The growth characteristics of bermudagrass (Cynodon spp.) promote its use across a wide range of environments. Its aggressive behavior, resistance to weed encroachment, traffic tolerance, and drought tolerance makes bermudagrass a desired species on golf courses, athletic fields, municipalities, and home lawns in North Carolina (Carrow and Petrovic, 1992; DiPaola and Beard, 1992). However, it is estimated that 20% to 25% of all turfgrass stands are under some kind of shade, primarily from trees and/or other structural objects (Beard, 1973) and of the warm-season turfgrasses typically used in the south, bermudagrass exhibits the poorest shade tolerance (Dudeck and Peacock, 1992).
The effects of shade can elicit profound physiological, morphological, and field performance effects on turfgrasses. McBee and Holt (1966) originally showed that decreases in light incidence drastically reduced percent TC, density, and color of bermudagrass. Schmidt and Blaser (1969) evaluated the effects of temperature, light, and nitrogen on growth and metabolism of ‘Tifgreen’ [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt Davy] bermudagrass. The results of their experiment suggested that low light intensity drastically inhibited nitrogen utilization for shoot growth and root development. Overall, the effects of shade can reduce TQ, turfgrass density, NDVI, chlorophyll content, root mass, pigment concentrations, total nonstructural carbohydrates, petiole length, internode diameter, number of stolons, total stolon lengths, and canopy photosynthetic rates (Baldwin et al., 2008; Bell and Danneberger, 1999b; Bunnell et al., 2005a; Peacock and Dudeck, 1981; Qian and Engelke, 1997; Sladek et al., 2009; Stuefer and Huber, 1998; Van Huylenbroeck and Van Bockstaele, 2001; Stuefer and Huber, 1998).
For breeding programs, turfgrass cultivars, and novel turfgrass accessions must be evaluated to improve on specific traits. Typically, these collections are subjected to rigorous selection pressure. Research to evaluate turfgrasses for shade tolerance has often used artificial shade to mimic environmental conditions (McBee and Holt, 1966; Peacock and Dudeck, 1981; Qian and Engelke, 1997; Sladek et al., 2009; Trenholm and Nagata, 2005; Van Huylenbroeck and Van Brockstaele, 2001; Wilkinson and Beard, 1975). Many forms of artificial shade have emerged for properly evaluating the trait. Baldwin et al. (2009) used various colors of shadecloth (65% light reduction in all treatments) to filter-specific wavelengths to determine their individual effects on TQ, relative clipping yield, relative chlorophyll concentration, relative shoot width, relative root biomass, relative root length density, relative specific root length, and root and shoot total nonstructural carbohydrates in bermudagrass. While each of the colored shade fabrics reduced specific measures, the black shadecloth provided the most detrimental effect across all response parameters. Varying levels of photosynthetic photon flux (PPF) allow for estimation of thresholds for acceptable TQ within turfgrass entries (Bunnell et al., 2005a; McBee and Holt, 1966; Miller et al., 2005; Sladek et al., 2009; Trenholm and Nagata, 2005; Van Huylenbroeck and Van Brockstaele, 2001).
Research has shown significant variation among bermudagrass genotypes in shade response. Under 90% uninterrupted shade, Gaussoin et al. (1988) noted the diversity in shade tolerance among 32 bermudagrass genotypes. Similarly, Baldwin et al. (2008) evaluated 42 bermudagrass cultivars collected from the National Turfgrass Evaluation Program (NTEP) and were able to group them into distinct classes (best, intermediate, and sensitive) based on TQ, shoot chlorophyll concentration, root length, and total root biomass. Finally, Hanna et al. (2010) evaluated the newly registered ‘ST-5’ [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt Davy] bermudagrass cultivar (later released as ‘TifGrand’—PP21017; Hanna and Braman, 2008) against other dwarf-type bermudagrass cultivars and showed 2-fold increases in TC under 70% continual shade.
Bermudagrass materials are often compared against other species to evaluate shade tolerance. Jiang et al. (2004) compared two hybrid bermudagrass cultivars to eight seashore paspalum cultivars and showed the bermudagrass hybrids maintained the lowest TQ throughout the study. However, Bunnell et al. (2005a) and Baldwin et al. (2009) showed recent gains in bermudagrass shade tolerance through the release of ‘Celebration’ (Riley, 2000), a common-type [Cynodon dactylon (L.) Pers.]. In the two studies, ‘Celebration’ exhibited superior shade tolerance compared with bermudagrass hybrids and performed similarly to ‘Sea Isle 2000’ seashore paspalum (Paspalum vaginatum Swartz.).
In addition to cultivar development through breeding and selection, further improvement in warm season turfgrass response to shade can be attained through cultural practices. Previous research suggests that reducing nitrogen fertility rates (Bell and Danneberger, 1999a; Bunnell et al., 2005b; Goss et al., 2002) and raising mowing heights (Bunnell et al., 2005b; Bell and Danneberger, 1999a; White, 2004) can improve TQ and overall turfgrass performance under low-light conditions.
The development of shade-tolerant cultivars, specifically TifGrand and ‘Celebration’, has shown the potential of bermudagrass to persist in low-light environments. However, with only these two commercially available shade-tolerant cultivars, there is great need for introducing new germplasm or cultivars with further enhanced shade tolerance. This should be possible through identification of new shade tolerance sources and breeding efforts. In the early 1990s, a collection trip to South Africa resulted in the introduction of nine common bermudagrass [Cynodon dactylon (L.) Pers.] accessions that showed promising shade tolerance in their native environment. The objectives of this research were to evaluate these accessions for shade tolerance under varying levels of PPF, and to estimate the effect of nitrogen fertilization on the performance of these materials under shade.
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Means comparisons for each entry within three shade treatments (0%, 63%, and 80%) for normalized difference vegetation index (NDVI), turfgrass cover (TC), and turfgrass quality (TQ) from data collected under a prolonged, shaded environment. The data shown represent 9 weeks after cup-cutter removal on 29 Sept. 2011 at the Lake Wheeler Turfgrass Field Laboratory, Raleigh, NC.
Means comparisons for each entry within three shade treatments (0%, 63%, and 80%) for normalized difference vegetation index (NDVI), turfgrass cover (TC), and turfgrass quality (TQ) from data collected under a prolonged, shaded environment. The data shown represent 9 weeks after cup-cutter removal on 11 Sept. 2012 at the Lake Wheeler Turfgrass Field Laboratory, Raleigh, NC.