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Abstract ［Objectives］ This study was conducted to explore the physiological mechanism of uniconazole in response to drought stress in Dahlia pinnata.
［Methods］With potted seedlings of D. pinnata variety ‘Danbanhuang’ as an experimental material, the effects of different concentrations (10, 20, 30 and 50 mg/L) of uniconazole on the photosynthetic characteristics of D. pinnata under drought stress and rewatering conditions were discussed.
［Results］ With the extension of drought time, the chlorophyll SPAD value, net photosynthetic rate, transpiration rate, and stomatal conductance of each treatment group decreased significantly, while the intercellular CO2 concentration showed a trend of first decreasing and then increasing; and after pretreatment of uniconazole spraying on leaves, it could alleviate the inhibition of drought stress on the photosynthesis of D. pinnata, and was beneficial to the recovery of photosynthetic capacity of D. pinnata after rewatering, and the effect of leaf application concentration of 30 mg/L was the best. It showed that exogenous uniconazole helps to improve the drought resistance of D. pinnata.
［Conclusions］This study provides a new experimental basis for the chemical regulation of stress resistance in D. pinnata and its production and application.
Key words Dahlia pinnata Cav.; Uniconazole; Drought stress; Photosynthetic characteristic
Received: June 19, 2021 Accepted: August 21, 2021
Supported by Applied Basic Research Program of Suzhou City (SYN201405).
Fangying XU (1997-), female, P. R. China, master, devoted to research about garden plant resources and application.
*Corresponding author. E-mail: [email protected].
Dahlia pinnata Cav. is a perennial bulbous flower belonging to Dahlia of Compositae. It has a wide variety of cultivars, rich colors, beautiful flower types, high ornamental value, and has high landscaping application value［1］. However, D. pinnata is very sensitive to soil moisture and not tolerant to drought and flood. It is often affected by drought stress in some cultivation areas in China. As a multi-dimensional abiotic stress, drought can cause a series of changes in plant phenotype, physiology, biochemistry, and molecular level. In severe cases, it can terminate photosynthesis, disorder metabolism, and ultimately cause plant death［2］. Therefore, studying the effects of chemical substances on the photosynthetic characteristics of D. pinnata can provide an important physiological index to judge their drought resistance regulation effect. The application of plant growth regulators is one of the important measures to alleviate plant abiotic stress［3］. Studies have shown that uniconazole not only has significant effects in regulating the growth and development of plants, but also has the effects of promoting root growth and enhancing stress resistance. For example, exogenous uniconazole can effectively improve the drought resistance in ornamental plants such as Trollius chinensis Bunge［4］, Wufeng Magnolia liliflora Desr.［5］ and Zinnia elegans Jacq.［6］, and reduce the adverse reactions caused by drought stress. In this study, the effects of uniconazole on the photosynthetic characteristics of D. pinnata under drought stress was investigated by the method of foliar spraying, so as to provide new experimental basis for the chemical regulation of stress resistance in D. pinnata and its production and application.
Materials and Methods
Experimental materials
The tested D. pinnata variety was ‘Danbanhuang’. Seedlings were raised in plug trays to the stage of six leaves and one core, and then transplanted into 15 cm×20 cm plastic pots. They were routinely conventionally cultivated and managed. Plants with healthy and similar growth were selected for the experiment.
Experimental methods
The concentration of uniconazole was set to 0, 10, 20, 30, 50 mg/L (denoted as S0, S1, S2, S3, S4, respectively). The solutions were sprayed to the leaf surface once every day at 9 oclock in the morning until water dripped from leaves. Each experimental treatment group had 12 pots, and after continuous spraying for 2 d, the drought stress treatment was carried out. The relative soil water content of the drought stress groups (S0-S4) and the normal control group (CK) were adjusted to 18% and 35%, respectively. During the stress period, the soil moisture content was measured every day around 18:00 by the weighing method combined with the TSZ-1 soil moisture rapid measuring instrument, and water was added in time to keep the relative soil moisture content within the set range.
Measurement indexes and methods
The SPAD value and photosynthetic indexes were measured at 0, 5, 10, and 15 d after drought stress. After 15 d of stress treatment, rewatering treatment was performed to restore the relative soil water content of the drought stress groups to about 75% of that of the control group, and the measurement was repeated after 5 d of rewatering (R5).
Determination of SPAD value Five D. pinnata plants with similar growth were selected from each group, and nine mature leaves were selected from each plant and measured for the SPAD value with an SPAD-502 chlorophyll meter.
Determination of photosynthetic indexes
Mature leaves of plants with relatively consistent growth were selected and determined with a Li-6400XT portable photosynthesis system for net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci) and transpiration rate (Tr) of the leaves. The lamination intensity from external sources in the culture environment was used for the measurement. The flow rate of the sample chamber was set to 500 μmol/s, and the atmospheric carbon dioxide concentration was about 400 μl/L. The measurement time was from 9:00 to 11:00 on a sunny and windless morning. Each group was measured repeatedly for 9 times.
Data analysis
EXCEL2013 was used for data statistics and icon drawing, and the LSD method of SPSS software was used for analysis on significance of difference.
Results and Analysis
Effects of exogenous uniconazole on SPAD value of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on the relative chlorophyll content (SPAD value) of D. pinnata are shown in Fig. 1. After 10 d of stress, the SPAD values of various stress treatment groups were significantly lower than that of the CK; and to 15 d of stress, they were 22.70%, 20.75%, 15.37%, 14.24%, and 20.55% lower than the CK, respectively. Among them, treatment groups S2 (20 mg/L) and S3 (30 mg/L) were significantly higher than S0 (spraying water). After rewatering, the SPAD value of each treatment group increased. Among them, the content of treatment group S3 was the highest, but was still significantly lower than the CK, and did not return to the level before stress. It showed that exogenous uniconazole could reduce the damage of drought stress to chloroplast structure to a certain extent and maintain relative stability of chlorophyll content, and the treatment with a uniconazole concentration of 30 mg/L had the best effect.
Effects of exogenous uniconazole on stomatal conductance of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on stomatal conductance (Gs) of D. pinnata are shown in Fig. 2. After the drought stress began, the Gs of D. pinnata continued to decrease. By the 15th day of stress, the Gs values of various treatment groups decreased by 61.25%, 57.71%, 43.63%, 45.36% and 56.98% respectively compared with the CK. After rewatering, the Gs values of various treatment groups increased, but were still significantly lower than the CK, and did not recover to the level before stress. However, the recovery effects of the uniconazole treatment groups were better than that of S0 (spraying with clear water), indicating that exogenous uniconazole could effectively slow down the decrease of D. pinnata stomatal opening under drought stress, and the application concentration of 30 mg/L was the best. Effects of exogenous uniconazole on the intercellular carbon dioxide concentration of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on the intercellular carbon dioxide concentration (Ci) of D. pinnata are shown in Fig. 3. During the drought stress period, the Ci of D. pinnata decreased first and then increased. By the 10th d of stress, the Ci values of various treatment groups decreased by 34.80%, 30.32%, 28.03%, 24.20% and 29.26% respectively compared with the CK in the same period. After rewatering, Ci increased in each treatment group, but still significantly lower than the CK, indicating that uniconazole pretreatment was beneficial to the accumulation of intracellular CO2 under drought stress conditions, and the application concentrations of 20 and 30 mg/L achieved a better effect.
Effects of exogenous uniconazole on intercellular transpiration rate of D. pinnata under drought stress
Under drought stress conditions, the transpiration rate (Tr) of D. pinnata leaves showed a continuous decreasing trend, and the values of various treatment groups were significantly lower than the CK. At 15 d of stress, the Tr values were 56.02%, 53.73%, 51.19%, 44.18% and 49.29% lower than the CK in the same period, respectively, and Tr of treatment S3 was significantly higher than those of other groups. After rewatering, Tr increased in each treatment group, and S3 was the highest among the treatment groups (Fig. 4), indicating that uniconazole pretreatment had a certain encouraging effect on the transpiration of D. pinnata under drought stress, and the application concentrations of 20 and 30 mg/L uniconazole achieved a better effect.
Agricultural Biotechnology2021
Effects of exogenous uniconazole on net photosynthetic rate of D. pinnata under drought stress
Under drought stress conditions, the net photosynthetic rate (Pn) of D. pinnata also showed a continuous downward trend, and the values of all treatment groups sprayed with uniconazole were significantly lower than the CK (Fig. 5). At 15 d of stress, the Pn values of various treatment groups decreased by 65.56%, 63.01%, 53.42%, 51.65%, and 60.69% respectively compared with the CK in the same period, and the decreases of S2 and
S3 among the treatment groups were relatively small, and were significantly higher than S0. After rewatering, Pn in each test group increased, and treatment groups S2 and S3 were significantly higher than S0, but still significantly lower than the CK, and did not return to the level before stress. It showed that drought stress seriously affected the photosynthesis of D. pinnata, while exogenous uniconazole could alleviate the inhibition of photosynthesis of D. pinnata by drought stress, and the appropriate concentration ranged from 20 to 30 mg/L. Conclusions and Discussion
Photosynthesis is the basis for the survival of plants, and it is more sensitive to changes in plant water. Under water shortage conditions, the stomata opening of plant leaves is reduced or even closed completely, resulting in a reduction in the absorption of carbon dioxide, thereby inhibiting the progress of photosynthesis. Meanwhile, it also reduces the content of chlorophyll, destroys the structure of thylakoid membrane, inhibits electron transfer and photosynthetic phosphorylation, and leads to a decline in the photosynthetic rate. Therefore, changes in photosynthetic physiological characteristics under drought stress conditions can reflect the drought resistance of plants［7］.
Previous studies have shown that exogenous uniconazole could increase the chlorophyll content in leaves of Lilium oriental hybrids. After being treated with 80 mg/L uniconazole at 14 weeks of planting, the chlorophyll b content increased to 2.26 times that of the control group［8］. In Petunia hybrida (J. D. Hooker) Vilmorin, uniconazole can increase leaf water content and chlorophyll content under drought stress, improve leaf structure, alleviate the inhibition of drought stress on photosynthesis, and help restore leaf water content and photosynthesis after rewatering［9］. In this study, under drought stress and rewatering conditions, the chlorophyll content and net photosynthetic rate after exogenous uniconazole treatment were higher than those in the clear water control, and the 30 mg/L treatment group was the most significant. It is because exogenous uniconazole slows down the damage of drought stress to chloroplasts and delays the degradation of chlorophyll. Meanwhile, uniconazole can increase chlorophyll content and photosynthetic efficiency by regulating endogenous hormones and key enzymes in the process of chlorophyll synthesis［10-11］.
Studies have found that the inhibition of leaf photosynthesis under drought stress is the result of the combined effect of stomata and non-stomata factors. Under mild or moderate drought stress, stomatal factors dominate; and under severe stress, non-stomatal factors caused by damage to photosynthetic organs play a leading role［12］. The results of this study showed that exogenous uniconazole effectively alleviated the decline of Pn, Gs and Tr in D. pinnata leaves under drought stress, but the effect on Ci differed depending on the stress time. In the early stage of drought stress, uniconazole pretreatment could alleviate the decrease of Ci, and in the later stage, it could inhibit the increase of Ci, indicating the decrease of Pn in D. pinnata leaves in the early stage of drought stress was the result of partial closure of stomata caused by water deficit, while in the late stage of drought stress, non-stomata factors gradually occupied the main reason, but the pretreatment with uniconazole could increase the utilization of intracellular carbon dioxide. Therefore, its photosynthetic rate was significantly higher than that of the control group, which is consistent with the results of Zhang et al.［13］. In summary, uniconazole spraying on leaves could effectively alleviate the effects of drought stress on photosynthesis of D. pinnata, and simultaneously help restore photosynthetic capacity after rewatering, and could improve the drought resistance of D. pinnata within a certain range, and the leaf application concentration in the range of 20-30 mg/L was appropriate.
References
［1］ YAN GX. Overview and enlightenment of Dahlia industry development［J］. Anhui Agricultural Science Bulletin, 2020, 26(10): 38-39. (in Chinese)
［2］ LAI JL, LI XX, XUE L, et al. Research progress of plant drought resistance［J］. Jiangsu Agricultural Sciences, 2018, 46(17): 23-27. (in Chinese)
［3］ GU DY, WANG XF, YANG FJ, et al. Research progress of exogenous substances on horticultural crops resistance［J］. Northern Horticulture, 2016(3): 195-198. (in Chinese)
［4］ ZHANG YY, LIU JX, XU TT, et al. Effects of exogenous hormones on the physiological characteristics of Trollius chinensis Bunge Seedlings under drought stress［J］. Bulletin of Agricultural Science and Technology, 2020(1): 88-90. (in Chinese)
［5］ SHI XD, CHEN SY, JIA ZK. The Dwarfing effects of different plant growth retardants on Magnolia wufengensis L.Y. Ma et L. R. Wang［J］. Forests, 2020, 12, 19 (DOI:10.3390/f12010019)
［6］ LI NY, SHI YP, WANG JZ. Effect of uniconazole on photosynthetic characters and leaf anatomical structure of Zinnia seedlings under water stress［J］. Acta Botanica Boreali-Occidentalia Sinica, 2012, 32(8):1626-1631. (in Chinese)
［7］ LI RX, SUN RJ, WANG TC, et al. Research progress on identification and evaluation methods, and mechanism of drought resistance in plants［J］. Biotechnology Bulletin, 2017, 33(7): 40-48. (in Chinese)
［8］ ZHENG RR, LIAO J, WU Y, et al. Effect of uniconazole on growth of Lilium oriental hybrids ‘Sorbonne’ ［J］. Northern Horticulture, 2012(3): 72-74. (in Chinese)
［9］ LI NY, SONG Y, HAN XF. Effects of uniconazole on water status and photosynthetic parameters of Petunias seedlings under drought stress［J］. Acta Agriculturae Universitatis Jiangxiensis: Natural Sciences Edition, 2011, 33(6): 1062-1066, 1076. (in Chinese)
［10］ HUANG YL, XIANG JL, CAI S, et al. Proteomic analysis of effect of uniconazole on response of Coix seedlings to low temperature stress［J］. Chinese Journal of Biologicals, 2019, 32(7):742-749. (in Chinese)
［11］ AHMAD I, AHMAD S, KAMRAN M, et al. Uniconazole and nitrogen fertilization trigger photosynthesis and chlorophyll fluorescence, and delay leaf senescence in maize at a high population density［J］. Photosynthetica, 2021, 59(1): 192-202.
［12］ SONG P, DING YF. Research progress on drought resistance of Euonymus［J］. Jiangsu Agricultural Sciences, 2019, 47(3): 11-15. (in Chinese)
［13］ ZHANG JZ, ZHANG QY, SUN GF, et al. Effects of drought stress and re-watering on growth and photosynthesis of Hosta［J］. Acta Prataculturae Sinica, 2014, 23(1):167-176. (in Chinese)
［Methods］With potted seedlings of D. pinnata variety ‘Danbanhuang’ as an experimental material, the effects of different concentrations (10, 20, 30 and 50 mg/L) of uniconazole on the photosynthetic characteristics of D. pinnata under drought stress and rewatering conditions were discussed.
［Results］ With the extension of drought time, the chlorophyll SPAD value, net photosynthetic rate, transpiration rate, and stomatal conductance of each treatment group decreased significantly, while the intercellular CO2 concentration showed a trend of first decreasing and then increasing; and after pretreatment of uniconazole spraying on leaves, it could alleviate the inhibition of drought stress on the photosynthesis of D. pinnata, and was beneficial to the recovery of photosynthetic capacity of D. pinnata after rewatering, and the effect of leaf application concentration of 30 mg/L was the best. It showed that exogenous uniconazole helps to improve the drought resistance of D. pinnata.
［Conclusions］This study provides a new experimental basis for the chemical regulation of stress resistance in D. pinnata and its production and application.
Key words Dahlia pinnata Cav.; Uniconazole; Drought stress; Photosynthetic characteristic
Received: June 19, 2021 Accepted: August 21, 2021
Supported by Applied Basic Research Program of Suzhou City (SYN201405).
Fangying XU (1997-), female, P. R. China, master, devoted to research about garden plant resources and application.
*Corresponding author. E-mail: [email protected].
Dahlia pinnata Cav. is a perennial bulbous flower belonging to Dahlia of Compositae. It has a wide variety of cultivars, rich colors, beautiful flower types, high ornamental value, and has high landscaping application value［1］. However, D. pinnata is very sensitive to soil moisture and not tolerant to drought and flood. It is often affected by drought stress in some cultivation areas in China. As a multi-dimensional abiotic stress, drought can cause a series of changes in plant phenotype, physiology, biochemistry, and molecular level. In severe cases, it can terminate photosynthesis, disorder metabolism, and ultimately cause plant death［2］. Therefore, studying the effects of chemical substances on the photosynthetic characteristics of D. pinnata can provide an important physiological index to judge their drought resistance regulation effect. The application of plant growth regulators is one of the important measures to alleviate plant abiotic stress［3］. Studies have shown that uniconazole not only has significant effects in regulating the growth and development of plants, but also has the effects of promoting root growth and enhancing stress resistance. For example, exogenous uniconazole can effectively improve the drought resistance in ornamental plants such as Trollius chinensis Bunge［4］, Wufeng Magnolia liliflora Desr.［5］ and Zinnia elegans Jacq.［6］, and reduce the adverse reactions caused by drought stress. In this study, the effects of uniconazole on the photosynthetic characteristics of D. pinnata under drought stress was investigated by the method of foliar spraying, so as to provide new experimental basis for the chemical regulation of stress resistance in D. pinnata and its production and application.
Materials and Methods
Experimental materials
The tested D. pinnata variety was ‘Danbanhuang’. Seedlings were raised in plug trays to the stage of six leaves and one core, and then transplanted into 15 cm×20 cm plastic pots. They were routinely conventionally cultivated and managed. Plants with healthy and similar growth were selected for the experiment.
Experimental methods
The concentration of uniconazole was set to 0, 10, 20, 30, 50 mg/L (denoted as S0, S1, S2, S3, S4, respectively). The solutions were sprayed to the leaf surface once every day at 9 oclock in the morning until water dripped from leaves. Each experimental treatment group had 12 pots, and after continuous spraying for 2 d, the drought stress treatment was carried out. The relative soil water content of the drought stress groups (S0-S4) and the normal control group (CK) were adjusted to 18% and 35%, respectively. During the stress period, the soil moisture content was measured every day around 18:00 by the weighing method combined with the TSZ-1 soil moisture rapid measuring instrument, and water was added in time to keep the relative soil moisture content within the set range.
Measurement indexes and methods
The SPAD value and photosynthetic indexes were measured at 0, 5, 10, and 15 d after drought stress. After 15 d of stress treatment, rewatering treatment was performed to restore the relative soil water content of the drought stress groups to about 75% of that of the control group, and the measurement was repeated after 5 d of rewatering (R5).
Determination of SPAD value Five D. pinnata plants with similar growth were selected from each group, and nine mature leaves were selected from each plant and measured for the SPAD value with an SPAD-502 chlorophyll meter.
Determination of photosynthetic indexes
Mature leaves of plants with relatively consistent growth were selected and determined with a Li-6400XT portable photosynthesis system for net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci) and transpiration rate (Tr) of the leaves. The lamination intensity from external sources in the culture environment was used for the measurement. The flow rate of the sample chamber was set to 500 μmol/s, and the atmospheric carbon dioxide concentration was about 400 μl/L. The measurement time was from 9:00 to 11:00 on a sunny and windless morning. Each group was measured repeatedly for 9 times.
Data analysis
EXCEL2013 was used for data statistics and icon drawing, and the LSD method of SPSS software was used for analysis on significance of difference.
Results and Analysis
Effects of exogenous uniconazole on SPAD value of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on the relative chlorophyll content (SPAD value) of D. pinnata are shown in Fig. 1. After 10 d of stress, the SPAD values of various stress treatment groups were significantly lower than that of the CK; and to 15 d of stress, they were 22.70%, 20.75%, 15.37%, 14.24%, and 20.55% lower than the CK, respectively. Among them, treatment groups S2 (20 mg/L) and S3 (30 mg/L) were significantly higher than S0 (spraying water). After rewatering, the SPAD value of each treatment group increased. Among them, the content of treatment group S3 was the highest, but was still significantly lower than the CK, and did not return to the level before stress. It showed that exogenous uniconazole could reduce the damage of drought stress to chloroplast structure to a certain extent and maintain relative stability of chlorophyll content, and the treatment with a uniconazole concentration of 30 mg/L had the best effect.
Effects of exogenous uniconazole on stomatal conductance of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on stomatal conductance (Gs) of D. pinnata are shown in Fig. 2. After the drought stress began, the Gs of D. pinnata continued to decrease. By the 15th day of stress, the Gs values of various treatment groups decreased by 61.25%, 57.71%, 43.63%, 45.36% and 56.98% respectively compared with the CK. After rewatering, the Gs values of various treatment groups increased, but were still significantly lower than the CK, and did not recover to the level before stress. However, the recovery effects of the uniconazole treatment groups were better than that of S0 (spraying with clear water), indicating that exogenous uniconazole could effectively slow down the decrease of D. pinnata stomatal opening under drought stress, and the application concentration of 30 mg/L was the best. Effects of exogenous uniconazole on the intercellular carbon dioxide concentration of D. pinnata under drought stress
The effects of drought stress and exogenous uniconazole on the intercellular carbon dioxide concentration (Ci) of D. pinnata are shown in Fig. 3. During the drought stress period, the Ci of D. pinnata decreased first and then increased. By the 10th d of stress, the Ci values of various treatment groups decreased by 34.80%, 30.32%, 28.03%, 24.20% and 29.26% respectively compared with the CK in the same period. After rewatering, Ci increased in each treatment group, but still significantly lower than the CK, indicating that uniconazole pretreatment was beneficial to the accumulation of intracellular CO2 under drought stress conditions, and the application concentrations of 20 and 30 mg/L achieved a better effect.
Effects of exogenous uniconazole on intercellular transpiration rate of D. pinnata under drought stress
Under drought stress conditions, the transpiration rate (Tr) of D. pinnata leaves showed a continuous decreasing trend, and the values of various treatment groups were significantly lower than the CK. At 15 d of stress, the Tr values were 56.02%, 53.73%, 51.19%, 44.18% and 49.29% lower than the CK in the same period, respectively, and Tr of treatment S3 was significantly higher than those of other groups. After rewatering, Tr increased in each treatment group, and S3 was the highest among the treatment groups (Fig. 4), indicating that uniconazole pretreatment had a certain encouraging effect on the transpiration of D. pinnata under drought stress, and the application concentrations of 20 and 30 mg/L uniconazole achieved a better effect.
Agricultural Biotechnology2021
Effects of exogenous uniconazole on net photosynthetic rate of D. pinnata under drought stress
Under drought stress conditions, the net photosynthetic rate (Pn) of D. pinnata also showed a continuous downward trend, and the values of all treatment groups sprayed with uniconazole were significantly lower than the CK (Fig. 5). At 15 d of stress, the Pn values of various treatment groups decreased by 65.56%, 63.01%, 53.42%, 51.65%, and 60.69% respectively compared with the CK in the same period, and the decreases of S2 and
S3 among the treatment groups were relatively small, and were significantly higher than S0. After rewatering, Pn in each test group increased, and treatment groups S2 and S3 were significantly higher than S0, but still significantly lower than the CK, and did not return to the level before stress. It showed that drought stress seriously affected the photosynthesis of D. pinnata, while exogenous uniconazole could alleviate the inhibition of photosynthesis of D. pinnata by drought stress, and the appropriate concentration ranged from 20 to 30 mg/L. Conclusions and Discussion
Photosynthesis is the basis for the survival of plants, and it is more sensitive to changes in plant water. Under water shortage conditions, the stomata opening of plant leaves is reduced or even closed completely, resulting in a reduction in the absorption of carbon dioxide, thereby inhibiting the progress of photosynthesis. Meanwhile, it also reduces the content of chlorophyll, destroys the structure of thylakoid membrane, inhibits electron transfer and photosynthetic phosphorylation, and leads to a decline in the photosynthetic rate. Therefore, changes in photosynthetic physiological characteristics under drought stress conditions can reflect the drought resistance of plants［7］.
Previous studies have shown that exogenous uniconazole could increase the chlorophyll content in leaves of Lilium oriental hybrids. After being treated with 80 mg/L uniconazole at 14 weeks of planting, the chlorophyll b content increased to 2.26 times that of the control group［8］. In Petunia hybrida (J. D. Hooker) Vilmorin, uniconazole can increase leaf water content and chlorophyll content under drought stress, improve leaf structure, alleviate the inhibition of drought stress on photosynthesis, and help restore leaf water content and photosynthesis after rewatering［9］. In this study, under drought stress and rewatering conditions, the chlorophyll content and net photosynthetic rate after exogenous uniconazole treatment were higher than those in the clear water control, and the 30 mg/L treatment group was the most significant. It is because exogenous uniconazole slows down the damage of drought stress to chloroplasts and delays the degradation of chlorophyll. Meanwhile, uniconazole can increase chlorophyll content and photosynthetic efficiency by regulating endogenous hormones and key enzymes in the process of chlorophyll synthesis［10-11］.
Studies have found that the inhibition of leaf photosynthesis under drought stress is the result of the combined effect of stomata and non-stomata factors. Under mild or moderate drought stress, stomatal factors dominate; and under severe stress, non-stomatal factors caused by damage to photosynthetic organs play a leading role［12］. The results of this study showed that exogenous uniconazole effectively alleviated the decline of Pn, Gs and Tr in D. pinnata leaves under drought stress, but the effect on Ci differed depending on the stress time. In the early stage of drought stress, uniconazole pretreatment could alleviate the decrease of Ci, and in the later stage, it could inhibit the increase of Ci, indicating the decrease of Pn in D. pinnata leaves in the early stage of drought stress was the result of partial closure of stomata caused by water deficit, while in the late stage of drought stress, non-stomata factors gradually occupied the main reason, but the pretreatment with uniconazole could increase the utilization of intracellular carbon dioxide. Therefore, its photosynthetic rate was significantly higher than that of the control group, which is consistent with the results of Zhang et al.［13］. In summary, uniconazole spraying on leaves could effectively alleviate the effects of drought stress on photosynthesis of D. pinnata, and simultaneously help restore photosynthetic capacity after rewatering, and could improve the drought resistance of D. pinnata within a certain range, and the leaf application concentration in the range of 20-30 mg/L was appropriate.
References
［1］ YAN GX. Overview and enlightenment of Dahlia industry development［J］. Anhui Agricultural Science Bulletin, 2020, 26(10): 38-39. (in Chinese)
［2］ LAI JL, LI XX, XUE L, et al. Research progress of plant drought resistance［J］. Jiangsu Agricultural Sciences, 2018, 46(17): 23-27. (in Chinese)
［3］ GU DY, WANG XF, YANG FJ, et al. Research progress of exogenous substances on horticultural crops resistance［J］. Northern Horticulture, 2016(3): 195-198. (in Chinese)
［4］ ZHANG YY, LIU JX, XU TT, et al. Effects of exogenous hormones on the physiological characteristics of Trollius chinensis Bunge Seedlings under drought stress［J］. Bulletin of Agricultural Science and Technology, 2020(1): 88-90. (in Chinese)
［5］ SHI XD, CHEN SY, JIA ZK. The Dwarfing effects of different plant growth retardants on Magnolia wufengensis L.Y. Ma et L. R. Wang［J］. Forests, 2020, 12, 19 (DOI:10.3390/f12010019)
［6］ LI NY, SHI YP, WANG JZ. Effect of uniconazole on photosynthetic characters and leaf anatomical structure of Zinnia seedlings under water stress［J］. Acta Botanica Boreali-Occidentalia Sinica, 2012, 32(8):1626-1631. (in Chinese)
［7］ LI RX, SUN RJ, WANG TC, et al. Research progress on identification and evaluation methods, and mechanism of drought resistance in plants［J］. Biotechnology Bulletin, 2017, 33(7): 40-48. (in Chinese)
［8］ ZHENG RR, LIAO J, WU Y, et al. Effect of uniconazole on growth of Lilium oriental hybrids ‘Sorbonne’ ［J］. Northern Horticulture, 2012(3): 72-74. (in Chinese)
［9］ LI NY, SONG Y, HAN XF. Effects of uniconazole on water status and photosynthetic parameters of Petunias seedlings under drought stress［J］. Acta Agriculturae Universitatis Jiangxiensis: Natural Sciences Edition, 2011, 33(6): 1062-1066, 1076. (in Chinese)
［10］ HUANG YL, XIANG JL, CAI S, et al. Proteomic analysis of effect of uniconazole on response of Coix seedlings to low temperature stress［J］. Chinese Journal of Biologicals, 2019, 32(7):742-749. (in Chinese)
［11］ AHMAD I, AHMAD S, KAMRAN M, et al. Uniconazole and nitrogen fertilization trigger photosynthesis and chlorophyll fluorescence, and delay leaf senescence in maize at a high population density［J］. Photosynthetica, 2021, 59(1): 192-202.
［12］ SONG P, DING YF. Research progress on drought resistance of Euonymus［J］. Jiangsu Agricultural Sciences, 2019, 47(3): 11-15. (in Chinese)
［13］ ZHANG JZ, ZHANG QY, SUN GF, et al. Effects of drought stress and re-watering on growth and photosynthesis of Hosta［J］. Acta Prataculturae Sinica, 2014, 23(1):167-176. (in Chinese)