Photosynthesis in Tall Fescue

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Abstract
The net photosynthetic rate of a decaploid genotype (I-16-2) of tall fescue (F. arundinacea Schreb.) was previously reported as 32-41 vs. 22 mg CO2/square decimeter per h in a hexaploid genotype (V6-802). The high rate was later correlated with increases in total ribulose 1,5-bisphosphate carboxylase protein (17%) and activity (27%). This report characterizes photosynthesis with respect to light saturation and early products of photosynthesis in an attempt to identify regulatory metabolic site(s) in these 2 genotypes. Analysis of the early products of photosynthesis indicated that both genotypes fixed CO2 via the Calvin-Benson cycle with phosphoglyceric acid as the initial primary product. Both genotypes had similar 14C-labeled intermediates. Sucrose was the primary sink of 14CO2 assimilation. After 10 min of 14CO2 assimilation with attached leaves, sucrose accounted for 89% (decaploid) and 81% (hexaploid) of the total 14C incorporated. In 10 min, this amounted to 1.3 (decaploid) and 0.8 (hexaploid) .mu.mol [14C]sucrose formed per gram fresh weight-1 and reflected the observed differences in photosynthetic rates. There was limited labeling of starch (1%) and fructan (1%). Results of total nonstructural carbohydrates and Pi analysis also demonstrated sucrose was the predominant carbohydrate in fescue leaves. Quantitative differences in sucrose and Pi between the 2 genotypes may reflect changes in partitioning and this possibility is discussed.