Abstract
This study examined the interaction between constant liquid fertilization (CLF) concentrations and plant growth regulator (PGR) application concentrations on petunia (Petunia ×hybrida) growth and flowering in the production and post-production environments. Paclobutrazol application is a common practice in bedding plant production to achieve a compact plant that increases greenhouse space-use efficiency, shipping density, and tolerance to physical handling stresses in the post-production environment. The objective of this research was to determine the best strategy for balancing CLF and PGR application concentration in the greenhouse environment so that growth and flowering can be maximized in the post-production environment. A two-factorial combination of four CLF concentrations [50, 100, 150, or 200 ppm nitrogen (N)] and four paclobutrazol drench concentrations (0, 5, 10, or 20 ppm) were provided to plants during the production phase, and plant growth and flowering were recorded in the production and post-production environments. From a sustainability perspective, the ideal PGR concentration was 5 ppm paclobutrazol, since this concentration resulted in the best combination of production and post-production characteristics and performance. At this PGR concentration, all plant growth and flowering measures increased as CLF increased from 50 to 200 ppm N; however, all CLF concentrations also produced commercially acceptable plants. Therefore, the ideal CLF concentration depends on the size of plant desired; that is, CLF concentrations as low as 50 to 100 ppm N can be provided depending on the market size requirements of the plants being grown. Based on our results, a combination of 50 ppm N CLF with 0 ppm paclobutrazol or 100 ppm N CLF with 5 ppm paclobutrazol both demonstrated adequate growth control during both production and post-production phases.
Annual ornamental plants produced in high-density situations tend to grow taller than desired and thus benefit from the use of PGRs to reduce the rate of stem elongation. From a production viewpoint, a high-quality plant is one that is compact, well-branched, and sufficiently rigid to allow for forceful handling and dense packing on shipping carts. Paclobutrazol is a widely used PGR in bedding plant production (Keever and Kessler 2008) that inhibits the synthesis of gibberellins, which are responsible for cellular elongation (Hedden and Graebe 1985). Besides reducing stem elongation, PGRs can promote uniform flowering, darken leaf color, decrease leaf expansion, and increase tolerance to handling stresses (Latimer and Whipker 2012). PGRs are most effective on bedding plants when applied immediately before rapid stem elongation. This is typically 1 to 3 weeks after transplanting a plug. Later applications should be avoided, especially in the case of triazoles, such as paclobutrazol, because they may delay flower opening when applied after flower initiation (Latimer and Whipker 2012).
While paclobutrazol is a useful tool for the production of high-quality bedding plants, it is important that plants resume normal growth once moved to a post-production environment, such as the consumer landscape (Latimer 1991; Ruter 1994). Residual effects of PGRs may be more important with the use of the triazole compounds, such as paclobutrazol, which are metabolized slowly and can affect post-production plant performance (Keever and Kessler 2008). The half-life of paclobutrazol in a plant or soil is in the range of several months (Rademacher 2000). For instance, when cranberry (Vaccinium oxycoccus) plants were treated with paclobutrazol as a soil drench, paclobutrazol was still detected in the soil 50 weeks after application (McArthur and Eaton 1989). Excessive height reduction and slow growth can result from excess paclobutrazol application, and stunting can be persistent in sensitive species (Latimer and Whipker 2012).
PGRs can be applied either as a foliar spray or a substrate drench. The benefits of drench applications include a more uniform and longer-lasting effect and a less negative effect on flowering and flower size (Latimer and Whipker 2012). In addition, the lower rates with drenches compared with sprays equate to less chemical used, meaning higher sustainability. For instance, Karaguzel et al. (2004) reported that drench was more effective than spray for reducing growth in potted lupine (Lupinus varius), stating that foliar sprays with ∼4-fold amount of a.i. are required to achieve growth retarding effects equal to that observed for media drench. In osteospermum (Osteospermum ecklonis), foliar sprays of ≤80 ppm paclobutrazol were ineffective in controlling plant growth; however, substrate drenches of paclobutrazol at 16 mg/pot limited plant height by 21% compared with the untreated control, and flowering was unaffected (Gibson and Whipker 2003). When potted black iris (Iris nigricans) plants were sprayed with 100, 250, 500, or 1000 ppm paclobutrazol, they resulted in curved leaves. However, when they were drenched with 0.25 or 1.0 ppm paclobutrazol, there was a significant reduction in plant height as well as an increase in flowering by 12% compared with untreated plants (Al-Khassawneh et al. 2006). Substrate drenches of paclobutrazol at 2 ppm reduced flowering by 80% and excessively reduced stem elongation.
Nitrogen (N) restriction, or lowering N fertilizer concentrations, is another method that can be used during commercial production to regulate plant growth. Over the past 2 decades, the standard industry fertilization practices that we have observed for petunia (Petunia ×hybrida) production have reduced from ≥200 to 100 to 150 ppm N CLF. However, poor N fertilization can negatively affect plant growth. For instance, low N decreases leaf area and chlorophyll content and results in decreased photosynthesis (Li et al. 2013; Radin 1983; Wu et al. 2019), which can be directly translated into plant productivity. With increasing N application, shoot growth is significantly favored over root growth (Cabrera and Devereaux 1998). In petunia and begonia (Begonia semperflorens-cultorum), root to shoot ratio decreased as fertilizer (20N–4.4P–16.6K) electrical conductivity (EC) increased from 0.12 to 2.77 dS·m−1 (Nemali and van Iersel 2004). Low fertilization concentrations can reduce growth and lower the requirement for paclobutrazol. However, this can cause the risk of supplying the retail customer with a plant that contains not enough nutrients to sustain further growth in the consumer environment, considering that consumers frequently fail to fertilize their plants (Park and Faust 2021). Therefore, finding an appropriate balance between CLF and PGR application concentrations is crucial for identifying sustainable production practices.
In this study, the interactive effects of PGR and CLF concentrations on the production and post-production performance of petunia were assessed. Petunia was chosen for the subject of the experiment because it is one of the most popular bedding plants, and paclobutrazol is commonly used to manage plant size on petunia as well as many other bedding plant species. Our hypothesis was that lower fertilization concentrations allow the use of lower concentrations of paclobutrazol application to achieve commercially acceptable plants; however, the lower fertilization concentrations during production negatively affect growth and flowering in the post-production environment. In contrast, high CLF concentrations create a requirement for high PGR concentrations to produce a high-quality flowering crop, and the high PGR concentrations negatively affect growing and flowering in the post-production environment. Thus, the objective of this research was to determine the best strategy for balancing CLF and PGR application concentration in the greenhouse environment so that growth and flowering can be maximized in the post-production environment.
Materials and methods
Two replications of the following experiment were conducted during Spring 2019 (22 Feb to 7 May for replication 1 and 1 Mar to 10 May for replication 2). Plug seedlings (288 cells/flat, four flats in total for both replications) of ‘Easy Wave Pink’ petunia were obtained from a commercial grower. Seedlings were transplanted into six-pack, plastic containers (six cells/container, 175 mL/cell, 80 containers for each replication), filled with a standard growing substrate that was custom formulated without a fertilizer starter charge (Fafard 3B; Sun Gro Horticulture, Anderson, SC, USA). The plants were placed in a glass greenhouse, and the initial irrigation event was performed with tap water after transplanting the seedlings.
The fertilizer and paclobutrazol treatments applied to the plants were defined by a two-factorial combination of four CLF concentrations (50, 100, 150, or 200 ppm N) and four PGR concentrations (0, 5, 10, or 20 ppm paclobutrazol), resulting in a total of 16 treatments. The experimental design was a randomized design, with five containers (six cells/container) per treatment for each of the replications. Among these, six plants per treatment were used for collecting weekly substrate pH/EC data during the production phase (3 weeks × two plants/week). Twelve plants per treatment were used for the first destructive harvest. Twelve plants per treatment were transplanted for the post-production phase (three plants per container).
The experiment was divided into two phases: the production phase and the post-production phase. The production phase consisted of the time from transplant to when at least five of the six plants in each six-pack container had at least one open flower, which occurred 38 d after transplant (DAT) for replication 1 and 34 DAT for replication 2. Paclobutrazol was applied 7 or 10 DAT for replications 1 and 2, respectively. During the production phase, air temperature averaged 68.4 ± 0.7 °F and daily light integral averaged 14.7 ± 7.3 mol·m−2·d−1 across both replications (Argus Control Systems, Surrey, BC, Canada).
The post-production phase began by placing the flowering plants in the dark at 71.6 °F for 2 d to simulate a shipping environment and then placing the plants in a greenhouse under a 50% light transmission shade curtain for 1 week to simulate a retail garden center environment. The temperature during the simulated retail environment averaged 71.4 ± 0.7 °F and daily light integral averaged 7.9 ± 3.5 mol·m−2·d−1 across both replications. The plants were then transplanted into 11-inch-diameter containers (1.1 L volume) with three plants per container and 6-inch spacing between the containers, using a commercial peat-based growing substrate with fertilizer starter charge (Fafard 3B), and grown in a greenhouse. The plants were located in the same greenhouse as the simulated retail environment, but the shade curtain was removed. This phase was meant to simulate the post-production environment and consequently the plants received no additional fertilizer during this time, except for the fertilizer starter charge in the growing substrate. The post-production phase lasted for 30 d for both replications. Air temperature averaged 71.7 ± 0.7 °F and daily light integral averaged 19.3 ± 6.4 mol·m−2·d−1 across both replications during the post-production phase.
Fertilization and paclobutrazol treatments
Plants were fertigated throughout the greenhouse production phase with a CLF solution using 15% N (1.1% ammoniacal-N, 11.8% nitrate-N, and 2.1% urea-N), 2.2% phosphorous, 12.5% potassium, 5% calcium, and 2% magnesium (Peters Excel Cal-Mag Special; ICL Fertilizers Co., Dublin, OH, USA) to deliver solutions containing 50, 100, 150, or 200 ppm N. Micronutrient concentrations were adjusted using a micronutrient blend containing 7% iron (Fe), 3.5% manganese, 3.5% zinc, 1.75% boron, 1.75% copper, and 0.7% molybdenum (Greencare Water-Soluble Micronutrient Blend; Blackmore Co., Belleville, MI, USA) so that all treatments received 1 ppm Fe with each fertigation application. The container capacity was 103 g/cell, which was determined by irrigating a container until the substrate was completely saturated and water stopped dripping from the container. Then, the substrate was placed in an oven at 70 °C for 1 week before re-weighing the substrate to calculate the weight of water held by the substrate at the container capacity. The six-pack containers were checked daily and manually fertigated when the substrate color started to turn into a lighter brown (about every 3–4 d) during the production phase. The fertigation volume was targeted to reach 80% of container capacity to minimize leaching of nutrients from the container. To accomplish this, the plant-container-substrate-water weight was measured at each fertigation event. The volume of solution to be added was calculated by subtracting the weight of the plastic, the dry substrate, and the plant from the total weight the plant-container-substrate-water weight assuming the substrate was at container capacity. The plant weight changed weekly, so a representative plant was removed from the substrate each week and weighed to account for this change. The volume of fertilizer solution applied at each irrigation to bring the container up to 80% container capacity was recorded for each six-pack container at each irrigation event.
Paclobutrazol (Bonzi; Syngenta, Greensboro, NC, USA) was applied as a drench at 7 or 10 DAT for replications 1 and 2, respectively. Four different concentrations (0, 5, 10, or 20 ppm paclobutrazol) were applied, and the drench volume applied was 87 mL/cell. For 0 ppm paclobutrazol treatment, plants were dosed with equivalent amounts of tap water. No additional fertilizer was applied after the greenhouse production phase except for the starter charge contained in the commercial substrate.
Data collection
Chlorophyll content of the petunia leaves was estimated weekly with a chlorophyll meter (Apogee Instruments, Logan, UT, USA). The leaves sampled were the largest uppermost leaves that would fit in the meter (e.g., 64 mm2, 12 samples per treatment). Substrate pH and EC were measured weekly with a pH/conductivity meter (Oakton PC 700; Cole-Parmer, Vernon Hills, IL, USA) using the substrate occupied by two plants per treatment. The plants in the containers sampled for pH and EC were discarded. Substrate EC was obtained using the 1:2 (growth substrate:deionized water, v/v) method by adding 100 g of deionized water to a 50-g substrate sample. A destructive harvest was performed at the end of the production phase (38 or 34 DAT for replications 1 and 2, respectively) and another at the end of the post-production phase (74 or 70 DAT for replications 1 and 2, respectively) for each of the two experimental replications. The following data were recorded on 12 plants per treatment for each harvest: shoot fresh weight, plant height, plant fullness (defined as shoot fresh weight divided by plant height), number of flowers per plant, and leaf area (LI-3100 area meter; LI-COR, Lincoln, NE, USA). Also, root fresh weight was recorded after the greenhouse production phase. Plant height was determined as the distance from the surface of the substrate to the base of the calyx of the uppermost flower. The number of flowers per plant included buds displaying flower color and senesced flowers.
Experimental design and data analysis
Data were analyzed using two-way analysis of variance to examine the main effects and interactive effects between CLF and PGR treatments, using statistical software (JMP Pro version 13; SAS Institute Inc., Cary, NC, USA). Regression lines were created with the Fit Curve option in JMP, and the best model fit was identified by the R2 value. Correlations with P < 0.05 were considered statistically significant. The presented results are the average of all samples from both replications.
Results and discussion
All of the growth and development parameters measured during the production phase displayed an interactive response to CLF and paclobutrazol; that is, growth and development increased as CLF increased, but decreased as paclobutrazol increased (Table 1, Fig. 1). The effect of fertilization on petunia growth during production was also reported by Klock-Moore and Broschat (1999), who found that shoot dry weight more than doubled as fertilizer concentration increased from 100 to 200 ppm N. Similar interactive responses to CLF and paclobutrazol were observed with maize [Zea mays (Iremiren et al. 1997)], strawberry [Fragaria ×ananassa (McArthur and Eaton 1988)], and ash [Fraxinus sp. (Tanis et al. 2015)].
Plant height (A), shoot fresh weight (B), leaf area (C), and flower number (D) recorded per plant at the end of the production phase (38 or 34 d after transplant for replications 1 and 2, respectively). Individual petunia plants were treated with 50, 100, 150, or 200 ppm nitrogen (N) constant liquid fertilization concentrations and 0, 5, 10, or 20 ppm paclobutrazol drench application concentrations. Lines indicate significant linear or quadratic effects; P < 0.05 (n = 24). Error bars represent ±1 SE (A) R2 = 0.960, 0.973, 0.915, or 0.973 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (B) R2 = 0.999, 0.999, 0.989, or 0.998 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (C) R2 = 0.982, 0.996, 0.996, or 0.999 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (D) R2 = 0.835, 0.567, 0.874, or 0.934 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; 1 ppm = 1 mg·L−1, 1 cm = 0.3937 inch, 1 g = 0.0353 oz, 1 cm2 = 0.1550 inch2.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05086-22
Statistical analysis for petunia growth and flowering measurements and media pH and electrical conductivity (EC) measurements made during the production and post-production phases. Plants were treated during the production phase with constant liquid fertilization (CLF) concentrations of 50, 100, 150, or 200 ppm nitrogen and plant growth regulator (PGR) concentrations of 0, 5, 10, or 20 ppm paclobutrazol as a drench. Measurements were made at the end of the production phase (38 or 34 d after transplant for replications 1 and 2, respectively) and were repeated at the end of the post-production phase (74 or 70 d after transplant for replications 1 and 2, respectively); 1 ppm = 1 mg·L−1.
We refer to the 100 ppm N CLF treatment as the current standard industry fertilization practice. When using the standard industry CLF concentration, increasing paclobutrazol to 5 ppm resulted in a large (53%) decrease in plant height, but increasing paclobutrazol from 5 to 20 ppm had a relatively small (18%) additional decrease in plant height. Reduction in plant height with paclobutrazol drench application also has been observed with many potted plants, such as sunflower and zinnia [Helianthus annuus and Zinnia ×hybrida (Ahmad et al. 2015)], hibiscus [Hibiscus rosa-sinensis (Ahmad Nazarudin 2012)], rose [Rosa ×hybrida (Carvalho-Zanão et al. 2018)], sea marigold [Borrichia frutescens (Carver et al. 2014)], and geranium [Pelargonium ×hortorum (Cox 1991)]. Dasoju et al. (1998) and Krug et al. (2005) also reported little or no further effect on height with additional increases in paclobutrazol drench dose, which is in agreement with our result. When using a lower CLF concentration than the industry standard, that is, 50 ppm N and 0 ppm paclobutrazol, height decreased 35%, but the plant quality was still acceptable, indicating that the low N concentration eliminated the need for PGR.
The CLF and paclobutrazol treatments applied during the production phase continued to affect growth and development during the post-production phase (Fig. 2). Petunia growth and flowering at the end of the post-production phase increased as CLF increased but decreased as paclobutrazol increased. Across all CLF treatments, plant height decreased 37% as paclobutrazol concentration increased from 0 to 5 ppm. The plant height then decreased by a further 36% as paclobutrazol increased from 5 to 20 ppm, indicating that the height reduction effect of paclobutrazol decreased with increasing concentration of paclobutrazol. The long-term efficacy of paclobutrazol with continued plant growth control was also reported with potted sunflower and zinnia during simulated shipping and marketing (Ahmad et al. 2015). Flower number continued to display a significant effect of CLF and paclobutrazol at the end of the post-production phase. Compared with the industry standard treatment, flower number increased 72% as CLF increased from 100 to 200 ppm N, but it decreased by 45% as paclobutrazol increased from 0 to 5 ppm. The observed reduction in flower number with paclobutrazol drench was consistent with previous findings on potted gardenia [Gardenia jasminoides (Kamoutsis et al. 1999)] and lupine (Karaguzel et al. 2004). However, there were also reports of no change in flower number (Carver et al. 2014) or increased flower number (Ahmad Nazarudin 2012; McArthur and Eaton 1989) with paclobutrazol drenches. Plants treated with paclobutrazol have been reported to synthesize more cytokinin, which could increase floral bud formation in some species (Suradinata et al. 2013; Xia et al. 2018). The decreased flower number with paclobutrazol treatment in this study is probably due to the significant reduction in the overall plant size.
Plant height (A), shoot fresh weight (B), leaf area (C), and flower number (D) recorded per plant at the end of the post-production phase (74 or 70 d after transplant for replications 1 and 2, respectively). Individual petunia plants were treated with 50, 100, 150, or 200 ppm nitrogen (N) constant liquid fertilization concentrations and 0, 5, 10, or 20 ppm paclobutrazol drench application concentrations during the production phase. Lines indicate linear effects between constant liquid fertilization and paclobutrazol drench application concentrations (n = 24). For statistical comparison results, refer to Table 1. Error bars represent ±1 SE (A) R2 = 0.898, 0.293, 0.568, or 0.999 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (B) R2 = 0.999, 0.677, 0.993, or 0.938 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (C) R2 = 0.026, 0.467, 0.828, or 0.828 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (D) R2 = 0.955, 0.947, 0.999, or 0.950 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; 1 ppm = 1 mg·L−1, 1 cm = 0.3937 inch, 1 g = 0.0353 oz, 1 cm2 = 0.1550 inch2.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05086-22
Chlorophyll measurements were affected by the interaction of paclobutrazol and CLF applications during both the production and the post-production phases (Table 1). During production, chlorophyll decreased linearly as the CLF concentration increased when no PGR was applied (Fig. 3A). This is counterintuitive, but this is likely due to the increased leaf expansion that occurred with increasing CLF that resulted in diluted chlorophyll concentrations. When PGR was applied, chlorophyll increased as CLF increased from 50 to 150 ppm N and then decreased as CLF increased further to 200 ppm N. This is probably because extra N promoted growth and as a result the plant grew beyond the PGR effect. The main effect of PGR continued throughout the post-production phase, whereas the main effect of CLF did not (Fig. 3B). In general, chlorophyll concentration was considerably higher during the production phase following PGR application rather than during the post-production phase for all PGR and CLF combinations. Increased chlorophyll concentration or darker foliage color with paclobutrazol application was also reported in many potted ornamental plants (Ahmad Nazarudin 2012; Ahmad et al. 2015; Carvalho-Zanão et al. 2018).
Chlorophyll recorded at the end of the production phase (A) and post-production phase (B). Individual petunia plants were treated with 50, 100, 150, or 200 ppm nitrogen (N) constant liquid fertilization concentrations and 0, 5, 10, or 20 ppm paclobutrazol drench application concentrations. Lines indicate either linear or quadratic effects between constant liquid fertilization and paclobutrazol drench application concentrations (n = 24). For statistical comparison results, refer to Table 1. Error bars represent ±1 SE (A) R2 = 0.813, 0.016, 0.001, or 0.263 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (B) R2 = 0.350, 0.594, 0.010, or 0.752 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; 1 ppm = 1 mg·L−1.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05086-22
Plant fullness, defined as shoot fresh weight divided by plant height, provides a measure that relates to overall plant quality because appearance is linked to the density of plant growth. Morel et al. (2012) stated that one of the main criteria for the visual quality of an ornamental potted plant is its compact shape. Plant fullness measured at the end of the production phase increased with increasing CLF and paclobutrazol (Fig. 4A), whereas only CLF continued to have an effect through the post-production phase (Table 1, Fig. 4B). The maximum fullness during production observed at 200 ppm N CLF and 20 ppm paclobutrazol was nearly twice that of the 100 ppm N CLF and 0 ppm paclobutrazol treatment. At the end of the post-production phase, the fullness measurement of the 200 ppm N CLF and 20 ppm paclobutrazol treatment increased only by 16% compared with 100 ppm N CLF and 0 ppm paclobutrazol.
Plant fullness (defined as shoot fresh weight divided by plant height) recorded at the end of the production phase (A) and post-production phase (B). Individual petunia plants were treated with 50, 100, 150, or 200 ppm nitrogen (N) constant liquid fertilization concentrations and 0, 5, 10, or 20 ppm paclobutrazol drench application concentrations. Plant fullness was not significantly affected by the plant growth regulator treatments in the post-production environment, so those data were pooled in (B). Lines indicate linear effects between constant liquid fertilization and paclobutrazol drench application concentrations (n = 24). For statistical comparison results, refer to Table 1. Error bars represent ±1 SE (A) R2 = 0.988, 0.980, 0.974, or 0.901 for 0, 5, 10, or 20 ppm paclobutrazol, respectively; (B) R2 = 0.545 across all paclobutrazol treatments; 1 ppm = 1 mg·L−1, 1 g·cm−1 = 0.0896 oz/inch.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05086-22
Root fresh weight at the end of the production phase generally decreased with increasing paclobutrazol and increased as CLF increased from 50 to 150 ppm N. In contrast, root fresh weight decreased as CLF was further increased to 200 ppm N (data not shown). Across all paclobutrazol treatments, root fresh weight increased from 3.28 to 3.99 g as CLF increased from 50 to 150 ppm N and it decreased to 3.45 g as CLF increased to 200 ppm N. When CLF applied was 150 ppm N, root fresh weight decreased from 4.58 to 3.48 as paclobutrazol increased from 0 to 20 ppm.
Substrate EC increased with increasing CLF, but paclobutrazol did not significantly affect EC at the end of the production phase (data not shown) (e.g., substrate EC increased from 0.8 to 3.1 dS·m−1 as CLF increased from 50 to 200 ppm N). At the end of the post-production phase, CLF was not significant, but substrate EC increased with increasing paclobutrazol. Across all CLF treatments, substrate EC increased from 1.9 to 3.5 dS·m−1 as paclobutrazol increased from 0 to 20 ppm. The increase in substrate EC with increasing paclobutrazol at the end of the post-production phase also poses the possibility that paclobutrazol enhances the longevity of petunia by reducing the rate of nutrient uptake. For example, paclobutrazol-treated plants are smaller, thus they require less water and fertilizer, resulting in more nutrients left in the growing substrate that can enhance their longevity in the post-production environment. Substrate pH decreased as CLF increased, and the effect of paclobutrazol was not significant at the end of the production. Across all paclobutrazol treatments, substrate pH decreased from 7.2 to 6.8. None of the treatments were significant at the end of the post-production phase for substrate pH. The total amount of N applied during the production phase averaged 20, 49, 81, or 118 mg/plant for each of the CLF treatments 50, 100, 150, or 200 ppm, respectively, across all paclobutrazol treatments. The total amount of N applied during the production phase averaged 92, 61, 58, or 56 mg/plant for 0, 5, 10, or 20 ppm paclobutrazol treatments, across all CLF treatments. The total N applied to the plants treated with 0 ppm paclobutrazol was higher because plants were much larger and thus required more frequent fertigation.
This project was undertaken with one petunia cultivar and the vigor among cultivars varies widely. Nonetheless, we expect the interactive effects of PGR and CLF to be similar across cultivars, while the targeted paclobutrazol concentration will surely vary. So, we expect the trends to hold true for many petunia cultivars, but further work with a wider population of genetics will be needed to verify the observations reported here. In addition, other factors influence the effects of fertilizer and PGRs on plant performance, such as temperature, leaching, application volumes, etc. Therefore, further research is required to determine the optimal application methods of fertilizer and PGRs.
Many combinations of CLF and PGR are available to produce a marketable petunia crop, but the best growth and flowering is achieved with relatively high CLF (200 ppm N) with no PGR. However, petunia crops grown without PGR are unlikely to meet commercial expectations for greenhouse space-use efficiency, appearance (plant size and fullness), and resilience during shipping. Therefore, from a sustainability perspective, the goal is to apply just enough PGR to produce a desirable plant while not restricting growth and flowering in the post-production environment. In this study, the ideal PGR concentration was 5 ppm paclobutrazol. At this PGR concentration, all CLF concentrations produced commercially acceptable plants; however, all plant growth and flowering measures improved as CLF increased. Therefore, from a sustainability perspective, the goal is to use just enough fertilizer to meet the quality expectations of the market. A CLF of as low as 50 ppm N may be acceptable depending on the size of plant being marketed, so it is possible that CLF practices may continue to drop from the 100 to 150 ppm N concentrations that are commonly used across the bedding plant industry. Based on our results, combinations of 50 ppm N CLF with 0 ppm paclobutrazol or 100 ppm N CLF with 5 ppm paclobutrazol both demonstrated adequate growth control during both production and post-production phases.
This approach will minimize fertilizer usage, which is desirable, but it also limits post-production performance, which may have implications for consumer satisfaction. Park and Faust (2021) proposed the application of fertilizer as a pulse immediately before shipping to allow the grower to minimize nutrient use during production while providing nutrients that will be beneficial for post-production performance for the consumer. The use of controlled-release fertilizers also allows the grower to reduce CLF concentrations, while providing nutrients for sustained growth in the post-production environment. These practices will allow the bedding plant industry to continue to provide high-quality, high-performance products while minimizing the environmental impact.
Units
References cited
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