Shade is a major challenge for turfgrass managers worldwide. The shade environment can be particularly challenging to warm-season turfgrasses, which have inherently higher light compensation points relative to cool-season grasses (Beard, 1973). Turfgrass physiological responses to shade include reduced pigment levels, nonstructural carbohydrates, increased levels of gibberellic acid, and decreased evapotranspiration, whereas morphological/developmental responses to shade include reduced tillering, thinner and narrower leaves, greater leaf extension rates, and reduced root mass (Dudeck and Peacock, 1992).
St. augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is considered one of the most shade-tolerant warm-season turfgrass species (Beard, 1973; Stier and Gardner, 2008); however, there is limited published information pertaining to comparative shade tolerance of st. augustinegrass. Trenholm and Nagata (2005) compared performance of five st. augustinegrass cultivars in mild to moderate shade ranging from 30% to 70% of full sunlight. Cultivars included ‘Floratam’, ‘Seville’, ‘Palmetto’, ‘Bitterblue’, and ‘1997-6’. With the exception of ‘Floratam’, all cultivars maintained acceptable quality and density up to and, in some cases, beyond 60% shade levels.
In recent years, newer cultivars with reportedly improved shade tolerance such as ‘Amerishade’ and ‘Captiva’ have been developed (Brosnan and Deputy, 2008; Trenholm and Kenworthy, 2009); however, published data on their comparative shade tolerance in relation to other commercially available cultivars are lacking. Because it is not uncommon for builders or homeowners to attempt to establish turfgrasses in densely shaded environments receiving 80% or greater reduction in PPF, determining the physiological limits of available cultivars by evaluating them in both moderate and heavily shaded environments would be valuable information.
In addition, for purposes of screening large collections of germplasm or segregating breeding populations for shade tolerance, breeders are continually in search of bioassays that could be used as a tool for more rapid selection of the most shade-tolerant lines. When compared with their less shade-tolerant counterparts, strategies used by shade-tolerant cultivars or species that enable greater tolerance to shade include greater photosynthetic rates (Miller et al., 2005; Wilkinson et al., 1975), reduced gibberellic acid production (Tan and Qian, 2003), improved carbohydrate allocation to roots (Wherley et al., 2005), and/or less upright leaf orientation (Wherley et al., 2011; Wilkinson and Beard, 1974).
The degree of shade intensity used to screen germplasm for shade tolerance can have a strong impact on the results of a screening. The light compensation point of most turfgrasses is ≈2% to 5% full sunlight (Beard, 1973). Therefore, it seems plausible that light intensity should be sufficient even in heavy shade (less than 20% PPF) to enable screening for shade tolerance differences among various germplasm. Winstead and Ward (1974) used 70% shade for determining selection indicators when comparing shade-intolerant bermudagrass vs. shade-tolerant st. augustinegrass but found no suitable bioassay that was useful for shade tolerance screening. The light quality (red:far-red) altering effects of vegetative shade are not easily reproduced in controlled environment or greenhouse studies, but reductions in light intensity (PPF) alone can cause many of the photomorphogenic responses observed in shade (Wherley et al., 2005).
The objectives of this study were to 1) compare performance and developmental responses among five commercial cultivars, one Plant Introduction (PI), and four experimental lines of st. augustinegrass to moderate and heavy shade; and 2) determine the appropriate shade intensity level and identify useful indicators for rapid shade screening of a diverse st. augustinegrass germplasm population.
Barrios, E.P., Sundstorm, F.J., Babcock, D. & Leger, L. 1986 Quality and yield response of four warm-season lawngrasses to shade conditions Agron. J. 78 270 273
Beard, J.B. 1973 Turfgrass science and culture. Prentice Hall, Englewood Cliffs, NJ
Bell, G.E. 2011 Turfgrass physiology and ecology: Advanced management principles. Cambridge University Press, Cambridge, UK
Brosnan, J. & Deputy, J. 2008 St. Augustinegrass. University of Hawaii Cooperati Extension Service Bulletin TM-3. 1 Apr. 2013. <http://www.google.com/url?sa=t&rct=j&q=brosnan%20deputy%20st%20aug&source=web&cd=1&ved=0CDEQFjAA&url=http%3A%2F%2Fwww.ctahr.hawaii.edu%2Foc%2Ffreepubs%2Fpdf%2FTM-3.pdf&ei=lgyNUYD0J7Hd4APBo4CYAw&usg=AFQjCNHdRS8zR7RQUrrjL0JQH_1gN9DUiQ>
Dudeck, A.E. & Peacock, C.H. 1992 Shade and turfgrass culture, p. 269–284. In: Waddington, D.V. R.N. Carrow, and R.C. Shearman (eds.). Turfgrass. Amer. Soc. Agron. Monograph No. 32. Madison, WI
Genovesi, A.D., Jessup, R.W., Engelke, M.C. & Burson, B.L. 2009 Interploid St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] hybrids recovered by embryo rescue In Vitro Cell. Dev. Biol. Plant 45 659 666
Miller, G.L., Edenfield, J.T. & Nagata, R.T. 2005 Growth parameters of Floradwarf and Tifdwarf bermudagrass exposed to various light regimes Intl. Turfgrass Soc. Res. J. 10 879 884
Moran, R. 1982 Formulae for determination of chlorophyllous pigments extracted with N,N- dimethylformamide Plant Physiol. 69 1376 1381
Stier, J.C. & Gardner, D.S. 2008 Shade stress and management, p. 447–471. In: Pessarakli, M. (ed.). Turfgrass management and physiology. CRC Press, Boca Raton, FL
Trenholm, L.E. & Kenworthy, K. 2009 ‘Captiva’ St. Augustinegrass. UF IFAS Extension Bulletin ENH1137. 1 Apr. 2013. <http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CD8QFjAB&url=http%3A%2F%2Fedis.ifas.ufl.edu%2Fpdffiles%2FEP%2FEP39800.pdf&ei=dg-NUcqQE4Lk9ATk0YHICA&usg=AFQjCNGKkOc_LpThf3LWL9WmV0uFEKPVow&bvm=bv.46340616,d.eWU>
Wherley, B.G., Gardner, D.S. & Metzger, J.D. 2005 Tall fescue photomorphogenesis as influenced by changes in the spectral composition and light intensity Crop Sci. 45 562 568
Wherley, B.G., Skulkaew, P., Chandra, A., Genovesi, A.D. & Engelke, M.C. 2011 Low-input performance of zoysiagrass (Zoysia spp.) cultivars maintained under dense tree shade HortScience 46 1033 1037
White, R.H. & Schmidt, R.E. 1989 Bermudagrass response to chilling temperatures as influenced by iron and benzyladenine Crop Sci. 29 768 773
Wilkinson, J.F. & Beard, J.B. 1974 Morphological responses of Poa pratensis and Festuca rubra to reduced light intensity, p. 231–241. In: Roberts, E.C. (ed.). Proc. 2nd Int. Turfgrass Res. Conf. ASA, CSSA, Madison, WI
Wilkinson, J.F., Beard, J.B. & Krans, J.V. 1975 Photosynthetic-respiratory responses of ‘Merion’ kentucky bluegrass and ‘Pennlawn’ red fescue at reduced light intensities Crop Sci. 15 165 168
Winstead, C.W. & Ward, C.Y. 1974 Persistence of southern turfgrasses in a shade environment, p. 221–230. In: Roberts, E.C. (ed.). Proc. 2nd Intl. Turfgrass Res. Conf. ASA, CSSA, Madison, WI