Carbon Dioxide Exchange Rate of Tall Fescue-Leaf Area vs. Leaf Weight Basis

The basis used for expression of carbon dioxide exchange rate (CER) of single leaves may alter the relative ranking of genotypes being selected for a breeding program. Therefore, field and growth chamber experiments were conducted on 16 genotypes of tall fescue (Festuca arundinacea Schreb.) to determine the relative merit of expressing CER data on a leaf area or leaf weight basis. Genotypes were grown in plots in the field and in pots in the growth chamber. The CER was measured using an infrared gas analyzer. Herbage yield was determined. Significant genotypic variation within experiments was found for CER when expressed on either a leaf area or leaf weight basis. The correlation coe5cient between CER on a leaf area and leaf weight basis ranged from 0.82 to 0.94 for the field experiments and was 0.76 for the growth chamber experiment. Comparision of coefficients of variation (CV's) associated with the two methods of expressing CER indicated slightly higher variation in data based on leaf weight. The CV's for the field experiments were between 11.5 and 16.5%, suggesting that either method of expression of CER for tall fescue could be satisfactorily used to detect genotypic differences. Leaf area is suggested as the preferred base because of its association with radiation interception. Further, genetic control of CER per unit leaf area appears to be independent from specific leaf weight. yield, the area or weight basis for expressing photosynthesis may be of prime importance. Sufficient genetic variation in photosynthesis was available and heritability great enough in tall fescue (Festuca arundinacea Schreb.) to facilitate progress by recurrent selection (Asay et al., 1974). Earlier, we (Wilhelm and Nelson, 1978a, 1978b) compared four genotypes of tall fescue selected for high and low CO, exchange rate (CER) on a leaf area basis in combination with high and low yield and found that CER ranking remained consistent over a range of radiation densities, leaf ages, and times of day. Further, the rate of canopy development was more critical in determining forage yield than was CER of single leaves (Nelson et al., 1977). Purposes of this study were to evaluate the influence of method of expressing leaf CER (leaf area or leaf weight basis) on variation among genotypes, the relationshi between leaf CER and yield, and the relationship f, etween SLW and leaf CER of tall fescue. Additional index words: Festuca arundinacea Schreb., Yield, Genetic variation, Specific leaf weight, Photosynthesis. R EPORTS of significant positive relationships between specific leaf weight (SLW, g m-2) and leaf photosynthesis (Pearce e t al., 1969; Wilson and Cooper, 1969; Topark-Ngarm et al., 1977) have intensified interest in evaluating the influence of method of ex ression of leaf photosynthetic rates. This is especia I' ly true in determining differences between genotypes and assessing photosynthesis relationship to yield. Ogren (1976) suggested that variation in leaf thickness or density (SLW) may account for much of the genetic variation reported in leaf photosynthetic rates of soybean [Glycine max (L.) Merr.]. With forages, where leaf tissue is the major component of Contribution from the Missouri Agric. Exp.Stn. Journal Series no. 9555. Received 3 Aug. 1984. Formerly aduate assistant (now lant hysiologist, USDAARS, Univ. oyNebraska-lincoln, ~l?685&) and rofessor of a onomy, Dep. of Agronomy, Univ. of Missouri, ~ o R m b i a , MO 6501 . MATERIALS AND METHODS