Introduction
Native plant production in Korea increased steadily from 2014 to 2018, following a sharp decrease in 2019, and recovered in 2020 to a total market of 40,920,000 plants (Korea Forest Service 2018 - 2020). One of the important strategies for increasing production is by introducing new species. Native plants have an advantage over imported plants as potential new commercial species because of their high environmental adaptability and pest resistance that they have developed over the long years in their natural habitat (Ryu 2004).
Minuartia laricina (L.) Mattf., commonly known as larch-like stitchwort, is one of the native plants in Korea with great potential as a commercial ornamental plant. It is an herbaceous perennial in the family Caryophyllaceae that grows in the alpine regions of Korea. It grows up to 10 cm in height and has terminal white flowers that are approximately 1.5 cm in diameter and bloom continuously from May to September. They are highly valuable as native ornamental plants due to their compactness and long duration of flowering.
Despite their strengths, they are a type of ground cover plant, and thus they are prone to droopy and leggy growth, which is an undesirable feature for potted plants. This feature can be observed in many floricultural crops that are naturally too tall for pot culture and require stem growth retardant treatments for adaptation as potted plants (Davis and Andersen 1989). Chemical plant growth retardants (PGRs) are the most common remedy for achieving compact, commercially valuable potted plants. Paclobutrazol [(2RS,3RS)-1-(4-Chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)- 3-pentanol] (PBZ) is a nitrogen heterocyclic compound and chlormequat chloride (2-Chloro-N,N,N-trimethylethan-1-aminium) (CCC) is an onium compound (Andersen and Andersen 2000). Both PBZ and CCC restrain stem elongation by inhibiting gibberellin (GA) biosynthesis (Andersen and Andersen 2000). Paclobutrazol is highly effective in small dosages, and therefore the suggested application rates are considerably lower than that of other PGRs such as CCC (Davis and Andersen 1989). Another type of PGR is daminozide [4-(2,2-Dimethylhydrazin-1-yl)-4-oxobutanoic acid] (DMZ), which is a hydrazine derivative (Davis and Andersen 1989) that reduces the conversion of GA to active forms of GA and reduces the translocation of GA to actively growing tissues (Rademacher 1991).
Few reports have investigated the cultivation of M. laricina (Gil et al. 2020;Shin et al. 2018;Yeon et al. 2019). According to the previous studies, its growth was improved under drier soil (Gil et al. 2020) and less shading (Shin et al. 2018). Optimal seedling growth conditions such as cell size and substrate type have been reported as well (Yeon et al. 2019). However, none of the previous studies have reported the use of growth retardants. Therefore, this research was conducted to assess the effects of PGRs on the growth and flowering of M. laricina and to determine whether this approach can improve the value of M. laricina as an ornamental potted plant.
Materials and Methods
Plant materials and growth conditions
The experiment was carried out in a greenhouse at the Useful Plant Resources Center (Yangpyeong, Korea) from June to August 2018. The plant materials used in this experiment were the cuttings of M. laricina originally propagated in 2017. When the cutting size averaged 11.1 cm in height with an average of 8.8 nodes and 7.3 shoots, they were transplanted into 11.5 x 11.5 x 10.5 cm3 plastic pots containing a commercial growing substrate (Baroker, Seoul Bio, Eumseong, Korea) with three plants per pot. The plants were irrigated daily using tap water. Average daily temperature inside the greenhouse during the experiment ranged from approximately 20°C to 32°C according to seasonal variations (Fig. 1).
Plant growth retardant (PGR) treatment
Immediately after transplanting, PGRs were applied once as a foliar spray at the amount of 10 mL per pot (n = 12 replicate pots per treatment). Three different PGRs were used including Trimmit (PBZ, 22.9% a.i., Syngenta Korea, Seoul, Korea), Cycocel (CCC, 11.8% a.i., OHP inc., Bluffton, SC, USA), and B-Nine (DMZ, 85% a.i., Enbio, Gunpo, Korea). Tested rates were PBZ at 50 and 100 mg·L-1 providing 0.5 and 1.0 mg a.i. per pot, CCC at 100, 500, and 1,000 mg·L-1 providing 1.0, 5.0, and 10.0 mg a.i. per pot, and DMZ at 500, 1,000, and 2,000 mg·L-1 providing 5.0, 10.0, and 20.0 mg a.i. per pot. Tap water without PGR was included as a control for comparison.
Data collection
The following data were measured monthly for three months after the PGR treatment: stem length, plant height, plant width, and the number of nodes, shoots, and flowers. The data were collected from the largest plant in each pot. Additionally, the date of the first visible bud and the first open flower were recorded for each pot.
Experimental design and statistical analysis
The experiment was designed as a completely randomized design. Analysis of variance (ANOVA) and mean separation by Duncan’s multiple range test at ɑ = 0.05 were performed using SAS 9.4. (SAS Institute Inc., Cary, NC, USA). The graph was created using SigmaPlot 10.0 (Systat Software, Inc., Chicago, IL, USA).
Results and Discussion
In this study, PBZ was the only PGR that exhibited effective dwarfing among the PGRs we tested in M. laricina (Fig. 2). Based on the assessment of M. laricina visual characteristics, PBZ at 50 mg·L-1 demonstrated an adequate degree of growth control, whereas 100 mg·L-1 resulted in an excessive growth suppression (Fig. 2A). Stem elongation of M. laricina treated with 50 or 100 mg·L-1 PBZ were 2.2 cm (13%) or 9.8 cm (57%) less, respectively, than plants sprayed with tap water only (Fig. 3). The stem length for CCC and DMZ treatment groups ranged from 17.0 cm to 18.4 cm and was not statistically different from the control group.
The restricting effect of PBZ spray on plant height also has been observed with bleeding heart (Dicentra spectabilis) (Kim et al. 1999), Dianthus caryophyllus (Bañón et al. 2002), black iris (Iris nigricans) (Al-Khassawneh et al. 2006), and red firespike (Odontonema strictum) (Rezazadeh et al. 2016). In addition, liner dip (Blanchard and Runkle 2007) or substrate drench (Ahmad et al. 2015;Lyons et al. 2018) of PBZ have demonstrated the stem growth inhibition on potted and bedding plants. Daminozide was reported to be effective at growth control when sprayed twice or more (Kim et al. 1999;Rezazadeh et al. 2016). When CCC was sprayed twice, it was effective at height control for baby primrose (Primula forbesii) (Zhang et al. 2020) but not for bleeding heart (Kim et al. 1999). One time foliar spray application of CCC on black iris did not result in height reduction, but drench application at relatively high rate (550 mg·L-1) resulted in 9.6% shorter stem (Al-Khassawneh et al. 2006).
Kim et al. (1999) states that two foliar applications of PGR is necessary because of uneven shoot emergence and growth. Furthermore, the ineffectiveness of CCC and DMZ treatment may be attributed to the fact that they degrade quickly and therefore are not persistent in the growth medium and may need to be applied more than once (Davis and Andersen 1989). On the other hand, PBZ is reported to be persistent in the growth medium and usually only requires one application (Davis and Andersen 1989). Similar to our results with M. laricina, PBZ at 10 to 40 mg·L-1 was more effective at growth control than CCC at 750 to 3,000 mg·L-1 or DMZ at 1,250 to 5,000 mg·L-1 for most kalanchoe species when applied once as a foliar spray (Currey and Erwin 2012).
The other growth parameters we measured in this study generally decreased with PBZ treatment compared to the control plants (Table 1). Plant height, node number, and flower number were significantly reduced when treated with 100 mg·L-1 PBZ, resulting in 5.3 cm (46%), 5.5 nodes (44%), or 139.7 flowers (59%) reduction, respectively, for each parameter, compared to the no PGR plants. Plant width and shoot number were significantly reduced when treated with 50 or 100 mg·L-1 PBZ, resulting in 3.2 cm (11%) or 13.7 cm (48%) reduction in plant height and 28 shoots (49%) or 36.9 shoots (64%) reduction in shoot number, respectively, for each PBZ rate, compared to the control group. Reduction in plant width up to 37% was also observed in sunflower and zinnia when they were drenched with PBZ at 0.5 to 4.0 mg a.i. per pot (Ahmad et al. 2015).
In contrast, these values either increased or remained constant with CCC or DMZ treatment compared to the control plants (Table 1). Chlormequat chloride at 1,000 mg·L-1 was the only PGR and rate that reduced the number of nodes, which had 3.8 (30%) less nodes than the control group. Still, growth parameters showed a decreasing tendency as the PGR rates increased in CCC and DMZ. For instance, plant width was increased by 12% or decreased by 9% or 3% for CCC at 100, 500, or 1,000 mg·L-1, respectively, and was increased by 3% or 1% or decreased by 2% for DMZ at 500, 1,000, or 2,000 mg·L-1, respectively, compared to the control plants.
The flower number is one of the most important parameters in floricultural crops. In this study, flower number significantly decreased by 59% when treated with 100 mg·L-1 PBZ, but it increased or remained constant with other PGR treatments (Table 1). However, the flower number per shoot significantly increased by 76% for both 50 mg·L-1 PBZ and 1,000 mg·L-1 DMZ treatments compared to the control plants. This was due to reduction in shoot number but constant flower number for 50 mg·L-1 PBZ and constant shoot number with increased flower number for 1,000 mg·L-1 DMZ. Higher number of flowers per shoot indicates higher flower density within a plant, which results in a higher quality product with abundant flowers.
The observed reduction in flower number with increasing PBZ concentration in this study was consistent with previous findings on Digitalis purpurea (Chon et al. 2014), Gardenia jasminoides (Kamoutsis et al. 1999), and lantana (Ruter 1996), but differs from reports on grevillea (Ben-Jaacov et al. 1989), lantana (Matsoukis et al. 2001), begonia (Suradinata et al. 2013), and garden mum (Oh et al. 2015), where flower number increased with the application of PBZ. Plants treated with PBZ have been reported to synthesize more cytokinin, which could increase floral bud formation in some plants (Suradinata et al. 2013;Xia et al. 2018). The decreased flower number with PBZ treatment in M. laricina is most likely due to the significant decrease in the number of shoots that was also observed with PBZ application (Table 1).
None of the PGR treatments significantly affected time to floral bud formation (Table 2). However, flowering was slightly accelerated with most PGRs except for 2,000 mg·L-1 DMZ, which delayed flowering by one week, speeding up flowering by 2 to 8 days (Table 2). Davis and Andersen (1989) stated that some plants respond to PBZ with earlier flowering, but most studies on PBZ application for potted plants report delayed flowering. Time to flowering was delayed up to 29 days as a result of PBZ application in bleeding heart (Kim et al. 1999), D. purpurea (Chon et al. 2014), black iris (Al-Khassawneh et al. 2006), zinnia (Ahmad et al. 2015), red firespike (Rezazadeh et al. 2016), and pineapple lily (Lyons et al. 2018). Delayed flowering observed for bleeding heart when sprayed twice with 3,000 mg·L-1 DMZ (Kim et al. 1999) is also in agreement with our finding that high rates of DMZ cause slower flowering.
In conclusion, stem growth of M. laricina was successfully inhibited with PBZ but not with CCC or DMZ. Paclobutrazol at 100 mg·L-1 excessively retarded plant growth, whereas 50 mg·L-1 PBZ demonstrated appropriate degree of growth control. In addition, the number of flowers per shoot significantly increased with 50 mg·L-1 PBZ because the number of shoots decreased when the flower number remained constant. The reduction in shoot number can be compensated by increasing the planting density for a fuller looking pot. Further research is required to determine more optimal PBZ concentration for potted M. laricina. Additional trials with repeated application of PGRs rather than once also may warrant evaluation considering the effect that varying persistence for different PGR types have on the efficacy.
