Annals of Botany 37: 501-518, 1973
© 1973 Annals of Botany Company
RESEARCH-ARTICLE |
The Growth and Development of Simulated Swards of Perennial Ryegrass
II. Carbon Assimilation and Respiration in a Seedling Sward
Botany Department, The Grassland Research Institute Hurley, Berkshire
Received: 14 September 1972
The rates of net photosynthesis (Pn,c) in the light (85 W m–2 visible), and respiration in the dark, of a simulated sward of S24 ryegrass were measured for 12 weeks during its development from a collection of two-leaved seedlings to a closed canopy with an LAI of 23 (15 of green leaf laminae).
By the sixth week light interception was complete (LAI = 10.6) and Pn,c had risen to 24 mg CO2 dm–2 h–1, similar to rates recorded in the field. Photosynthetic functions (light response curves) showed that the swards remained unsaturated up to energy receipts of almost 400 W m–2, whereas single leaves were light saturated at about 130 W m–2. Early in the development of the sward LAI had a greater effect on Pn,c than radiation receipt, later the reverse was true. The growth habit of the sward ranged from moderately erect (an S value of 0.72) to moderately prostrate (‘S’ = 0.37), while the ability of the two youngest fully expanded leaves on a tiller to make use of light in photosynthesis declined as the sward increased in density from values of A max of 20 to 5 mg CO2 dm–2 h–1. By varying the values of S and A max fed into a model of canopy photosynthesis, within the above limits, it was demonstrated that, in practice, A max is a greater determinant of canopy photosynthesis than S, except at low LAI where a prostrate sward has a marked advantage over an erect one.
The rate of dark respiration rose as the swards increased in weight, although not in proportion to it, until the ninth week when a ceiling yield of live plant tissue was reached. Respiratory losses from the sward came almost equally from a component associated with maintenance (Rm) and one associated with growth (Rg). The rate of Rm was estimated to be about 0.014 g day–1 per gram of plant tissue, and that of Ra about 0.25 g per gram of new tissue produced—both close to theoretical values. The measured dry matter production curve of the swards was compared with that estimated from the gas analysis data. Similarly the rates of gross photosynthesis estimated from the gas analysis data were compared with the predictions of the mathematical model. In both cases the fit was reasonably good. A balance sheet was drawn up; of every 100 units of carbon fixed, 45 were lost in respiration and 16 as dead leaf, 5 ended up in the root and 6 in the stubble; only 28 remained as harvestable live leaf tissue.