Rooting characteristics and turfgrass quality of three bermudagrass cultivars and a zoysiagrass

Abstract

Warm season turfgrasses are increasingly used throughout the transitional areas of Mediterranean countries due to their low water requirements. A two-year field study was carried out in north-eastern Italy to evaluate the turf canopy quality and root system architecture of three seeded bermudagrass cultivars (La Paloma, Princess-77 and Yukon), and a zoysiagrass (Companion) under non-irrigated conditions. Plots (1.6×4.5 m) were established in July 2005 and arranged in a randomized complete block with three replicates. Visual quality ratings were assessed weekly from April to November in 2006 and 2007. Soil cores were collected at depths of 0–3, 3–8, 8–15, 15–25 and 25–40 cm in August 2006, January 2007, May 2007 and September 2007 to measure main rooting characteristics: root length density (RLD), root mass density (RMD) and root diameter. Companion zoysiagrass had higher canopy quality than bermudagrass cultivars in early spring and in the late growing season, while it displayed poorest quality from June to October. Among the bermudagrass cultivars, Yukon showed the highest quality in both years. Differences in RLD among grasses appeared to be related to seasonal changes in air temperature and precipitation. In August 2006 Companion zoysiagrass averaged 3.5–4.4 cm cm⁻³ less RLD than bermudagrass cultivars La Paloma and Princess-77. Visual turf quality was positively correlated to the RLD and RMD in the 25–40 cm soil layer, whereas no correlations were found with root diameters nor any parameter collected in the upper soil layers. Deep-rooting cultivars may have an advantage in deficit soil moisture periods resulting in higher quality turf throughout the year.

Keywords:

• Cynodon dactylon
• drought tolerance
• root distribution
• root length density
• root mass density
• root system
• soil layer
• warm season grasses
• water conservation
• Zoysia japonica

Introduction

Turfgrasses are integral parts of European city architectural landscapes and their maintenance leads to important challenges in term of water use for irrigation (Leinauer et al. 2010). Growing cool season grasses in the transitional zones of the Mediterranean Europe has been questioned due to their high water use relative to warm season species (Volterrani et al. 2010, Rimi et al. 2011). In fact, the evapotranspiration rate for warm season turfgrasses, such as bermudagrass [Cynodon dactylon (L.) Pers.] and zoysiagrass (Zoysia japonica Steud.), ranges from 2 to 5 mm d⁻¹ compared with 3 to 8 mm d⁻¹ for cool season grasses (Beard 1994). However, differences in water use rates among warm season grasses have been further described, and large variation has been observed between cultivars within species (Huang 2008). Efficiency of the turfgrass canopy in using water is influenced by several epigeal traits, including leaf morphology, growth habitus, and turfgrass density, and also by rooting characteristics (Beard 1973, Shearman 1986).

Rooting depth and root density are critical for an efficient plant capacity of water supply from soil, especially under non-irrigated conditions. It is widely recognized that roots of various turfgrasses are able to spread in lower soil profiles as a response to loss of soil moisture in the upper layers (Beard 1973, Huang et al. 1997a, Huang 1999). Drought resistance mostly depends on the ability of plants to deepen roots for taking up water in the lower soil layers where moisture is more reliable (Huang 2008). Accordingly, differences in root system architecture among turfgrasses have been investigated with regard to drought avoidance and turf quality (Carrow and Duncan 2003, Suplick-Ploense and Qian 2005, Su et al. 2007). Su et al. (2008) found that tall fescue (Festuca arundinacea Schreb.) had higher root length density (RLD) than Kentucky bluegrass (Poa pratensis L.), together with a better visual quality under water deficit conditions. In water stressed warm season grasses, reductions of shoot growth have been attributed more to root viability than to root length (Huang et al. 1997b). In addition, wide differences in spatial root distribution have been observed in several species (Huang et al. 1997b, Suplick-Ploense and Qian 2005, Su et al. 2007, 2008), suggesting that influence of rooting characteristics on turf quality should be considered on a depth by depth basis.

Under limiting irrigation conditions, plant strategies for drought tolerance are also called to meet a human requirement of desirable turf quality. Turf breeders and researchers have been working to obtain warm season turfgrass with a deep color, high density and uniformity, in addition to early spring green-up and late fall color retention (Munshaw et al. 2006, Patton et al. 2008, Schiavon et al. 2011). Moreover, several studies have documented that warm season grasses interacted with seasonal weather patterns in determining turfgrass quality (Kopec et al. 2007, Sevostianova et al. 2011) and root growth (Ueno and Yoshihara 1967, Beard 1973). However, the relationships occurring between root system architecture and turf quality are still unclear. A field experiment was conducted to obtain information on differences in visual quality and rooting characteristics of warm season grasses in a transition zone environment.

Materials and methods

Site, plant material, and management practices

A field study was conducted at the experimental agricultural farm of Padova University in Legnaro, north-eastern Italy (45° 20′ North, 11° 57′ East, elevation 8 m) during the years 2006–2007. The area has a subtropical climate with hot, humid summers and average annual minimum temperature range of 9.4 to 12.2 °C, with an annual rainfall of 820 mm (Figure 1). The soil at the site was a silty loam (16% clay, 66% silt, and 18% sand), with a pH of 8.1, 2.3% organic matter, an available phosphorus content of 38.3 mg kg⁻¹, and an exchangeable potassium content of 178.1 mg kg⁻¹. Grasses used in this study were seeded on 4 July 2005 and included bermudagrass cultivars La Paloma, Princess-77, and Yukon, and Companion zoysiagrass. A slow-release fertilizer (20 N – 5 P₂O₅ – 8 K₂O) was applied monthly from May to September at a rate of 4 g m² of N. Plots were mowed weekly with a rotary mower at a height of 32 mm and clippings were removed. Except the first month of the establishment period (July 2005), no supplemental irrigation to natural precipitation was provided during the study. No pesticides, insecticides, or herbicides were used during the present study.

Figure 1.  Monthly mean air temperatures and monthly precipitation from 2006 to 2007 and long-term averages (1963–2007) at the agricultural experimental farm of Padova University, Legnaro, north-eastern Italy.

Turf quality and rooting characteristics

The experimental design was a randomized complete block with three replicates. Grasses served as the whole plots (4.5×1.6 m) treatment and soil depth as subplots. Turf quality ratings were collected on a weekly schedule from April to November of 2006–2007. Plots were visually rated for quality using a 1–9 scale, where 1 = dead, 6 = acceptable and 9 = ideal. Turfgrass quality ratings were taken according to the guidelines of the National Turfgrass Evaluation Program (NTEP), based on combination of color, density, uniformity, texture, weed infestation, and susceptibility to environmental stresses (Krans and Morris 2007). Bermudagrass and zoysiagrass had their own standard (National Turfgrass Evaluation Program 2011).

Within each plot, three root cores (40×5 cm diameter) were collected on 28 August 2006, 31 January, 14 May, and 12 September 2007 using a split tube sampler (04.17; Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands). Soil cores were cut into five sections representing the 0–3, 3–8, 8–15, 15–25, and 25–40 cm layers. To disperse soil particles, a 2:98 (v/v) oxalic acid/deionized water solution was used (Heringa et al. 1980) and successively roots were thoroughly washed. After washing, roots were placed in a 12:88 (v/v) ethanol/deionized water solution and stored at 4 °C to be subsequently analysed by means of a computerized root measuring system, WinRHIZO (Ver. 2003b; Regent Instruments, Quebec City, Canada). Roots were washed using deionized water and placed on an image scanner (Epson Expression 1600; Epson America, Long Beach, CA, USA) equipped with a water-filled tray to minimize root overlap (Beasley and Branham 2007). Rhizomes and roots were identified according to their growth habit, and rhizomes were discarded from the measurements. Roots were analysed for total root length (cm) and average diameter (mm), at a resolution of 1016 dots per cm. After image analysis, roots were dried in a forced air oven at 105 °C for 2 h and weighed to determine total root mass (mg). For each core section, RLD and root mass density (RMD) were respectively calculated as the ratio of total root length and total root mass to the volume of soil (Baldwin et al. 2009).

Statistical analysis

The turfgrass quality ratings that were collected weekly were averaged every month prior to data analysis. The rooting characteristics measured in the three root cores collected within each plot were averaged to obtain one value (Su et al. 2008). For the data, homogeneity of the error variance was verified based on graphic diagnostics of studentized residual plots. Turf quality, RLD, RMD, and root diameter were statistically analysed using a repeated measures analysis of variance with SAS Proc Mixed (version 9.2; SAS Institute, Cary, NC). Three types of covariance structures were compared for the repeated measures: compound symmetry, autoregressive order one, and spatial power, using the Akaike information criterion (AIC) and the REPEATED statement in Proc Mixed (Littell et al. 1996). A compound symmetry covariance structure generally resulted in the best fit for the data (lowest AIC values); therefore repeated measures were analysed using the compound symmetry structure. Tukey's honestly significant difference test was used at the 0.05 probability level to identify significant differences among means. Proc Corr and Proc Reg (version 9.2; SAS Institute, Cary, NC) were used to correlate rooting characteristics (RLD, RMD and average root diameter) of 28 August 2006, 14 May, and 12 September 2007 with turf quality ratings assigned to plots within 24 h of the root samples being taken (Table I). Diagnostics for linear relationships were performed using the Shapiro Wilk test at P>0.05 to ensure that residuals were normally distributed (Shapiro and Wilk 1965).

Table I. Turf visual quality of three bermudagrass (BD) cultivars and a zoysiagrass (ZY) on three evaluation dates in Legnaro, north-eastern Italy. Data are averaged over three replicates (means, standard deviations in parentheses).

	Visual quality ratings*
Cultivar	28 August 2006	14 May 2007	12 September 2007
Companion (ZY)	5.0 (0.0)	4.7 (0.6)	3.7 (0.6)
La Paloma (BD)	6.3 (0.6)	5.3 (0.6)	6.0 (0.0)
Princess-77 (BD)	7.0 (0.0)	6.3 (0.6)	6.0 (0.0)
Yukon (BD)	8.0 (0.0)	6.7 (0.6)	7.3 (0.6)
*Visual quality ratings on a 1 to 9 scale; where 1 = dead, 6 = acceptable and 9 = ideal.

Results and discussion

Turf quality

The analysis of variance for turf visual quality revealed significant two-way interaction between cultivars and months (Table II). Zoysiagrass Companion reached or exceeded the acceptable visual quality of 6.0 in May and June 2006, being in the top statistical group in April, May, and November 2006, and April and November 2007 (Figure 2). These results are in agreement with previous reports from Turkey, documenting better visual quality for zoysiagrasses in comparison with bermudagrass cultivars in early spring (Severmutlu et al. 2011). Companion zoysiagrass had lower visual quality than bermudagrass cultivars from June to September of both years, which suggests poor adaptability to non-irrigated conditions. These findings couple with those of Sevostianova et al. (2011), who reported that zoysiagrass De Anza exhibited lower summer quality than bermudagrass cultivars in a semiarid environment.

Figure 2.  Turf quality (visual ratings on a 1–9 scale; where 1 = dead, 6 = acceptable and 9 = ideal) of bermudagrass (BD) and zoysiagrass (ZY) cultivars from April to November of 2006–2007. Data points indicate weekly ratings that were averaged every month and over three replicates. Within each month, different letters denote statistical differences according to Tukey's honestly significant difference test (p<0.05).

Table II. Results of analysis of variance for root length density (RLD), root mass density (RMD) and root diameter of four warm season turfgrasses.

Effect	Turf visual quality	RLD	RMD	Root diameter
	p>F
Cultivar (C)	<0.0001	<0.0001	0.0012	0.6346
Sampling date (S)*	<0.0001	<0.0001	<0.0001	<0.0001
C×S	<0.0001	0.0013	0.0419	0.2087
Depth (D)		<0.0001	<0.0001	<0.0001
C×D		0.0011	0.7811	0.9987
D×S		<0.0001	<0.0001	0.0161
C×D×S		0.4210	0.1078	0.9942
	%
Coefficent of Variation	28	88	113	15
*Sampling date for ‘Turf visual quality’ indicates weekly ratings that were averaged every month from April to November of 2006–2007.

Among bermudagrass cultivars, Yukon showed highest turf quality in spring 2006 (April and May) (Figure 2), which may be explained by early spring green-up compared with the other bermudagrasses in the study area (Macolino et al. 2010, Rimi et al. 2011). In addition, Yukon generally displayed better visual quality than La Paloma in summer 2006, and better than La Paloma and Princess-77 in summer 2007 (Figure 2). These results are similar to those reported by Patton et al. (2008), indicating that Yukon had better turf quality than Princess-77 based upon data collected in ten locations across the USA.

Rooting characteristics

Significant two-way interactions cultivar×sampling date, cultivar×depth, and depth×sampling date affected RLD of the warm season turfgrasses studied (Table II). However, the three-way interaction was not significant (Table II), consequently the data were pooled over sampling dates (Table III), depths (Table IV), or cultivars (Figure 3). Averaged over sampling dates, RLD decreased for all the grasses tested when passing from the upper soil layer (0–3 cm) to the 3–8 cm, then to the 8–15 cm layer (Table III). This was expected, as it has been widely reported that the root system of turfgrasses is primarily superficial and dramatically decreases toward the upper 20 cm soil layer (Huang et al. 1997b, Suplick-Ploense and Qian 2005, Su et al. 2008). Differences in RLD among the lower soil layers (8–15 cm, 15–25 cm, 25–40 cm) were observed only for Companion zoysiagrass, such that RLD in the 25–40 cm layer was ≈ five times smaller compared with the 8–15 cm layer (Table III). Sustained 25–40 cm RLD of bermudagrass cultivars could explain their better turf quality compared with Companion zoysiagrass under drought stress (Figures 1 and 2).

Figure 3.  Root length density (RLD), root mass density (RMD), and root diameter of warm season turfgrasses at five soil depths as affected by sampling date. Data were averaged over four grasses (three bermudagrass cultivars and a zoysiagrass) and three replicates. Within each date, different letters denote statistical differences according to Tukey's honestly significant difference test (p<0.05).

Table III. Root length density of bermudagrass (BD) and zoysiagrass (ZY) cultivars at five soil depths. Data were averaged over four sampling dates (28 August 2006, 31 January, 14 May, and 12 September 2007) and three replicates.

	Root length density (cm cm^−3)
Soil Depth	Companion (ZY)	La Paloma (BD)	Princess-77 (BD)	Yukon (BD)
0–3 cm	17.8 a*	24.3 a	24.1 a	19.7 a
3–8 cm	10.5 b	13.7 b	9.3 b	9.9 b
8–15 cm	5.9 c	6.9 c	5.5 bc	6.0 c
15–25 cm	3.0 cd	5.6 c	3.8 c	3.9 c
25–40 cm	1.1 d	3.6 c	2.9 c	2.4 c
*Within each column, means sharing a letter are not statistically different according to Tukey's honestly significant difference test (p<0.05).

Table IV. Root length density and root mass density of bermudagrass (BD) and zoysiagrass (ZY) cultivars on four sampling dates. Data were averaged over five depths (0 to 3, 3 to 8, 8 to 15, 15 to 25, and 25 to 40 cm soil layers) and three replicates.

	Root length density (cm cm^−3)				Root mass density (mg cm^−3)
Cultivar	28 August 2006	31 January 2007	14 May 2007	12 September 2007	28 August 2006	31 January 2007	14 May 2007	12 September 2007
Companion (ZY)	9.0 b*	6.5 a	6.8 a	8.4 b	0.74 b	0.76 b	0.95 ab	1.11 b
La Paloma (BD)	13.4 a	8.0 a	8.2 a	13.7 a	1.35 a	1.19 a	1.06 a	1.42 a
Princess-77 (BD)	12.5 a	6.1 a	6.7 a	11.1 ab	1.07 a	0.66 b	0.67 b	1.23 ab
Yukon (BD)	11.4 ab	6.2 a	6.3 a	9.6 b	1.13 a	0.73 b	1.01 a	1.07 b
*Within each column, means sharing a letter are not statistically different according to Tukey's honestly significant difference test (p<0.05).

When the data were pooled over the five depths, differences in RLD among grasses were observed on 28 August 2006 and 12 September 2007 (Table IV). In August 2006, Companion zoysiagrass averaged 3.5–4.4 cm cm⁻³ less RLD than bermudagrass cultivars La Paloma and Princess-77. In September 2007, La Paloma had an average RLD 4.1–5.3 cm cm⁻³ higher compared with Companion and Yukon. These results, low root exploration capacity for Companion zoysiagrass relative to bermudagrasses, agreed with Huang et al. (1997b), who reported that roots of zoysiagrass Emerald had lower vitality compared with common bermudagrass. No statistical differences in RLD between the grasses tested in January and May 2007 (Table IV) could be due to abnormal root activity, as the result of winter dormancy or spring green-up under drought stress (Figure 1). When the data were averaged over cultivars, differences in RLD among the 8–15 cm, 15–25 cm, and 25–40 cm soil layers were not observed on 31 January 2007 (Figure 3). Thus, the magnitude of difference in RLD between soil depths was greater during the active growing season than during dormancy. From a general standpoint, RLD revealed a seasonal trend with reduced root exploration capacity during dormancy and renewed RLD after spring green-up, in accordance to previous studies (Kennedy 1929, Beard 1973).

The analysis of variance for the RMD revealed significant two way interactions between cultivars and sampling dates and between depths and sampling dates (Table II). In the average of the five depths, the differences between cultivars varied widely across the study period, leading to an unclear trend in RMD when cultivars were compared (Table IV). Cultivar responses for RMD partially contrasted with those for RLD (Table IV), as RMD seemed more sensible than RLD in responding to winter cultivar differences. This difference between the two parameters could be due to contrasting associations to the root vitality, with root length being more important than mass in maintaining physiological function. Moreover, increases in RMD were relatively high for Companion and Yukon in May 2007 (Table IV), which coupled with high turf quality (Figure 2) and can be explained by early recovery from winter dormancy.

The significant interaction depth×sampling date was related to the environmental differences within and between years and affected RMD similarly to RLD (Figures 1 and 3). However, averaged over cultivars, on 12 September 2007 there were no differences in RMD among the 8–15 cm, 15–25 cm, and 25–40 cm soil layers, differing from the RLD responses (Figure 3). On all sampling dates, a neat decrease in RMD occurred from the 0–3 cm to the 3–8 cm soil layer, passing from 2.5–3.5 to 0.8–1.3 mg cm⁻³. Compared to RLD, the extent of difference between these two soil layers appeared of greater importance for RMD (Figure 3), suggesting that the two traits studied may have different biological implications. These results are similar to those of Su et al. (2008), who pointed out that RLD and RMD of several cool season grasses were unlikely related to the soil depth.

Significant interaction between depths and sampling dates affected root diameter, whereas the main effect of cultivar and all interaction terms including cultivar were not significant (Table II), therefore the data were averaged over cultivars (Figure 3). The average diameter of the grasses studied was ≈ 0.2 mm, however differences between the soil depths occurred on 28 August 2006 and 12 September 2007 (Figure 3). Root diameter in the 0–3 cm soil layer was 0.04 mm larger compared with the deeper layers in August 2006, and 0.06–0.08 mm larger in September 2007. These findings indicate that root diameter relatively increased in the superficial soil layer during the active growing season, which may be due to a high presence of parenchymatous cells (Beard 1973, Taiz and Zeiger 1998).

Correlations between turf quality and rooting characteristics

Root diameter and RLD were negatively correlated in the 3–8 cm and 8–15 cm soil layers (Table V), confirming that root diameter of turfgrasses decreases as primary elongation advances (Beard 1973, Taiz and Zeiger 1998). Root diameter was also positively correlated with RMD in the 0–3 cm and 3–8 cm soil layers (Table V), suggesting a prominent influence of parenchymatous cells on rooting structures. As expected, the RLD and RMD were positively correlated at each depth (Table V), such that root yield was greater as root exploration capacity increased. In addition, visual turf quality was significantly and positively correlated with both RLD and RMD in the 25–40 cm soil layer (Table V). Correlations between turf quality and rooting characteristics were not detected in the upper soil layers.

Table V. Correlation coefficients relating root diameter, root length density (RLD), root mass density (RMD), and turf visual quality for warm season turf grasses. Data were collected on four grasses over three sampling dates and three replicates (n=36).

		Pearson correlation coefficient (r)
Depth	Parameter	RLD	RMD	Turf quality
0–3 cm	Root diameter	0.20 ns	0.44 **	0.03 ns
	RLD		0.70 ***	0.33 ns
	RMD			0.10 ns
3–8 cm	Root diameter	−0.46 **	0.36 *	−0.29 ns
	RLD		0.51 **	0.12 ns
	RMD			−0.10 ns
8–15 cm	Root diameter	−0.49 **	0.13 ns	−0.22 ns
	RLD		0.45 **	0.15 ns
	RMD			0.01 ns
15–25 cm	Root diameter	−0.18 ns	0.29 ns	0.03 ns
	RLD		0.75 ***	0.33 ns
	RMD			0.22 ns
25–40 cm	Root diameter	−0.20 ns	−0.03 ns	0.01 ns
	RLD		0.85 ***	0.46 **
	RMD			0.41 *
*p<0.05, **p<0.01, ***p<0.001, ns = not significant, p≥0.05.

These results suggest that the cultivar differences in root density found in the deepest layer were of greater interest for turf quality. Visual turf quality takes in account several parameters, including leaf color and texture, turf density and uniformity; each of them differently influences the individual making the evaluation (Krans and Morris 2007). Results of this study show an association between visual turf quality and root density in mass and length at 25–40 cm. Further research is needed to learn more about the implications of rooting characteristics on the basic components of turfgrass quality, especially for species displaying a broad range of quality such as zoysiagrass.

Acknowledgements

Appreciation is given to Marisa Cossu for her technical assistance in the performance of the experiments. The authors are also grateful to two anonymous reviewers and the associate editor for their comments and corrections on the manuscript. This research was supported by the Italian Ministry of Instruction, University and Research (MIUR).