Canopy elongation rate (CER) for various genotypes of bermudagrass at ambient conditions and different shade treatments averaged for 6 weeks. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment. Shade 30 = shade at 30% of ambient conditions; Shade 60 = shade at 60% of ambient conditions; Shade 90 = shade at 90% of ambient conditions.
Normalized canopy elongation rate (NCER) (measured in millimeters per day) representing percent change in canopy elongation rate from control for various genotypes of bermudagrass for 6 weeks averaged for 30%, 60%, and 90% shade. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment.
Scatterplot of 20 group centroids from discriminant analysis using eight morphological parameters specific leaf area, leaf area ratio, relative water content, leaf angle, leaves per stem, leaf weight ratio, leaf width, and root-to-shoot ratio as predictors of five bermudagrasses (green color refers to TifB16119; blue color refers to TifB16108; orange color refers to TifB16117; cyan color refers to Tifway; and purple color refers to TifGrand) grown in a greenhouse under four irradiance levels. Numbers 0% (represented by Δ) and 90% (represented by □) indicate the ambient and heavy shade, respectively.
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Beard (1997) estimated that 20% to 25% of cultivated turfgrass encounters shade. Shade occurs when light is blocked by a nearby building, cloud cover, or tree canopy, thereby reducing the quantity of photosynthetic photon flux (PPF) available to the plant and, in some instances, changing the spectral quality of light. Such change in light quantity and quality results in photomorphogenic responses in plants, which are mediated through specialized photoreceptors such as phytochromes (Gautier et al. 1999; Stuefer and Huber 1998). This change in canopy morphology is often termed shade avoidance syndrome (Smith and Whitelam 1997) and is largely attributed to increased gibberellins, resulting in stem elongation and reduction in stem diameter (McBee and Holt 1966; Tan and Qian 2003). Shade avoidance syndrome results in carbohydrates being partitioned preferentially to the developing leaf tissue (Pierik and de Wit 2014), which in the case of turf is removed quickly through mowing. Wherley et al. (2005) demonstrated that changes in both light quality and quantity could induce shade avoidance responses in tall fescue (Schedonorus arundinaceus). Studzinska et al. (2012) reported gibberellin 2-oxidase overexpression muted the shade avoidance response in creeping bentgrass (Agrostis stolonifera), resulting in improved shade resistance compared with the wild type. Under shaded conditions, increasing the mowing height is standard practice to increase the leaf area retained, resulting in increased carbon assimilation (Beard 1973; Dudeck and Peacock 1992). There is a normal decreasing light gradient from the tip of the leaf to the base, and photosynthetic rates follow this same pattern; frequent mowing eliminates the most photosynthetically active leaf region (Prioul et al. 1980). Turfgrass leaves become more vertically oriented as a result of repeated mowing, requiring more light to reach their light compensation point (Beard 1973; Gardner and Goss 2013). Increased mowing heights, on the other hand, can have a negative effect on turfgrasses by increasing respiration rates, increasing intershading of the turfgrass canopy, reducing leaf evaporation, and reducing resistance to traffic stresses (Beard 1973; Gardner and Goss 2013; Singh et al. 2024a, 2024b).
Although these responses to shade are intended as a defense mechanism in high plant populations, in a mowed turf these responses result in excessive foliage loss, decreased ability to recover from wear, and poor turfgrass quality (Aldahir 2015). Managing turf under shade requires adjustments to cultural practices and selection of shade-resistant species and cultivars (Gardner and Goss 2013). Interspecific variation in shade resistance has been largely reported, although it can be cultivar dependent (Zhang et al. 2017). Barrios et al. (1986) identified ‘Oklawn’ centipede [Eremochloa ophiuroides (Munro) Hack.] to be more shade resistant than ‘Floratam’ St. Augustinegrass (Stenotaphrum secundatum). Similarly, Jiang et al. (2004) considered seashore paspalum (Paspalum vaginatum) to be more shade resistant compared with bermudagrass (Cynodon dactylon). Zhang et al. (2017) used minimum daily light integrals to quantify shade resistance of various warm-season turfgrass species, reporting bermudagrasses as having among the worst shade resistance.
Despite its poor shade resistance in general, bermudagrasses are an important species in the southern and transition zone climates of the United States. Enhancement in shade resistance within this species would have meaningful impacts on sustainability of golf, lawn, and sports field sectors. Schwartz et al. (2020) reported ‘ST-5’ (hereafter referred to as TifGrand®) as having superior shade resistance compared with ‘Tifway’ (Hanna et al. 2010). Similarly, ‘Riley’s Super Sport’ and ‘TiftNo.2’ were identified as more shade resistant than other bermudagrasses, as defined by greater root biomass and root length under low irradiance (Baldwin and McCarty 2007). Zhang et al. (2017) and Chhetri et al. (2019) both reported smaller minimum daily light integrals for ‘Riley’s Super Sport’ than several bermudagrasses tested.
Turfgrasses that maintain relatively lower vertical growth rates under shade have been associated historically with superior shade resistance. McBee and Holt (1966) reported the excellent shade resistance of a ‘No-Mow’ bermudagrass. Tegg and Lane (2004) observed a small increase in vertical shoot elongation rate for supine bluegrass (Poa supina Schrad.), indicative of a shade-resistant response, whereas relatively large increases for kentucky bluegrass (Poa pratensis) were indicative of shade sensitivity. Meeks et al. (2015) reported leaf extension rate was a good indicator of shade resistance among various bluegrasses (Poa spp.). ‘Patriot’ bermudagrass has been reported to have a high tendency for vertical growth, which has been linked to poor performance in reduced irradiance environments (Chhetri et al. 2019; Trappe et al. 2011). More recent research suggests there is a balance between a muted vertical growth rate and general productivity of the plant, resulting in the most shade-resistant genotypes also having strong growth rates in low-irradiance environments (Meeks and Chandra 2021). These light-induced morphological responses are also dependent on the severity of shade, with St. Augustinegrass leaf extension rates well correlated to shade resistance under moderate shade but not severe shade (Wherley et al. 2013).
Shade resistance is a highly sought-after trait for the breeding and development of new cultivars (Ghimire et al. 2016). Understanding how canopy morphology influences the shade response of resistant genotypes within a species may contribute to more efficient selection and improvement of the shade-resistance trait. The major goal of our study was to quantify the avoidance mechanisms and growth patterns of hybrid bermudagrasses under shade to identify key traits that contribute to shade resistance within these species.
This study was conducted at the Oklahoma State University Horticultural Research Greenhouse Complex, Stillwater, OK, USA. The first experiment was conducted from 12 Jun to 30 Sep 2020 (hereafter referred to as Expt. 1), and the second experiment from 5 Oct 2020 to 14 Mar 2021 (hereafter referred to as Expt. 2). Four 400-W high-pressure sodium lights (Rudd Lighting Inc., Racine, WI, USA), one over each treatment and control, were used to create a 14-h photoperiod and to fulfill the minimum light requirements for the control, which resulted in an average daily light integral of 26.86 µmol·m–2·d–1 during Expt. 1, whereas 20.34 µmol·m–2·d–1 was used during Expt. 2. The average daily temperature was 30.7 °C during Expt. 1 and 14.6 °C during Expt. 2.
The experimental design for each experiment was a modified split-plot design with four replications of each treatment combination (genotype × shade treatment). Genotypes of each species were selected on the basis of previous field studies to create a gradient of shade resistance (Schwartz B, personal communication). The selected hybrid bermudagrasses (C. dactylon × Cynodon transvaalensis) were TifB16108 (shade resistant), TifGrand (shade resistant), TifB16117 (moderately resistant), TifB16119 (shade sensitive), and ‘Tifway’ (shade sensitive). The experimental units were shaded using a poly-woven fabric nominally rated to reduce PPF by 30%, 60%, and 90% of ambient conditions (which hereafter are referred to as shade conditions). Grasses were also grown under ambient greenhouse conditions as a control. The shade fabric was mounted over a polyvinylchloride (PVC) frame measuring 150 × 150 × 120 cm.
Growth tubes were created by capping 20-cm-long sections of PVC pipe and filling them with a peat-based media (Metro-Mix® 36; Sun Gro Horticulture, Agawam, MA, USA). Tubes were 10 cm in diameter, in which genotypes were planted as plugs and allowed to establish within the greenhouse for 6 weeks. Plants were fertilized with a commercial biosolids product (6 N–2 P2O5–0 K2O; Harrell’s, Lakeland, FL, USA) at a rate of 48 kg·ha–1 N at planting and subsequently 4 weeks later. During the shade treatment period, liquid fertilization was applied using 20 N–20 P2O5–20 K2O soluble fertilizer (Jr Peters Inc., Allentown, PA, USA) at the rate of 146 kg·ha–1 in split doses (36.6 kg·ha–1 per week) at weekly intervals. Preventive applications of two different insecticide mixtures were made in rotation at an interval of 15 d to control Rhodesgrass mealybugs (Antonina graminis) and bermudagrass mites (Eriophyes cynodoniensis Sayed). During the first week, a mixture of bifenthrin (Talstar P, 7.9% a.i.; FMC Corp., Philadelphia, PA, USA) at 0.128 kg·ha–1, abamectin (Avid 0.15 EC, 2% a.i.; Syngenta, Greensboro, NC, USA) at 0.002 kg·ha–1, and Prefer 90 Surfactant (0.25% v/v; CHS inc., Inver Grove Heights, MN, USA) was used, and the next application used imidacloprid (Mallet® 2F T&O, 21.4% a.i.; Nufarm, Melbourne, Australia) at 0.479 kg·ha–1. Grasses were clipped once per week at 2 cm to maintain the mowing height. Plants were hand-watered daily to prevent drought stress.
Vertical leaf extension was measured from the soil line to the tip of the two longest leaves in each pot, and the daily canopy elongation rate (CER) was calculated by dividing by the number of days between mowing events. The CER data were normalized (normalized canopy elongation rate (NCER)] by dividing each shaded experimental unit with the CER under ambient conditions for a specific genotype in a replication.
Mowing of grasses was withheld during the final week of each study [8 weeks after treatment (WAT)] for measurement of canopy morphology. Ten shoots were selected randomly from the middle portion of each pot (to avoid the edge effect), and leaf (lamina) and pseudostems were separated (Fontanier and Steinke 2017). Subsequently, all the leaves were placed on a plain white sheet marked with a ruler. The leaves were pressed against the sheet completely using a clear acrylic plate, and an image was taken using a digital camera (Powershot G5; Canon, Tokyo, Japan) mounted on a stand in ambient light conditions in the laboratory. The images were then analyzed using a custom macro in ImageJ 1.52a software (National Institutes of Health, Bethesda, MD, USA) to measure the green area after calibrating with the ruler in the image. The leaf count per shoot and the leaf widths were calculated using the Analyze particles option in Image J. The leaves used for measuring leaf area were dried in an oven for 48 h at 80 °C and weighed. The remaining leaves and stems were dried in an oven for 48 h at 80 °C and weighed. The leaf canopy fraction (LCF) in relation to the total canopy weight was calculated as LCF = Leaf weight (g) ÷ Canopy weight (g), where canopy weight is Leaf weight + Pseudostem weight.
SLA was calculated as the ratio of leaf area to the leaf dry weight using the following formula: SLA = Leaf area (m2) ÷ Leaf dry weight (kg).
After collecting the subsample, the remainder of the canopy (leaves and pseudostems) was harvested and dried in an oven for 48 h at 80 °C and weighed. The canopy yield was calculated as the ratio of the whole shoot weight (including the subsample weight) to the total area (81 cm2) of the pot (Wolf et al. 1972). The LAI was calculated using the following formula: LAI = Canopy yield (kg·m–2) × LCF × SLA (m2·kg–1).
The roots, rhizomes, and stolons were harvested separately, dried in an oven for 48 h at 80 °C, and weighed. This weight plus the canopy dry weight constituted the whole-plant dry weight. The LWR was calculated using the following formula (Kvet 1971): LWR = Leaf dry weight (kg) ÷ Whole-plant dry weight (kg).
The LAR was calculated using the following formula (Radford 1967): LAR = Total leaf are (cm2) ÷ Whole-plant dry weight (kg).
The leaf angle was measured between the pseudostem and the leaf blade of the second or third fully expanded leaf on the previously selected 10 shoots. The shoots were first excised from the pot and an image was then collected using the method described for leaf area. The angle was measured manually for each shoot using the measure angle tool within ImageJ 1.52a software.
Ten leaves were selected randomly from the middle portion of each pot for relative water content (RWC) analysis. Leaves were weighed individually for determining fresh weight and were then immersed in water for 24 h until fully hydrated under room temperature. After hydration, the samples were removed from the water and any surface moisture was dried lightly by using filter paper. The leaves were reweighed to determine turgid weight. The samples were then oven-dried for 48 h at 80 °C and weighed for the third time to determine the dry weight. The percentage RWC was calculated by using the formula: RWC (%) = [(Fresh weight – Dry weight) ÷ (Turgid weight – Dry weight)] × 100.
A combined analysis was performed for both experimental runs using SAS ver. 9.4 (SAS Institute Inc., Cary, NC, USA), and all data were subjected to PROC GLIMMIX with means separated using Fisher’s protected least significant difference (α = 0.05). For the CER and NCER data, a repeated-measures model was used to test the effects over time.
The CER exhibited a significant genotype × treatment interaction (Table 1). At 0%, 30%, and 60% shade treatments, TifB16119 showed the greatest CER among all the genotypes except for TifB16117 under 60% shade (Fig. 1). No significant differences among entries were observed under 90% shade. When pooled across all measurement dates, TifB16117 was the only entry to exhibit a significantly greater CER in 60% or 90% shade compared with the control (Fig. 1).
Citation: HortScience 60, 5; 10.21273/HORTSCI18169-24
A significant genotype × week interaction was observed for the NCER (Table 2). TifB16119 generally exhibited the lowest NCER among entries, but this varied with week (Fig. 2). Specifically, ‘TifGrand’, ‘Tifway’, and TifB16617 were greater than TifB16119 at 1 WAT. TifB16108 and TifB16117 had a greater NCER than ‘TifGrand’ or TifB16119 at 2 WAT. In contrast, ‘TifGrand’ exhibited a greater NCER than each other entry at 3 and 6 WAT. At each measurement date, the NCER exceeded 100% for each entry, suggesting a shade avoidance response was universally present. However, entries demonstrated two general patterns of shade avoidance over time. First, TifB16108 and TifB16117 exhibited a sharp peak at 2 WAT followed by a return to near-baseline levels (still 110%–140% of the control). The second pattern (exhibited by ‘Tifway’ and ‘TifGrand’) resulted in a decline in the NCER from 1 WAT to 4 WAT followed by a sudden return to baseline or greater levels. The most shade-sensitive entry, TifB16119, was more similar to the second pattern, but the magnitude of the response was muted to the point that no differences over time were significant.
Citation: HortScience 60, 5; 10.21273/HORTSCI18169-24
The genotype main effect was significant for LAI, SLA, LAR, LWR, leaf angle, shoot and root biomass, and leaf count per shoot (Table 3). The shade treatment main effect was significant for SLA, leaf width, LWR, shoot biomass, root biomass, and leaf count per shoot. The interaction between genotype and shade treatment was not significant for any of the parameters.
TifB16117 (2.0) and TifB16119 (2.0) exhibited 26% to 29% lower LAIs than each other genotype (Table 4). Similarly, TifB16117 (21.5 m2·kg–1) had the lowest SLA compared with other genotypes, which ranged from 25.6 to 28.6 m2·kg–1. ‘TifGrand’ demonstrated the greatest LAR (2.4 m2·kg–1) and LWR (0.074 g·g–1) among genotypes. Leaf angle was smallest for TifB16119 (109.4°), followed by TifB16108 (115.0°), which was less than other genotypes (range, 126.2–129.7°) (Table 4). TifB16119 had the largest leaf width (1.04 mm), whereas TifB16108 (0.75 mm) had smaller leaf widths than ‘TifGrand’ (0.88 mm), but similar widths to other genotypes (Table 4). ‘Tifway’ (n = 3.8) had a greater leaf count per shoot than other genotypes except ‘TifGrand’ (n = 3.5). TifB16119 (1.4 g) had less shoot biomass than other genotypes except TifB16117 (1.64 g). TifB16108 had the largest root biomass (21.1 g), whereas ‘TifGrand’ had the lowest root biomass (9.5 g), resulting in ‘TifGrand’ also having the smallest root-to-shoot ratio (7.5 g·g–1).
Shade increased SLA (pooled across entry) from 22.8 to 30.0 m2·kg–1 for the 0% and 90% shade treatments, respectively (Table 5). The 60% and 90% shade treatments also reduced leaf width (by 18% to 19%), shoot biomass (by 117% to 160%), and root biomass (by 59% to 129%) compared with the control. Only the 90% shade treatment (n = 2.7) reduced leaf count per shoot compared with the control (n = 3.8).
Linear correlation analysis was performed to assess the interdependence of derived morphological parameters. The greatest correlation was observed between the LWR and the LAR (r = 0.840), which was consistent with the analysis of variance (Table 6). The increased LAR of ‘TifGrand’ could be related to the higher LWR, rather than the greater SLA, considering a relatively lower correlation of LWR with SLA (r = 0.501). The LWR correlated negatively with the root-to-shoot ratio (r = –0.696), and correlated positively with shoot biomass (r = 0.627), whereas both the RWC and leaf count per shoot were not strongly correlated to any of the tested parameters. Despite a strong correlation between the LAI and leaf width (r = 0.624), and the LAI and shoot biomass (r = 0.792), the relationship between the leaf width and shoot biomass was observed to be relatively weaker (r = 0.454), although still statistically significant (P < 0.001) (Table 6).
Canonical discriminant analysis resulted in eight statistically significant functions (variates), of which two had eigenvalues greater than or approaching 1.0. The first variate explained 39.6% of the variance and the second variate explained 28.4% of the variance. Therefore, 68% of the total variance was explained by the first two functions (Table 7). Canonical loadings described how well the original canopy parameters correlated to the function variate scores. The first variate demonstrated high positive loadings for leaf angle (0.940) and LAR (0.416), whereas high negative loadings for leaf width (–0.970) compared with other parameters. In contrast, the second variate had high positive loadings for the number of leaves per stem (0.845) and leaf width (0.625), and higher negative loadings for the LAR (–0.732), RWC (–0.306), and root-to-shoot ratio (–0.705) (Table 7).
Graphical examination of multivariate space indicated genotypes clustered together closely along the x-axis, suggesting function 1 largely identified traits that differentiated genotypes (Fig. 3). TifB16119 demonstrated a large negative score for function 1, which can be attributed to a smaller leaf angle and LWR, and a large leaf width, whereas ‘TifGrand’ showed a large positive score for function 1, resulting in part from a high LWR. ‘TifGrand’ was located most closely to ‘Tifway’, which is surprising considering the differences in their shade resistance. This suggests factors other than morphology (e.g., light use efficiency) can drive shade resistance within the species.
Citation: HortScience 60, 5; 10.21273/HORTSCI18169-24
Function 2 largely discriminated the shade response, with increasing scores corresponding to increasing light availability. Based on loadings, lower function 2 scores are indicative of a leafier canopy, corresponding to a higher LAR and lower root-to-shoot ratios. Surprisingly, most genotypes exhibited a similar pattern for function 2, with the exception of the shade-sensitive TifB16119, which demonstrated minimal variation in function 2 regardless of shade severity.
Increased shoot elongation is a shade avoidance mechanism that enhances the competitiveness of unmowed plants by increasing leaf area and ability to intercept light. In a turfgrass system, this response has historically been viewed as negative, because regular mowing would reduce photosynthate more quickly. As a result, shade resistance in turfgrasses has been characterized by low rates of increase in vertical shoot elongation in the shade (Tegg and Lane 2004). Results from our study are generally in agreement with those of Tan and Qian (2003), who observed that reducing light intensity from 52% to 13% on three different kentucky bluegrass (P. pratensis L.) cultivars increased shoot elongation by 33% for ‘Kenblue Times’, 32% for ‘Livingston’, and 26% for ‘NuGlade’. However, not all genotypes performed similarly in displaying a strong shade avoidance response. For example, TifB16119 (shade sensitive) had a relatively large baseline CER, but did not vary with shade treatment, suggesting an inherently faster growing habit and lack of sensitivity to shade under the provided growing conditions. All other entries, including the shade-resistant ones, were somewhat similar in their shade avoidance response. Thus, the theory that shade resistance is related to the lack of a shade avoidance response may not be true for all bermudagrasses (Noor et al. 2023). Furthermore, variation in the response over time reinforces the work by Meeks and Chandra (2021), who reported similar findings in St. Augustinegrass. Last, the severity of shade used in various studies likely affects the timing and magnitude of response. Wherley et al. (2013) suggested moderate shade to be the optimal environment to test relative shade resistance for grasses because of the general collapse in growth observed as severe shade intensities.
A larger LAI presumably promotes light absorption by the leaves. Plants that are able to maintain LAI even at severe shade intensities would thus be better able to maintain a positive carbon balance. Healey et al. (1998) observed a decrease in LAI in Panicum maximum cv. Petrie (green panic) and Bothriochloa insculpta cv. Bisset (creeping bluegrass) under 25% shade. The lack of a shade treatment effect on LAI suggests increasing SLA and leaf length may be able to compensate for reductions in leaf count per shoot. A larger SLA under low light is regarded as an acclimating characteristic and is indicative of morphological plasticity, as it increases the chance of receiving light per unit of leaf mass (Gong et al. 2015). Plants that are grown in low light tend to increase SLA by expanding their leaf area to capture more light (Duan and Fontanier 2022; Reich et al. 1998).
The LWR has been reported to decrease in shade-sensitive Phaseolus vulgaris L. plants when grown under shaded conditions (Hadi et al. 2006). Shade-resistant plants have the ability to maintain a constant LWR over a range of light intensities as an indication of normal plant development patterns (Hadi et al. 2006). In our study, each genotype responded similarly by reducing the LWR with moderate and severe shade conditions. The shade-resistant entry TifB16108 demonstrated a growth habit that favored root production, resulting in relatively low LWR, despite a high LAI and SLA. In contrast, ‘TifGrand’ (also shade resistant) had the highest LWR and LAR, suggesting a leafier canopy. Eriksen and Whitney (1981) found the leafiness index (i.e., LAR) increased under reduced light intensity. Similar results were observed in soybean (Glycine max) by Gong et al. (2015) as a result of shade in intercropping. Hadi et al. (2006) observed an increasing LAR with increasing shade and suggested it was a result of increasing SLA. Similar results were observed by Loach (1970) in Liriodendron tulipifera, in which he observed an increase in LAR under shade, but again attributed it to increased SLA rather than LWR. Our study provides evidence that more than one canopy strategy can result in increased shade resistance in bermudagrasses.
Leaf angle is an important plant characteristic for distribution of light in a canopy. Horizontal leaves intercept more light in upper layers of the canopy, resulting in heavy shading of the lower leaves. Leaf orientation revealed that leaf angles are fairly large (some nearly vertical) in full sun and more nearly horizontal in the shade (Mc Millen and Mc Clendon 1979). TifB16108 (shade resistant) demonstrated a relatively horizontal leaf angle, but it was less than that of TifB16119 (shade sensitive). Similarly, the shade-sensitive ‘Tifway; and shade-resistant ‘TifGrand’ exhibited comparable leaf angles, suggesting this trait alone may not be useful in predicting sensitivity to shade in bermudagrasses. Leaf count per shoot did not show consistent relationships with known shade resistances of the selected genotypes.
Wilkinson and Beard (1974) observed a greater shoot weight in red fescue (shade resistant) than kentucky bluegrass (shade sensitive) under low light conditions, suggesting higher shoot growth under low photosynthetic irradiance as a characteristic of shade resistance. In our study, the lower shoot biomass for TifB16119 followed by TifB16117 is in agreement with their apparent shade resistance. Baldwin (2008) reported shade-reduced root biomass among all bermudagrasses tested, although shade-resistant cultivars Riley’s Super Sport and TiftNo.2 had a greater root biomass than the shade-sensitive cultivars Arizona Common and GN-1 under shade. In our study, TifB16108 showed the greatest root biomass, indicating shade-resistant behavior. Concurrent studies have shown this may be a result of greater photosynthetic efficiency within this genotype (Kajla 2021). Surprisingly, ‘TifGrand’, which maintained a high shoot biomass and was generally of good quality, had the lowest root biomass among all genotypes. This behavior demonstrated a preference for shoot growth over root growth, suggesting ‘TifGrand’ shade resistance may be related to maintenance of light-harvesting tissues when light becomes limiting (Hebert et al. 2001). In contrast, TifB16108 may derive shade resistance from high light use efficiency or tiller density.
In conclusion, genetic differences in morphology had a stronger influence on apparent resistance than their response to shade itself. TifB16108 had a higher LAI but a lower LWR and LAR, which suggests resistance was derived from maintenance of tiller density. Canonical discriminant analysis suggested leaf width, LAR, and root-to-shoot ratio are morphological factors that best differentiated the bermudagrass genotypes. Morphological parameters provide insight into how genotypes avoid mowing stress under shaded turf systems. However, resistance of a genotype depends on both physiological tolerance and morphological adaptation to shade and can be better explained by combining their physiological and morphological adaptations in shade.
Canopy elongation rate (CER) for various genotypes of bermudagrass at ambient conditions and different shade treatments averaged for 6 weeks. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment. Shade 30 = shade at 30% of ambient conditions; Shade 60 = shade at 60% of ambient conditions; Shade 90 = shade at 90% of ambient conditions.
Normalized canopy elongation rate (NCER) (measured in millimeters per day) representing percent change in canopy elongation rate from control for various genotypes of bermudagrass for 6 weeks averaged for 30%, 60%, and 90% shade. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment.
Scatterplot of 20 group centroids from discriminant analysis using eight morphological parameters specific leaf area, leaf area ratio, relative water content, leaf angle, leaves per stem, leaf weight ratio, leaf width, and root-to-shoot ratio as predictors of five bermudagrasses (green color refers to TifB16119; blue color refers to TifB16108; orange color refers to TifB16117; cyan color refers to Tifway; and purple color refers to TifGrand) grown in a greenhouse under four irradiance levels. Numbers 0% (represented by Δ) and 90% (represented by □) indicate the ambient and heavy shade, respectively.
Contributor Notes
Funding for this project was provided by a Specialty Crop Research Initiative grant (award no. 2019-51181-30472) from the US Department of Agriculture National Institute for Food and Agriculture.
Canopy elongation rate (CER) for various genotypes of bermudagrass at ambient conditions and different shade treatments averaged for 6 weeks. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment. Shade 30 = shade at 30% of ambient conditions; Shade 60 = shade at 60% of ambient conditions; Shade 90 = shade at 90% of ambient conditions.
Normalized canopy elongation rate (NCER) (measured in millimeters per day) representing percent change in canopy elongation rate from control for various genotypes of bermudagrass for 6 weeks averaged for 30%, 60%, and 90% shade. Means with the same letter are not significantly different at P = 0.05. Bars above each treatment represent the standard error of the mean of four replications of a genotype under each shade treatment.
Scatterplot of 20 group centroids from discriminant analysis using eight morphological parameters specific leaf area, leaf area ratio, relative water content, leaf angle, leaves per stem, leaf weight ratio, leaf width, and root-to-shoot ratio as predictors of five bermudagrasses (green color refers to TifB16119; blue color refers to TifB16108; orange color refers to TifB16117; cyan color refers to Tifway; and purple color refers to TifGrand) grown in a greenhouse under four irradiance levels. Numbers 0% (represented by Δ) and 90% (represented by □) indicate the ambient and heavy shade, respectively.
