Effects of Uniconazole on Stem and Leaf Growth and Endogenous Hormone Contents of Dahlia ( Dahlia pinnata Cav.)

Fangying XU Yanhua MIAO Chi CHEN Xianfeng YUAN Jianzhong TAN

Abstract    [Objectives]  This study was conducted to investigate the physiological mechanism of the response of  Dahlia pinnata  to exogenous uniconazole during vegetative growth.

[Methods] The effects of different concentrations （20， 30， 60 and 80 mg/L） of foliar fertilization on stem and leaf growth， chlorophyll and endogenous hormone contents were investigated using dahlia seedlings （cultivar ‘Shiwucaiqiu’） cultivated in the open field.

[Results]  Different concentrations of uniconazole could make dahlia plants dwarf， shorten internodes， increase stem diameter， and deepen leaf color， and the higher the application concentration， the more significant the effects. Compared with the control group， the relative contents of chlorophyll （SPAD value） in various treatment groups increased， the contents of endogenous GA3 and IAA decreased， and the contents of ABA increased. Among them， the contents of IAA and ABA also showed a trend of increasing first and then decreasing.

[Conclusions] The results indicated that uniconazole could affect the growth of stems and leaves and plant type by regulating photosynthetic pigments and the levels of endogenous hormones in dahlia， and good dwarfing effect and ornamental value could be obtained under the condition of foliar application concentration of 30 mg/L. This study provides a new experimental basis for the cultivation and application of dahlia.

Key words   Dahlia; Uniconazole; Vegetative growth; Plant type; Endogenous hormones

Dahlia （ Dahlia pinnata  Cav.） is a perennial bulbous flower of  Dahlia  in Asteraceae. It has many varieties， gorgeous colors， beautiful flower shapes， wide application range and high ornamental value. It is the most widely cultivated ornamental plant in the world  [1] . However， most dahlia varieties are tall and prone to lodging and stem breakage during open field cultivation， which has attracted much attention in production practice. At present， there have been many research reports on plant dwarfing technologies. Among them， the use of plant growth retarders is a simple and effective control method  [2] ， which can achieve the purposes of dwarfing plant height and improving plant type according to different needs.

As a new type of plant growth retarder， uniconazole can effectively improve plant type， enhance resistance， and regulate flowering period by inhibiting the biosynthesis of endogenous gibberellins in plants， thereby affecting the levels of other endogenous hormones  [3] . It has been applied to a variety of plants. The effect of uniconazole in dwarfing dahlia plants has been reported several times  [4-5] ， but the regulatory mechanism of uniconazole on the growth and development of dahlias is still seldom studied. In this study， with dahlia cultivar ‘Shiwucaiqiu’ as the material， the effects of foliar application of different concentrations of uniconazole on the growth of dahlia stems and leaves and the content of endogenous hormones were investigated， hoping to provide a new experimental basis for its cultivation and application.

Materials and Methods 

Experimental materials

The tested dahlia cultivar was ‘Shiwucaiqiu’. After pre-germination in early March， it was planted in the garden and horticulture training base of Architecture College， Soochow University in early April 2021 by dividing the tuberous roots into 40 cm×60 cm spacing， and conventional cultivation and management measures were taken. The tested uniconazole was analytically pure （purchased from Shanghai Yuanye Bio-technology Co.， Ltd.）.

Experimental methods

The concentration of uniconazole for foliar spraying was set to 20， 30， 60 and 80 mg/L （respectively marked as S1， S2， S3， and S4）， and a clear water treatment was set as the control check （CK）. The dahlia plants were sprayed for the first time when the average plant height was 20 cm， and sprayed for the second time  7 d  later. Each experimental group included 12 plants.

Determination indices and methods

The morphological indices were investigated at two stages before the treatment with uniconazole and when the first flower opened. The SPAD value and endogenous hormone contents were determined at 0， 7， 14， 21， and 28 d after uniconazole treatment.

Investigation of morphological indices

Plant height： The plant height was measured from the surface soil to the highest point of the above-ground stem of a plant， with a meter ruler. The calculation of plant height increment and the dwarfing rate were calculated according to following equations： Plant height increment=Plant height at the opening of first flower-Initial plant height before treatment， and Dwarfing rate=（Plant height increment of the control group-Plant height increment of the treatment group）/Plant height increment of the control group.

Stem thickness： The stem thickness was the diameter of the third stem from the root of the plant upwards， which was measured with a vernier caliper： Stem thickness increment=Stem thickness at the  opening  of the first flower-Initial stem thickness before treatment.

Number of internodes： It was the number of stem nodes from the root of a plant to the growing point， and the length of internode was calculated according to the equation： The length of internode=Height of the first flower/Number of internodes.

Determination of leaf color and SPAD

Fully expanded mature leaves were selected at 14 d after the treatment with uniconazole， and the leaf color was identified using the RHS color chart of the Royal Horticultural Society， followed by the determination of the RHSCC value. Meanwhile， 9 mature leaves were selected from 4 plants with similar growth vigor， and the SPAD value was measured by an SPAD-502 chlorophyll meter.

Determination of endogenous hormones

At 0， 7， 14， 21， and 28 d after treatment with uniconazole， the fully expanded leaves of the second node were taken， wrapped with tin foil and marked， and quickly frozen in liquid nitrogen and stored at -80 ℃ for later use. The endogenous hormones GA3， IAA and ABA were determined according to the operation method of the ELISA kit （Shanghai Keshun Biotechnology Co.， Ltd.）.

Data analysis

Microsoft Excel 2016 was used for data entry and drawing， and SPSS 17.0 software was used to perform one-way analysis of variance （ANOVA）.

Results and Analysis 

Effect of foliar application of uniconazole on plant height of dahlia

The effect of uniconazole on the growth of dahlia plants is shown in Table 1. Compared with the control group （CK）， the plant height and internode length of each uniconazole treatment group were significantly reduced， and the dwarfing rates reached 23.86%， 38.21%， 43.27% and 52.05%， respectively， while the stem diameter increased significantly during the same period. Meanwhile， with the increase of the uniconazole treatment concentration， the plant height increment and internode length gradually decreased， while the stem diameter increment showed a trend of first increasing and then decreasing. It could be seen that the application of uniconazole could significantly reduce the elongation of dahlia stems， but the effect on the number of internodes had little effect.

Effect of foliar application of uniconazole on leaf color of  dahlia

Uniconazole also had a significant effect on the leaf color of dahlia during vegetative growth. Compared with CK， the appearance of the leaf color after spraying with uniconazole for 1 week was significantly darker， from green in the CK （RHSCC value 137） to dark green （RHSCC value NN137）， while no significant differences were found between the treatment groups. Meanwhile， the leaves thickened， but with the increase of the treatment concentration， the leaves gradually became smaller and shrunken. When the foliar spraying concentration of uniconazole exceeded  60 mg/L，  the leaf deformity was more significant.

Secondly， from the change of relative chlorophyll content （SPAD value） （Fig. 1）， within 4 weeks after treatment， the SPAD values of the uniconazole treatment groups were always significantly higher than that of the control group. For example， the values increased by 33.33%， 39.14%， 42.19% and 45.64%， respectively after 21 d of treatment. It showed that the chlorophyll content of dahlia leaves could be increased by uniconazole treatment， and the leaf color was deepened with the increase of the application concentration. The leaves with the application concentrations of 60 and 80 mg/L were the darkest.

Effects of uniconazole on accumulation of three endogenous hormones in dahlia

The main mechanism of uniconazole is to affect the activity of kaurene oxidase， reduce the formation of GA precursor raw materials， and reduce plant height by inhibiting the biosynthesis of GA  [6] ， while the increase of GA content is often accompanied by the elongation of shoots  [7] . In this study， the GA3 content of each treatment group was always lower than that of the CK during the vegetative growth of dahlia. For example， the GA3 contents of various treatment groups decreased by 20.33%， 25.22%， 30.05% and 41.97%， respectively， on the 14  th  d after treatment. After that， the differences in GA3 content between the treatment groups gradually decreased （Fig. 2）. It indicated that there was a strong correlation between plant height and the accumulation of endogenous GA3 after foliar application of uniconazole.

Secondly， GA can promote the conversion of L-tryptophan to the precursor D-tryptophan of IAA synthesis， and promote the synthesis of D-tryptophan to IAA， while IAA can also promote the synthesis of GA， inhibit the decomposition of GA， and maintain a constant level of GA content  [8] . Therefore， this study also explored the effect of uniconazole on the IAA content of dahlia， and the results are shown in Fig. 3. After uniconazole treatment， the content of endogenous IAA in leaves generally increased first and then decreased. Among the various treatments， treatment groups S2-S4 were significantly lower than the CK. For example， their values deceased by 30.78%， 29.04% and 33.51%， respectively at 21 d of treatment. The results indicated that the uniconazole decreased the content of endogenous GA and also decreased the content of IAA. It was speculated that the combined effect of the two inhibited the elongation of dahlia stems.

Fangying XU  et al.  Effects of Uniconazole on Stem and Leaf Growth and  Endogenous  Hormone Contents of Dahlia （ Dahlia pinnata  Cav.）

It has been clear that GA3 and ABA have the same synthetic precursor farnesyl pyrophosphate （FPP）， and the blockage of GA3 synthesis will increase the content of ABA  [9] . This study showed （Fig. 4） that the ABA content of dahlia showed a trend of first increasing and then decreasing after treatment with uniconazole. On the 14  th  d of treatment， the ABA content peaked， and the values of various treatment groups increased by  12.98% ，  27.92% ，  33.72%  and 44.96% compared with the CK， respectively， but from the 21  st  to the 28  th  d of treatment， the ABA content began to decrease， and the S3 and S4 treatment groups were still significantly higher than the CK. It indicated that uniconazole treatment could significantly increase the endogenous ABA content in dahlia leaves， and the 80 mg/L treatment group had the highest content.

Conclusions and Discussion

Uniconazole is an efficient plant growth retardant， which can affect the physiological processes such as photosynthesis， endogenous hormone synthesis and nutrient absorption， and has various effects such as controlling plant height and adjusting plant type  [10] . In this study， uniconazole could significantly dwarf the plant height， shorten the internodes， and thicken the stem of dahlia， and the higher the application concentration， the more significant the regulation effects. It was also found that the reduction of dahlia plant height was mainly caused by the shortening of internode length， while the number of internodes was not affected. However， when the concentration of uniconazole exceeded 60 mg/L， the stem elongation was excessively inhibited， resulting in too short internodes and compact plant type， and the height of the plants at the first flowering stage was less than 40 cm， which significantly reduced the ornamental value of dahlia cultivated in the open field.

Chlorophyll is the most important photosynthetic pigment in the process of plant photosynthesis， and it is an important indicator to reflect the strong photosynthetic capacity and growth and development  [11] . Uniconazole can improve the expression of the chlorophyll gene SGR2 by regulating cytokinin， and increase the content of chlorophyll and inhibit the fading of chlorophyll， thereby improving the photosynthetic efficiency  [12] . The results of this study showed that after 14 d of uniconazole treatment， the leaves of dahlia were significantly thickened， the leaf color became dark green， and the chlorophyll content increased; and the higher the concentration of foliar application， the more significant the regulation effects. However， when the foliar application concentration exceeded 60 mg/L， the phenomenon of leaf size reduction and leaf shrinkage aggravated. Therefore， considering the practicability of cultivating dahlia in the open field， the effect of foliar application of 30 mg/L uniconazole was appropriate， which was reflected by dwarfed plants， compact plant shape， dark green leaves， normal leaf shape， and remarkably improved ornamental effect.

There is a complex synergistic and antagonistic relationship between plant endogenous hormones. Uniconazole treatment affects the biosynthesis of GA， while GA3 and IAA work together to promote plant cell division and elongation growth， and ABA inhibits cell elongation and promotes lateral expansion  [8，13] . In this study， the application of uniconazole in dahlia leaves reduced the content of endogenous IAA and GA3， and increased the level of endogenous ABA， and the higher the application concentration， the more significant the regulation effects， which is relatively similar to the situation of using paclobutrazol  [14-15] . It could be considered that these changes in endogenous hormone levels are also the physiological basis for uniconazole to regulate the stem and leaf growth and plant type of dahlia.

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 Editor： Yingzhi GUANG   Proofreader： Xinxiu ZHU