Warming effect of night warming system
During growth period of wheats, the change of canopy temperature at night is shown in Fig. 1-A, the temperature ranged for normal and nighttime warming were − 1.88°C to 26.25°C, -1.75°C to 26.62°C, respectively; and their average temperature was 9.46°C and 9.73°C, respectively. Additionally, as the growth period advances, the temperature continued to drop, reaching its lowest temperature at the end of January and early February, and then gradually rising. The increment of night canopy temperature under nighttime warming is displayed in Fig. 1-B. Except for rainy and snowy days, the canopy temperature under WT were higher than that under NT, indicating that the passive warming device had a significant warming effect. The warming effect was similar with that from Chen et al. (2017), which also showed a warming effect by using a passive warming device. Throughout the growth period of wheats, the average temperature increment was 0.28°C. The soil-plant system temperature increases under the passive nighttime warming system, mainly by intercepting long-wave radiation on the ground at night to achieve the purpose of warming (Beier et al. 2004). From October to May of the following year, the average increment during warming was 0.28°C and the increment of each month was 0.26, 0.12, 0.20, 0.18, 0.17, 0.46, 0.57 and 0.26°C, respectively. Considering these results, the warming effect was best around the flowering and booting stages. Prior reports also found that the warming effect of passive warming device was obvious from booting stage (Beier et al. 2004; García et al. 2015; Tian et al. 2012), and high night temperature during grain number determination might be not helpful to grain filling (García et al. 2015), which may be the main reason for the decrease in wheat production under nighttime warming. Overall, warming effect of night warming system was prominent.
Available Cu content in soil
Soil available Cu is the form of Cu that can be absorbed by plants, so this indicator has been used to evaluate the level of soil Cu availability (Ghori et al. 2019; Marschner 2012). According to the soil available Cu abundance standard of farmland in China, (< 0.5 mg/kg is lack, 0.5-1.0 mg/kg is on the verge of deficiency, > 1.0 mg/kg is sufficient) (Huang et al. 2019), the soil available Cu level in no-Cu applications (NT-CK and WT-CK) was in the range of 0.57–0.95 mg/kg (Fig. S1), indicating that Cu was required in this soil. The lower available Cu content was mainly due to soil type and higher pH (Marschner 2012). Because carbonate cinnamon soil itself has low Cu content according its parent material and higher pH can affect the solubility and then effectiveness of Cu (Zhang 2016). As the growth period advances, the available Cu content in the soil showed a trend of first increasing, then decreasing and then increasing. Moreover, the available Cu content from the treatments without Cu applications (NT-Cu and WT-Cu) reached the lowest level during the booting stage; the available Cu concentration from the Cu applications reached the lowest and highest level during the seeding stage and maturation stage, respectively. According to two-way ANOVA analysis, the interaction of warming and Cu factors, and the single factor of warming had not reached a significant level of the available Cu concentration in soil. However, the Cu factor had a very significant impact on the available Cu concentration in soil throughout wheat growth period. Moreover, it could also be concluded from the Fig. S1 that after Cu application, the available Cu content was significantly higher (352%-768%) than that from CK, regardless of warming or no-warming environment. For the treatments with the same amount of Cu application, no significant effect was observed throughout wheat growth period, except that the available Cu contents were significantly increased by WT-CK than NT-CK at jointing stage, by NT-Cu than WT-Cu at booting stage, respectively.
Overall, Cu applications resulted in significantly higher available Cu concentration in soil and almost no effect on soil Cu concentration by nighttime warming was observed throughout wheat growth period.
Soil urease and catalase activities
Soil enzymes are an important part of soil ecosystem, and their activity can sensitively reflect the direction and extent of biochemical reactions in soil. Soil enzymes activities are susceptible to heavy metal pollution and temperature (Duan et al. 2018; Xiao et al. 2018), while prior studies have not documented the changes in their activities under the dual background of Cu pollution and warming. In this study, the changes in soil urease and catalase activities throughout the growth stages were tested and are respectively displayed in Fig. S2-A and S2-B. The activity of urease can reflect the supply capacity of soil inorganic nitrogen; catalase can promote the decomposition of the metabolic intermediate product hydrogen peroxide and alleviate the toxicity of heavy metals, and its activity reflects the redox ability of soil biochemical process to some extent (Guzel and Terzi 2013; Lai 2019).
For soil urease activity (Fig. S2-A), according to two-way ANOVA analysis, the interaction of warming and Cu factors, and the single factor of warming had not reached a significant level on it, while the Cu factor had a significant impact on it throughout wheat growth period (P < 0.01), especially at the seedling stage and jointing stage. Soil urease activities were lower in all Cu applications than CK during wheat growth stages, the specific analysis is as follows: At no-warming environment, urease activities were remarkedly lower by NT-Cu than NT-CK, except jointing stage; at warming environment, NT-Cu significantly reduced urease activities than WT-CK. These results indicated that Cu pollution but not warming significantly affected urease activities. For soil catalase activity (Fig. S2-B), the interaction of warming and Cu factors, and the single factor of Cu had not reached a significant level of it, except it was significantly affected by Cu at seedling stage, and the warming factor had a significant impact on it only at jointing and booting stages. A decreasing trend on soil catalase activities were observed by all Cu application than by CK, but a significant level was found only at seeding stage. Simultaneously, a decreasing trend on it was observed by all warming applications (WT-CK and WT-Cu) than by no-warming applications (NT-CK and NT-Cu), but a significant level was reached only at jointing and booting stages. A possible explanation was that soil catalase activities were sensitive to temperature, due to the higher increment of canopy temperature during the two stages (Fig. 1; Xiao et al. 2018). Meanwhile, these results also revealed that nighttime warming did not appear to have a trend towards mitigating the toxicity of soil Cu pollution by regulating enzyme activities (Guzel and Terzi 2013; Xiao et al. 2018).
Overall, the interaction of warming and Cu factors had not reached a significant level on soil urease and catalase activities. Cu applications reduced the two enzymes activities and only urease activities reached the significantly level throughout wheat growth period. Warming applications had no significant effect on urease activities, but significantly reduced catalase activities in the middle growth stages.
Yield and biomass of different tissues in wheats
The change of biomass of different tissues in wheats throughout the whole growth period is displayed in Fig. 4. During this process, single factor of warming or Cu had a significant impact on the total biomass of wheats, except Cu factor at booting stage; a significant difference on the total biomass by the interaction effect of warming and Cu was also observed at vegetative period.
Throughout the sampling points, the total biomass of wheats were significantly lower by warming treatments than that by no-warming treatments, except at booting stage. At booting stage, the warming effect was best (Fig. 1). Moreover, at this stage, warming treatments resulted in a higher total biomass of wheats than no-warming treatments, which may be explained by that higher temperature increment at this stage promoted wheats growth, especially aboveground parts (Fig. 1; Hatfield and Prueger 2015) and to some extent, the increase of carbohydrates was a response of plants to temperature stress (Seginer and Gent 2014). For the treatments with the same amount of Cu application, WT-Cu significantly increased total biomass of wheats than NT-Cu, mainly due to that the significant enhancement in stem biomass (Fig. 1). For other stages, wheats treated with WT-Cu had significantly enhanced total and aboveground biomass of wheats than that of wheats treated with NT-Cu. Moreover, for the roots biomass, WT-Cu resulted in a decrease than NT-Cu and a significant level was observed at stages of reproductive development, except that it was significantly enhanced by WT-Cu than NT-Cu at seeding stage. These findings could be due to that the temperature was relatively low and the wheat roots were thin and few at seeding stage, an increase in the appropriate temperature under warming treatments was conducive to its growth and development (Pregitzer et al. 2010). At maturation stage, the biomass of each tissue was higher by no-warming applications than that by warming applications, except that wheats in WT-Cu exhibited higher leaves biomass and significant influence than other treatments.
For the treatments with the same amount of Cu application, the difference of the biomass of roots and stems and yield between warming and no-warming treatments were significant. Among them, the highest yield was achieved in NT-Cu, which showed significant influence than other applications, followed by NT-CK, which significantly increased yield than other applications. Moreover, no significant difference on yield was observed between no-warming treatments. These outcomes of the decrease in wheat production under warming treatments could be explained by that (1) high temperature, especially at the booting stages (Fig. 1), might be not helpful to grain filling (García et al. 2015; Hu and Yin 2014), by accelerating the growth of aboveground parts (Fig. 2) or shortening the duration of critical period of wheat grain formation García et al. (2015). (2) average temperature during winter wheat growth season in trail site was 9.52°C and was surpassed 8.6°C (Chen et al. 2018). Because by carrying out long-term research from 1980 to 2013 in Huang-Huai-Hai plain in China, the report from Chen et al. (2018) found that an average temperature of 8.6°C during the winter wheat growing period was the yield variation threshold, and nighttime warming would reduce wheat yield above this threshold. Moreover, Zhao et al. (2016) also conducted a large number of warming experiments in China and revealed that in areas where the temperature during the wheat growing season was low and the water supply was not restricted, the wheat yield would increase; in other cases, warming would lead to a decrease in wheat production. These results were also similar with that from García et al. (2015), who found that an accelerated development under nighttime warming shortened the duration of critical period of wheat grain formation, reduced solar radiation capture with negative consequences for biomass production and therefore, grain yield. However, there were many studies, that have found that nighttime warming could increase wheat yield in China, such as the results of reports from Chen et al. (2018), Zheng et al. (2017), and Zhou et al. (2021). Based on the results of these experiments, it can be drawn that the changes in wheat yield were mainly affected by changes in temperature, regions and soil moisture. In addition, the trial found that there was no significant difference on yield between WT-Cu and WT-CK, indicating an increasement trend of yield by Cu treatment was suppressed by warming treatment in this trial.
Regards with Cu factor, total biomass of wheats were remarkedly higher by Cu applications than no-Cu applications, whether it was aboveground or underground parts, except for jointing stage. These results could be mainly explained by that Cu deficiency existed in the no-Cu applications soil (Table S1) and then limited the wheat growth (Adrees et al. 2015; Sağlam et al. 2016). Moreover, the pH in this soil was higher (Table S1), the solubility and uptake of Cu by plant roots was limited in high pH (Huang et al. 2019). For Cu applications, Cu was applied in soil, which was urgently needed for wheat growth in this trail. So growth and yield in wheats were relatively improved in Cu-contaminated soil than that in soil of urgently imperative Cu element, due to that Cu is an essential micronutrient element affecting the growth and metabolism progress of plants (Ghori et al. 2019; Marschner 2012).
In the whole, nighttime warming decreased the total biomass and yield of wheats grown in Cu-contaminated soil. The highest yield was achieved in NT-Cu, followed by NT-CK. The lower yield was observed in WT-Cu and WT-CK. Though the highest yield was achieved in NT-Cu, there was a hidden danger of Cu contamination in wheats.
Yield components in wheats
From the perspective of yield components, since spikes per pot, grains per spike and 1000-grain weight are the main factors that constitute the wheat yield, any change in any factor will definitely affect the wheat yield (Hu and Yin 2014). By two-way ANOVA analysis (Table 1), spikes per pot and grains per spike were significantly affected by the interaction of warming and Cu factors and the single factor of warming/Cu, except for spikes per pot affected by Cu factor and grains per spike affected by the interaction factors; no significant influence on 1000-grain weight were observed. With regards to spikes per pot and grains per spike, warming treatments significantly reduced them than no-warming treatments, while Cu treatments remarkedly increased them than no-Cu treatments, except for spikes between WT-CK and WT-Cu. Among them, the highest spikes and grains per spike were achieved in NT-Cu, which may be probably the main reason for its high yield (Fig. 2). Compared with NT-CK, warming treatments significantly reduced spikes per pot. This result might be mainly due to that the temperature directly affect the formation of tiller spikes or indirectly reduce tiller spikes formation by reducing suitable soil moisture and shortening the growth period (Tian et al. 2012) and accelerating the nutrient consumption of tiller nodes at night in the pre-growing stage (Ji et al. 2017). Because the number of spikes per pot is determined by tiller spikes and the main stems, the tiller spikes are more sensitive to the external environment than the main stems (Hu and Yin 2014). So the number of tiller spikes determines spikes per pot to a certain extent. For 1000-grain weight, no prominent effect was observed among all treatments.
Table 1
Yield components in wheats
|
Spikes per pot
|
Grain per spike
|
1000-grain weight (g)
|
NT
|
CK
|
89.64 ± 3.61b
|
30.67 ± 1.53bc
|
43.90 ± 0.40a
|
Cu
|
102.37 ± 7.21a
|
42.00 ± 2.65a
|
44.37 ± 0.50a
|
WT
|
CK
|
68.53 ± 3.00c
|
27.22 ± 2.34c
|
44.87 ± 0.31a
|
Cu
|
65.83 ± 1.89c
|
34.22 ± 1.69b
|
44.33 ± 0.21a
|
Sig.
|
W
|
***
|
***
|
ns
|
Cu
|
ns
|
***
|
ns
|
W×Cu
|
*
|
ns
|
ns
|
Note: D Data were analyzed by two-way ANOVA (W/WT, warming treatment; NT, no-warming treatment; CK, no-Cu application; Cu, Cu application; Sig., significant; ns, not significant; *, p < 0.05; **, p < 0.01; ***, p < 0.001). Different letters at the same growth stage represent a significant difference among treatments (p < 0.05). |
Table 2
Cu translocation factors in different tissues of wheats
|
Translocation factor
|
|
Seeding stage
|
Jointing stage
|
Booting stage
|
Maturation stage
|
Root to leaf
|
Root to stem
|
Stem to leaf
|
Root to stem
|
Stem to leaf
|
Stem to spike
|
Root to stem
|
Stem to leaf
|
Stem to glume
|
Stem to grain
|
NT
|
CK
|
0.02 ± 0.00c
|
0.16 ± 0.04a
|
2.24 ± 0.14a
|
0.11 ± 0.03b
|
3.96 ± 1.02a
|
4.25 ± 1.19a
|
0.19 ± 0.03a
|
0.90 ± 0.10b
|
0.47 ± 0.13b
|
1.42 ± 0.26a
|
Cu
|
0.08 ± 0.01b
|
0.05 ± 0.01c
|
2.20 ± 0.17a
|
0.10 ± 0.01b
|
1.19 ± 0.18b
|
1.52 ± 0.32b
|
0.10 ± 0.02b
|
1.83 ± 0.38a
|
1.07 ± 0.10a
|
1.45 ± 0.15a
|
WT
|
CK
|
0.11 ± 0.01a
|
0.11 ± 0.01b
|
2.00 ± 0.06a
|
0.17 ± 0.03a
|
2.37 ± 0.68b
|
1.66 ± 0.53b
|
0.12 ± 0.02b
|
0.73 ± 0.04b
|
1.57 ± 0.42a
|
1.27 ± 0.32ab
|
Cu
|
0.02 ± 0.01c
|
0.05 ± 0.00c
|
1.58 ± 0.32b
|
0.12 ± 0.01b
|
2.03 ± 0.29b
|
2.24 ± 0.51b
|
0.05 ± 0.01c
|
1.76 ± 0.41a
|
1.60 ± 0.37a
|
0.91 ± 0.12b
|
Note: Different letters at the same growth stage represent a significant difference among treatments (p < 0.05). |
Overall, Nighttime warming decreased spikes per pot and grains per spike, while soil Cu pollution enhanced spikes per pot and grains per spike. The highest/lowest spikes and grains per spike may be explained the high/low yield of NT-Cu/WT-CK.
Cu contents in different tissues in wheats
Figure 3 and Table S2 showed the heatmaps and accurate value of Cu concentrations in different tissues of wheats, respectively. Among all the tissues, the Cu contents in roots were the highest (30.85-176.66 mg/kg, Table S2), and it gradually decreased with the growth period, which may transfer to the tissues in the aboveground. Moreover, the Cu contents of roots were markedly higher in Cu applications than no-Cu applications, due to higher Cu applied in soil (Marschner 2012); for the treatments with the same amount of Cu application, the Cu concentrations of roots were significantly higher in warming applications than no-warming applications, except at seeding and booting stages. These findings could be explained by that the absorption of Cu by wheat roots and the transfer from root to aboveground were affected by temperature (Turner and Lahav 1985).
For leaf, no significant difference among treatments at jointing and booting stages. At other growth stages, NT-Cu, NT-CK had the highest and the lowest Cu contents of leaves, respectively; WT-Cu led to lower Cu contents of leaves than NT-Cu, indicating that temperature affected Cu contents of leaves (Marschner 2012). In the warming environment, an increase and reduce in Cu contents of leaves by WT-Cu was observed than that by WT-CK at reproductive stages and at vegetative stages, respectively. For stem, there was no remarkable variation among treatments at jointing stage. At booting and maturation stages, NT-Cu showed the highest Cu content of stems and prominent influence than other applications; no significant effect on Cu content of stems was observed between WT-Cu and WT-CK, which may be caused by the transmission of Cu to the reproductive tissues (Hu and Wu 2014; Marschner 2012).
From booting period, Cu applications could result in an enhancement in Cu content of spikes than no-Cu applications. For the treatments with the same amount of Cu application, Cu content of spikes was significantly increased under warming treatments than that under no-warming treatments and WT-Cu had the highest content. Regards with glume and grain at maturation stage, warming applications remarkedly increased Cu content of glume than no-warming applications, while warming applications significantly decreased Cu content of grain than no-warming applications. Among them, the Cu content in grains treated NT-Cu was highest (9.33 mg/kg). These results indicated that the transfer of Cu from the glume to the grain was restricted in the warming environment (Turner and Lahav 1985). However, it is worth noting that the Cu content in grains treated NT-Cu was close to the national standards threshold (10 mg/kg) of Cu content in foodstuff (GB 15199-94), which presented a potential risk to food security. For crops grown in Cu-contaminated farmland, this may be a very good signal to adapt to the warming environment.
Overall, warming treatments increased the Cu content in the roots and reduced the Cu content in the aboveground. In warming environment, the transfer of Cu from glumes to grains was limited under Cu application and then the Cu content of grains in WT-Cu application was significantly lower than other applications.
Cu accumulation in different tissues in wheats
The change of Cu accumulation in different tissues of wheats during its growth period is displayed in Fig. 4, which characterizes the ability of Cu transport and redistribution in tissues (Marschner 2012). By two-way ANOVA analysis, Cu accumulation in the whole plant were significantly influenced by two-factor interaction of Cu and warming and single factor of Cu or warming, except for single factor of Cu and two-factor interaction at booting stage and two-factor interaction at maturation stage. With the advancement of the growth period, Cu accumulation in underground and aboveground tissues of wheats showed a trend of continuous increase. During the growth period, the Cu accumulation of aboveground tissues and whole wheats were significantly lower in warming treatments than no-warming treatments, except booting stage, which are mainly due to the fact that the biomass and Cu content under the warming treatments were higher than those under the non-warming treatments (Fig. 2, Fig. 3 and Table S2). Cu applications led to a higher Cu accumulation of roots than no-Cu applications, and NT-Cu had the highest Cu accumulation, which are mainly due to the higher biomass and Cu content under the NT-Cu (Fig. 2, Fig. 3 and Table S2). Throughout the growth period of wheats, the Cu accumulation of roots were remarkedly lower in WT-Cu than that in NT-Cu, except booting stage. At booting stage, Cu accumulation of whole wheats were significantly higher by Cu applications than that by no-Cu applications, which mainly due to the higher biomass and Cu in wheats, especially in roots, stems and leaves (Fig. 2, Fig. 3 and Table S2). Regards to nighttime warming, warming treatments increased Cu accumulation of aboveground tissues than no-warming treatments, mainly contributed by that higher temperature stimulated the plant growth (Hu and Wu 2014; Marschner 2012) and caused the increasement of biomass of leaves and stems (Fig. 2). However, WT-Cu resulted in a significant decrease in Cu accumulation of roots than NT-Cu, indicating higher nighttime warming might promoted Cu accumulation shifts to aboveground at booting stage. At maturation stage, Cu accumulation of whole wheats were also significantly higher by Cu applications than that by no-Cu applications, while warming treatments led to a significant decrease in Cu accumulation of underground and aboveground tissues of wheats. At warming condition, through WT-Cu significantly decreased the Cu accumulation of whole wheats than that in WT-CK, Cu accumulation of grains were remarkedly lower in WT-Cu than that in WT-CK. These results mainly due to that Cu mainly accumulated in roots and leaves, and less transferred to grains. For the two Cu applications, WT-Cu significantly decreased the Cu accumulation of whole wheats than NT-Cu, mainly because that Cu accumulation of grains and stems were significantly lower in WT-Cu than that in NT-Cu. These outcomes revealed that nighttime warming could inhibit the Cu accumulation of wheats grown on Cu-contaminated soil, especially in grains.
Cu translocation factors in different tissues in wheats
Cu translocation factors between adjacent tissues of wheats are shown in Table 1, characterizing the transfer and redistribution ability of Cu between organizations (Marschner 2012). During all the sampling points, Cu translocation factors from roots to aboveground parts treated by no-warming treatments, warming treatments reached the highest value in the maturation stage and booting stage, respectively. These results also show that the absorption of Cu by aboveground parts could be promoted in the reproductive stages (Hu and Wu 2014; Marschner 2012). The Cu translocation factors were significantly lower in Cu applications than that of no-Cu applications, mainly due to the fact that more Cu was absorbed by the roots under Cu applications (Ghori et al. 2019; Marschner 2012). Moreover, WT-Cu didn’t lead to a significantly higher in this index than NT-Cu. These results indicated that nighttime warming did not promote the Cu transfer from root to aboveground parts in Cu-contaminated soil. In other words, Cu pollution in soil, to some extent, relatively reduced the risk of Cu pollution above ground under nighttime warming environment.
At jointing sampling point, WT-Cu had the lowest Cu translocation factors from stems to leaves and showed remarkedly influence than other applications; no significant effect among other treatments was observed. At booting sampling point, NT-CK resulted in a significantly higher Cu translocation factors from stems to leaves than other applications; no noteworthy effect among other treatments was found. NT-CK had the highest Cu translocation factors from stems to leaves during the jointing and booting stages. At maturation sampling point, Cu applications led to a significantly higher the Cu translocation factors than no-Cu applications, and there was no significant level between NT-CK and WT-CK, and between NT-Cu and WT-Cu. These outcomes revealed that nighttime warming did not promote the Cu translocation factors from stems to leaves in Cu-contaminated soil.
At booting sampling point, NT-CK led to a significantly higher Cu translocation factors from stem to spike than other applications; no noteworthy effect among other treatments was found. At maturation stage, NT-CK significantly reduced Cu translocation factors from stem to glume than other treatments; there was no noteworthy influence among other treatments. Warming treatments decreased Cu translocation factors from stem to grain than no-warming treatments. Among them, the Cu translocation factor was significantly lower in WT-Cu than that in NT-CK and NT-Cu. Previous studies have also shown that Cu migration between adjacent tissues was significantly affected by temperature (Pregitzer et al. 2010).
Overall, Cu applications limited the Cu translocation factors from underground to aboveground tissues than no-Cu applications. Nighttime warming significantly decreased Cu translocation factors from underground to aboveground tissues and from stems to grains in Cu-contaminated soil. That is, Cu pollution in soil, to some extent, relatively reduced the risk of Cu pollution aboveground in wheats under nighttime warming environment.