Pour Some Light on Me

With winter approaching and associated cooler temperatures how bermudagrass sustains itself is largely dependent on light.

It's quite apparent from traveling through the tropical, sub-tropical, and semi-arid to desert regions of the world that light greatly influences the health and quality of warm season turfgrasses like bermudagrass (Cynodon dactylon). With winter approaching how bermudagrass sustains itself is largely dependent on light. This is especially true for non-overseeded bermudagrass in regions where true dormancy does not occur (ex. south Florida, Hawaii).

Figure 1. Photosynthetic photon flux (PPF) taken in full sun under variable skies in Columbus, OH. Results are an average of biweekly scans from vernal equinox to autumnal equinox (Bell & Danneberger, 2000)

Solar Irradiance

Solar radiance for plant growth occurs in a spectral band from 400 to 700-nm wavelengths and is referred to as the photosynthetically active radiation (PAR). Plant pigments, including chlorophylls and carotenoids, absorb PAR best at specific wavelengths. PAR is further divided into high-activity and low-activity wavelengths based on pigment absorption bands. Photosynthetically active radiation occurs within 400 to 500 nm referred to as blue light, and within 600 to 700 nm referred to as red light. The energy units for PAR are watts per meter squared (W m-2).

To quantify the brightness or intensity of light within PAR, the term Photosynthetic photon flux (PPF) is used. The unit for PPF is micromole per square meter per second (µmol m-2 s-1). This quantity is the number of photons, which would fall on this given area within each second.

The amount of PPF can vary during the day (Figure 1). Notice the diminished amount of photon flux available in the afternoon in full sun illustrated by a vertical line drawn at solar noon (1330 h). Normally, you would expect to see an increase in PPF after solar noon on a clear day. Looking at an entire growing season in a continental climate like Columbus, Ohio the reduction in PPF in the afternoon measured a few hours before solar noon and a few hours after solar noon can be reduced by 25%. This is most likely due to cloud cover. Cool season turfgrasses (C3) like creeping bentgrass reach maximum photosynthetic capacity at 1/2 full sunlight (saturation point ranges from 534 (500 to 600 for Kentucky bluegrass) to 1072 (1000 for creeping bentgrass) µmol m-2 s-1) (1,5). The impact of cloudy afternoon skies on cool season turfgrasses is minimal(1). In contrast warm season turfgrasses (C4) like bermudagrass reach maximum photosynthetic capacity at full sunlight (saturation point for warm season turfgrasses 1794 to 2139 µmol m-2 s-1) a reduction in afternoon light from continual cloud cover or shade causes a decline in turf quality (3).


Figure 2. Photosynthetic photon flux (PPF) taken in full sun, decidiuous and building shade under variable skies in Columbus, OH. Results are an average of hourly biweekly scans from vernal equinox to autumnal equinox (Bell & Danneberger, 2000.)

As an additional comment, tree shade and building shade reduces the amount of PPF signifcantly as much as 10-fold (Figure 2).


Quantifying Daily Solar Irradiance

Figure 3: Average yearly Daily Solar Irradiance from selected southern cities in the United States (Faust)

Although light measurements are made instantaneous at a specific time, the total amount of that energy for the day maybe useful. For example, on a clear December day in Minneapolis the PPF may be the same as a clear day in June, but the total PPF for those days, due in part to daylength, would be quite different. The term daily light integral (DLI) is used to describe the total quantity of light delivered over an entire day (3). Daily light integrals units are moles per meter squared per day (mol m-2 d-1) and can be plotted over a calendar year for various locations (Figure 3). The greatest difference in DLI during the winter occurs between northern and southern United States while the greatest difference in summer is between eastern and western United States.

At Clemson University (3) researchers determined that "a DLI of 32.6 mol m-2 d-1 was required to provide a commercially acceptable Tifeagle turf quality". Given that threshold it is no surprise that the for the months of December, January and February - on average - that the radiant energy is below this level in Atlanta, Miami, and Houston; and for Phoenix - December and January.

Implications for non-overseeded semi-dormant to growing bermudagrass is that the low total amount of light most likely has changed the morphological characteristics of the bermudagrass. Turfgrass plants in response to shade or low light levels become more upright in growth habit including a thinner, longer leaves, less tillering, shallower rooting, and less total root mass (2,4,7). Overall, the turf is subject to a decline in both density and quality. If the winter months are more cloudy and rainy than normal, the detrimental changes would be more dramatic.


Figure 4. Comparison of selected cities percent total sunshine to the United States average

Although discussion has primarily centered on winter conditions, reductions in light could be expected on bermudagrass in the summer especially in tropical areas close to the equator like Southeast Asia or parts of the Middle East like Dubai, United Arab Emirates. For example, in Figure 4 Dubai has half the amount of sunshine of Miami during the summer. Additionally, Dubai has extremely high temperatures and humidity during the summer.


Photograph 5. The Montgomerie Golf Club in Dubai, UAE taken July 26, 2007. (photograph courtesy of Colin Thorsborne, golf course manager)

Photograph 5 and 6 show the changing light conditions in Dubai during the summer (Photograph 5) compared to fall (Photograph 6). In photograph 6 the golf course has been overseeded, which is becoming a more common practice the last few years.


Photograph 6. The Montgomerie Golf Club in Dubai, UAE taken November 7, 2007. The course has been overseeded the last week of October (photograph courtesy of Colin Thorsborne, the golf course manager)


Photograph 7. Photograph showing the shade adaptation of Tifeagle compared to Tifdwarf on a shaded putting green in Hawaii (photograph courtesy of Jon Scott, Jack Nicklaus Design)

Management Considerations

By its nature, bermudagrass has poor shade tolerance. As winter overseeding becomes less popular, low light affects on slow growing or semi-dormant bermudagrass increase in importance. One attribute to look for in hybrid bermudagrass is the shade tolerance. Variation does occur. Regarding the ultradwarfs, Tifeagle has shown good shade tolerance compared to the industry standard Tifdwarf (6, Photograph 7).

Cultural practices need to take into account the changes in growth habit of bermudagrass under low light conditions. These practices should be initiated prior to the onset of low light stress.

* Raise the mowing height. Traffic or wear stress is significant problem on bermudagrass greens that are under low light stress. Due to the lack of tillering, growth, and recovery potential wear contributes to thinning and loss of density. Even slight increases 1 mm or so can have a positive impact. Increasing the height improves wear tolerance and helps minimize scalping.

* Switch to solid front rollers. On greens subjected to low light solid rollers reduce wear injury and scalping.

* If possible use walkbehind mowers. Less wear is associated with walkbehind mowers especially when it comes to clean-up mowings. The clean-up pattern should be staggard so that the mowers do not travel the same path daily.

* Topdress frequently and lightly. Light topdressings where the material quickly moves down into the canopy may help protect the crown, and provide a firmer surface to mow. Thus, reducing the potential for gouging or scalping.


* Monitor for potential diseases, including algae. The best control with fungicides is through preventative treatments under low light conditions.


References

1. Bell, G.E., T.K. Danneberger, and M.J. McMahon. 2000. Spectral irradiance available for turfgrass growth in sun and shade. Crop Science 40:189-195.

2. Bell, G.E. and T.K. Danneberger. 1999. Temporal shade on creeping bentgrass turf. Crop Science 39:1142-1146

3. Bunnell, B.T., L.B. McCarty, J.E. Faust, W.C. Bridges, Jr., and N.C. Rajapakse. 2005. Quantifying a daily light integral requirement of a 'Tifeagle' bermudagrass golf green. Crop Science 45:469-574.

4. Danneberger, T.K. 1994. Light, as a resource. in Turfgrass Ecology and Management. G.I.E., Inc. Publishers. Cleveland, Ohio. p. 25-35.

5. Fry, J. and B. Huang. 2004. Shade. in Applied Turfgrass Management and Physiology. John Wiley and Sons, Hoboken, New Jersey. p. 111-123

6. Miller, G.L., and J.T. Edenfield. 2002. Light intensity and duration influence growth of ultradwarf bermudagrass. Golf Course Management 70(9): 111-113.

7. Wherley, B.G., D.S. Gardner, and J.D. Metzger. 2005. Tall fescue photomorphogenesis as influenced by changes in the spectral composition. Crop Science 45:562-568.

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