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Leaf growth and leaf aging both influence the total amount of CO, fixed by a particular leaf in the canopy during its photosynthetically productive life. Four genotypes of tall fescue (Festuca arundinacea Schreb.) were selected for all combinations of high and low COB exchange rates (CER) and yield. The purpose of this study was to determine CER during aging and leaf growth rates of tall fescue genotypes in growth chambers and in the field on a Mexico silt loam (Udollic Ochraqualfs; fine, montmarillonitic, mesic). Leaf growth of vegetative tillers in controlled environments was continuous throughout the day for all genotypes, regardless of CER or yield performance; however, leaf growth rate during a 24-hour period changed diurnally in response to altered temperature and plant water status. Average leaf elongation rates were greater in the field during autumn (8.2 mm/ day) than in summer (4.2 mm/day). In both field and growth-chamber studies, high-yielding genotypes exhibited approximately 50% greater rates of both leaf elongation and leaf area expansion than did low-yielding genotypes. During 4 weeks in a growth chamber and 6 weeks in the field, CER of all four genotypes decreased after collar formation at a rate of about 15 to 20% per week. Leaf diffusive resistances were similar for all genotypes and increased during aging from 1.42 sec/cm to 3.56 sec/cm in both studies. Genotypes with faster rates of leaf area development should reach critical leaf area index (LAI) faster than those with slow rates. They also would have relatively younger leaf tissue in the upper canopy, which is more efficient photosynthetically than older leaf tissue.