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Abstract [Objectives] This study was conducted to investigate the effect of paclobutrazol （PBZ） on heat tolerance of dahlia. [Methods] A dahlia variety Danbanhuang was selected as the experimental material in this study. After 100， 200， 300 or 400 mg/L paclobutrazol was sprayed evenly to the leaves of dahlia seedlings， they were cultured in an incubator at high temperature （35 ??/30 ??， day/night） for two days， and then transferred to an incubator at normal temperature （25 ??/20 ??， day/night）. The physiological and biochemical parameters of the dahlia seedlings were measured before exposure to high temperature （D0）， after exposure to high temperature for two days （D2） and exposure to normal temperature for one day （R1）. [Results] Compared to the negative control， foliar application of paclobutrazol decreased the content of MDA， increased the contents of chlorophyll， proline， soluble protein， and the activity of SOD， POD and CAT in dahlia plants under heat stress. The contents of MDA， soluble protein and proline in PBZ treated dahlia plants increased when they were subject to high temperature stress （35 ??/30 ??， day/night）， and then decreased when the temperature returned to normal （25 ??/20 ??， day/night）， and CAT activity decreased at high temperature， and then increased at normal temperature， while the activity of SOD and POD kept rising during the entire experimental period. [Conclusions] Foliar application of paclobutrazol can alleviate the adverse effect caused by high temperature to dahlia plants， and the experimental data provide a theoretical basis for the application of paclobutrazol in dahlia cultivation in South China.
Key words Dahlia； Heat stress； Paclobutrazol； Physiological response； Regulation
Dahlia （Dahlia pinnata Cav.） should be planted in a sunny， cool and well??ventilated environment， and its optimal growth temperature is 15-25 ??C[1]. However， the high summer temperature in South China has a significant adverse effect on the growth and development of dahlia[2]. Therefore， it is of great significance to explore the heat tolerance and regulatory mechanism of dahlia. Previous studies have shown that there are various exogenous substances that can be used to improve the heat resistance of plants. For example， 0.1 mmol/L salicylic acid （SA） improves the heat tolerance of Viola ?? writtrockiana and Celosia crassipes[3-4]； 0.5 mmol/L SA improves the heat tolerance of Lilium canadensis[5]. Zhang[2] reported that 0.5 mmol/L SA improves the heat tolerance of dahlia. Paclobutrazol （PBZ） is effective not only in controlling plant height[6]， but also in improving plant heat tolerance. For example， paclobutrazol at a suitable concentration can alleviate the damage caused by heat stress to wheat and ryegrass leaves[7-8]. However， the effects of paclobutrazol in regulating heat tolerance of dahlia have not been reported so far. Therefore， in this present study， the seedlings of a dahlia variety Danbanhuang were treated with different concentrations of paclobutrazol and subject to high temperature， and the changes in the physiological indices related to heat tolerance were measured， to evaluate the effect of paclobutrazol on heat tolerance of dahlia， and to provide a theoretical basis for its application in dahlia cultivation in South China. Materials and Methods
Materials
A dahlia variety Danbanhuang was used as the experimental material. Dahlia seedlings were grown from seeds in the field， and transplanted at seven??leaf stage to large pots of 15 cm?? 20 cm. Then， the healthy and similar??sized seedlings （about 20 cm in height） were selected and used in this experiment.
Methods
The selected potted dahlia plants were pre??cultured in an incubator with temperature at 25 ??/20 ??， a photoperiod of 12 h light/12 h dark （day/night）， and relative humidity of about 80% for 2 d. Then， 100 （Treatment PP1）， 200 （Treatment PP2）， 300 （Treatment PP3） or 400 mg/L （Treatment PP4） paclobutrazol was sprayed evenly over the surface of all leaves of each seedling till leaf surfaces were fully wet and the solution began to drip. Paclobutrazol was replaced by water in negative control （HT） and 0.5 mmol/L SA in positive control （SA）. There were three replicates for each treatment， and six seedlings in each replicate. Paclobutrazol， water or SA was sprayed twice a day （at morning and evening） for two days. Twelve hours after the last spraying， the dahlia seedlings were transferred to an incubator with temperature at 35 ??/30 ??， a photoperiod of 12 h light/12 h dark （day/night）， and relative humidity of about 80%. Two days later， the seedlings were transferred to the incubator with temperature at 25 ??/20 ?? （day/night）. The physiological and biochemical parameters of the dahlia seedlings were measured before exposure to high temperature （D0）， after exposure to high temperature for two days （D2） and exposure to normal temperature for one day （R1）.
Measurement of physiological and biochemical indices
The physiological and biochemical indices related to heat injury and heat tolerance of dahlia were determined. Among them， the content of chlorophyll was determined using acetone??acetone mixture[9]， the content of malondialdehyde （MDA） was quantified using thiobarbituric acid method[10]， the content of soluble protein content was assayed using Coomassie Brilliant Blue G??250[10]， and the content of proline content was measured using ninhydrin method[10]. The crude enzyme solutions of superoxide dismutase （SOD）， peroxidase （POD） and catalase （CAT） were prepared by referring to Shi??s method[11]， SOD activity was determined using nitro blue tetrazolium （NBT）， POD activity was measured based on the determination of guaiacol oxidation and CAT activity was measured spectrophotometrically according to Li[10]. Data analysis
Data were sorted and histograms were plotted in Excel. The differences between means were evaluated using least significant difference （LSD） test in SPSS 20.0.
Results and Analysis
Effects of paclobutrazol on heat injury of dahlia under heat stress
According to wilting index， the six treatments were SA< PP1< PP2< PP3< HT < PP4. Compared with the control HT， the wilting index of Treatment PP4 （400 mg/L paclobutrazol） was increased by 10.29%， while that of the other four treatments was decreased by 44.94%， 41.85%， 35.68% and 12.18%， respectively， suggesting that paclobutrazol could alleviate the adverse effects caused by high temperature on dahlia plants to a certain extent， and the effects of 100-200 mg/L paclobutrazol were better.
Effects of paclobutrazol on chlorophyll content of dahlia under heat stress
After two days of exposure to high temperature at 35 ??/30 ?? （day/night）， the symptoms of heat injury appeared： the leaves of dahlia began to turn brown and wilt. But foliar application of paclobutrazol at appropriate concentrations alleviated such symptoms of dahlia under heat stress， and the partially wilted leaves returned to normal one day after the seedlings were transferred to normal temperature condition. However， high concentrations of paclobutrazol aggravated the heat damage to dahlia. Under heat stress， significant difference in chlorophyll content among the six treatments was noted （Fig.1）， and paclobutrazol and SA treatments all had higher chlorophyll content than control （P
Effects of paclobutrazol on MDA content of dahlia under heat stress
Plant stress is associated with cell membrane lipid peroxidation， and malondialdehyde （MDA） is a product of lipid peroxidation[7]. Our results showed that after two days of exposure to high temperature （Fig. 2）， no significant difference was observed in the MDA content among all the treatments except Treatment SA. However， when the temperature returned to normal， the MDA content of paclobutrazol treatments dropped nearly to the level before exposure to high temperature， and was significantly lower than that of control HT. Among all paclobutrazol treatments1， PP1 and PP2 （100-200 mg/L paclobutrazol） treatment were better， showing similar effect to Treatment SA.
Effects of paclobutrazol on proline content in dahlia under heat stress
Proline is an important osmolyte in plants. So， the effects of paclobutrazol on proline content of dahlia under heat stress were evaluated in this study. As shown in Fig. 3， the proline content of all paclobutrazol and SA treatment increased under heat stress， and decreased when the temperature returned to normal， and was always significantly lower than that of the control HT. The results showed that foliar application of paclobutrazol can enhance the thermal stability of dahlia cells， so that the proline content changes little before and after exposure to high temperature. Effects of paclobutrazol on soluble protein content of dahlia under heat stress
Soluble protein is also an osmolyte associated with plant stress response[8]. The change in soluble protein content of dahlia leaves is shown in Fig. 4. There was significant difference in soluble protein content among the six treatments at both D2 and R1， and all paclobutrazol and SA treatments had higher soluble protein content than the control HT， suggesting that foliar application of paclobutrazol can increase the accumulation of osmotic adjustment??related proteins to resist heat injury， and reduce the accumulation when heat stress is eliminated.
Chuanle ZHAO et al. Effects of Paclobutrazol on Physiological Parameters of Dahlia under Heat StressEffects of paclobutrazol on SOD activity of dahlia under heat stress
Reactive oxygen species （ROS） such as O-2?¤， H2O2， OH-?¤ increases in plants under high temperature stress， and excessive free radicals will damage cells[5]. The antioxidant enzymes such as SOD， POD and CAT in plants can effectively remove excessive reactive oxygen. As shown in Fig. 5， there was significant difference in SOD activity among all the six treatments at both D2 and R1， and the SOD activity of all the paclobutrazol treatments was higher than that of the control HT and Treatment SA， which explains why paclobutrazol can enhance the heat resistance of dahlia.
Effects of paclobutrazol on POD activity of dahlia under heat stress
As shown in Fig. 6， there was significant difference in POD activity between treatments SA and PP2 at D2 and R1， but no significant difference among other treatments. The POD activity of all paclobutrazol and SA treatments was higher than that of the control HT. Among them， the POD activity of PP1 and PP2 treatments （100-200 mg/L paclobutrazol） was the highest， which was even higher than that of Treatment SA. POD activity in these treatments changed in a similar pattern to SOD activity.
Effects of paclobutrazol on CAT activity of dahlia under heat stress
As shown in Fig. 7， after two days of exposure to high temperature， the CAT activity of all paclobutrazol treatments （except PP1） was significantly higher than that of the control HT. When the temperature returned to normal， the CAT activity among all paclobutrazol treatments showed no significant difference， but was significantly higher than that of the control HT， and close to that of Treatment SA. The results proved that 100 to 400 mg/L paclobutrazol can improve the CAT activity of dahlia under heat stress. Discussion
Temperature is an important factor influencing plant growth. The physiological metabolism of plants will be suppressed when the air temperature exceeds a certain point， causing heat injury or even death of plants[12]. Paclobutrazol， as a plant growth retardant， can be used not only for controlling plant height[6]， but also for regulating plant stress response[7-8]. However， the application of paclobutrazol in dahlia has been rarely reported. Treatment with paclobutrazol at a suitable concentration can alleviate the symptoms caused by heat injury in dahlia， and greatly reduces chlorophyll degradation， which is conducive to plant photosynthesis.
The heat tolerance??related physiological indices of dahlia treated with paclobutrazol changed in different patterns. Among them， the contents of chlorophyll， MDA， soluble protein and proline increased after exposure to high temperature for two days and then decreased after exposure to normal temperature for one day. The activity of SOD and POD kept rising during the whole experimental period， while the activity of CAT decreased after exposure to high temperature for two day and then increased after exposure to normal temperature for one day. As a product of membrane lipid peroxidation， the content of MDA can reflect the extent of membrane damage caused by lipid peroxidation[13]. Our results showed that the MDA content in paclobutrazol??treated dahlia plants was decreased significantly， compared with that of the negative control， indicating that paclobutrazol alleviated membrane lipid peroxidation of dahlia leaf cells caused by heat stress. Moreover， SOD， POD and CAT are important antioxidant enzymes in plants， which play an important role in plant defense and stress response[5]. Our data showed that the activity of SOD， POD and CAT was significantly increased by foliar application of paclobutrazol， which is beneficial to the scavenging of reactive oxygen species， thus reducing the damage caused by heat stress to dahlia plants. According to the changes of leaf morphology and physiological and biochemical indices， it can be concluded that foliar application of 100 to 200 mg/L paclobutrazol can significantly improve the heat tolerance of dahlia， and thus can be used for the cultivation of dahlia in South China.
References
[1] LIU XY. How to manage potted dahlia[J]. Auhui Agricultural Science Bulletin， 2007， 13（8）： 148-148.
[2] ZHANG PP. Basic study on mechanisms of thermotolerance and chemical regulation in dahlia[D]. Suzhou： Soochow University， 2016. [3] DU XH， LIU HC， CUI XN， et al. Effects of salicylic acid on heat resistance of Viola ?? writtrockiana seedlings[J]. Guihaia， 2016， 36（6）： 728-734.
[4] LI YH， WEI YX， GU M. Physiological effect of salicylic acid on heat stress of Celosia cristata[J]. Acta Botanica Boreali??Occidentalia Sinica， 2008， 28（11）： 2257-2262.
[5] CHEN QM， YIN H， LI XY， et al. Effects of salicylic acid on the activities of antioxidant systems in lily plants under high temperature stress[J]. Journal of China Agricultural University， 2008， 13（2）： 44-48.
[6] JIA HT， DANG JD， LIU FL. Physiological function mechanism and application of multi??effect triazole[J]. Journal of Anhui Agricultural Sciences， 2003， 31（2）： 323-324.
[7] LIAN HC， XU ZC， DONG XC， et al. Effect of placlobutrazol on wheat seedling resistance to high temperature[J]. Journal of Shandong Agricultural University， 1994， （2）： 233-235.
[8] MA BY， XU LG， JIN SH， et al. Effects of paclobutrazol on thermotolerance of perennial ryegrass[J]. Acta Horticulturae Sinica， 2005， 32（6）： 1118-1120.
[9] ZHANG XZ. Determination of plant chlorophyll content with acetone??ethanol mixture[J]. Liaoning Agricultural Sciences， 1986， （3）： 28-30.
[10] LI HS. Experimental theory and technology of botanic physiology and biochemistry[M]. Beijing： Higher Education Press， 2000：134-261.
[11] SHI LH， MOU ZM， YAO J， et al. Cell Membrane damage and change of protective enzymes?? activity in different mulberry varieties under soil water stress[J]. Acta Sericologica Sinica， 2005， 31（1）： 13-17.
[12] ZHANG ZS， LI RQ， WANG JB. Effects of Ca2+ pretreatment on plasmalemma permeability， GSH and ASA contents， and calcium distribution in pepper mesophyll cells under heat stress[J]. Acta Phytoecologica， 2001， 25（2）： 230-234.
[13] LI G， WAN S， ZHOU J， et al. Leaf chlorophyll fluorescence， hyperspectral reflectance， pigments content， malondialdehyde and proline accumulation responses of castor bean （Ricinus communis L.） seedlings to salt stress levels[J]. Industrial Crops & Products， 2010， 31（1）：13-19.
[14] MA ZH， JIA RZ. Effects of PP333 on growth and physiology of carnation[J]. Journal of Shenyang Agricultural University， 2000， 31（6）： 570-571.
Key words Dahlia； Heat stress； Paclobutrazol； Physiological response； Regulation
Dahlia （Dahlia pinnata Cav.） should be planted in a sunny， cool and well??ventilated environment， and its optimal growth temperature is 15-25 ??C[1]. However， the high summer temperature in South China has a significant adverse effect on the growth and development of dahlia[2]. Therefore， it is of great significance to explore the heat tolerance and regulatory mechanism of dahlia. Previous studies have shown that there are various exogenous substances that can be used to improve the heat resistance of plants. For example， 0.1 mmol/L salicylic acid （SA） improves the heat tolerance of Viola ?? writtrockiana and Celosia crassipes[3-4]； 0.5 mmol/L SA improves the heat tolerance of Lilium canadensis[5]. Zhang[2] reported that 0.5 mmol/L SA improves the heat tolerance of dahlia. Paclobutrazol （PBZ） is effective not only in controlling plant height[6]， but also in improving plant heat tolerance. For example， paclobutrazol at a suitable concentration can alleviate the damage caused by heat stress to wheat and ryegrass leaves[7-8]. However， the effects of paclobutrazol in regulating heat tolerance of dahlia have not been reported so far. Therefore， in this present study， the seedlings of a dahlia variety Danbanhuang were treated with different concentrations of paclobutrazol and subject to high temperature， and the changes in the physiological indices related to heat tolerance were measured， to evaluate the effect of paclobutrazol on heat tolerance of dahlia， and to provide a theoretical basis for its application in dahlia cultivation in South China. Materials and Methods
Materials
A dahlia variety Danbanhuang was used as the experimental material. Dahlia seedlings were grown from seeds in the field， and transplanted at seven??leaf stage to large pots of 15 cm?? 20 cm. Then， the healthy and similar??sized seedlings （about 20 cm in height） were selected and used in this experiment.
Methods
The selected potted dahlia plants were pre??cultured in an incubator with temperature at 25 ??/20 ??， a photoperiod of 12 h light/12 h dark （day/night）， and relative humidity of about 80% for 2 d. Then， 100 （Treatment PP1）， 200 （Treatment PP2）， 300 （Treatment PP3） or 400 mg/L （Treatment PP4） paclobutrazol was sprayed evenly over the surface of all leaves of each seedling till leaf surfaces were fully wet and the solution began to drip. Paclobutrazol was replaced by water in negative control （HT） and 0.5 mmol/L SA in positive control （SA）. There were three replicates for each treatment， and six seedlings in each replicate. Paclobutrazol， water or SA was sprayed twice a day （at morning and evening） for two days. Twelve hours after the last spraying， the dahlia seedlings were transferred to an incubator with temperature at 35 ??/30 ??， a photoperiod of 12 h light/12 h dark （day/night）， and relative humidity of about 80%. Two days later， the seedlings were transferred to the incubator with temperature at 25 ??/20 ?? （day/night）. The physiological and biochemical parameters of the dahlia seedlings were measured before exposure to high temperature （D0）， after exposure to high temperature for two days （D2） and exposure to normal temperature for one day （R1）.
Measurement of physiological and biochemical indices
The physiological and biochemical indices related to heat injury and heat tolerance of dahlia were determined. Among them， the content of chlorophyll was determined using acetone??acetone mixture[9]， the content of malondialdehyde （MDA） was quantified using thiobarbituric acid method[10]， the content of soluble protein content was assayed using Coomassie Brilliant Blue G??250[10]， and the content of proline content was measured using ninhydrin method[10]. The crude enzyme solutions of superoxide dismutase （SOD）， peroxidase （POD） and catalase （CAT） were prepared by referring to Shi??s method[11]， SOD activity was determined using nitro blue tetrazolium （NBT）， POD activity was measured based on the determination of guaiacol oxidation and CAT activity was measured spectrophotometrically according to Li[10]. Data analysis
Data were sorted and histograms were plotted in Excel. The differences between means were evaluated using least significant difference （LSD） test in SPSS 20.0.
Results and Analysis
Effects of paclobutrazol on heat injury of dahlia under heat stress
According to wilting index， the six treatments were SA< PP1< PP2< PP3< HT < PP4. Compared with the control HT， the wilting index of Treatment PP4 （400 mg/L paclobutrazol） was increased by 10.29%， while that of the other four treatments was decreased by 44.94%， 41.85%， 35.68% and 12.18%， respectively， suggesting that paclobutrazol could alleviate the adverse effects caused by high temperature on dahlia plants to a certain extent， and the effects of 100-200 mg/L paclobutrazol were better.
Effects of paclobutrazol on chlorophyll content of dahlia under heat stress
After two days of exposure to high temperature at 35 ??/30 ?? （day/night）， the symptoms of heat injury appeared： the leaves of dahlia began to turn brown and wilt. But foliar application of paclobutrazol at appropriate concentrations alleviated such symptoms of dahlia under heat stress， and the partially wilted leaves returned to normal one day after the seedlings were transferred to normal temperature condition. However， high concentrations of paclobutrazol aggravated the heat damage to dahlia. Under heat stress， significant difference in chlorophyll content among the six treatments was noted （Fig.1）， and paclobutrazol and SA treatments all had higher chlorophyll content than control （P
Effects of paclobutrazol on MDA content of dahlia under heat stress
Plant stress is associated with cell membrane lipid peroxidation， and malondialdehyde （MDA） is a product of lipid peroxidation[7]. Our results showed that after two days of exposure to high temperature （Fig. 2）， no significant difference was observed in the MDA content among all the treatments except Treatment SA. However， when the temperature returned to normal， the MDA content of paclobutrazol treatments dropped nearly to the level before exposure to high temperature， and was significantly lower than that of control HT. Among all paclobutrazol treatments1， PP1 and PP2 （100-200 mg/L paclobutrazol） treatment were better， showing similar effect to Treatment SA.
Effects of paclobutrazol on proline content in dahlia under heat stress
Proline is an important osmolyte in plants. So， the effects of paclobutrazol on proline content of dahlia under heat stress were evaluated in this study. As shown in Fig. 3， the proline content of all paclobutrazol and SA treatment increased under heat stress， and decreased when the temperature returned to normal， and was always significantly lower than that of the control HT. The results showed that foliar application of paclobutrazol can enhance the thermal stability of dahlia cells， so that the proline content changes little before and after exposure to high temperature. Effects of paclobutrazol on soluble protein content of dahlia under heat stress
Soluble protein is also an osmolyte associated with plant stress response[8]. The change in soluble protein content of dahlia leaves is shown in Fig. 4. There was significant difference in soluble protein content among the six treatments at both D2 and R1， and all paclobutrazol and SA treatments had higher soluble protein content than the control HT， suggesting that foliar application of paclobutrazol can increase the accumulation of osmotic adjustment??related proteins to resist heat injury， and reduce the accumulation when heat stress is eliminated.
Chuanle ZHAO et al. Effects of Paclobutrazol on Physiological Parameters of Dahlia under Heat StressEffects of paclobutrazol on SOD activity of dahlia under heat stress
Reactive oxygen species （ROS） such as O-2?¤， H2O2， OH-?¤ increases in plants under high temperature stress， and excessive free radicals will damage cells[5]. The antioxidant enzymes such as SOD， POD and CAT in plants can effectively remove excessive reactive oxygen. As shown in Fig. 5， there was significant difference in SOD activity among all the six treatments at both D2 and R1， and the SOD activity of all the paclobutrazol treatments was higher than that of the control HT and Treatment SA， which explains why paclobutrazol can enhance the heat resistance of dahlia.
Effects of paclobutrazol on POD activity of dahlia under heat stress
As shown in Fig. 6， there was significant difference in POD activity between treatments SA and PP2 at D2 and R1， but no significant difference among other treatments. The POD activity of all paclobutrazol and SA treatments was higher than that of the control HT. Among them， the POD activity of PP1 and PP2 treatments （100-200 mg/L paclobutrazol） was the highest， which was even higher than that of Treatment SA. POD activity in these treatments changed in a similar pattern to SOD activity.
Effects of paclobutrazol on CAT activity of dahlia under heat stress
As shown in Fig. 7， after two days of exposure to high temperature， the CAT activity of all paclobutrazol treatments （except PP1） was significantly higher than that of the control HT. When the temperature returned to normal， the CAT activity among all paclobutrazol treatments showed no significant difference， but was significantly higher than that of the control HT， and close to that of Treatment SA. The results proved that 100 to 400 mg/L paclobutrazol can improve the CAT activity of dahlia under heat stress. Discussion
Temperature is an important factor influencing plant growth. The physiological metabolism of plants will be suppressed when the air temperature exceeds a certain point， causing heat injury or even death of plants[12]. Paclobutrazol， as a plant growth retardant， can be used not only for controlling plant height[6]， but also for regulating plant stress response[7-8]. However， the application of paclobutrazol in dahlia has been rarely reported. Treatment with paclobutrazol at a suitable concentration can alleviate the symptoms caused by heat injury in dahlia， and greatly reduces chlorophyll degradation， which is conducive to plant photosynthesis.
The heat tolerance??related physiological indices of dahlia treated with paclobutrazol changed in different patterns. Among them， the contents of chlorophyll， MDA， soluble protein and proline increased after exposure to high temperature for two days and then decreased after exposure to normal temperature for one day. The activity of SOD and POD kept rising during the whole experimental period， while the activity of CAT decreased after exposure to high temperature for two day and then increased after exposure to normal temperature for one day. As a product of membrane lipid peroxidation， the content of MDA can reflect the extent of membrane damage caused by lipid peroxidation[13]. Our results showed that the MDA content in paclobutrazol??treated dahlia plants was decreased significantly， compared with that of the negative control， indicating that paclobutrazol alleviated membrane lipid peroxidation of dahlia leaf cells caused by heat stress. Moreover， SOD， POD and CAT are important antioxidant enzymes in plants， which play an important role in plant defense and stress response[5]. Our data showed that the activity of SOD， POD and CAT was significantly increased by foliar application of paclobutrazol， which is beneficial to the scavenging of reactive oxygen species， thus reducing the damage caused by heat stress to dahlia plants. According to the changes of leaf morphology and physiological and biochemical indices， it can be concluded that foliar application of 100 to 200 mg/L paclobutrazol can significantly improve the heat tolerance of dahlia， and thus can be used for the cultivation of dahlia in South China.
References
[1] LIU XY. How to manage potted dahlia[J]. Auhui Agricultural Science Bulletin， 2007， 13（8）： 148-148.
[2] ZHANG PP. Basic study on mechanisms of thermotolerance and chemical regulation in dahlia[D]. Suzhou： Soochow University， 2016. [3] DU XH， LIU HC， CUI XN， et al. Effects of salicylic acid on heat resistance of Viola ?? writtrockiana seedlings[J]. Guihaia， 2016， 36（6）： 728-734.
[4] LI YH， WEI YX， GU M. Physiological effect of salicylic acid on heat stress of Celosia cristata[J]. Acta Botanica Boreali??Occidentalia Sinica， 2008， 28（11）： 2257-2262.
[5] CHEN QM， YIN H， LI XY， et al. Effects of salicylic acid on the activities of antioxidant systems in lily plants under high temperature stress[J]. Journal of China Agricultural University， 2008， 13（2）： 44-48.
[6] JIA HT， DANG JD， LIU FL. Physiological function mechanism and application of multi??effect triazole[J]. Journal of Anhui Agricultural Sciences， 2003， 31（2）： 323-324.
[7] LIAN HC， XU ZC， DONG XC， et al. Effect of placlobutrazol on wheat seedling resistance to high temperature[J]. Journal of Shandong Agricultural University， 1994， （2）： 233-235.
[8] MA BY， XU LG， JIN SH， et al. Effects of paclobutrazol on thermotolerance of perennial ryegrass[J]. Acta Horticulturae Sinica， 2005， 32（6）： 1118-1120.
[9] ZHANG XZ. Determination of plant chlorophyll content with acetone??ethanol mixture[J]. Liaoning Agricultural Sciences， 1986， （3）： 28-30.
[10] LI HS. Experimental theory and technology of botanic physiology and biochemistry[M]. Beijing： Higher Education Press， 2000：134-261.
[11] SHI LH， MOU ZM， YAO J， et al. Cell Membrane damage and change of protective enzymes?? activity in different mulberry varieties under soil water stress[J]. Acta Sericologica Sinica， 2005， 31（1）： 13-17.
[12] ZHANG ZS， LI RQ， WANG JB. Effects of Ca2+ pretreatment on plasmalemma permeability， GSH and ASA contents， and calcium distribution in pepper mesophyll cells under heat stress[J]. Acta Phytoecologica， 2001， 25（2）： 230-234.
[13] LI G， WAN S， ZHOU J， et al. Leaf chlorophyll fluorescence， hyperspectral reflectance， pigments content， malondialdehyde and proline accumulation responses of castor bean （Ricinus communis L.） seedlings to salt stress levels[J]. Industrial Crops & Products， 2010， 31（1）：13-19.
[14] MA ZH， JIA RZ. Effects of PP333 on growth and physiology of carnation[J]. Journal of Shenyang Agricultural University， 2000， 31（6）： 570-571.