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TURFGRASS SCIENCE

Tall Fescue Photomorphogenesis as Influenced by Changes in the Spectral Composition and Light Intensity

^a Dep. of Crop Science, Campus Box 7620, North Carolina State Univ., Raleigh, NC 27695
^b Dep. of Horticultural and Crop Science, 2021 Coffey Road, The Ohio State Univ., Columbus, OH 43210-1086

^* Corresponding author (gardner.254{at}osu.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The influence of deciduous foliage shade on turfgrass development has not been fully investigated. Previous research neglects changes in spectral distribution, e.g., red:far-red light (R:FR) ratios common of foliage shade. Turfgrass plants may respond simultaneously but in different ways to changes in light intensity and spectral composition. A field study was conducted in 2001–2002 at the Ohio Turfgrass Research and Educational Facility, Columbus, OH. Two tall fescue (Festuca arundinacea Schreb.) cultivars of differing shade tolerance were established under low photosynthetic photon flux (PPF) in approximately 8% of full sunlight with high (>1) and low (<1) R:FR ratios to distinguish between developmental effects of R:FR ratio (spectral composition) and PPF (light intensity) on turfgrass photomorphogenesis. Few morphological differences in shade tolerance between the two cultivars were observed during the 2-yr study. However, under low PPF, high R:FR ratios led to increased tillering, leaf blade width and thickness, and chlorophyll contents. Root mass declined under reduced PPF regardless of R:FR ratio. Results suggest that while turfgrass photomorphogenesis in shade is influenced by changes in PPF, many characters are further influenced by changes in the R:FR ratio.

Abbreviations: DS, deciduous shade • FS, full sunlight • NS, neutral shade • PPF, photosynthetic photon flux • R:FR, Red:far-red

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
DECIDUOUS TREE CANOPIES significantly alter the spectral composition and PPF of solar radiation available to plants in shade. PPF in the shade of fully leafed trees can be reduced to 1 to 5% full sunlight and this significantly affects plant growth and development (Shirley, 1945). Common cool-season turfgrass species, such as Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue (Festuca arundinacea Schreb.), exhibit more upright growth habits, thinner and longer leaves, shallow root systems, lower plant carbohydrate levels, reduced tillering, and decreased stand density when grown in shade (Dudeck and Peacock, 1992). Although a number of factors, such as tree root competition, may contribute to decline of turfgrass quality in shade, two of the most influential are the reduction of PPF and R:FR ratio occurring under vegetative canopies. R:FR is the ratio of the photon flux ratio of light in the 10-nm band around 660 nm to light around 730 nm.

Reduction of PPF alters the plant's photosynthetic-respiratory balance resulting in lower carbohydrate levels compared to grasses grown in open sun (Blackman and Templemen, 1940). Carbohydrate allocation favors shoot growth over root development (Dudeck and Peacock, 1992). The reduction in carbohydrate levels results in reduced tillering and stand density. Changes in PPF initiate altered leaf size and structure, chloroplast ultrastructure, and photosynthetic and respiratory metabolism (Smith, 1982).

Plant perception of the R:FR ratio is an important aspect of shade acclimation (Holmes and Smith, 1975). Changes in R:FR ratio influence plant development by altering phytochrome equilibrium. Responses to low R:FR ratio include increased stem elongation, reduced leaf area, reduced branching/tillering, and changes in chlorophyll content (Dudeck and Peacock, 1992).

The effects of foliage shade on the spectral composition, and in particular, the R:FR ratio of light has been well documented (Federer and Tanner, 1966; Vezina and Boulter, 1966). Tree leaves alter R:FR ratio of light by selectively attenuating red and blue quanta while transmitting far-red. Both deciduous and coniferous canopies filter significant amounts of red and blue quanta relative to building shade (Bell et al., 2000). Holmes and Smith (1977) found the R:FR ratio of sunlight, irrespective of time of year and weather, to be nearly constant, averaging 1.15. Reported R:FR ratios in deciduous foliage shade range from 0.36 to 0.97 (Goodfellow and Barkham, 1974). Bell et al. (2000) reported R:FR ratios of 0.91 and 0.80 for deciduous and coniferous shade, respectively.

The light compensation point of most turfgrasses is around 2 to 5% full sunlight (Beard, 1973). The most common procedure for mitigating vegetative shade problems has been removal of tree limbs. Changes in R:FR ratios alter plant development before any serious reduction in light intercepted per plant is detectable (Ballaré et al., 1987). As a result, "shade-avoidance" symptoms in turf may occur under shade environments despite adequate levels of PPF and this could divert resources that would otherwise be used for more agriculturally productive activity, such as root and leaf growth (Ballaré et al., 1997).

Minimal turfgrass research has focused on the influence of altered R:FR ratios under shaded environments and their impacts on turfgrass physiology. Neutral density shade fabrics are often used in shade research for ease of use and manipulation of light intensity. These materials do not alter the spectral composition of vegetative shade light and therefore do not accurately reproduce the natural foliage shade environment. A significant amount of shade research has underestimated the extent and array of developmental responses to foliage shade (Buisson and Lee, 1993). Knowledge of plant development in shade would therefore be improved by understanding the relative contributions of PPF and R:FR ratios to turfgrass photomorphogenesis. The objective of this research was to investigate the effects of altered R:FR ratios and PPF on chlorophyll content, leaf and chloroplast development, tillering, and root mass of tall fescue.

	MATERIALS AND METHODS

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The study was initiated on 15 June 2001 and concluded on 26 Jan. 2003 at the Ohio Turfgrass Foundation Research and Education Facility, Columbus, OH. Plots were constructed by excavating to a 30-cm root zone depth and backfilling with a 90% sand and 10% peat (v/v) mixture overtop an existing native clay loam soil. Replicate studies consisting of three 5.8- x 4.0-m main plots was established in full sun, deciduous tree shade (an east to west running row of mature hardwoods, primarily Acer spp. and Fraxinus spp. that are 20–25 m in height), and neutral density shade of a fiberglass shade structure. The fiberglass panels were mounted on a frame approximately 3 m above the turf. Panels were removed in the fall to coincide with leaf drop in the deciduous shade plots. Treatment plots were established in full sunlight and shade. Full sunlight (FS) plots received high PPF and high R:FR ratio characteristic of natural daylight while neutral shade (NS) and deciduous shade (DS) plots received similar PPF (7 and 8% of full sun, respectively) but different R:FR ratios (Table 1).

Within each main plot were six 1.5- x 1.5-m subplots. Two cultivars of tall fescue, ‘Plantation’ (Pennington Seed, Madison, GA), and ‘Equinox’ (Turf Merchants, Inc., Tangent, OR) were established by seed on 28 June 2001 at a rate of 29.3 g m^–2. The cultivars were chosen for their differences in subjective evaluation of shade tolerance (Plantation = high, Equinox = low) as reported in the results of a three-year dense shade study (NTEP, Beltsville, MD).

Rainfall was monitored and differential irrigation was provided to each of the light treatment plots to compensate for plot-to-plot differences in the amount of water received. The perimeter of the deciduous foliage shade plots was root pruned to 30-cm depth monthly to prevent encroachment of tree roots.

An 18-3-18 granular fertilizer (United Horticultural Supply, Maumee, OH) was applied at 24 kg N ha^–1 monthly from April through October (excluding July) during each season of the study. A granular application of dithiopyr [3,5-pyridinedicarbothioic acid, 2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-S,S-dimethyl ester] was made in early April of 2001 and 2002 to suppress annual weeds. Preventative fungicide spray applications of propiconazole [1-((2-(2,4-dichlorophenyl)-4-propyl-11,3-dioxolan-2-yl)methyl)–1H-1,2,4-triazole] were made on a monthly basis throughout the growing season. Mefenoxam [(R)-2-((2,6-dimethylphenyl)-methoxyacetylamino)-propionic acid methyl ester] was applied in July of both years for prevention of Pythium spp. disease. Plots were mowed at the same time as needed with a rotary push mower to a height of 7.62 cm and the clippings collected.

Plant samples were collected for analysis of the various physiological and morphological parameters in September of 2001 and 2002. No sampling occurred within 10 cm of the plot borders. Chlorophyll content of the newest fully expanded leaf blades of five random plants in each subplot was determined by the method of Moran and Porath (1980). A 3-mm diameter disk was removed from the center of each leaf and placed in 5 mL of N,N-dimethylformamide in the dark at 4°C for 72 h. For leaves with widths less than 3 mm, 1.59-mm disks were removed from 18 leaf blades for an equivalent leaf tissue area. The absorbance at 664 and 647 nm was measured with a Hitachi U-2000 spectrophotometer (Hitachi Instruments, Inc., Japan). Chlorophyll content (µg cm^–2) was then determined by the Moran formula (1982).

Leaf widths were determined by removing 2.5 cm from the middle of the newest most fully expanded leaf blades of 10 random plants in each subplot with a razor blade. Cut leaf samples were immediately placing in water to prevent wilt. These samples were then blotted dry and the average leaf area determined with a Li-Cor model 3100 leaf area meter (LI-COR, Lincoln, NE).

The number of tillers per plant was determined from an 81 cm² (10.2-cm diam) round plug from a randomly determined location within each subplot. A single mother plant with no daughter plants was determined to have zero tillers.

A hole drill (Black and Decker, Towson, MD) was used to remove a 4.5 cm in diameter x 15-cm deep root plug to collect root samples. Roots were separated from the thatch layer and the sand/peat root zone mix. Roots were then oven dried at 105°C for 48 h in a VWR 1350 F drying oven (VWR Scientific, West Chester, PA) and weighed.

Chloroplast ultrastructure and leaf histology were analyzed by transmission electron microscopy (TEM) at the Molecular and Cellular Imaging Center (Ohio Agricultural Research and Development Center, Wooster, OH). Plant tissue was infiltrated and fixed in 3% (v/v) glutaraldehyde, 1% (v/v) paraformaldehyde, in 0.1 M potassium phosphate buffer (pH 7.4) for 4 h at room temperature. Samples were washed three times with 0.1% (w/v) potassium phosphate buffer (pH 7.4), post-fixed in 2% (w/v) OsO₄ in 0.1 M potassium phosphate buffer (pH 7.4) for 30 min at room temperature, washed three times with distilled water, and dehydrated in a graded ethanol–acetone series. Samples were then embedded in Spurr's resin following manufacturer instructions (Electron Microscopy Sciences, PA). Ultra-thin sections were counterstained with 3% uranyl acetate in 50% ethanol for 35 min, and 2% aqueous bismuth sulfate stain for 15 min. Samples were then viewed on a Hitachi H-7500 transmission electron microscope. Samples were viewed at 150 and 300x magnification for measurement of mesophyll thickness and air space, epidermal cell structure and size, and chloroplast orientation while 4000x was used for viewing chloroplast ultrastructure. SigmaScan scientific software (SPSS Science, Chicago, IL) was used to measure various components of chloroplast ultrastructure and leaf histology from three to five 19- x 24-cm micrograph images per cultivar and light treatment combination.

The data were analyzed as three randomized complete block studies combined over locations (light environments), by the analysis of variance procedure of SAS (SAS Institute, 1990). Data are presented as means with standard deviations. Orthagonal contrasts were used to hypotheses of the different light environments.

	RESULTS

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The various light treatments affected tiller production in both years of the study (Table 2). Plants of both cultivars grown in FS had significantly greater tillering rates than those grown in either shade treatment. No tillering was observed in either cultivar under DS in the first year. The shade tolerant cultivar Plantation had higher tillering rates than Equinox. Plantation tiller production was two times that of Equinox in FS, but there were no differences between the two cultivars in either shade environment (Table 2).

Accumulation of dry matter in the roots was influenced by reducing PPF. However, R:FR ratios had no effect (Table 3). On average, FS plants had about five times more root mass than plants grown in either NS or DS. In shade, changes in spectral composition did not result in changes in root mass. There were no differences in root mass between the two cultivars in any treatment.

In Year 1, NS plants contained slightly higher amounts of chlorophyll than DS plants (Table 2). However, in Year 2, even greater differences were observed between the light treatments as FS and NS promoted overall higher chlorophyll contents than DS (Table 3). In shade, high R:FR ratios in NS led to higher chlorophyll contents than low R:FR ratios of DS. Again, Plantation and Equinox chlorophyll contents did not differ in Year 2.

Leaf thickness varied among the three treatments, but not between cultivars (Fig. 1 , Fig. 2) . PPF had the greatest influence on leaf thickness. FS produced considerably thicker leaf blades than those grown in either shade treatment. However, in low PPF, NS plants had significantly thicker leaves than DS plants (Table 4).

View larger version (48K): [in this window] [in a new window]   Fig. 1. Leaf transverse sections of cultivar Plantation from growth in full sunlight (FS), neutral shade (NS), and deciduous shade (DS). Bar = 500 µm.

View larger version (43K): [in this window] [in a new window]   Fig. 2. Leaf transverse cross-sections of cultivar Equinox from growth in full sunlight (FS), neutral shade (NS), and deciduous shade (DS). Bar = 500 µm.

In all treatments and cultivars, the proportion of mesophyll contributing to differences in leaf thickness did not change (Table 4). Mesophyll occupied approximately 85% of the cross-sectional area of the leaf whereas epidermis occupied roughly 15%, regardless of light treatment or cultivar (Fig. 3 , Fig. 4) .

View larger version (34K): [in this window] [in a new window]   Fig. 3. Leaf transverse sections of cultivar Plantation illustrating leaf anatomy in full sunlight (FS), neutral shade (NS), and deciduous shade (DS). Bar = 100 µm.

View larger version (36K): [in this window] [in a new window]   Fig. 4. Leaf transverse sections of cultivar Equinox illustrating leaf anatomy in full sunlight (FS), neutral shade (NS), and deciduous shade (DS). Bar = 100 µm.

Neither chloroplast dimensions nor the number of grana per chloroplast were influenced by PPF or R:FR ratio (Table 5). Grana thickness was influenced by PPF (Table 5). Chloroplasts from plants grown under FS had thicker grana stacks than those grown in either shade environment. No other differences were seen with regard to the influence of light treatments on grana thickness.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

In both years, light intensity was the most influential factor controlling tiller production. However, in the shade, high R:FR ratios enhanced tiller production. In the initial 3 mo of the study, tillering occurred in NS (high R:FR), but none was observed in DS (low R:FR). Because plots were established in June, nearly 6 wk following tree leaf development, DS plots were exposed to low R:FR ratios for the entire period from germination (June) to Year 1 sampling (September). A noticeable increase in tillering occurred from Year 1 to Year 2 in all treatments, especially in DS (Table 3). This is probably because grass plants must reach a minimum level of maturity before tillering is possible (Beard, 1973). There is a strong correlation between leaf thickness and photosynthetic capacity (Oguchi et al., 2003). Therefore, it is possible that there is a relationship between the increase in leaf thickness observed in the NS compared to DS and tiller production.

A relationship between PPF and chlorophyll content was not observed in Year 1 of this study (Table 2). However, the effects of increased R:FR ratios in this study were reflected in the higher chlorophyll contents of plants in NS relative to plants in DS (Table 2, 3). Transferring plants of many species to far-red-attenuated light results in greater chlorophyll per unit area (McMahon and Kelly, 1995; Rajapakse et al., 1999). Buisson and Lee (1993) report that papaya (Carica papaya L.) leaves grown under high R:FR ratios had greater chlorophyll contents (µg chlorophyll cm^–2) than those grown in low R:FR ratios (FR-filtered shade) at similar levels of PPF. This supports the role of phytochrome in initiating increased chlorophyll production despite the low PPF in NS. In Year 2, the relationship between PPF and chlorophyll per unit area was difficult to interpret (Table 3). However, as in Year 1, chlorophyll content was higher in the NS plants. Because shaded turf generally has thinner leaves than leaves in full sun, results may have differed had chlorophyll been measured on a weight or volume basis. Leaves grown at low light intensities have more chlorophyll per unit weight or unit volume of leaf, but the chlorophyll content per unit area of leaf surface is often lower than leaves grown at higher light intensities due to differences in leaf morphology (Boardman, 1977).

Turfgrass plants in NS had the highest chlorophyll contents in both years. High R:FR ratios presumably promoted greater chlorophyll production by influencing phytochrome equilibrium. High light intensities have been shown to promote breakdown of chlorophyll through pigment photooxidation (Demmig-Adams and Adams, 1996; Beard, 1973). Therefore high light intensities may have contributed to a degradation of chlorophyll contents in full sunlight, whereas low light intensities likely prevented breakdown of chlorophyll.

The two cultivars did not differ in many of the measurements made despite the difference in relative shade tolerance of the cultivars. This may have occurred because we are comparing cultivars of similar and relatively shade-tolerant turfgrass species. However, the differing shade tolerances of the two cultivars could be seen in the chlorophyll production by each cultivar in response to changes in R:FR ratios in shade. Chlorophyll production by the shade-tolerant Plantation decreased by 24% in NS compared to DS. However, chlorophyll production by the shade-intolerant Equinox was 56% less under DS compared with NS. This would appear to be a significant factor contributing to the increased shade tolerance reported for Plantation relative to Equinox, since shade-intolerant taxa respond most strongly to reduced R:FR ratio (Lee et al., 1996).

Only minor differences in chloroplast development were observed in this study. The number of grana per chloroplast remained consistent throughout the treatments for Plantation but not Equinox. A significant increase in the number of grana from FS to shade in Equinox is a response expected of less shade tolerant plants. This is in contrast to the more shade-tolerant Plantation, which did show a change in grana number between FS and shade. Similarly, Wilkinson and Beard (1975) observed that relatively shade intolerant Kentucky bluegrass ultrastructure was altered by shade, whereas that of relatively shade tolerant fine fescue was not.

Grana thickness was greater in FS than either shade treatment. FS plants had planar, more uniform orientation of grana stacks whereas chloroplasts of plants grown in shade were more spread out across the chloroplast and appeared less organized. Staggered, interconnected organization of chloroplasts is thought to be an adaptation by plants in shade for maximizing light absorption under low PPF (Boardman, 1977).

In general, the response by chloroplast ultrastructure to shading and R:FR was minimal. Wilkinson and Beard (1975) found that shade tolerant ‘Pennlawn’ red fescue chloroplast ultrastructure to be similar under various PPF. This response is therefore characteristic of a species having good shade tolerance. However, the shade intolerant Kentucky bluegrass exhibited different chloroplast ultrastructure depending on the level of PPF. Results from our experiment imply differences in chloroplast ultrastructure at varying PPF levels may not exist between cultivars within a shade tolerant species.

In summary, our results suggest that while turfgrass photomorphogenesis in shade is influenced by changes in PPF, many characters are further influenced by changes in the R:FR ratio. Under low PPF, high R:FR ratios led to increased tillering, leaf blade width and thickness, and chlorophyll contents. However, other characters such as root mass do not appear to be influenced by light quality and would be influenced any time PPF is reduced, regardless of R:FR.

	ACKNOWLEDGMENTS

 
The authors wish to thank Karli Fitzelle and Tea Meulia at the Ohio Agricultural Research and Development Center Transmission Electron Microscopy Laboratory at Wooster, Ohio for their assistance.

	NOTES

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Salaries and research support provided in part by State and Federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Journal Article Number HCS03-36.

Received for publication April 22, 2004.

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