Climatic variation in forage grasses. 3. Seasonal changes in growth and assimilation in climatic races of Lolium, Dactylis and Festuca.
Seasonal changes in relative growth rate (RGR) and its components, net assimilation rate (NAR) and leaf area ratio (LAR), were measured in Mediterranean and N. European varieties of perennial ryegrass, cocksfoot and tall fescue grown in an unheated glasshouse through the year. In all varieties, RGR reached a minimum of 0.2-0.4 mg/mg/week during the winter and increased to a maximum of 1.0/1.2 mg/mg/week in the summer. NAR was lowest (about 1 mg/cm2/ week) in winter and highest (about 6 mg/cm2/week) in summer. LAR was greatest (25-35 mm2/mg) in Nov. and decreased to below 20 mm2/mg in Mar. and Apr. Over the whole year RGR was greater in cocksfoot (0.76-0.77) than in ryegrass (0.70-0.74) or fescue (0.68/0.70); in the winter, though not at other seasons, it was greater in the Mediterranean variety of each species (mean 0.46) than in the northern variety (0.41). Both these differences were based on variation in LAR rather than in NAR. The greater LAR of the Mediterranean varieties (31.1) than of the northern varieties (23.7) in the winter was associated with a large shoot/root ratio; that of cocksfoot (27.0) compared with the other species (22.5-23.0), however, was not associated with differences in shoot/root ratio. NAR over the whole year was greatest in the Lithuanian ryegrass (3.61) and least in the Portuguese cocksfoot (2.79), but these differences made little contribution to seasonal variation in RGR. NAR increased up to the highest energy input recorded, over 300 cal/cm2/ day of total radiation. At low levels of energy income in the winter, the efficiency of conversion of light energy was high, up to 16%, but this fell to 2-3% at energy inputs of above 250 cal/cm2/day of total radiation in the summer.-R.B.