Irradiance Response of Tall Fescue Genotypes with Contrasting Levels of Photosynthesis and Yield

The purpose of this study was to determine if genotype by irradiance interactions or genetic variation in diurnal trends of CO, exchange rate (CER) influenced the CERyield relationship in tall fescue (Festuca mundinucea Schreb.). Genotypes were selected in the field for high CER-high yield, high CER-low yield, low CER-high yield, and low CER-low yield. Responses of CER to photosynthetic photon flux density (PPFD) of these genotypes were determined using both growth chamberand fieldgown material. The shapes of the PPFD response curves were similar for all genotypes, and saturation occurred at approximately 1,200 pE m-a sec-l (400 to 700 nm) under both growth conditions. In a growth chamber at 25/20 C (light/dark), all genotypes achieved near maximal CER and photorespiration within 1 hour of the beginning of the photoperiod and maintained these rates throughout the rest of the photoperiod, with high CER genotypes exhibiting about 30% higher rates than low CER genotypes. Photorespiration rates were significantly correlated (r = 0.80) with CER. Leaf diffusive resistance were similar for all genotypes throughout the photoperiod. Genotypic differences did not occur in PPFDresponse curves or diurnal patterns of CER. This suggests that differences in these two factors among genotypes are not the underlying cause of the poor relationship reported between short term measurements of leaf photosynthesis on recently collared leaves and forage yield. Other factors must be limiting genetic expression for CER in terms of yield. Additional index words: Leaf diffusive resistance, Pho. torespiration, Diurnal trends, Festum arundinucea Schreb., Photosynthetic photon flux density. G ENETIC variability has been reported in tall fescue (Festuca arundinacea Schreb.) for both C02 exchange rate (CER) of single leaves and yield (Nelson et al., 1975a). Similar findings have been reported for other cool-season perennial forage grasses (Wilson and Cooper, 1970; Carlson et al., 1971; Rhodes, 1972; Borrill et al., 1974). Although genetic variability has been found, a positive relationship between CER of single leaves and yield has not been well documented. Increased forage yield and decreased CER were associated with selection for small mesophyll cells when compared to selections made for large mesophyll cells in perennial ryegrass (Lolium perenne L.) (Wilson and Cooper, 1970). Nelson et al. (1975a) reported an inconsistent relationship between CER and yield in clonal lines of tall fescue. Individual harvest period correlation coefficients between CER and yield ranged from -0.60 to +0.60 in three field studies, but the overall correlation coefficient was nonsignificant and negative. 'Contribution from the Missouri Agric. Exp. Stn. Journal Series 7704. Received 15 July 1977. 'Graduate assistant (now plant physiologist, Science and Education Administration-Federal Research, USDA, Lincoln, NE 68583) and professor of agronomy, Dep. of Agronomy, Univ. of Missouri-Columbia, MO 65201. Canopy shading and development (Rhodes, 1973) and dry matter distribution (Treharne and Eagles, 1970) have been suggested as factors influencing the relationship between leaf photosynthesis and yield. Nelson et al. (1975a) also suggested that genotype by irradiance interactions may influence the CER-yield relationship. In addition, genetic variation in diurnal trends in CER could affect this relationship (Pallas,