Effects of foliar silicon and nitrogen applications on winter color retention and spring green-up of zoysia grass

Çakır, M., Sever Mutlu, S., and Dönmez, Ş. (2024). "Effects of foliar silicon and nitrogen applications on winter color retention and spring green-up of zoysia grass," BioResources 19(4), 8577–8593.

Abstract

Zoysia japonica Steud. (zoysia grass), with its high drought, shade, and salt tolerance, it is an excellent choice for green areas. However, in regions with subtropical climates, it goes into winter dormancy with a loss of green color and functionality, which is a main barrier to its widespread use. The application of silicon and nitrogen in fall was hypothesized to enhance winter color retention of Z. japonica. This study assessed the impact of fall (late-season) nitrogen (0, 2.5, or 5.0 g/m² ammonium sulfate) and foliar silicon (0, 3, or 6 mL/L potassium silicate) applications on the winter color retention of Z. japonica grown in the field. The experiment was conducted over two consecutive growing seasons in Antalya, Türkiye. Turfgrass quality, color, chlorophyll content, shoot density, and winter dormancy were all improved by late-season nitrogen application. Overall, two sequential nitrogen applications at 5 g/m² in fall provided 65% to 100% green coverage with acceptable turfgrass quality during fall and winter, indicating the possibility of maintaining the year-round green color of Z. japonica in subtropical climates. However, the silicon treatment did not affect the winter color retention of Z. japonica. The apparent lack of a beneficial response of Z. japonica to the silicon application might be due to the dose, application methods, and silicon source.

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Effects of Foliar Silicon and Nitrogen Applications on Winter Color Retention and Spring Green-Up of Zoysia Grass

Mert Çakır,^a,* Songül Sever Mutlu,^b and Şirin Dönmez ^a

DOI: 10.15376/biores.19.4.8577-8593

Keywords: Turfgrass management; Dormancy; Stress tolerance; Silicon fertilization

Contact information: a: Department of Landscape Architecture, Faculty of Architecture, Süleyman Demirel University, 32200, Isparta, Türkiye; b: Department of Horticulture, Faculty of Agriculture; Akdeniz University, 07059, Antalya, Türkiye; *Corresponding author: pmmertcakir@gmail.com

INTRODUCTION

Zoysia japonica, a warm-season turfgrass, is an excellent choice for Mediterranean areas. It is commonly used in warm-humid and warm-arid areas because of its desirable characteristics, such as high tolerance to heat and drought stresses and low maintenance requirements. However, winter dormancy, resulting in a decline in color and overall aesthetic and developmental stagnation, is the main barrier to the widespread use of Z. japonica in the region (De Luca et al. 2008), where chilling, referring to a non-freezing range of temperatures (0 to 12 °C), is common in winter and early spring. When the average minimum air temperature falls below 15 °C for 15 consecutive days, Z. japonica discoloration begins, followed by yellowish color and withering during the winter season (Wei et al. 2008). The complete loss or decline in winter color poses a challenge for turf managers who want year-round visual appeal and functionality in their landscapes. Therefore, improving Z. japonica’s color retention in response to sub-optimal temperatures is important.

In the 1970s, dormant lawns were dyed with special coloring dyes. However, this method was not very popular because it could not increase playability on sports fields and could not provide a natural green color. To create a temporary, green, and playable field, cool-season grasses are frequently overseeded into warm-season turf in the fall. This expensive practice has yielded quite successful results for Cynodon spp. However, overseeding has not been as successful with Zoysia spp. due mainly to the difficulty of overseeded Zoysia spp. recovery in the spring (Hurley et al. 1989; Razmjoo et al. 1996).

Applying late-season (fall) nitrogen (N) or iron applications to increase the winter color retention of Zoysia spp. has been shown to be effective to varying degrees (Gibeault et al. 1997; Munshaw et al. 2006; Patton et al. 2010; Volterrani et al. 2010). Studies have indicated that applying N near the end of the growing season to Cynodon dactylon, a warm-season turfgrass species, can enhance winter color retention and boost spring greening without compromising resistance to cold (Richardson 2002). However, discrepancies exist in how late N treatment affects the winter and spring performances of Z. japonica. Z. japonica may respond to late-season N application by prolonging fall color without promoting early spring green-up (Dunn et al. 1993). Fall N fertilization did not cause winter injury in Z. japonica ‘Meyer’, but because of application, slower spring green-up and brown patch development in spring were observed in Z. japonica ‘FZ-102’ (Volterrani et al. 2010). Late N application to Z. matrella ‘Zeon’ was also effective in delaying low-temperature discoloration; however, the timing of application was found to be a potential risk for delaying spring green-up (Volterrani et al. 2010). These results support the need for further research to determine if late-season N can enhance winter color retention of Z. japonica without delaying spring green-up. Understanding the effects of N fertilization on winter color retention can help turfgrass managers optimize fertilizer applications for improved winter performance in the Mediterranean environment.

Silicon (Si) is a potentially beneficial nutrient to enhance plant resistance to various stresses such as low temperature (Epstein 2009), salinity tolerance (Liang et al. 2003), ultraviolet radiation (Shen et al. 2010), and high temperature (Hattori et al. 2005). The foliar applications of Si have been shown to influence chilling tolerance in turfgrasses by enhancing physiological processes and stress tolerance mechanisms (He et al. 2010). Therefore, the application of exogenous Si can be an alternative approach to alleviating low-temperature stress in plants (Epstein 1999, 2009). Si is widely promoted and marketed as a turfgrass management supplement (Datnoff and Rutherford 2003). Three main areas have dominated research on the application of Si to turfgrass: reducing salinity and drought stress, improving wear tolerance, and suppressing plant diseases (Guertal and Datnoff 2021). However, its effects on the winter color retention ability of warm-season turfgrasses remain relatively unexplored.

The fall-winter green color of Z. japonica could potentially be maintained for a longer period with Si and N treatments in the fall. To date, research on Si along with N to enhance the winter color retention of Z. japonica is scant. Therefore, this study aimed to investigate the effects of foliar Si and N applications on winter color retention and spring green-up of Z. japonica under Mediterranean climate conditions. The results may promote wider use of environmentally friendly Z. japonica in subtropical regions.

EXPERIMENTAL

Materials and Methods

The experiment was conducted over two consecutive growing seasons (2018-2019) at Akdeniz University, located between 36° 53′ 59″ north latitude and 30° 38′ 25″ east longitude in Antalya, Türkiye. The study was established on a mature sward of ‘El-Toro’ Z. japonica growing on a silty clay loam (Typic Xerochrept/Paralithic Xerortent). The soil’s physical and chemical properties were determined according to Jackson (1962). The soil had 1.49% organic matter (OM), an electrical conductivity (EC) of 0.21 dS/m, a pH of 8.2, an N level of 399 ppm, a P level of 8.5 ppm, a K level of 5963 ppm, and 27.21% CaCO₃. The climate in the research region is typical of the Mediterranean. The meteorological data are presented in Table 1.

Table 1. Monthly Mean Minimum and Maximum Air Temperature and Monthly Mean Precipitation Values

The field design was a split plot with three replications. Main plots, 2 m × 4.5 m, were fertilized at 0 (N₀), 2.5 (N₁), or 5.0 (N₂) N g/m² from ammonium sulfate [(NH₄)₂SO₄; N: 21% and S: 24%]. The application levels for N were based on Bowman et al. (2002), Oh et al. (2015), and Miller et al. (2016). The application levels for Si were based on Ibrahim et al. (2015). Subplots, 2 m × 1.5 m, were fertilized at 0 (Si₀), 3.0 (Si₁), or 6.0 (Si₂) mL/L Si from potassium silicate (K₂SiO₃; K₂O: 12.79% and SiO₂: 26.19%). Hereafter, N₀ will be referred to as no N application, N₁ as low rate N application, N₂ as high rate N application, Si₀ as no Si application, Si₁ as low rate Si application, and Si₂ as high rate Si application. Weighted amounts of N were applied to plots by hand. Approximately 10 mm of irrigation was applied following each N fertilization. Using a spray shield and a single-nozzle CO₂-pressurized sprayer, the potassium silicate solutions were administered in a 10-L spray volume the day following N fertilization, ensuring complete plot coverage and promoting foliar uptake. The treatments were applied first on September 15^th and then four weeks later on October 13^th, 2018. When the plots reached a height of 7.5 cm, they were mowed at a height of 5 cm with a reel mower. Irrigation was provided throughout the study to maintain adequate soil moisture. During the experiment, no chemicals were applied to the plots, and weed control was mechanically completed. The study was repeated in 2019.

Measurements of turfgrass quality, color, grass index, and chlorophyll content were taken biweekly. The turfgrass quality, as an integral of color, uniformity, and density, was visually assessed on a scale of 1 (the worst, dead, or completely dormant and brown turf) to 9 (the best and fully green turf), with 6 being acceptable turf (NTEP 2024). Turfgrass color, a measure of overall plot color, was visually rated on a scale of 1 to 9, with 1 being straw brown and 9 being dark green (NTEP 2024). The grass index (or green color index) value was collected with a handheld turf NDVI color meter (TCM 500 NDVI Spectrum Technologies Inc., Aurora, IL, USA). The turf color meter measures reflected light from grass in the red (600 nm) and near-infrared (850 nm-NIR) spectral bands. The device was placed in contact with the grass and pressed, which generated a grass index ranging from 1.0 to 9.0. The chlorophyll content value was collected using a handheld chlorophyll meter (Field Scout CM 1000 Chlorophyll Meter; Spectrum Technologies, Inc., Plainfield, IL, USA) measuring the reflected red and far-red light ratios to calculate the relative chlorophyll content at 80 cm above the plot surface. The outcome is a chlorophyll content index that is unitless and ranges from 0 to 999. The grass index and chlorophyll values of each plot were averaged from ten readings per plot.

Fall/winter dormancy, or color retention, is used to assess the ability of a genotype to retain color in response to temperature changes or frost events during the fall (Sever Mutlu et al. 2020). The dormancy was assessed weekly each winter from December, when the plants began to enter dormancy, to March, when they began to spring green-up, using a visual scale of 0% (no dormancy) to 100% (complete dormancy, the entire plots covered with straw brown colored vegetation). Turfgrass spring green-up was rated using a visual scale of 0% to 100%, with 100% = green vegetation over the entire plot and 0% = no green vegetation. Spring green-up ratings were made every week each spring from March 15 to May 30 until all the plots had achieved full green-up.

Turf density was determined by manually counting the shoots on three 108 mm diameter (A = 91.61 cm²) plugs randomly collected with a cup cutter from different locations in each treatment plot in June. The same plugs were used to determine roots and shoot dry weights. They were rinsed from the soil with tap water. Shoots were harvested, and roots were clipped and placed in paper bags and dried in an oven at 72 °C for 72 h (until the weight stabilized) to determine shoot and root dry weights.

Leaf samples were collected from each plot eight weeks after the initial application, in November 2018 and 2019, to assess plant nutrient contents. Total N content was determined as a percentage using the Kjeldahl method (Trikilidou et al. 2022). The mineral content (K, Ca, Mg, Fe, Cu, Mn, and Zn) of the plant materials was determined by atomic absorption spectrometry (AAS) (Mahood 2021). The amount of P was determined using modifications of the spectrophotometric method developed by Olsen (1954). The method described by Elliott and Snyder (1991) was used to determine the Si content of plant tissue.

All observations that were collected on a weekly or biweekly basis (e.g., quality, color, grass index, chlorophyll content) were averaged on a plot-by-plot basis over the season (winter or spring) to condense and simplify the data presentation. An analysis of variance was performed using SPSS (IBM, Armonk, NY, USA), and means were compared using the Duncan test at the 0.05 probability level. Hartley’s F_max test (Hartley 1950) was performed to determine the homogeneity of variance between the first and second trials of the experiments.

RESULTS AND DISCUSSION

The results of the F_max test (Hartley 1950) supported the hypothesis that there was homogeneity of variance between the two trials; therefore, combined data were analyzed. The N × Si interaction was not significant for any observed parameter at any evaluation date, but the N and Si main effects were significant (Table 2). The application of N treatments significantly affected every parameter examined, except for shoot and root dry weights and leaf Ca, Fe, Si, and Mn contents. However, the effects of Si treatment were limited to shoot density, root dry weight, and leaf K and Mg contents.

Table 2. Analysis of Variance for Observed Parameters as Influenced by Nitrogen and Potassium Silicate Treatments

Quality, Color, and Grass Index

There was a significant increase in turfgrass quality and color with increased N, both in the winter and spring seasons (Table 3). Only the plots that received a high rate of N demonstrated acceptable turfgrass quality (6.0) and retained their green color in winter. The spring turfgrass quality values of the N treatments were above the acceptable range (Table 3), consistent with the previous reports (Patton 2010; Trappe and Patton 2013). N is the primary nutrient that influences turfgrass plants’ responses to color and quality (Beard 1973). The N application intensified the green color, where the color differences between the high and low N doses were significant. The N application is known to enhance the color values of various turfgrasses (Dunn et al. 1993; Gibeault et al. 1997; Richardson 2002; Volterrani et al. 2010). The leaf N content and winter quality (P < 0.001; r = −0.81) and color (P < 0.001; r = −0.74) were significantly and positively correlated, confirming the relevant impact of N on the turfgrass quality and color (Table 1A, Appendix).

In contrast to N, the effect of Si applications on quality and color was not significant. Information regarding the Si effects on the response of a warm-season turfgrass species to low temperatures in a field setting is limited. However, in a study by He et al. (2010), in which plants were grown in pots in a controlled environment, foliar Si application enhanced the growth and color of Paspalum vaginatum under chilling stress by maintaining membrane stability. Trenholm et al. (2001) reported that potassium silicate, applied as a soil drench in summer, significantly increased color and quality in two different P. vaginatum ecotypes. However, qualitative responses to foliar Si treatments were found to be less consistent compared to drench Si treatments (Trenholm et al. (2001). Grass index values that are calculated from the normalized difference in vegetation index have been used to assess turfgrass quality and color in various stress conditions as a means of reducing subjectivity (Keskin et al. 2008; Gopinath et al. 2021). Plots treated with N showed greater grass index values than controls, while the Si treatment had no effect, which was consistent with the visually evaluated quality and color results (Table 3).

Table 3. Turfgrass Quality, Color, and Grass Index Values of Z. japonica as Influenced by Si and N Applications in Fall Season

Chlorophyll Content

The chlorophyll content increased linearly with an increasing N rate both in winter and spring (Table 4). The high- and low-dose N treatments increased the average winter chlorophyll contents by 35% and 17%, respectively. Similarly, in a study by Pompeiano et al. (2013), in which the timing and source of N on freezing tolerance were investigated, N (NH₄) sharply increased the chlorophyll content of both Z. japonica and C. dactylon, especially four weeks after application. Nitrogen is essential for the synthesis of chlorophyll within chloroplasts. Furthermore, correlation analysis revealed a substantial positive association between leaf N content and relative chlorophyll content in the winter (P < 0.001; r = −0.73) and spring (P < 0.001; r = −0.67) (Table 1A, Appendix). This relationship is primarily attributable to the fact that 50% to 70% of the total N in the leaves is incorporated into enzymes that are associated with chloroplasts (Taiz and Zeiger 2009; 2010).

In contrast to N, chlorophyll contents were not influenced by Si applications. Comparative research on the effects of fall Si treatment on the chlorophyll concentration of Z. japonica at suboptimal temperatures is lacking. Nevertheless, Si treatment had no influence on chlorophyll content in salt-stressed Triticum aestivum (Ahmad et al. 1992) and in Glycine max (Shen et al. 2010), which is consistent with our findings. In contrast, Si application enhanced chlorophyll content in Agrostis palustris (Schmidt et al. 1999) and Poa pratensis exposed to NaCl stress (Bae et al. 2012).

Table 4. Relative Chlorophyll Content of Z. japonica as Influenced by Si and N Applications in Fall Season

Dormancy and Spring Green-Up

Warm-season turfgrasses, with an optimum growth temperature between 27 and 35 °C, experience discoloration and the onset of dormancy in the fall when temperatures fall below 15 °C (Beard 1973; Wei et al. 2008). This is because the rate of chlorophyll degradation seems to be accelerating faster than the rate of chlorophyll synthesis at low temperatures. Winter dormancy causes not only a loss of aesthetic value but also functional growth. Therefore, dormant Z. japonica will not resist traffic in functional green areas subjected to daily use (Pompeiano et al. 2014). Thus, it is highly desirable that warm-season grasses exhibit as little dormancy as possible.

The winter dormancy of Z. japonica was considerably impacted by the N treatment. Silicon, however, had no such effect (Table 5). High-dose N treatment provided significantly better winter color retention, as demonstrated by the lower ratio of dormancy compared to low-dose N and control treatments throughout the winter. In February, the dormancy ratio reached 73% in non-treated control plots, while the plots that received a high dose of N were still covered with 74% green grass. Correlation analysis also showed that there was a significant negative relationship between leaf N content and dormancy ratio in December (P < 0.001; r = −0.84) and February (P < 0.001; r = −0.77), confirming the relevant impact of N on winter color retention (Table 8, appendix). Results indicate that winter color retention of Z. japonica can be enhanced by a high-dose N application with a relatively functional turf cover.

When soil temperatures rise above 10 °C in the spring, warm-season grasses break dormancy (Beard 1973). They start fresh growth in the form of roots and shoots from the existing nodes of rhizomes, stolons, and crown meristematic regions. The N treatments significantly enhanced the spring greening rate of Z. japonica, while the Si treatments had no effect at all. In March, the spring green-up rate, which was 59% in control plots, increased to 73% and 85% in plots treated with low and high doses of N, respectively (Table 5). Overall results indicated that Z. japonica plots treated with high-dose N applications maintained 65% to 100% green cover throughout the year. Whereas, in non-treated control plots, that level of green cover was maintained for only eight months.

Table 5. Winter Dormancy (December, January, and February) and Spring Green-Up Rates (March, April, and May) of Z. japonica as Influenced by N and Si Applications in Fall Season

Turf Density, Root, and Shoot Dry Weights

Turfgrass density, an important component of turfgrass quality, is under genetic control and strongly affected by environmental factors, nutrient availability, and biotic and abiotic stresses. Both N and Si applications have significant effects on turf density (Table 6). There was a significant increase in shoot density with an increase in N dose. After averaging the Si treatment, the high and low doses of N increased shoot density by 22% and 10%, respectively, consistent with earlier reports by Oral and Açıkgöz (2001) and Badra et al. (2005). Shoot density values also differed among Si treatments. After averaging the N treatment, the high-dose Si yielded a higher shoot density than the low-dose Si treatment but remained in the same statistical group with plots that received no Si. Trenholm et al. (2001) reported that shoot density responses to Si were inconsistent, and foliar potassium silicate slightly enhanced or provided similar density compared to the control in P. vaginatum during two consecutive field trials. Overall, shoot density was greatest in the plots that received high doses of both Si and N, which provided 21% more shoots than the control plots, which did not receive either of the two nutrients. The tillering that contributes to turfgrass density is controlled by a complex network of genetic, hormonal, and environmental factors (McSteen and Leyser 2005). Studies have revealed the importance of balance, particularly between auxin and cytokinin, in tillering (Shang et al. 2021). Zinc has long been associated with the maintenance of a healthy hormonal balance in plants (Hull 2001). Reduced growth and other signs of a zinc shortage have been associated with a sharp decrease in the concentration of auxin (Hull 2001). Thus, it may be useful to evaluate shoot density and leaf tissue Zn content together to make more accurate determinations. Jones (1980) indicated that Zn tissue concentrations ranging from 20 to 55 ppm should be sufficient for most turfgrasses, which were reported to be 35 ppm for Z. japonica by Butler and Hodges (1967). The plots that received high doses of N were able to provide optimal Zn content, exhibiting the highest shoot density (Table 6). A significant positive correlation existed between shoot density and tissue content of N (r = 0.51), Zn (r = 0.49), and Si (r = 0.34), further confirming the effects of Zn and N on shoot density.

The root dry weight was not influenced by N treatments; however, it was significantly impacted by Si applications (Table 6). Low doses of Si application provided the highest root dry weight, which was followed by high-dose Si and non-treated control (Si₀). These results are consistent with those of Eneji et al. (2008) who reported that Si application increased root dry weight in four different water-stressed grass species. Si treatment is known to increase photosynthetic efficiency and water utilization, which may be the cause of the increased root development seen in this study in response to Si application (Schmidt et al. 1999).

Although Si and N applications resulted in enhanced shoot biomass compared to untreated control plots, this difference was not statistically significant (Table 6). Similarly, shoot growth of P. vaginatum was unaffected by Si application (Trenholm et al. 2001). Datnoff and Rutherford (2003) reported that no linear relationship was found between increasing concentrations of Si applied to C. dactylon and leaf dry weight. In contrast, the addition of Si was reported to increase shoot weight in several cool-season grasses including, P. pratensis (Bae et al. 2012), A. palustris (Gussack et al. 1998), and Festuca arundinacea (Eneji et al. 2008).

Table 6. Average Values for Turf Density, Root, and Shoot Dry Weights of Z. japonica as Influenced by N and Si Applications in Fall Season

Nutrient Content of the Plants

Leaf N concentrations differed significantly among N treatments (Table 7). As expected, tissue N concentration was greatest in the high-dose N-treated plots (2%). Bryson et al. (2014) reported the reference range of leaf N content in Z. japonica as 2.04% to 2.36%. Results showed that only the plots that received high doses of N had a tissue N content that was close to the sufficiency range. This finding may help to explain why, in the winter, only high-dose N treatments offered acceptable turfgrass quality. The effect of Si application on the tissue N content, however, was not significant. Adams et al. (2020) also reported that tissue N content did not change as a result of Si application to Z. japonica.

The N application caused a significant decrease in tissue P levels, whereas the Si application had no discernible effect (Table 7). Similarly, the Si application did not change the tissue P content in Z. japonica (Adams et al. 2020), and Lolium perenne (Nanayakkara et al. 2008). The results also showed that leaf P contents of all treatment plots were higher than the sufficiency range (0.19% to 0.22%) for Z. japonica, as reported by Bryson et al. (2014).

The tissue K content increased significantly in response to N applications (Table 7). After averaging the Si treatment, the increase in tissue K content of the low and high N treatments was 21% and 32%, respectively. The Si application, however, significantly decreased the tissue K content. The tissue K content of high-dose Si-treated plots was 19% lower than that of control plots (Si₀) after averaging the N treatment. The current results were consistent with those of Şen et al. (2011), in which Si application to P. vaginatum under salinity stress decreased leaf K concentration. In contrast, Eneji et al. (2008) reported that Si application significantly increased K uptake in four different grass species. Trenholm et al. (2001) reported that the leaf K content of P. vaginatum was increased by Si application in the first year, while no difference was observed in leaf K content in the second year of their two-year study. There have also been reports of no variations in the leaf K contents of Z. japonica and Oryza sativa leaves in response to Si treatment (Chen et al. 2010; Adams et al. 2020). Bryson et al. (2014) reported the sufficiency range of K content for Z. japonica as 1.05% to 1.27%. According to the findings, all treatments’ tissue K levels ranged from 1.09 to 1.77, which was either within or somewhat above Z. japonica‘s K sufficiency range (Table 7).

The content of Mg, the central core of the chlorophyll molecule, increased linearly with the increase in N application dose (Table 7). However, Si application significantly decreased the leaf Mg content. The results are consistent with those of Ayres (1966), in which Si application suppressed Mg uptake in Saccharum officinarum. However, Adams et al. (2020) reported no variation in the leaf Mg content of Z. japonica in response to Si application. Results also indicated that, except for plots that received only a high dose of N, the leaf Mg content of all plots was below the sufficiency range of 0.13% to 0.15% as reported for Z. japonica (Bryson et al. 2014). This may further help to explain why high-dose N treatments provided the best winter color retention.

The leaf Ca contents of all treatments ranged between 0.52 and 0.83 ppm, higher than the optimum values (0.44% to 0.56%), as reported by Bryson et al. (2014). The N application decreased leaf Ca content, while the effect of the Si application was insignificant. The findings agree with those of earlier studies, in which Si treatment did not significantly change leaf Ca concentrations in Z. japonica (Chen et al. 2010) and O. sativa (Adams et al. 2020).

Tissue Si concentrations did not significantly differ due to N fertility treatments (Table 7). Contrary to expectations that control plots (Si₀) would have lower Si levels than Si-treated plots, the Si application led to a non-significant decrease in leaf Si content. Gussack et al. (1998) also found a similar response in A. stolonifera, which had the lowest Si content in plots receiving the highest rates of Si. Decreased leaf Si content in response to Si application might be related to the Si doses applied. Solutions containing more than 100 mg/L Si were reported to cause the H₄SiO₄ monomer condensation process to begin (Iler 1979; Chan 1989). This process speeds up with increasing pH and Si concentration and can eventually lead to polymerization, which reduces Si availability and absorption (Chan 1989; Gallinari et al. 2002). The current findings, however, disagree with those of Saigusa et al. (2000), who found that applying porous hydrate calcium silicate to Z. japonica enhanced the concentration of Si in the leaves.

The effect of N and Si applications on the Fe content of plants was not significant (data not shown). The findings are consistent with those of previous studies in which leaf tissue Fe concentrations did not change in response to Si application in Z. japonica (Adams et al. 2020) and O. sativa (Chen et al. 2010).

Tissue Cu contents enhanced significantly in response to an increase in N dose, and the plots that received the high dose of N had 53% more Cu content than control plots (Table 7). However, the differences in Cu contents in response to Si treatments were not significant, which agrees with those of Adams et al. (2020). The Cu content values of all treatments in the present study were higher than the reference values (2 to 4 ppm), as stated by Bryson et al. (2014).

The leaf Zn content was highest in the high-dose N treatment, but the low-dose N and control plots remained in the same statistical group. In contrast to the effect of N, Si treatment had no significant effect on leaf Zn content. Similarly, Adams et al. (2020) found that applying Si to Z. japonica did not influence the quantity of Zn in the leaves.

Table 7. Shoot Tissue Contents of N, P, K, Mg, Ca, Si, Cu, and Zn in Z. japonica in Response to Si and N Applications in Fall Season

CONCLUSIONS

Zoysia japonica, with its high drought, shade, and salt tolerance and low maintenance requirements, is an excellent choice for use in green areas in tropical climates. However, in regions with subtropical and temperate climates, it goes into winter dormancy, resulting in a decline in aesthetic value and functionality. The decline in winter color retention poses a challenge for turf managers who want year-round green grass coverage in their landscapes. This study focused on determining the effectiveness of fall N and Si applications on the winter color retention of Z. japonica in the Mediterranean climate of Türkiye.

Fall N application enhanced turfgrass quality, color, chlorophyll content, shoot density, and reduced winter dormancy. Overall, two sequential N applications at 5 g/m² in the fall provided 65% to 100% green cover with acceptable turfgrass quality during the fall and winter, indicating the possibility of maintaining the year-round green color of Z. japonica in subtropical climates.
The Si treatment had no effect on the winter color retention of Z. japonica. The apparent lack of response of Z. japonica to the Si application may be explained by the application dose, methods, or source of Si used in the present study. The effectiveness of Si in alleviating stress is related to its accumulation in plant tissues (Epstein 2009). However, in this study, foliar potassium silicate application at 3.0 or 6.0 mL/L Si did not increase leaf Si content. It might possess positive responses in lower concentrations. Some studies have reported that soil Si application is more successful than foliar application in increasing Si content in plant tissues and producing a stimulatory effect (Haynes 2014; Zajaczkowska et al. 2020). Therefore, it is important to investigate the potential of potassium silicate to boost Si accumulation in Z. japonica at various dosages and methods. The time of application and especially the source of Si used might also affect the lack of responsiveness to Si. The new Si sources, such as stabilized silicic acid and nano-silica (silicon oxide nanoparticles), are more efficient, even at small concentrations (Prakash et al. 2011; Laane 2017; Ismail et al. 2022). Therefore, the color retention and spring green-up response of Z. japonica to different sources of Si may also differ from the results found in this study, which also warrants further studies.

ACKNOWLEDGMENTS

This paper originated from the doctoral thesis titled “The Effects of Potassium Silicate and Nitrogen Applications on Grass Performance of Zoysiagrass (Zoysia japonica Steud.),” which was conducted by the first author under the guidance of the second and third authors. The authors are grateful for the support of the Süleyman Demirel University Faculty Member Training Program Coordination Unit, Grant No. ÖYP06723-DR-16 and Süleyman Demirel University Scientific Research Projects Coordination Unit, Grant No. FYL-2019-7005.

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Article submitted: April 6, 2024; Peer review completed: June 8, 2024; Revised version received: August 7, 2024; Accepted: September 11, 2024; Published: September 24, 2024.

DOI: 10.15376/biores.19.4.8577-8593

APPENDIX

Supplementary Information

Table 1A. Correlation Coefficients between Observed Parameters