3.1 Differences in the Responses of Net Photosynthetic Rate (Pn) of Different Winter Wheat Cultivars
Figure 1a is the change curve of Pn (µmol CO2 m− 2 s− 1) of each cultivar from May 21st to May 31st. As shown in Fig. 1a and Table 2, Pn value of HN6119 showed a decrease trend from May 21st to May 28th and an increase trend from May 28th to May 31st. F test result of HN6119 was extremely significant from May 21st to May 22nd, and May 28th to May 31st, significant from May 24th to May 28th. Pn value of GY5218 showed no obvious change from May 21st to May 23rd and showed significant decrease from May 23rd to May 24th and extremely significant decrease from May 24th to May 31st. Pn value of JM325 showed increase trend from May 21st to May 23rd, and the change trend after May 23rd is similar to GY5218. F test result of JM325 was extremely significant from May 22nd to May 23rd .
Under the influence of dry hot wind, the variation characteristics of Pn showed differences among cultivars, which indicating that the responses of different cultivars to dry hot wind were different. For HN6119, the Pn on May 21st was in a high level; when happened, the crop responded to the dry hot wind by decreasing the Pn. While it showed a significant 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 Pn on May 21st also was in a high level; when dry hot wind happened, it showed no significant change and remained in a high level; when dry hot wind ended, there were no signs of recovery and showed a significant 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 Pn; 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 Pn on the day suffering from dry hot wind. For HN6119, the daily change curve of Pn 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, Pn 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 significant effect on this cultivar. For GY5218 and JM325, peak of Pn showed at 12:00 pm On May 22nd ; the peak time showed a significant 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, Pn 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 Pn 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.
3.2 Differences in the Responses of Transpiration Rate (Tr) of Different Winter Wheat Cultivars
Figure 2a is the change curve of Tr (mmol H2O m− 2 s− 1) of each winter wheat cultivar from May 21st to May 31st. As shown in Fig. 2a and Table 2, Tr value of HN6119 showed a decrease trend from May 21st to May 28th, and an increase trend from May 28th to May 31st. F test result of HN6119 was extremely significant from May 23rd to May 24th, and May 28th to May 31st. The change trend of GY5218 and JM325 is similar, both showed continuous increase from May 21st to May 23rd, significant decrease from May 23rd to May 24th and May 28th to May 31st. The increase trend of GY5218 and JM325 from May 21st to May 23rd is significantly different, but the decease trend is almost the same, the overall change trend from May 23rd to May 31st is decreasing.
Under the influence of dry hot wind, the variation characteristics of Tr showed differences among cultivars, which indicating that the responses of different cultivars to dry hot wind were different. For HN6119, the variation characteristics of Tr is similar to Pn, the crop responded to the dry hot wind by decreasing the Tr, 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 Tr; when dry hot wind ended, it showed a significant decrease trend, it indicates that dry hot wind had effect on it. The value of Tr was closely related to the amount of water loss in leaves. According to the difference of relative water content of leaves on the first 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 Tr decrease of HN6119 resulted in less water reduction of leaves, while the Tr increase of GY5218 and JM325 resulted in more water loss from leaves.
Figure 2b is the daily variation curve of Tr on the day suffering from dry hot wind. For HN6119, Tr 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, Tr 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 Tr of HN6119 was decreased in order to reduce the water loss of leaves; while Tr of GY5218 and JM325 was increased leading to the increased water loss of leaves.
Therefore, the characteristics differences of Tr 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.
3.3 Differences in the Responses of Stomatal Conductance (Gs) of Different Winter Wheat Cultivars
Figure 3a shows the change curve of Gs (mol H2O m− 2 s− 1) of each winter wheat cultivars from May 21st to May 31st. It shows that the variation tendency Gs of different cultivars was significantly different. For HN6119, Gs showed a decreasing trend from May 21st to May 28th, and an increase trend from May 28th to May 31st. The variation characteristic is similar to Pn and Tr, and the significant changes were observed from May 21st to May 22nd, May 24th to May 28th and May 28th to May 31st. For GY5218 and JM325, the variation characteristic is similar, both showed a very significant increase trend from May 22nd to May 23rd and a decreasing trend after May 23rd. However, the significant decrease was showed in different period for GY5218 and JM325 as it is showed in Table 2.
Under the influence of dry hot wind, the variation characteristics of Gs showed the responses of different cultivars to dry hot wind is different. For HN6119, the crop responded to the dry hot wind by decreasing the Gs. Gs was recovered during the recovery time after dry hot wind conditions, 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 Gs; 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.
From Fig. 3b, it can be seen that Gs of HN6119 remained a low level from the beginning, which was consistent with the variation characteristic of Pn and Tr. HN6119 showed strong resistance to dry hot wind by reducing the stomatal conductance to reduce Tr and Pn, in order to reduce the damage caused by dry hot wind. For GY5218 and JM325, Gs remained a high level when affected by dry hot wind, which was consistent with the variation characteristic of Tr. However, the increase of Gs and Tr at 12:00 pm, 14:00 pm, and 16:00 pm on the day affected by severe dry hot wind was significant than the increase of Pn. It indicates that the increase of Tr did not cause the response of Pn, 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 Gs 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.
3.4 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 first 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.
Table 3
Correlation coefficient test of photosynthetic physiological parameters of different winter wheat cultivars
DATE | HN6119 | GY5218 | JM325 |
Pn | Tr | Pn | Tr | Pn | Tr |
5/21 | Tr | 0.912*** | | 0.781*** | | 0.905*** | |
Gs | 0.891*** | 0.957*** | 0.862*** | 0.964*** | 0.874*** | 0.993*** |
5/22 | Tr | 0.971*** | | 0.952*** | | 0.837*** | |
Gs | 0.993*** | 0.973*** | 0.928*** | 0.997*** | 0.852*** | 0.996*** |
5/23 | Tr | 0.981*** | | 0.756** | | 0.473 | |
Gs | 0.989*** | 0.998*** | 0.707** | 0.997*** | 0.531 | 0.998*** |
5/24 | Tr | 0.991*** | | 0.861*** | | 0.990*** | |
Gs | 0.993*** | 0.999*** | 0.876*** | 0.999*** | 0.992*** | 0.999*** |
Note: *, ** and *** indicate that the correlation coefficient test reaches the significance level of 0.05, 0.01, and 0.001 respectively. |
As shown in Table 3 (intensive observations not included), the correlation coefficients of Tr and Gs of three cultivars on all measurement date showed significant positive correlation (P < 0.001); the correlation coefficients of Pn, Tr and Gs before and after suffering from dry hot wind also showed significant positive correlation (P < 0.001). While the correlation coefficient tests of Pn, Tr, and Gs on the day suffering from dry hot wind were different. For HN6119, the correlation coefficients at all measurement time showed significant positive correlation (P < 0.001). For GY5218, the correlation coefficients significant level declined (P < 0.05). For JM325, the correlation coefficients showed significant positive correlation except the correlation coefficients of Pn and Tr at 16:00 pm and 18:00 pm on May 22nd and the correlation coefficients of Pn, Tr, and Gs at 14:00 pm and 16:00 pm on May 23rd. The correlation coefficients between some parameters showed nonsignificant correlation, it is closely related to the effect of dry hot wind. According to the hourly meteorological element values, the dry hot wind lasted from 13:00 pm to 18:00 pm on May 22nd and lasted from 13:00 pm to17:00 pm on May 23rd. The correlation of Pn and Tr of JM325 was affected by the dry hot wind at 16:00 pm on May 22nd and continued to be affected until May 23rd. The affection on correlation of Pn and Tr was exacerbated when a severe dry hot wind occurred as the temperature reached 35.7 ℃ at 14:00 pm and 37.1 ℃ at 15:00 pm on May 23rd and affected the correlation of Pn and Gs. The difference in correlation of these parameters discerned that HN6119 showed better self-stability when affected by dry hot wind, the correlation coefficients of Pn and Tr showed significant correlation. The metabolism balance was not disturbed and still maintain normal regulation. For GY5218 and JM325, the correlation coefficients of Pn, Tr and Gs 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.
3.5 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 deficiency 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.
Table 4
SI (%) of photosynthesis physiological parameters of different winter wheat cultivars
Stress treatment | HN6119 | GY5218 | JM325 |
Pn | Tr | Gs | Pn | Tr | Gs | Pn | Tr | Gs |
5/23 | Mild | 14 | 43 | 54 | 27 | 56 | 66 | 37 | 56 | 65 |
5/24 | Mild | -- | 15 | 17 | 16 | 49 | 35 | 24 | 47 | 37 |
Severe | 49 | 50 | 46 | 51 | 45 | 42 | 71 | 56 | 57 |
5/25 | Severe | 26 | 46 | 36 | 31 | 48 | 35 | 40 | 42 | 21 |
5/26 | Mild | -- | 8 | 16 | 8 | 19 | 32 | 4 | 41 | 54 |
5/27 | Severe | -- | 27 | 30 | 21 | 47 | 46 | 29 | 48 | 57 |
Note: --indicates that nb>na, there is no significant 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 Pn showed the highest decreasing speed, the stress value reached the control level on the first 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, Pn showed the most significant 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. Tr and Gs 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 influence 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.