Climatic Variation in Forage Grasses. III. Seasonal Changes in Growth and Assimilation in Climatic Races of Lolium, Dactylis and Festuca

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Abstract
The seasonal changes in relative growth rate and its components, net assimilation rate and leaf area ratio, have been examined in Mediterranean and north European varieties of Lolium perenne, Dactylis glomerate and Festuca arundinacea grown in an heated glasshouse through the year. In all varieties the relative growth rates (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. Similarly, the net assimilation rates (NAR) were least (about 1 mg/cm2/week) in the winter, and greatest (about 6 mg/cmV week) in the summer. The leaf area ratios (LAR), however, were greatest (25-35 mm2/mg) in Nov., and decreased to below 20 mm2/mg in March to April. Over the whole year RGR was greater in the varieties of Dactylis (0.76-0.77) than in Lolium (0.70-0.74) or Festuca (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. At low levels of energy income in the winter, the efficiency of conversion of light energy is high, up to 16%, but this falls to 2-3% at energy inputs of above 250 cal/cm2/day of total radiation in the summer.

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