that the regulation effect had been disrupted.
It can be seen from the correlation analysis that HN6119 showed strong resistance to dry hot wind, while JM325 and GY5218 showed weak resistance to dry hot wind.

Difference in the Test of Resistance to Dry Hot Wind of Different Winter Wheat Cultivars
As the dry hot wind is sporadic, in order to recover the de ciency of natural dry hot wind samples, the dry hot wind stress simulation experiment was carried out in 2020 and the samples were used to verify the reliability of above conclusions. Table 4 shows the stress value of photosynthesis physiological parameters of each cultivar after dry hot wind stress. Note: --indicates that n b >n a , there is no signi cant impact after the dry hot wind stress.
As shown in Table 4, for HN6119, the variation characteristics of parameters under mild and severe dry hot wind are similar,the stress value reached the highest level on the day suffering from dry hot wind and then showed a decrease trend, especially P n showed the highest decreasing speed, the stress value reached the control level on the rst day after mild dry hot wind stress and third days after severe dry hot wind stress. The stress value of mild and sever dry hot wind on thousand kernel weight (TKW) was 0.01% and 0.36% respectively, indicating that dry hot wind had little damage to HN6119 and showed the characteristics of resistance to dry hot wind.
For GY5218 and JM325, P n showed the most signi cant variation characteristics, the stress value showed a decrease trend, but the stress value still didn't reach the control level on the third days after dry hot wind stress. T r and G s showed no obvious consistent variation characteristics after dry hot wind stress. The stress value was still in a high level on the third days after dry hot wind stress, indicating that dry hot wind had caused damage to the crop. The stress value of mild and sever dry hot wind on thousand kernel weight (TKW) was 3.51%, 3.57% for GY5218, and 8.12%, 8.84% for JM325, respectively. It indicated that GY5218 and JM325 showed weak resistance to dry hot wind.
Under the in uence of simulated dry hot wind, the experimental parameters can't be measured, so the analysis procedure of 2020 is different from 2019. However, the results obtained by two experiment methods are consistent, indicating that the conclusion of "HN6119 showed strong resistance to dry hot wind, while JM325 and GY5218 showed weak resistance to dry hot wind" is credible.

Discussion
Under the combined conditions of eld natural dry hot wind and arti cially simulated dry hot wind experiments, differences in the responses of physiological parameters such as photosynthesis and transpiration were analyzed, in order to discern the differences in the responses to dry hot wind of each cultivar.
In this study, it was found that the cultivar with strong resistance to dry hot wind showed stronger stomatal regulation ability on the natural dry hot wind day. Through stomatal conductance adjustment, the transpiration rate and leaf water loss were reduced in order to avoid serious damage to plants. The physiological functions could be gradually recovered after the end of dry hot wind. While the cultivar with weak resistance to dry hot wind showed more water loss of leaves by increasing the stomatal conductance and transpiration rate, and the physiological parameters showed signi cant decrease and no signs of recovery when the dry hot wind ended. As shown in the previous literature (Xu, et al. 1984), the transpiration intensity and stomatal aperture under dry hot wind conditions were observed, the results showed that dry hot wind increased the transpiration intensity and stomatal aperture, which resulted in water loss of leaf cells. It is consistent with the response of cultivars with weak resistance to dry hot wind on hot wind day in this study. However, the conclusion in the previous literature was for a certain cultivar, it didn't discern the response of cultivars with stronger resistance to dry hot wind on the day suffering from dry hot wind. Few research results about the recovery of physiological functions after suffering from dry hot wind were mentioned in previous literature.
This study shows that physiological parameters of cultivars with strong resistance to dry hot wind could maintain a signi cant positive correlation when affected by dry hot wind, new balance would be established between the parameters, it indicates that these cultivars have better self-regulation ability. While physiological parameters of cultivars with weak resistance to dry hot wind showed worse correlation when affected by dry hot wind, especially the severe dry hot wind. It indicates that the regulation function of the plant was damaged and affected the normal physiological functions such as photosynthesis and transpiration. This nding has not been involved in previous studies.
In this study, the photosynthetic physiological parameters of different cultivars under the condition of natural dry hot wind were measured on different dates. The results show that the net photosynthetic rate, transpiration rate and stomatal conductance of three cultivars all decreased on the rst day after the end of dry hot wind. Under the condition of arti cial simulated dry hot wind, the photosynthetic physiological parameters of different cultivars all decreased compared with the value without stress, it indicates that dry hot wind has retarding effect on photosynthetic physiological parameters. As shown in the previous literature (Zhao, et al. 2013), under the condition of arti cial simulated dry hot wind, the photosynthetic transpiration of wheat ag leaves at grain lling stage were signi cantly retarded and caused partial closure of stomata, while a similar study (Zhang, et al. 2015) showed that the net photosynthetic rate, transpiration rate, and stomatal conductance of wheat ag leaves on the rst day after suffering from dry hot wind were lower than the control group. The conclusion is similar to our study. As shown in the previous literature (Zhao, et al. 2013), the observation of stomatal opening and closing and the determination of transpiration intensity under arti cial simulated dry hot wind experiment condition were performed, the results showed that the maximum value of stomatal opening of cultivars with no resistance to dry hot wind showed a trend of earlier appearance, the duration was signi cantly prolonged and the transpiration intensity had no signi cant change. The maximum value of stomatal opening of cultivars with strong resistance to dry hot wind showed no signi cant change, and the transpiration intensity had a signi cant trend of decreasing. There were some differences with the conclusions of this study, which may be related to the difference in resistance of cultivars.
In conclusion, the environmental factors such as CO2 concentration and soil moisture of each measurement date were consistent in this study.

Conclusion
The experimental result in a condition of natural dry hot wind conditions in 2019 showed that: (1) the similarities in the impacts of dry hot wind on photosynthetic physiological parameters of different cultivars: the net photosynthetic rate, transpiration rate, and stomatal conductance of all cultivars showed decreasing trend on the rst day after suffering from day hot wind; the differences: the net photosynthetic rate, transpiration rate, and stomatal conductance of HN6119 was recovered during the recovery time after dry hot wind conditions, while GY5218 and JM325 still showed a continuous decrease trend; (2) there were differences in responses of different cultivars to dry hot wind; HN6119 showed less water loss of leaves by reducing the stomatal conductance and transpiration rate, indicating that HN6119 showed the characteristics of resistance to dry hot wind. GY5218 and JM325 showed more water loss of leaves by increasing the stomatal conductance and transpiration rate,both stomatal conductance and transpiration rate signi cantly decreased when the dry hot wind ended, indicating that GY5218 and JM325 were damaged and showed weak resistance to dry hot wind; (3) both the correlation coe cients of T r and G s on all measurement date and the correlation coe cients of P n , T r and G s on the day without dry hot wind for three cultivars showed signi cant positive correlation (P < 0.001); the correlation coe cients of P n , T r , and G s for HN6119 at all measurement time on the day suffering from dry hot wind showed signi cant positive correlation, the correlation coe cient signi cant level for GY5218 declined; the correlation coe cients of P n and T r for JM325 showed nonsigni cant correlation at 16:00 pm and 18:00 pm on May 22nd, and also the correlation coe cients of P n , T r , and G s at 14:00 pm and 16:00 pm on May 23rd. It indicated that HN6119 showed better self-stability when affected by dry hot wind, and the metabolism balance was not disturbed and still maintain normal regulation, while the regulation effect of GY5218 and JM325 had been disrupted; (4) the difference in characteristics and difference in correlation of P n , T r and G s among different cultivars showed that HN6119 showed strong resistance to dry hot wind, while JM325 and GY5218 showed weak resistance to dry hot wind.
The results of arti cially simulated dry hot wind experiment in 2020 showed that the stress of severe dry hot wind on thousand kernel weight (TKW) of HN6119, GY5218 and JM325 is 0.01%, 3.51%, 3.57%, respectively. The stress of mild dry hot wind on thousand kernel weight (TKW) of HN6119, GY5218 and JM325 is 0.36%, 8.12%, 8.84%, respectively.
In conclusion, HN6119 showed strong resistance to dry hot wind, while JM325 and GY5218 showed weak resistance to dry hot wind.  Change curves of Gs at different winter wheat cultivars affected by dry hot wind