Plant material, growing conditions, and experimental treatments.

A study was conducted in the greenhouse at Pacini Horticultural Nursery located in Rigoli (Pisa), central Italy (lat. 45°45′N, long. 10°26′E, 6 m elevation) under 22 ± 3 °C minimum air temperature range, 42 ± 3 °C maximum air temperature range, 56.6% average relative humidity, and natural sunlight. On 7 June 2011, plants derived from one-node sprigs of ‘Patriot’ bermudagrass, a recently developed high-quality hybrid cultivar (Taliaferro et al., 2006), were transplanted into plastic pots (17-cm deep, 20-cm diameter, 3.8-L volume) placing three plants per pot. Plants were composed by one stolon 10–12 cm long and one shoot 3–5 cm high. A nonamended, nonfertilized sphagnum peat was used as substrate after being adjusted to pH 5.9 with calcium oxide (2.5 g·L⁻¹) and fertilized with macronutrients and micronutrients. Nitrogen, P, and K were supplied at different rates. Nitrogen was supplied as ammonium nitrate (34.0% N), P as triple superphosphate (20.1% P), and K as potassium sulfate (42.3% K). As control rates, N, P, and K levels normally imposed by the nursery to bermudagrass donor plants (314, 52, and 198 mg·L⁻¹ substrate, respectively) were adopted. Starting from the control rates, each element was reduced to zero, halved, doubled, or tripled while the other two were kept unchanged (13 treatments in all). Fertilization treatments and corresponding N, P, and K rates are listed in Table 1. Magnesium and micronutrients [iron (Fe), copper (Cu), molybdenum (Mo), manganese (Mn), boron (B), and zinc (Zn)] were supplied at only one rate adding to the substrate: 732 mg·L⁻¹ magnesium sulfate (9.8% Mg), 14 mg·L⁻¹ iron chelate (6.0% Fe), 7.8 mg·L⁻¹ copper sulfate (23.0% Cu), 4 mg·L⁻¹ sodium molybdate (39.6% Mo), 2.7 mg·L⁻¹ manganese sulfate (32.5% Mn), 2.7 mg·L⁻¹ borax (11.3% B), and 0.8 mg·L⁻¹ zinc sulfate heptahydrate (22.7% Zn).

Table 1.Nitrogen (N), phosphorus (P), and potassium (K) rates applied to ‘Patriot’ hybrid bermudagrass grown in pots.

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Pots were arranged in a completely randomized design with four pots per treatments. During the trial, plants were manually irrigated as necessary.

Data collection and statistical analysis.

After 1 month of growth in pots, stolons were developed enough to be used for propagation and plants were harvested. The following data were collected from each pot: dry weight (after oven-drying at 80 °C until constant weight) of aerial part (shoots plus stolons); number of primary stolons; and, on three primary stolons per pot, length, number of nodes, and number of stolons developing from the nodes (ramifications or secondary stolons). In addition, aerial dry matter of three pots per treatment was analyzed for N, P, K, Ca, Mg, sulfur (S), Fe, Mn, and Zn content. Nitrogen and S were determined using a Flash Elemental Analyzer 1112 NC (Thermo Fisher Scientific, Waltham, MA) according to the manufacturer’s instructions. The other mineral elements (P, K, Ca, Mg, Fe, Mn, and Zn) were extracted by nitric/perchloric acid digestion (Manzelli et al., 2010) and measured by inductively coupled argon plasma emission spectroscopy (IRIS Intrepid II XSP Radial, Thermo Fisher Scientific).

Data were analyzed separately for N, P, or K. Regression analysis was adopted and the coefficients of determination (R²) were calculated. When treatment effect was significant, means were separated using the Tukey’s test.

Plant growth.

As expected, N, P, and K proved to be all necessary for the growth and the development of ‘Patriot’ hybrid bermudagrass grown in the nursery. In fact, when not supplied with N, P, or K, plants showed a general reduction in growth and pale green color. Nevertheless, the assessed parameters exhibited a different response to increasing rates of nutrients depending on the element.

The role of N fertilization in enhancing the production of aboveground matter in bermudagrasses is widely known (Overman et al., 1990; Sartain and Dudeck, 1982; Snyder and Cisar, 2000; Stanford et al., 2005; Trenholm et al., 1998). In this study, N rate influenced aerial dry weight, the number of primary stolons, and their ramifications, but not their length or their number of nodes (Table 2). On the contrary, other authors found an increasing effect of N on stolon length of ‘Tifdwarf’ (Stanford et al., 2005; Trenholm et al., 1997) and ‘Floradwarf’ (Trenholm et al.,1997) bermudagrass.

Table 2.Aerial dry weight, number of primary stolons, primary stolon length, number of nodes, and number of secondary stolons in ‘Patriot’ hybrid bermudagrass plants subjected to different nitrogen (N), phosphorus (P), and potassium (K) rates. Values are means (± SE) of four replications.

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Aerial dry weight increased with increasing N rate with a nonlinear less than proportional trend (Table 2). As shown by R² values, N exhibited a greater effect on this parameter than P or K (Table 2). In absence of N fertilization (zero N rate) aerial dry weight was significantly lower than with any other N rates, and increased by 144% passing from zero N to half N rate; its highest value was obtained with triple N rate, that was not different from double N and control rate (Table 2). The number of primary stolons was negatively affected by the absence of N fertilization (zero N rate), while no differences were observed among the other N rates despite a significant nonlinear regression (Table 2). The number of secondary stolons increased along with N increase fitting a linear regression model, but only the difference between zero N and triple N was significant (Table 2). Trenholm et al. (1997) found both linear and quadratic components of N in the response model for the number of stolons produced by hybrid bermudagrasses.

As observed by Guertal (2006), although P is one of the nutrients most commonly applied to turfgrass, research on it is somewhat limited. This author found that shoot density and clipping dry weight of ‘TifEagle’ bermudagrass increased linearly with increasing P rate (32, 64, 96, 128 kg·ha⁻¹ P). On the contrary, Rodriguez et al. (2002) obtained an increase in cover rate and shoot weight during the establishment of four hybrid bermudagrass cultivars passing from 1N:0P:0.66K or 1N:0P:1.41K to 1N:0.17P:0.66K fertilization ratio, but further increasing of P rate to 1N:0.39P:0.66K or 1N:0.57P:0.66K ratios did not produce additional enhancement or even caused a decrease in plant growth. In this study, aerial dry weight, the number and the length of primary stolons, and the number of secondary stolons were influenced by P rates. They varied with increasing P according to a nonlinear regression model and showed an initial increase followed by a decrease (Table 2). The increase that occurred passing from zero P rate to half P was significant for all the P-influenced parameters (Table 2). The highest values were observed with half P rate for both aerial dry weight, with no difference compared with control and double P rate, and the number of secondary stolons, which did not varied further with higher P rates (Table 2). The highest number of primary stolons was obtained with control P rate, but without differences as compare with half P and double P rate, and the longest stolons derived from double P rates, but without differences compared with half P, control, and triple P (Table 2). Phosphorus had a larger effect than N and K on the number of primary stolons, as shown by R² values (Table 2).

In a field study conducted by Peacock et al. (1997) growth rates of ‘Tifgreen’ bermudagrass were unaffected by K fertilization. In contrast, in the absence of any K application, Sartain (2002) and Snyder and Cisar (2000) noticed a reduction in bermudagrass growth; however, above a threshold rate, K did not produce any additional growth. In this study the absence of K caused a decrease in all measured growth parameters, while no differences were noticed among the other K rates except for the number of nodes (Table 2). In studies that are aimed to turf fertilization management this parameter is not evaluated, while it is very important for nursery purposes. The highest number of nodes was obtained with control K rate, which was comparable to only double K (Table 2). The values of R² showed that K affected the characteristics of primary stolons (length, number of nodes, and ramifications) more than N and P (Table 2). The variation, due to K rate, of aerial dry weight, primary stolon length, and nodes fits a nonlinear regression model, while the number of both primary and secondary stolons increased with increasing K rate according to a linear regression model (Table 2). Trenholm et al. (1997, 1998) found both no effect and linear and/or quadratic components in the response of bermudagrass to K rate depending on cultivar and daylength. In cultivar FloraDwarf, shoot and stolon growth, stolon number, and stolon length were unaffected by K rates under long-day (>13 h) while a linear effect of K was observed under short-day (<13 h); for the same parameters, cultivar Tifdwarf showed a linear response to increasing K rates under long-day and both linear and quadratic effect under short-day (Trenholm et al., 1997, 1998).

Nutrient content.

Tissue nutrient concentration in ‘Patriot’ hybrid bermudagrass grown in pots was affected by N, P, and K rates. Since no information is available about bermudagrass elemental composition in nursery conditions, representative sufficiency ranges as presented by McCrimmon (2001) and Jones et al. (1991) for this species in field conditions were considered in the present study for macronutrients and micronutrients, respectively (Table 3). The analyzed micronutrients (Fe, Mn, and Zn) resulted within the stated sufficiency ranges at any applied fertilization treatment, while insufficient levels were sometimes detected for macronutrients (Tables 4 and 5).

Table 3.Representative sufficiency ranges for macronutrient and micronutrient content in bermudagrass tissue.

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Table 4.Macronutrient [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)] content in aerial part of ‘Patriot’ hybrid bermudagrass plants subjected to different N, P, and K rates. Values are means (± SE) of three replications.

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Table 5.Micronutrient [iron (Fe), manganese (Mn), zinc (Zn), and sodium (Na)] content in aerial part of ‘Patriot’ hybrid bermudagrass plants subjected to different nitrogen (N), phosphorus (P), and potassium (K) rates. Values are means (± SE) of three replications.

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When N was not provided (zero N rate), N, K, and Mg concentrations in plant tissue were below the stated sufficiency ranges; magnesium concentration was below the sufficiency range also when N rate was tripled, while Ca concentrations did not reach the stated sufficiency range irrespective of the N rate applied (Table 4). All the nutrients whose tissue concentration was influenced by N fertilization (N, K, Mg, Mn, and Zn) varied with increasing N according to a nonlinear regression model: nitrogen, K, and Mg increased significantly passing from zero N to control rate and did not show any further increase; Mn and Zn concentrations were significantly higher when control rate was applied in comparison with both lower and higher N rates (Tables 4 and 5). As obvious, tissue N content was especially affected by N rate (R² = 0.922), but also Mn concentration was influenced by N more than by P or K (Tables 4 and 5). Nitrogen rate did not affect P, Ca, S, and Fe (Tables 4 and 5).

Phosphorus rate influenced the tissue content of all the analyzed nutrients except for Ca and Fe (Tables 4 and 5). The variation of N fit a linear regression model, while the other P-influenced nutrients changed according to a nonlinear regression model (Tables 4 and 5). Plants that were not supplied with P (zero P rate) showed a significant reduction in N concentration compared with plants fertilized with double P and triple P rates (Table 4). Phosphorus content in control plants was significantly higher than in plants not supplied with P or fertilized with half P rate, but significantly lower compared with double P rate (Table 4). When P was not provided, K concentration was significantly lower than that observed with any other P rate (Table 4). Differences were noticed between zero P rate and control, double, and triple P rates, and between half P and double P in Mg concentration (Table 4). Control plants and plants supplied with double P rate showed a significantly higher S concentration than plants that were not provided with P (Table 4). Control P rate resulted also in higher Mn concentration than half P rate and higher Zn concentration compared with any other P rate (Table 5). Ca concentrations did not reach the stated sufficiency range irrespective of the P rate applied, and other values below the sufficiency range were observed for P, K, and Mg when P was not provided, and for Mg when P was halved in respect to the control rate (Table 4). As obvious, tissue P content was especially affected by P rate (R² = 0.932), but also S concentration was influenced by P more than by N or K (Table 4).

Nitrogen, S, and Fe concentrations in plant tissue were unaffected by K rates, while significant effects were observed for all the other analyzed elements (Tables 4 and 5). Phosphorus, Ca, and Mg decreased with increasing K rates according to a linear regression model (Table 4). For P, a significant difference was detected between zero and double K rates; zero K rate differed from double and triple K in Ca concentration; magnesium concentration in plants not supplied with K was higher than in plants provided with control, double, and triple K rates, and half K differed from triple K rate (Table 4). Potassium competition with both Ca and Mg was previously observed in bermudagrass by Miller (1999). The zero K rate was the only treatment in the whole experiment to ensure a Ca concentration within the stated sufficiency range. Nevertheless, no symptoms of Ca deficiency were observed in this study. That could suggest that the representative sufficiency ranges for Ca as presented by McCrimmon (2001) are possibly too high. In fact, McCrimmon, who reported the mineral composition of 12 bermudagrass cultivars under different N:K treatments, found lower Ca values in most cases. Moreover, according to Turgeon (2007), Ca is absorbed by turfgrass at levels equal to or even below those of P, that is, usually, not over 0.5%.

Potassium, Mn, and Zn varied with increasing K according to a nonlinear regression model (Tables 4 and 5). Potassium concentration was below the sufficiency range in plants not supplied with K (zero K rate) and significantly increased passing to control rate, but did not show further increase (Table 4). Manganese and Zn concentrations were higher when control rate was applied in comparison with both lower and higher K rates (Table 5). On the basis of R² values, K was the nutrient showing the main effect not only on K concentration in plant tissues, but also on their Ca, Mg, and Zn contents (Tables 4 and 5).

Most of the studies about the mineral composition of bermudagrass concern N, P, and K levels. Nitrogen content detected in this study was consistent with that found by several authors in bermudagrass turf clippings obtained under field conditions (Barrios et al., 1979; Goatley et al., 1994; Peacock, 2001; Petrovic et al., 2005; Snyder and Cisar, 2000; Walworth and Kopec, 2004).

The foliage of adequately fertilized turfgrass usually contains about 10% as much P as N (Snyder et al., 2007), and P absolute concentration of dried turfgrass clippings is usually less than 0.5% (Mc Crimmon, 2001; Petrovic et al., 2005; Turgeon, 2007; Walworth and Kopec, 2004). In this study, P content in aerial part of ‘Patriot’ hybrid bermudagrass ranged from 7% to 14% of N content with the exceptions of plants not provided with P (4% as much P as N) or with N (23% as much P as N). Phosphorus content above 0.5% was detected in plants fertilized with double and triple P rates, zero K, and half K.

Although in turfgrass tissue K levels may reach up to 5% of dry weight where liberal quantities have been supplied through fertilization, normal percentages are about half that much (Turgeon, 2007), corresponding about to half N percentage (Snyder et al., 2007). In many studies on bermudagrass, K levels in clippings were close to 2% or slightly lower (Barrios et al., 1979; Goatley et al., 1994; McCrimmon, 2001; Peacock, 2001; Petrovic et al., 2005; Snyder and Cisar, 2000; Walworth and Kopec, 2004). Potassium content found in ‘Patriot’ hybrid bermudagrass ranged on average from 1.78% to 3.06%, but values lower than 2% were detected only in plants not supplied with N, P, or K. These plants excluded, ‘Patriot’ contained from 59% to 80% as much K as N. This result may indicate that stolons, which represented the prevalent material in the samples analyzed in this study, contain higher K amounts than leaves, prevailing in clippings.