Response of Kentucky Bluegrass Turfgrass to Plant Growth Regulators

Abstract

Plant growth regulators (PGRs) are widely used in turfgrass management. This study aimed to find the influence of different rates of PGRs on Kentucky bluegrass visual quality. Six PGRs were applied, Trinexapac Ethyl, Paclobutrazol, Flurprimidol, Mefluidide, Ethephon, and Gibberellic Acid. The measurement of the leaf color was performed using a spectrophotometer. The turfgrass visual quality was judged using a visual rating system. Trinexapac Ethyl and Flurprimidol applications improved the overall appearance of turfgrass. Paclobutrazol and Gibberellic Acid decreased the overall appearance of investigated Kentucky bluegrass cultivars. The leaf texture assessment was only improved by Gibberellic Acid. The color assessment was enhanced with Paclobutrazol but deteriorated with Gibberellic Acid. PGRs affected the wavelength in a range of 520 to 630 nm. Gibberellic Acid caused lighter leaves with higher green and yellow hues. Paclobutrazol caused darker leaves with a lower green and a reddish hue. Trinexapac Ethyl application resulted in a more reddish hue. PGR rates also affected the visual quality of Kentucky bluegrass turfgrass.

1. Introduction

Mechanical mowing is routine practice in turfgrass management. This operation is the most important cost factor and the most time-consuming step in turfgrass maintenance. Moreover, mowing creates additional waste disposal costs [1]. Thus, chemical methods for the inhibition of grass growth have aroused great interest. Plant growth regulators (PGR) are natural or synthetic phytohormones controlling the growth or other physiological functions of plants [2]. PGRs are increasingly used to suppress seedhead production of warm- and cool-season grasses and reduce annual bluegrass Poa annua L. growth and development. [3]. Reduced vertical grass growth leads to a higher quality of playing surfaces on sport turfgrass facilities [2]. PGR application also improves turfgrass visual quality, salt tolerance, resistance to drought, and poor light conditions [4]. For those tasks, the most common plant growth regulators widely used in the turfgrass industry are Trinexapac Ethyl, Paclobutrazol, Flurprimidol, Mefluidide, Ethephon, and Gibberellic Acid [1,5,6].

Trinexapac Ethyl blocks the final stage of gibberellin biosynthesis. This results in decreasing the growth of grasses [7,8,9]. Furthermore, Trinexapac Ethyl resulted in higher turfgrass quality and visual turfgrass assessment [10,11]. The application of Trinexapac Ethyl on Kentucky bluegrass reduced clipping biomass and resulted in better visual quality [12]. Trinexapac Ethyl is also used to reduce the population of annual bluegrass [4].

Paclobutrazol and Flurprimidol reduce plant growth by blocking the synthesis of gibberellins [13,14]. Flurprimidol reduces grass growth by internode shortening. It is also used as a fungicide to prevent grass diseases such as dollar spot or brown patch, resulting in leaf discoloration [15,16]. Paclobutrazol is also used on turfgrass to reduce the growth of weeds, including annual bluegrass [17,18]. Applications of Paclobutrazol sometimes increase the relative index of chlorophyll and concentration of leaf nitrogen. However, Paclobutrazol also decreases turfgrass density and consequently the aesthetics of the turfgrass, and reduces traffic tolerance [19,20].

Ethephon affects grass growth by stimulating the production of ethylene. Ethephon is mainly used in turfgrass management to suppress floral stem development. However, it was also reported that Ethephon reduced bermudagrass (Cynodon dactylon (L.) Pers.) quality, discoloration of leaves, and shoot thinning [19,21]. Gibberellic Acid accelerates regrowth after mechanical damage caused by sport usage [22,23]. The application of gibberellins affects turfgrass growth throughout the vegetation season. However, this effect is lower during summer [24]. Gibberellin application resulted usually in the yellowing of grass leaves. This effect can be reduced by higher nitrogen fertilization. Gibberellic Acid is used in sport facilities to extend the period of exploitation from early spring to late autumn.

PGR applications sometimes resulted in leaf discoloration of turfgrass [25]. The visual quality of turfgrass is the most important characteristic of sports facilities. Visual quality assessment is widely used not only by turfgrass managers but also in research investigations and breeding. The quality of turfgrass takes into account aesthetic characteristics, i.e., turf density and uniformity, leaves texture and smoothness, and turfgrass cover and color. Turfgrass color is the main indicator of aesthetic quality. It is affected by species and cultivar composition, soil water availability and nutrient status, light conditions, and diseases [26,27].

The widely used method for turfgrass quality assessment is a visual score system. Turfgrass quality is assessed by an evaluator visual rating using a nine-point scale [28,29]. Regarding the color rating, one point indicates light yellow or brown turfgrass while nine points indicate dark green turfgrass. This is a fast method without requiring any equipment [30]. However, this method strongly depends on the training and experience of the evaluators [31,32]. Quantitative methods are definitely more expensive and time consuming. However, obtained data do not have an error of subjective assessment and are repetitive. In recent years image analysis has been considered very reproducible and less time consuming than conventional visual rating system [31,33]. Thus, image analysis is increasingly used to evaluate the color pattern of ornamental plants [34,35] and the color quality of turfgrass [25].

There is limited scientific information on the consequences of PGR application on turfgrass facilities due to the reduction in grass growth. There is some evidence that PGR application may lead to side effects such as the decreased visual quality of turfgrass. The main issue is to adjust the PGR rate to reduce the cost of maintenance while meeting visual aesthetic and quality. We hypothesize that (i) the PGR affects the color spectrophotometric characteristics and visual quality of Kentucky bluegrass, and (ii) discoloration depends on the PGR rate. The objective of this study was to evaluate the effect of six PGRs applied at five different application rates on Kentucky bluegrass visual quality.

2. Materials and Methods

2.1. Experimental Design

The pot experiment was carried out in the period from 2019 to 2021. Plastic pots 15 cm in diameter were filled with loamy sand soil (80% sand, 15% silt, and 5% clay) and a peat addition (15% by volume) (according to the ASTM standard F2396-04). The pots were placed in a rain-out shelter with no walls and a transparent glass roof. Soil water content was established at maximum water holding capacity using the Rain Bird sprinkler system (Rain Bird Inc., Tucson, AZ, USA). A total of 279 pots were arranged in a completely randomized design with three replications. Three cultivars of Kentucky bluegrass (Poa pratensis L.), i.e., Limousine, Niweta, and Baronial, were sown at a rate of 50 g m⁻² (1.125 g per pot). The turfgrass was clipped weekly to 25 mm. Pots were fertilized in monthly intervals (from April to October) with mineral fertilizers at rates: 180 kg N ha⁻¹, 60 kg P₂O₅ ha⁻¹, and 120 kg K₂O ha⁻¹. Six PGRs were applied in June 2019, 2020, and 2021 at five rates (

Table 1. PGR rates applied during the experiment.

PGR	Rate (kg PGR ha−1)
	R1	R2	R3	R4	R5
Trinexapac Ethyl	0.02	0.04	0.08	0.16	0.32
Paclobutrazol	0.06	0.12	0.24	0.48	0.96
Flurprimidol	0.0005	0.0010	0.0020	0.0040	0.0080
Mefluidide	0.005	0.010	0.020	0.040	0.080
Ethephon	1.0	2.0	4.0	8.0	16.0
Gibberellic Acid	0.01	0.02	0.04	0.08	0.16

). Untreated control without PGR application with three replications was also included.

2.2. Measurement

Leaf color was measured using a Konica Minolta CM 600d spectrophotometer (Konica-Minolta, Osaka, Japan) with an 11 mm diameter viewing area and 10° observation angle. Visible spectrum (380–720 nm) transmittance was measured with a resolution of 10 nm in three replications per pot. Measurements were repeated every 2–3 days from application day to 30 days after application.

Color parameters were calculated using the Commission Internationale de l’Eclairage (CIE) Lab color system relative to the D65 illuminant standard. This CIELab system represents color components’ projection on three axes i.e., L, a, and b. The L value represents lightness with 100 for white and 0 for black. The a and b values describe the green-red and blue-yellow chromatic coordinates, respectively. A positive a or b represents a red or yellow hue, respectively, and a negative a or b represents a green or blue hue, respectively. The composite color difference (ΔE) was calculated (Equation (1)):

$$\mathrm{\Delta }E=\sqrt{{\left(\mathrm{\Delta }L\right)}^2+{\left(\mathrm{\Delta }a\right)}^2+{\left(\mathrm{\Delta }b\right)}^2}$$

(1)

where ΔL, Δa, and Δb are the subtraction of L, a, and b of the treated turfgrass and control turfgrass with no PGRs.

Turfgrass quality was determined on a 1–9 scale according to the National Turfgrass Evaluation Program (NTEP) in the United States [26]. Three turfgrass characteristics were assessed, leaf texture, overall appearance, and turfgrass color. The lowest score indicated broad leaf, very poor appearance, and light green turfgrass. The highest score indicated thin leaves, a very good appearance, and dark green turfgrass.

2.3. Statistical Analyses

Statistical analyses were performed using Statistica v.13.1 (StatSoft Inc., Tulsa, OK, USA). The Shapiro–Wilk test was used to determine the distribution of normality. Levene’s test was performed for checking the homogeneity of variance. Three-way ANOVA (cultivar/PGR/rate) was used in a completely randomized experimental design. The effect of year and interactions of the year by cultivar, PGR, and rate were non-significant. Thus, the effect of the year was ignored in the ANOVA analysis. The ANOVA was followed by a Bonferroni test with a level of significance of p < 0.05. The relationships between PGR rates vs. color parameters were quantified using regression models.

3. Results

3.1. Turfgrass Color

Kentucky bluegrass cultivars had significantly different color characteristics. However, these cultivars presented a similar response upon PGR application. Limousine leaves (L = 40.40) were darker than the other two cultivars. The lightest leaves were those of Niweta (L = 40.88) (

Table 2. Effect of PGR application on Kentucky bluegrass color CIELab parameters.

PGR	Rate	L	a	b
Ethephon	R1	40.60 fg	−6.48	17.46
	R2	40.27 fg	−6.65	17.54
	R3	40.74 fg	−6.56	17.70
	R4	40.67 fg	−6.59	17.88
	R5	40.42 fg	−6.42	17.91
Flurprimidol	R1	40.87 fg	−6.68	17.52
	R2	40.88 fg	−6.54	17.72
	R3	40.53 fg	−6.65	17.65
	R4	40.88 fg	−6.44	17.82
	R5	40.74 fg	−6.63	17.74
Gibberellic Acid	R1	41.90 efg	−6.88	19.01
	R2	42.45 de	−6.90	19.81
	R3	43.21 cd	−7.03	20.56
	R4	44.27 bc	−7.05	21.65
	R5	45.39 a	−6.84	22.34
Mefluidide	R1	40.98 fg	−6.62	17.70
	R2	40.41 fg	−6.78	17.71
	R3	40.94 fg	−6.47	17.51
	R4	40.28 fg	−6.20	16.61
	R5	40.76 fg	−6.02	17.43
Paclobutrazol	R1	40.36 fg	−6.28	16.95
	R2	40.63 fg	−6.25	16.97
	R3	40.63 fg	−5.96	17.30
	R4	40.43 fg	−5.72	16.43
	R5	39.91 f	−5.35	15.74
Trinexapac Ethyl	R1	41.19 fg	−6.35	17.50
	R2	40.63 fg	−6.20	16.75
	R3	40.72 fg	−6.33	17.34
	R4	40.57 fg	−6.07	16.69
	R5	40.80 fg	−5.92	16.97
Control		40.69	−6.37	16.56
Means for PGR
Ethephon		40.54 b	−6.54 b	17.70 b
Flurprimidol		40.78 b	−6.59 b	17.69 b
Gibberellic Acid		43.45 a	−6.94 a	20.68 a
Mefluidide		40.67 b	−6.42 bc	17.39 b
Paclobutrazol		40.39 b	−5.91 d	16.68 b
Trinexapac Ethyl		40.78 b	−6.18 cd	17.05 b
Means for Cultivars
Limousine		40.44 c	−6.30 b	16.74 b
Niweta		41.20 b	−6.69 a	18.22 a
Baronial		41.62 a	−6.29 b	18.50 a

For each column, different superscripts letters indicate significant differences (p < 0.05).

). Niweta was also characterized by higher green (a = −6.66) and yellow hues (17.11). Limousine had the lowest yellow (b = 15.93) hue in relation to other cultivars.

CIELab parameters were modified by all experimental factors, i.e., cultivar, PGR, and rate (Table 2). Furthermore, the PGR×rate interaction was significant for the L parameter and differential color parameters ΔL, Δb, and ΔE.

The reflectance characteristic of leaves after PGR application is presented mainly in the wavelengths from 520 to 630 nm (Figure 1). This wavelength responds with the colors of green, yellow, orange, and red. Gibberellic Acid application resulted in the greatest differences in the reflectance spectrum. These differences in wavelength range are described in detail using CIELab parameters (Table 2). Gibberellic Acid resulted in lighter leaves (L = 43.45) with higher green (a = −6.94) and yellow (b = 20.68) hues. Paclobutrazol application made leaves darker in every cultivar (L = 39.91), but this effect was noticeable only when the R5 rate was applied. Any other PGRs did not affect the L parameter. Paclobutrazol and Trinexapac Ethyl resulted in a lower green hue (a = −5.91 and −6.18, respectively) compared to the control treatment (a = −6.43).

Marked differences were noticed for color parameters: ΔL, Δa, Δb, and ΔE (

Table 3. The ΔL, Δa, Δb, and ΔE color parameters of Kentucky bluegrass leaves after PGR application.

PGR	Rate	ΔL	Δa	Δb	ΔE
Ethephon	R1	−0.086 e	−0.105	0.898 fg	2.565 d
	R2	−0.426 e	−0.277	0.982 fg	2.726 d
	R3	0.048 e	−0.185	1.140 fg	2.587 d
	R4	−0.023 e	−0.216	1.315 fg	2.789 d
	R5	−0.270 e	−0.044	1.348 fg	2.550 d
Flurprimidol	R1	0.174 e	−0.304	0.957 fg	2.426 d
	R2	0.185 e	−0.167	1.161 fg	2.388 d
	R3	−0.159 e	−0.275	1.087 fg	2.403 d
	R4	0.193 e	−0.069	1.256 fg	2.503 d
	R5	0.046 e	−0.256	1.179 fg	2.396 d
Gibberellic Acid	R1	1.206 cde	−0.511	2.448 ef	3.705 cd
	R2	1.762 cd	−0.522	3.251 de	4.679 bc
	R3	2.523 bc	−0.653	3.997 cd	5.626 b
	R4	3.582 ab	−0.679	5.087 bc	7.036 a
	R5	4.700 a	−0.469	5.781 ab	8.325 a
Mefluidide	R1	0.290 e	−0.245	1.133 fg	2.573 d
	R2	−0.286 e	−0.408	1.143 fg	2.659 d
	R3	0.251 e	−0.097	0.951 fg	2.377 d
	R4	−0.411 e	0.170	0.044 fg	2.309 d
	R5	0.068 e	0.353	0.871 fg	2.623 d
Paclobutrazol	R1	−0.336 e	0.096	0.385 fg	2.504 d
	R2	−0.059 e	0.127	0.405 fg	2.311 d
	R3	−0.061 e	0.418	0.735 fg	2.572 d
	R4	−0.260 e	0.650	−0.131 gh	2.878 d
	R5	−0.780 e	1.026	−0.818 h	3.261 d
Trinexapac Ethyl	R1	0.500 e	0.024	0.939 fg	2.711 d
	R2	−0.065 e	0.170	0.190 fg	2.311 d
	R3	0.028 e	0.039	0.781 fg	2.854 d
	R4	−0.126 e	0.302	0.130 fg	2.527 d
	R5	0.105 e	0.455	0.403 fg	2.318 d
Means for PGRs
Ethephon		−0.151 b	−0.165 d	1.137 b	2.643 b
Flurprimidol		0.088 b	−0.214 d	1.128 b	2.423 b
Gibberellic Acid		2.755 a	−0.567 e	4.113 a	5.874 a
Mefluidide		−0.018 b	−0.045 cd	0.828 bc	2.508 b
Paclobutrazol		−0.299 b	0.463 a	0.115 d	2.705 b
Trinexapac Ethyl		0.088 b	0.198 b	0.489 cd	2.544 b
Means for Cultivars
Limousine		0.041 b	−0.111	0.841 c	2.405 c
Niweta		0.335 b	−0.029	1.147 b	3.222 b
Baronial		0.856 a	−0.025	1.917 a	3.722 a

For each column, different superscripts letters indicate significant differences (p < 0.05).

). The interaction between the PGR and rate significantly affected the ΔL parameter. However, the difference was only noticed for Gibberellic Acid. A higher rate of Gibberellic Acid resulted in higher ΔL, from 1.21 at R1 to 4.70 at R5. Any other PGR did not affect the ΔL parameter. Paclobutrazol and Trinexapac Ethyl applications resulted in positive Δa, which means that grass leaves had a reddish hue. Other PGRs resulted in negative Δa values, with the lowest for Gibberellic Acid (Δa = −0.57). The parameter Δb was mostly affected by Gibberellic Acid with a mean value of 4.11, which means that leaves were more yellowish. However, these changes were related to the PGR rate. Paclobutrazol reduced Δb to −0.82 at R5. The greatest differences in composite color difference were between the control and Gibberellic Acid (ΔE = 5.87 on average). The ΔE increased from 3.71 at R1 to 8.32 at R5. No other PGR influenced the ΔE parameter.

The Δa and Δb color parameters affected by PGR application are presented in Figure 2. Data points presenting results obtained for Ethephon, Flurprimidol, Mefluidide, and Trinexapac Ethyl are in the center of the coordinate system. This means that these PGRs did not change blue-yellow and green-red hues in comparison with untreated control.

However, Gibberellic Acid application resulted in positive values of Δb and negative values of the Δa parameter. This indicates that Gibberellic Acid application changed the leaf color to a more yellowish and greenish hue. Paclobutrazol treatments resulted in positive values of Δa and negative values of the Δb parameter. Thus, Paclobutrazol resulted in a more bluish and reddish hue than the untreated control turfgrass. The changes in leaf color also depended on PGR rates. However, this effect was statistically significant for Gibberellic Acid and Paclobutrazol. Higher rates of Gibberellic Acid resulted in increased ΔL, Δb, and ΔE values, indicating that leaves were lighter and had a bluish hue. A higher rate of Paclobutrazol only decreased the Δb parameter.

The relationships between PGR rates and CIELab color parameters were significant for Paclobutrazol, Gibberellic Acid, and Mefluidide (

Table 4. Correlations matrix for relationships of PGR rates vs. turfgrass color parameters.

PGR Rate	L	a	b	ΔL	Δa	Δb	ΔE
Flurprimidol	−0.008	0.007	0.010	−0.011	0.012	0.034	0.002
Ethephon	−0.008	0.032	0.026	−0.011	0.050	0.084	−0.012
Trinexapac Ethyl	−0.024	0.107	−0.020	−0.028	0.177	−0.065	−0.072
Paclobutrazol	−0.090	0.232	−0.086	−0.105	0.337 *	−0.240 *	0.197 *
Gibberellic Acid	0.339 *	0.011	0.183	0.337 *	0.017	0.313 *	0.378 *
Mefluidide	−0.015	0.171	−0.026	−0.018	0.303 *	−0.083	0.002

* Significant at p < 0.05.

). These relationships are presented as non-linear regression models in Figure 3. The highest coefficient of determination (R² = 0.774) was determined for the relationship between the Δa color parameter and the Paclobutrazol rate.

Turfgrass discoloration changed over time during the 30 days after application (DAA) (Figure 4). When Gibberellic Acid was applied, the ΔE parameter rapidly increased until 9 DAA up to a value of 7.54 on average for three cultivars. After this time, ΔE slowly decreased and in 27 DAA reached the initial value in 1 DAA. For other PGRs, the highest values of ΔE were obtained for 8 DAA, however, the maximum ΔE value did not exceed 3.0.

Overall, Gibberellic Acid application resulted in the greatest changes in color characteristics of Kentucky bluegrass leaves. Moreover, the nature of these changes indicated deterioration aesthetic value of turfgrass.

3.2. Turfgrass Quality

Kentucky bluegrass quality was only affected by the cultivar and PGRs. Only overall appearance was also affected by the interaction of PGRs and rate. Limousine had a higher rating for overall appearance, color, and leaf texture than other cultivars (

Table 5. Turfgrass quality score rating: overall appearance, turfgrass color, and leaf texture.

PGR	Rate	Overall Appearance	Turf Color	Leaf Texture
Ethephon	R1	7.86 ab	7.77	7.46
	R2	7.62 bc	7.79	7.50
	R3	7.60 bc	7.79	7.50
	R4	7.84 ab	7.77	7.46
	R5	7.76 b	7.79	7.50
Flurprimidol	R1	7.45 c	7.88	7.50
	R2	7.74 b	7.79	7.50
	R3	7.88 ab	7.85	7.50
	R4	7.88 ab	7.79	7.50
	R5	8.02 a	7.85	7.50
Gibberellic Acid	R1	7.79 b	7.77	7.46
	R2	7.82 abc	7.73	7.43
	R3	7.71 b	7.73	7.79
	R4	7.57 bc	7.55	7.83
	R5	7.05 d	7.48	7.83
Mefluidide	R1	7.88 ab	7.73	7.50
	R2	7.79 b	7.79	7.50
	R3	7.86 ab	7.79	7.50
	R4	7.50 bc	7.70	7.50
	R5	7.69 b	7.70	7.50
Paclobutrazol	R1	7.86 ab	7.96	7.46
	R2	7.55 bc	7.97	7.50
	R3	7.64 bc	8.00	7.50
	R4	7.33 cd	8.03	7.50
	R5	7.14 cd	8.00	7.42
Trinexapac Ethyl	R1	7.88 ab	7.79	7.50
	R2	7.79 b	7.79	7.25
	R3	7.83 ab	7.67	7.25
	R4	7.67 bc	7.82	7.42
	R5	7.81 b	7.85	7.50
Control		7.62	7.67	7.29
Means for PGR
Ethephon		7.73 ab	7.78 ab	7.49 ab
Flurprimidol		7.80 a	7.83 ab	7.50 ab
Gibberellic Acid		7.59 ab	7.65 b	7.67 a
Mefluidide		7.74 ab	7.74 ab	7.50 ab
Paclobutrazol		7.51 b	7.99 a	7.48 b
Trinexapac Ethyl		7.80 a	7.78 ab	7.38 b
Means for Cultivars
Limousine		8.26 a	7.99 a	8.64 a
Niweta		7.71 b	7.93 a	7.64 b
Baronial		7.11 c	7.47 b	6.24 c

For each column, mean values with different letters are significantly different (p < 0.05); superscripts are used only for significant differences according to ANOVA.

). The worst turfgrass overall appearance and leaf texture assessment was recorded for Baronial. Trinexapac Ethyl and Flurprimidol application improved overall appearance assessment compared with the untreated control. Paclobutrazol decreased the overall appearance of the investigated Kentucky bluegrass cultivars. However, Paclobutrazol significantly improved the color assessment. Gibberellic Acid application resulted in lower overall appearance and color assessment. The best leaf texture was identified when Gibberellic Acid was used. Overall appearance assessment was modified by PGRs in relation to PGR rates. Higher rates of Flurprimidol increased the overall appearance of Kentucky bluegrass turfgrass. However, higher rates of Gibberellic Acid and Paclobutrazol decreased the overall appearance. In general, PGRs affected Kentucky bluegrass visual quality. However, this effect varied and depends on the kind of PGR and PGR rate. This effect is also different for different visual features, i.e., overall appearance, color, and leaf texture.

4. Discussion

Kentucky bluegrass quality was significantly affected by the PGR application. A similar side effect of PGR application in turfgrass has been widely reported [21,36]. The main aim of the Trinexapac Ethyl application is to reduce clipping yield and increase canopy photochemical efficiency and better shade tolerance. Thus, Trinexapac Ethyl is recommended for turfgrass under low-light conditions [11,37]. Trinexapac Ethyl has not only been shown to affect grass growth but also to improve leaf color [38]. This effect is ascribed to higher chlorophyll-b concentration and cell density and delayed ageing [39]. Our research confirmed this beneficial effect of Trinexapac Ethyl on the color of Kentucky bluegrass turfgrass. However, this effect was only shown in a red hue. Research by Lickfeldt et al. [12] also showed that Kentucky bluegrass turfgrass after Trinexapac Ethyl application presented better visual quality than untreated turfgrass despite the application rate. Stier and Rogers [40] observed that Trinexapac Ethyl enhanced the color and increased chlorophyll levels of supina bluegrass (Poa supina Schrad.) and Kentucky bluegrass. Research on Trinexapac Ethyl treatment on perennial ryegrass (Lolium perenne L.) color showed a more pronounced effect [25].

In this research, Paclobutrazol application on Kentucky bluegrass also improved turfgrass color but reduced overall appearance assessment. A similar effect was also reported in research with other grass species [25]. Miller [18] observed that Paclobutrazol application improved the quality of creeping bentgrass (Agrostis stolonifera) turfgrass. Paclobutrazol also raised chlorophyll levels of zoysia grass (Zoysia japonica) under field conditions [41]. Better color quality after Paclobutrazol application is usually ascribed to greater resistance to diseases. Miller [18] and Fidanza et al. [7] reported that Paclobutrazol treatments resulted in lower dollar spot (Sclerotinia homoeocarpa F.T. Bennett) severity. On the other hand, McCarty et al. [5] reported no effect of Paclobutrazol on St. Augustine grass (Stenotaphrum secundatum (Walt.) Kuntz.). A similar effect was observed by Melero et al. [42] in an experiment with common carpetgrass (Axonopus fissifolius (Raddi) Kuhlm.). Moreover, Kopec et al. [43] reported that Paclobutrazol caused a decrease in bermudagrass color. This effect lasted 5 months before the turfgrass reached a satisfactory color and visual quality.

Gibberellic Acid influences above-ground grass features, such as germination, plant height, and tillering, and below-ground biomass, i.e., root dry matter [44]. However, it is also reported that Gibberellic Acid causes a more yellowish color of turfgrass [45]. The present study also reported negative discoloration of Kentucky bluegrass after the addition of Gibberellic Acid. Matthew et al. [24] ascribed this effect to lower chlorophyll content. Thus, they recommended a higher N fertilizer rate to reduce this problem.

Flurprimidol, Mefluidide, and Ethephon did not change the green-red and blue-yellow hue with respect to the control. Similar results were also obtained by Głąb et al. [25] in their research with perennial ryegrass. However, Brosnan et al. [19], in research with bermudagrass (Cynodon dactylon), observed that Ethephon and Flurprimidol decreased color and quality assessment. In the current research, Flurprimidol application resulted in better quality assessment but with no effect on leaf color. McCullough et al. [46] reported that Ethephon deteriorated bermudagrass root dry matter, root length, and turfgrass quality. In this study, Mefluidide and Ethephon rates did not affect Kentucky bluegrass turfgrass quality. Mefluidide worsened the green color of centipedegrass (Eremochloa ophiuroides (Munro) Hack.) turfgrass [47]. According to Dernoeden [48], Mefluidide may cause a red or bronze color of bentgrass at low temperature [48]. Some research reports showed the contradictory effects of PGR application on turfgrass visual quality and color. It can be summarized that the visual quality of turfgrass treated with PGRs depends on treatment conditions, i.e., grass species, PGR dose, time of application, light, temperature, availability of mineral nutrients, and the vigor of the plant and its endogenous hormonal content [49].

When the problem of turfgrass discoloration after PRG application appears, one of the common solutions is higher nitrogen fertilization. Akdeniz and Hosaflioglu [50] noticed that the higher and uniform visual quality of perennial ryegrass was a result of the higher rate of nitrogen fertilization. This effect can be enhanced by supplemental iron fertilization [51]. Caturegli et al. [52] recommended nitrogen fertilization but also with an additional irrigation rate.

5. Conclusions

Summarizing the results of this study, we found that PGRs used in turfgrass management may have a side effect reflected in the visual quality of the turfgrass. Trinexapac Ethyl and Flurprimidol applications resulted in a better overall appearance assessment, while Paclobutrazol decreased the overall appearance but enhanced visual color assessment. Gibberellic Acid decreased the overall appearance and color assessment but improved leaf texture.

Visual turfgrass color assessment corresponded with the results of the spectrophotometric measurements of leaf color. The changes linked to different PGRs were observed mainly in the wavelength from 520 to 630 nm. Gibberellic Acid application resulted in lighter leaves with higher green and yellow hues. Paclobutrazol caused darker leaves with lower green and more reddish hues. Trinexapac Ethyl application resulted in a more reddish hue. The changes in leaf color were related to PGR rates. However, this effect was only noticed for Gibberellic Acid and Paclobutrazol. Flurprimidol, Mefluidide, and Ethephon did not affect the green-red and blue-yellow hue in comparison with the control. Spectrophotometric measurement of leaf color is more accurate than a visual rating system and explains in detail the character of differences. This research provides useful information for turfgrass managers concerning the selection of PGRs and the positive and negative consequences of their application. Future research should focus on developing methods of prevention of visual quality deterioration.