Canopy Performance and Root System Structure of New Genotypes of Zoysia spp. During Establishment Under Mediterranean Climate

Abstract

In a hypothetical climate change scenario, zoysiagrass species could be a good choice for turfgrass areas due to their adaptation to heat conditions and the great variability in species and cultivars. Knowledge of the root system’s characteristics is paramount for predicting cultivar adaptation to different heat–drought scenarios and therefore for designing proper turfgrass management programs, especially irrigation. A field experiment was conducted in the Mediterranean environment of Valencia (Spain) to study the root weight density (RWD), root length density (RLD), and average root diameter (RDI) at three different soil depths (0–5, 5–15, and 15–30 cm) of five new zoysiagrass genotypes (Zoysia matrella (L.) Merr. cultivars, Zoysia japonica Steud., and their hybrid), relating these parameters to the performance of these cultivars during their establishment. All the tested cultivars had a higher RWD and RLD in the upper soil layer (0–5 cm), while the RDI was higher in the lowest layer of the sampled soil profile (0.269 mm compared with 0.249 mm and 0.241 mm in the upper layers). All the tested cultivars showed the same RWD and RLD, while the Zoysia matrella cultivar ZG18003 showed a higher RDI value (0.2683 mm) than those for the Z. japonica cultivar (0.2369 mm) and the hybrid (0.2394 mm). This last finding could have influenced its more rapid establishment, although it was not linked to its NDVI values during autumn. In conclusion, different morphological root characteristics were detected among new zoysiagrass genotypes and soil depths, which could have affected their canopy performance, and they are expected to affect irrigation management in a possible future drought scenario.

1. Introduction

In some areas of the world, turfgrass is becoming the primary land cover within urban and suburban pervious environments [1], producing real benefits that may be divided into functional, recreational, and aesthetic components [2]. However, turfgrass cultivation is not easy; the final user would prefer to have a permanent green area throughout the year, but turfgrass species typically thrive only in specific seasons and suffer from abiotic stresses during the rest of the year. Heat is probably one of the most troublesome abiotic stresses affecting turfgrass systems as it limits the growth of turfgrasses causing cellular and physiological damage [3]. Heat stress can be mitigated by proper irrigation management, but, in many urban and suburban areas of the world, water is scarce and in high demand. Besides this, some of the predicted changes in climate such as higher summer temperatures and prolonged droughts will influence turfgrass water demand [4]; therefore, turfgrass cultivation will be more challenging in the future. In this adverse predicted scenario, the careful selection of turfgrass species and cultivars will be essential, with warm-season species playing an important role despite their lack of low-temperature tolerance. The Mediterranean region lies almost entirely within the transition zone [5], in which both cool-season and warm-season grasses can be successfully cultivated. Warm-season species are typically favored over cool-season species in these regions due to their lower water consumption, more efficient soil moisture use, and reduced evapotranspiration rates [6]. However, due to the increasing need to minimize water consumption driven by climate change and extended drought periods, the success of these species largely relies on their drought stress tolerance [7,8].

Zoysiagrass (Zoysia spp. Willd.) is a perennial warm-season grass adapted to the tropical and southern temperate regions of the world [8]. The genus Zoysia contains eleven species, but only three of them (Z. japonica Steud; Z. matrella (L.) Merr.; Z. pacifica (Goudswaard) Hotta & Kuroki) and their interspecific hybrids have been used for various turfgrass applications [9]. These species require minimal maintenance inputs, including water; therefore, they could play a key role in transition zone environments, making sports turfs and lawns more environmentally friendly and sustainable [10]. A 3-year study with 21 zoysiagrass genotypes concluded that the mean supplemental irrigation required to prevent water stress ranged from an average of 93 to 488 mm per year [11], which is low compared with that needed for cool-season turfgrass species and for some warm-season turfgrass species. Several glasshouse and field studies on zoysiagrass [12,13,14] reported that morphological root characteristics, such as maximum root depth, total root weight, or root number, were positively correlated with turf canopy quality. The rooting depth and the root density are critical for the plant to efficiently access water from the soil, especially under non-irrigated conditions [15]. The distribution of root density within the soil profile is crucial for assessing drought stress tolerance. This is because drought conditions usually affect the upper layers of the soil due to evaporation. Deep roots enable plants to access water that remains available deeper in the soil profile for a longer period [16]. Previous research has demonstrated that zoysiagrass is characterized by a relatively shallow root system, which contributes to its diminished drought tolerance when compared with other warm-season grasses and tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) [15,17]. Since these rooting characteristics will likely contribute to drought resistance [12,13,18], the selection of the zoysiagrass cultivar becomes paramount, as it has been reported to significantly affect all rooting parameters [13]. There is currently no available information regarding the characteristics of the root system in newly developed zoysiagrass cultivars.

The hypothesis of this research was that the differences in the root system structure among zoysiagrass genotypes could influence their canopy performance. Therefore, the objective of this study was to detect the root characteristics of five different modern genotypes of zoysiagrass and their turfgrass quality during establishment in a Mediterranean environment.

2. Materials and Methods

2.1. Experimental Site

A field experiment was conducted from January to December 2024 on five Zoysia spp. vegetative genotypes: Z. matrella (‘ZG18003’, ‘ZG09004’, and ‘ZG09037’), Z. japonica (‘XZ11199’), and a hybrid Z. matrella × Z. japonica (‘ZG09070’). All five cultivars were planted by spreading the appropriate volume of sprigs in 6 m² plots on 11 of July 2023, at the Polytechnical University of Valencia research farm in Valencia, Spain (39°48′ N, 0°34′ W; 5 m asl). The soil in the experimental area was an alluvial soil, Calcaric Fluvisol according to the World Reference Base [19], characterized by a sandy loam texture consisting of 76.7% sand, 13.3% silt, and 10% clay, with only 3% of stones. It was alkaline (pH 8.5, 1:2.5 soil/water suspension), non-saline (EC 0.167 dS m⁻¹, 1:5 soil/water suspension), with low organic matter (24.9 g kg⁻¹), and a medium content of carbonates (29.6% CaCO₃). The monthly precipitation and air temperature in the study area during the evaluation period are reported in

Table 1. Monthly maximum, mean, and minimum temperature (°C) and monthly precipitation (mm) over the study period at the experimental agricultural farm of the Polytechnic University of Valencia in Valencia, Spain (39°48′ N, 0°34′ W; 5 m asl).

	Air Temperature (°C)			Precipitation
	Maximum	Mean	Minimum	(mm)
January	17.6	13.1	9.2	7
February	19.0	13.9	9.6	8
March	18.6	14.5	10.6	8
April	21.2	16.4	11.6	19
May	24.5	20.0	15.2	4
June	27.1	23.4	19.3	22
July	30.1	26.5	22.3	47
August	31.8	27.8	23.6	0
September	28.0	23.7	19.6	119
October	24.9	20.7	16.9	81
November	20.9	16.8	13.4	13
December	17.7	12.5	8.2	13

.

2.2. Plots Management and Determinations

The trial plot was irrigated as needed until full establishment, which was achieved between July and October 2024. Fertilization was performed monthly during the growing season with a 24-5-11 + 3 CaO complex fertilizer (Sportsmaster CRF Mini High N, AICL, Barcelona, Spain) at 30 g m⁻². Mowing was conducted twice a week with a rotary mower (Sterwins GCV160, Ronchin, France) at 3 cm height.

Two types of determinations were performed: weekly evaluations of the turfgrass canopy during 2024 and root characteristics through turfgrass root samples collected in October 2024. The weekly turfgrass evaluations consisted of (i) visual turfgrass coverage (%) from January to December 2024; (ii) visual turfgrass color based on a 1 to 9 scale with 1 representing light green and 9 representing dark green [20] from March to December 2024; (iii) visual turfgrass quality based on a 1 to 9 scale with 1 being the poorest, 6 acceptable, and 9 excellent [20] from September to December 2024; and (iv) Normalized Difference Vegetation Index (NDVI) of the canopy measured using a handheld crop sensor unit Model 505 (Trimble Corp., Sunnyville, CA, USA) held 70 cm above the turf canopy, producing NDVI values ranging from 0.00 to 0.99, from July to the end of 2024. All four mentioned determinations were plotted against time, and the area under the progress curve (AUPC) was calculated and compared among the Zoysia genotypes. Additionally, from each plot, three soil cores were collected on 2 October 2024, using a 30 mm diameter gouge auger (Royal Eijkelkamp, Giesbeek, The Netherlands). The cores were sectioned into three layers corresponding to depths of 0–5, 5–15, and 15–30 cm. To disperse soil particles, a 2:98 (v/v) oxalic acid/deionized water solution was applied following Heringa et al. (1980) [21], and the roots were then thoroughly washed. After washing, the roots were immersed in a 12:88 (v/v) ethanol/deionized water solution and stored at 4 °C for subsequent analysis using the WinRHIZO technology (Regent Instruments, Quebec City, QC, Canada) to determine, for each cultivar, the total root length and the average root diameter (RDI). The roots were oven-dried at 105 °C for 36 h and then weighed to determine the total root weight for each cultivar. Finally, the root weight density (RWD) and the root length density (RLD) were calculated based on the volume of the soil core for each depth layer.

2.3. Statistical Analysis

Plots were arranged in a randomized block design with three replicates. Different types of analysis of variance (ANOVA) tests were employed. A repeated-measures ANOVA was performed to test the effect of the Zoysia genotype, sampling dates, and their interaction on turfgrass coverage, NDVI, color, and turfgrass quality. Only the turfgrass coverage and NDVI results are presented; data on the color and quality are not shown for conciseness. An ANOVA was performed to test the effect of the Zoysia genotype on the area under the progress curve (AUPC) of turfgrass coverage, turfgrass color, turfgrass quality, and NDVI; and a multifactorial ANOVA was performed to test the effects of the genotype, soil depth, and their interaction on RWD, RLD, and RDI. The data were transformed when necessary to ensure the normality and homoscedasticity of the residuals and then back-transformed to obtain the final results. Statistical analysis was performed with R, Version 4.3.2 (Copyright © 2022 by Posit Software, Version 2024.12.0+467, PBC, Boston, MA, USA). Additional packages were used, including lm for fitting the model with single, multifactorial, or repeated-measures analysis of variance, and predictmeans for Fisher’s protected least-significance-difference test at p = 0.05.

3. Results

3.1. Canopy Assessment Evolution

The evolution of turfgrass coverage and NDVI is shown in Figure 1 and Figure 2, respectively, while the evolution of the turfgrass’s green color and quality is not shown.

The cultivar, sampling date, and the interaction between cultivar and sampling date were significant for turfgrass coverage (p < 0.001). The Z. matrella cultivars were more rapid in covering the plot as they achieved 50% coverage between January and late April, while Z. japonica and the hybrid reached this 50% coverage from May to July (Figure 1). The Z. matrella cultivar (ZG18003) was the fastest, reaching almost 100% coverage, while the hybrid (ZG09070) was the slowest.

The interaction between cultivar and sampling date was significant for NDVI. The NDVI evolution of the five genotypes (Figure 2) shows differences among cultivars during autumn and winter in which the Z. japonica cultivar (XZ11199) on some evaluation dates (mostly in December) had a higher NDVI than all the other cultivars, especially the Z. matrella (ZG18003) cultivar, which had the lowest values from late September until the end of the study.

The area under the progress curve of the two significant turfgrass canopy determinations is shown in

Table 2. Area under the progress curve of the evolution of four determined parameters over the turfgrass canopy (coverage and color) at the experimental agricultural farm of the Polytechnic University of Valencia in Valencia, Spain (39°48′ N, 0°34′ W; 5 m asl). Different letters within a column indicate statistically significant differences (Tukey test, p = 0.05 for comparing means).

Species	Cultivar	Coverage	Color
Z. matrella	ZG18003	31,503.0 a	1875.5 a
Z. matrella	ZG09004	28,151.7 ab	1846.6 ab
Z. matrella	ZG09037	26,058.0 ab	1927.3 a
Z. japonica	XZ11199	23,454.2 b	1696.3 b
Z. matrella × Z. japonica	ZG09070	21,655.0 b	1804.1 ab

. The cultivar effect was significant for coverage (p = 0.01) and color (p = 0.001) but not for quality and NDVI (p-values = 0.32 and 0.1, respectively). Plot coverage after the turf implantation in July 2023 showed that the ZG18003 cultivar of Z. matrella had a higher AUPC than the other genotypes including the Z. japonica XZ11199 and Z. matrella × Z. japonica ZG09070. Regarding the turf’s green color, the ZG18003 and ZG09037 cultivars of Z. matrella had a higher AUPC than the Z. japonica cultivar (XZ11199). The turfgrass quality and NDVI were determined from July to December 2024, with no differences among cultivars. However, as previously mentioned, during late season, the Z. japonica cultivar (XZ11199) obtained higher NDVI values than most of the other genotypes (Figure 2).

3.2. Root System Parameters

The RWD, RLD, and RDI comparisons among cultivars and depths are shown in

Table 3. Multifactorial ANOVA results (p-values) for root weight density (RWD), root length density (RLD), and root diameter (RDI) when comparing five Zoysia genotypes (Z. matrella (ZG18003, ZG09004, and ZG09037), Z. japonica (XZ11199), and Z. matrella × Z. japonica (ZG09070)) and three soil depths (0–5, 5–15, 15–30 cm) at the experimental agricultural farm of the Polytechnic University of Valencia in Valencia, Spain (39°48′ N, 0°34′ W; 5 m asl).

Effect	RWD	RLD	RDI
cultivar	0.198 (ns)	0.206 (ns)	0.008 (**)
depth	0.000 (***)	0.000 (***)	0.003 (**)
cultivar × depth	0.216 (ns)	0.176(ns)	0.749 (ns)

** p < 0.01; *** p < 0.01; ns: not significant at the 0.05 probability level.

and in Figure 3, Figure 4 and Figure 5, respectively.

The ANOVA (Table 3) displayed that the genotype factor was significant for RDI; the depth factor was significant for all measured parameters; and the interaction of both factors was not significant for any measured parameter.

There were no differences among cultivars for RWD, which averaged 0.0113 g cm⁻³ for the five cultivars. Regarding the RWD among soil depths, the 0–5 depth had more RWD (0.0223 g cm⁻³) than the other two explored soil depths, with no differences between 5–15 cm (0.0082 g cm⁻³) and 15–30 cm (0.0035 g cm⁻³) (Figure 3).

With respect to RLD, all the five tested cultivars again had the same length density, averaging 89.3 cm cm⁻³. However, the RLD among soil depths was different. The upper layer (0–5 cm) had 154.4 cm cm⁻³ of RLD, higher than the RLD of the 5–15 cm layer (74.4 cm cm⁻³), with the 15–30 cm layer obtaining the lowest RLD (39.3 cm cm⁻³) (Figure 4).

Figure 5 shows the RDI comparison among cultivars and depths, with statistical differences among both factors. One of the Z. matrella cultivars (ZG18003) had a greater RDI (0.2683 mm) than the hybrid (ZG09070) and the Z. japonica cultivar, with 0.2369 and 0.2394 mm, respectively. The depth was also significant, and roots in the 15–30 soil profile had a greater average diameter (0.268 mm) than roots in the 0–5 and 5–15 cm soil profiles, averaging 0.249 and 0.241 mm, respectively, which did not differ from each other.

4. Discussion

Little field research has been conducted on the root system characteristics of zoysiagrass cultivars and their relation to canopy performance. In this study, five zoysiagrass genotypes were compared during 2024 after being planted in July 2023.

In the Mediterranean environment of Valencia, the Z. matrella cultivars’ establishment, especially that of cultivar ZG18003, was faster than the establishment of Z. Japonica and the hybrid. This result differed from those in the previous studies conducted by Patton et al. 2007 [22] in West Lafayette, IN, USA, and by Volterrani et al. [23] in Pisa, Italy, who found faster establishment for Z. japonica than for Z. matrella. The green color of two out of three Z. matrella cultivars (ZG18803 and ZG09037) was also more pronounced throughout the year than that of the Z. japonica cultivar (XZ11199). However, when the turfgrass was evaluated for NDVI and turf quality, all the tested genotypes performed the same in terms of AUPC, although with some relevant NDVI differences among genotypes in autumn. The analysis of root characteristics in the soil profile showed that all the zoysiagrass cultivars had the same RWD and RLD, with no differences between Z. matrella and Z. japonica cultivars that could have explained the Z. matrella cultivars’ better performance in terms of coverage and green color during the first part of the year. However, ZG18003 Z. matrella had more RDI than XZ11199 Z. japonica and the hybrid cultivar (ZG09070). Nevertheless, Fuentealba et al., 2015 [12], reported that Z. japonica had a higher rate of root depth development (3.32 cm d⁻¹) than that of Z. matrella (2.50 cm d⁻¹) with differences among cultivars in both species; this positive trait of Z. japonica could be particularly useful in a drought scenario.

All five zoysiagrass tested cultivars showed higher RWD and RLD values at the 0–5 cm depth than at the 5–30 cm soil profile, which indicates that they will require slightly greater and more frequent supplemental irrigation to produce acceptable quality than other turfgrass species, such as common bermudagrass (Cynodon dactylon (L.) Pers.), that have a more uniform RLD through the soil profile [12,22]. The root diameter values of all the cultivars were higher at the deepest soil layer (10–15 cm). This result could be linked to the physical properties of the soil, which may have greater porosity in the upper layers than in the lower part of the soil (below 15 cm), which tends to increase root diameter [24].

5. Conclusions

These preliminary results reveal variations in the performance of the five tested zoysiagrass cultivars. Notably, Z. matrella cultivars exhibited superior ground coverage and green color during the early part of the year. For all the cultivars, the root system was primarily concentrated within the top 5 cm of the soil, confirming the relatively shallow rooting typical of these warm-season species. The diameter of the roots deserves further investigation, as it may be a crucial factor in selecting cultivars. We found that the Z. matrella cultivar ZG18003 exhibited a greater root diameter than both the Z. japonica cultivar and the hybrid.