Introduction
The name ‘succulent’ was derived from a Latin word ‘succulentus’ which translates as fleshy or juicy (Chidamian 1984). This definition is especially true to the plants that are classified as succulents. Most of these plants are popular due to their water-efficient characteristics. They are xerophytes or plants that require minimum watering (Nyffeler et al. 2008). This generation of young adults finds succulents appropriate to fashion their home, office and other small spaces are because these plants need less attention compared to other ornamental plants. Aside from their extensive care requirements, succulents have gained popularity due to their unique geometrical shapes with vibrant colors which change every season (Altman and Waisel 2001).
Echeveria is one of the largest genera of Crassulaceae with more than 160 which was reported to have originally come from Mexico (Kimnach 2003;Uhl 2007). Species under this genus are have varying shape and size which comes in a wide array of colors of green and blue, and accents of red purple and pink. In spring or summer, these species are in active growth showing more of a greener hue. But, during dormant conditions, they become richly colored (Hewitt 1993). Most of the succulents are rosette shaped and are expected to be compact. However, leaf structure or formation easily changes depending on the season and their intercalary meristems may increase in response to these environmental changes (Fisher and French 1976). When ornamental plants lose their structure, their aesthetic value lessens due to lanky structures produced by lengthened stems and would, later on, show diminishing color hues which may be caused by environmental factors or hormone response (Cutler and Schneider 1990).
An important component in horticulture is the use of plant growth regulators (PGRs) which are useful in plant modification and development. These PGRs are able to do so as it interferes with the metabolism, biosynthesis, and translocation of plant hormones as well as regulate endogenous hormones within the plant (Salisbury and Ross 1992). There are five PGR groups that are popularly categorized namely auxins, cytokinins, gibberellins, ethylene, and abscisic acid. However, there are many chemical alternatives that effectively synthesizes these hormones that can regulate and affect certain changes (George et al. 2008).
Aside from the endogenous hormones, another group of bio-regulators that can alter a plant’s growth and development is the growth retardants. This technique is most commonly used to compensate for gibberellic acid which is responsible for bolting or elongation of stems (Grossman 1990). Popularly used growth retardants include chlormequat, daminozide, and paclobutrazol (Wood 2018). Chlormequat (2-chloroethyl trimethylammonium chloride), daminozide (N-dimethylamino succinamic acid), and paclobutrazol (PBZ) are plant growth retardants that are generally applied to inhibit the biosynthesis of gibberellin in plants (FAO 2017;Rademacher 2000). These retardants have been studied dealing with growth and development of several crops such as potato (Bhattrai 2017), citrus (Tsagkarakis et al. 2012), wheat and rye (Sorensen and Danielsen 2006), sunflower (Koutroubas and Damalas 2016), marigold (Maslanka and Magdziarz 2017) and zinnia (Pinto et al. 2005), and petunia (van der Poll and Allan 1990). Thus, this study aims to determine the effects of these plant growth retardants on the onset of colder conditions where irregular growth of Echeveria species are prominent and which growth inhibitor would best suit the said species.
Materials and Methods
Planting Materials
Two Echeveria species namely E. ‘Little Rose’ and E. ‘A Grimme One’ (Fig. 1). These succulents which belong to the Crassulaceae family. Succulent plants were taken from the Plant Factory, Sahmyook University with the same size and maturity, which were about a month old. These plants were repotted and transplanted to uniform pots with a size of 8 cm x 8 cm x 7.5 cm with the same amount of soil. The media consisted of 2:1 of commercial soil (Seoul Bio Inc., South Korea) and vermiculite.
Experimental Design
The study was conducted using a 3 x 3 factorial arrangement in completely randomized design (CRD) with four replications with eight plants per treatment, thus having 80 plants per species. The three plant growth retardants namely chlormequat, daminozide and paclobutrazol, their different concentration levels of 5, 25, and 100 ppm, and their combination served as treatments.
Application of Growth Retardants and Experimentation Management
The growth retardants were prepared separately based on their respective treatment concentration and applied using the drench method. Based on the pot size, 40 ml was drenched for each pot containing one plant. The application was repeated twice, and the second drench was applied two weeks after the start of the experiment. The plants were placed inside the greenhouse with an average temperature of 15°C (± 2°C) with a 75 - 80% humidity and an average light intensity of 110 μmol.m-1s-1. Plants were equally given with 10 ml of water once a week. Fig. 2, 3
Data gathered and Statistical Analysis
Vegetative parameters that were collected are plant height and diameter, and, leaf length and width in the study with the use of a digital caliper. Initial data gathering was done before the application of the growth retardants and final data collection was done at the end of the study. The data was analyzed using SPSS v. 22 (SPSS Inc., Japan). Means of the data was analyzed through analysis of Variance (ANOVA) and the differences between means were tested using Tukey’s Test (p < 0 .05) .
Results
Echeveria ‘Little Rose’
Results of the analysis of variance suggested that the parameters of E. ‘Little Rose’ was significantly affected by the different growth regulators except for those of leaf width as shown in Table 1.
Statistical analysis revealed that between the different growth regulators that were applied, succulent species’ plant height was highly affected by paclobutrazol. Those treated with chlormequat and daminozide had no significant difference in the plant height between concentration levels. It can be noted that those applied with chlormequat (22.79 mm) and daminozide (21.00 mm) had significantly taller plants compared to that of the control (20.06 mm). Paclobutrazol was able to inhibit plant height of E. ‘Little Rose’. A concentration of 100 ppm was observed to be the most effective garnering the shortest plants (16.79 mm). As the concentration decreased, it was noted that the plants had higher plant height (Fig. 1). The data trend for plant height of this species was likewise observed for other growth parameters such as that of plant diameter, leaf length, and width.
Echeveria ‘A Grimme One’
Based on the analysis of variance for the growth data of E. ‘A Grimme One’, the effect of growth regulators were found to have highly affected the growth parameters except for that of the leaf width. The results and its statistical analysis on this said species is presented in Table 2.
The plant height and diameter, and the leaf length was significantly affected by the selected growth regulators. Results suggest that the trend that was previously observed from the other succulent species was similar to E. ‘Grimme One’. Chlormequat treatments on their concentration did not significantly differ from each other. The use of 5, 25, and 100 ppm had statistically had a similar plant height with 35.35 mm, 35.33 mm and 34.86 mm, respectively. This was, likewise, the same condition of the results for Daminozide where the levels of concentration did not differ from each other. However, it may be noted that despite the similarity between the levels of concentration, both the treatment of chlormequat and daminozide had significantly shorter plants, smaller in plant diameter and leaf length.
Among the three growth regulator, the plant height, diameter and leaf length was significantly affected the paclobutrazol concentration levels. Compared to the control, the 5 ppm concentration was found to be similar in plant diameter and leaf length with that of the control with 39.84 mm and 19.03 mm, respectively. On the other hand, the use of 100 ppm gave the shortest plants (21.74 mm), smallest plant diameter (23.81 mm) and shortest leaf length (14.01 mm) compared to all the concentration levels of paclobutrazol as well as with other plant growth regulator treatments.
It was noted that the leaf width was not significantly affected by the treatment of plant growth inhibitors.
Discussion
Plant growth regulators (PGRs) have been used over the years due to their contribution to manipulate the growth and allow plants to adapt to specific conditions. This achieved by interfering with the gibberellin synthesis (Karimi et al. 2014). In this study, the use of the growth inhibitors, namely chlormequat, paclobutrazol, and daminozide, has been found to control the growth of Echeveria species.
Chlormequat (2-chloroethyl trimethylammonium chloride) is a member of the quaternary ammonium compound that acts by reducing the cell elongation and at the same time lowering the rate of cell division. This growth regulator has been used in controlling agronomic field crops such as oats, wheat, and rye as well as for pear which are usually prone to lodging (Sorensen and Danielsen 2006) However, in this study, when chlormequat was applied to E. ‘Little Rose’, results revealed that the PGR was not able to control its growth and had taller plants compared to those of the control. Plant hormones or phytohormones are known to cause effects and act at low concentration to regulate many aspects of plant growth and development, however, there are still many unanswered questions as to these hormones’ interaction within plants (Gray 2004). It may be said that despite the primary purpose of chlormequat to control the growth of plants, the response of the internal hormones, and the entry of growth retardants may cause varying or unexpected effects (Grossmann 1990). The effects of chlormequat on E. ‘Little Rose’ had significantly increased the plant height and diameter, on the other hand, E. ‘Grimme One’ was affected in a contrasting manner in which the treatment decreased the plant height.
Daminozide (Succinic acid-2,2-dimethyl hydrazide) is a plant growth inhibitor or a retardant that is used to regulate shoot growth of commercialized crops (Chauhan et al. 1987;Latimer and Whipker, 2013). This plant growth regulator was previously applied and studied with other economically important crops such as on wheat (Miroshnichenko et al. 2017), herbs (Liao et al. 2017;Karimi et al. 2015) and fruit trees e.g. apple (Petracek et al. 2003). Based on the results, the use of daminozide significantly reduced the rate of growth of two Echeveria species regardless of the amount of concentration that was applied. This growth retardant has been effective in the respective ornamental application of potted plants in just small and minimum amounts (Latimer and Whipker 2013). Reports also showed that the use of this gibberellin inhibitor has successfully reduced plant height in ornamental potted plants such as in potted red fire spike (Odontonema strictum) where they were applied as drenches (Rezazadeh et al. 2016).
Like that of the studies of Rezazadeh et al. (2016), the application of daminozide was effective and provided significant differences between the control plants. However, when compared with the use of paclobutrazol, the latter was found to be more effective in reducing plant when applied as drenches at higher concentrations. Paclobutrazol [(2RS, 3RS)-1-(4-chlorophenyl)-4, 4-dimethyl-2-(1H-1, 2, 4-trizol-1-yl)- pentan-3-ol] is considered as a plant growth regulator as well as a multi-stress ameliorant (Soumya et al. 2017). It has been famously used in varied plants such as that of Syzygium campanulatum (Nazarudin et al. 2007) and flowering plants such as sunflowers (Spitzer et al. 2011), lotus (Li and Hill 1989), larkspur (Mansuroglu et al. 2009), and pansy (Biswas et al. 2018). This growth inhibitor has been continuously used due to its properties to inhibit plant growth but as well as act as a stress protectant through maintaining water relativity and membrane stability of the plant (Soumya et al 2017). Numerous studies suggest that drenching method has been more effective compared to using it as a spray. Other studies also reported that the use of paclobutrazol, not only regulated plant height but, also enhanced leaf color and quality of the plants (Lenzi et al. 2015). Hence, the effects of paclobutrazol from other plant species were likewise evident in selected Echeveria species in this study.
