I always like to end one year and start the next with a look at an old research article — perhaps one that introduced a well-known turfgrass cultivar, or one that first reported on a technique for improved turfgrass maintenance.
This year, I went to our top turfgrass research journal, Crop Science, and I searched for the oldest journal article, using the search term “turfgrass.” That brought me to a 1962 journal article on a topic that we still research today: the response of bermudagrass under varying light intensity.
The general objective of this work was to examine the effects of light intensity on single leaves of bermudagrass [Cynodon dactylon (L.) Pers.] and compare that with the effect of light intensity when all the leaves were in a community (overlapping and affecting each other, for example). To study this, the researchers planted common bermudagrass into big boxes (2 feet × 2 feet in size; 0.6 × 0.6 meter) and applied daily cutting height treatments of 1, 2 or 8 inches (2.5, 5 or 20 centimeters). There were also treatments where the bermudagrass was allowed to grow up to 26 inches (66 centimeters), after which it was lopped back to a height of 2 or 8 inches.
Net photosynthesis and respiration were measured at several light intensities on both single isolated leaves and on the bermudagrass community. Light intensities were measured in foot-candles(!). A quick browse of conversion factors reveals that the foot-candle values cited in the paper (1,000 to 6,000 foot-candles) convert to PAR (photosynthetic active radiation) values of 8.4, 17, 25, 33, 42 and 50 moles/square meter/second. For reference, recent work shows that hybrid bermudagrass requires a daily PAR of around 24 to 26 moles/square meter/second.
The researchers found that, for an individual leaf, uptake of carbon dioxide was maximized at a light intensity of 3,000 foot-candles (~25 moles/square meter/second). In the leaf community, however, there were some differences. First, to maximize photosynthesis in the lower leaves, light intensity needed to be greater, an effect of shading from the stacked leaves. Because it is not possible to change ambient light intensity, the authors considered whether leaf orientation could be adjusted to better allow lower leaves to catch sunlight. With swards that were cut every day, photosynthetic efficiency was in the order of (from better to worse): 8-inch cut > 1-inch cut > 2-inch cut.
Why was the daily cut at 1 inch more photosynthetically effective than the bermudagrass at the 2-inch cut? It came down to leaf orientation. When cut at 2 inches (daily), much of the newest verdure was vertically oriented and was inefficient at absorbing light. The 1-inch height of cut produced a surface that was closely overlapping yet more horizontal in growth, enabling it to trap more light. Even less effective were the treatments where tall plants were mowed down to 2 or 8 inches. This simply eliminated most of the leaf tissue, and no leaves were left for photosynthesis.
This early work clearly showed that defoliation of tall growth is inefficient for photosynthesis, that leaf orientation within the plant community is important, and that overlapping of leaves creates a need for higher light intensities. This work was one of the first to clearly quantify a light requirement for bermudagrass, and its accuracy has not been successfully challenged for well over 50 years.
Source: Alexander, C.W., and D.E. McCloud. 1962. CO2 uptake (net photosynthesis) as influenced by light intensity of isolated bermudagrass leaves contrasted to that of swards under various clipping regimes. Crop Science 2:132-135.
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 21-year member of GCSAA.