Photosynthesis and Growth of Bermudagrass Swards. I. Carbon Dioxide Exchange Characteristics of Swards Mowed at Weekly and Monthly Intervals1

Abstract

Bermudagrass (Cynodon dactylon L. Pers.) is a highly productive subtropical species widely grown in the southern United States for pasture and hay. This work was undertaken to determine the response of CO2 exchange rate (CER) of the canopy to photosynthetic photon flux density (PPFD) and to leaf area index (LAI) as influenced clipping frequency. An experiment was conducted during the growing season on an established sward of ‘Coastal’ bermudagrass. One series of plots was mowed to a height of 6 cm every week and another series every month. Quadrats, 0.25 m2 in size, were hand harvested weekly to ground level to estimate phytomass yields and LAI. Carbon dioxide exchange rates were measured weekly over a wide range of PPFD with a chamber technique. The response of CER of bermudagrass canopies to PPFD followed a rectangular hyperbola with the curves becoming more linear and maximum CER increasing as LAI increased. Rates at near full sunlight (2,000/μmol‐2 sec‐1 PPFD) w ere about 9.5 g m−2 hour−1 for plots with the highest LAI. Dark respiration rates of the canopy plus soil ranged from 1.3 to 1.9 g m−2 hour−1 and were not influenced by LAI. Optimum LAI was 4.7 at 1,600‐2,000 μ m−2 sec−1 PPFD; the critical LAI (95% PPFD interception based on a canopy PPFD extinction coefficient of 0.62) was 4.8. Swards mowed weekly had much lower CER than plots mowed monthly, except just after the monthly mowing when LAI was low and the treatment effect was reversed. For weekly‐mowed plots LAI usually ranged from 1 to 3, whereas on monthly‐mowed plots LAI ranged from near zero to 6.5. Lower CER of swards mowed weekly was attributed mostly to the lower LAI, although at comparable LAI values CER was also lower in weekly mowed plots. Plants in weeklymowed plots had lower dark respiration rates and lower initial slopes of the light response curves than monthly‐mowed plants. These observations confirm the lower productivity often reported for frequently harvested grass swards and indicate the cause to be low LAI.