Field Evaluation of Daily Light Integral Requirements for Zoysiagrass and Bermudagrass Cultivars.

Maintaining acceptable turf in shade is a significant management challenge. Proper turfgrass selection for shade can be challenging due to difficulty in quantifying necessary light levels particular to each environment. Biologically speaking, rather the responding to number of hours of sunlight or percent shading, plants ultimately respond to the cumulative daily total number of photons (mols m^-2 d^-1) received within the photosynthetically active wavelengths (400-700 nm), or ‘daily light integral’ (DLI).  Minimal DLI needed for acceptable quality could vary by month of year, and could be influenced simultaneously by temperature effects on photosynthetic and respiratory balance. Currently there is growing interest by turfgrass researchers in quantifying DLI requirements as they relate to turf quality of cultivars and species, however, the primary work in this area has been with bermudagrass putting greens. More recent work involving numerous warm-season grasses has been conducted, but these have been shorter-term greenhouse studies.  This field study is being conducted over multiple seasons in College Station, TX under replicated treatments offering 0 to 90% reductions in photosynthetic photon flux (PPF).  Project objectives are to 1) determine minimal daily light integral (DLI) requirements for acceptable turf quality of 8 zoysiagrass and bermudagrass turfgrass cultivars, 2) evaluate whether minimal DLI requirements change seasonally (spring, summer, and fall months), 3) assess effects of lower vs. upper-end heights of cut for each species on DLI requirement, and 4) determine whether monthly application of trinexepac-ethyl reduces DLI requirement.  Results to date indicate 1) taller cutting height (5 cm) turf displays noticeably improved tolerance to low light conditions compared to lower-end height (2 cm), 2) shade tolerance of some cultivars is improved by application of plant growth regulator, and 3) zoysiagrass cultivars generally show improved shade tolerance compared to bermudagrass, with ‘Zorro’ and ‘Tifgrand’ exhibiting the best shade tolerance for each species.