Physiological comparisons of two soybean cultivars differing in canopy photosynthesis.  I. Variation in vertical (14)CO 2 labelling and dry weight partitioning. 
Differences in canopy apparent photosynthesis (CAP) among soybean [Glycine max (L.) Merr.]  genotypes have been shown to be correlated to seed yields.  Since the physiological basis for such differences in CAP is unknown, two cultivars known to differ in CAP, 'Tracy' and 'Davis', were studied during the 1978-1980 growing seasons.  The CAP and dry weights of component plant parts were determined.  In 1978 and 1979, (14)CO2 uptake by vertical leaf strata was determined and specific leaf weight (SLW) and leaf area index (LAI) were determined for corresponding strata in 1979 and 1980.  Measurements were taken on several dates during reproductive growth.  With the exception of CAP, all measurements ((14)C uptake, dry weights) were made in layers within the canopy.  CAP on some dates were significantly higher in Tracy than in Davis and integrated CAP values from a certain growth period, labeled as R5 to R7, averaged 16 percent higher in Tracy for the three years studied.  No differences in the relative recovery of (14)C from different layers of leaves in the canopy were found.  This indicates that variations in canopy structure or leaf orientation did not play a major role in the CAP differences between cultivars.  The differences seem related to variations in leaf dry weights.  Overall, Tracy exhibited 13.5, 19.2, and 13.2 percent greater leaf dry weights than Davis during 1978, 1979, and 1980, respectively.  These differences in leaf dry weight seem largely due to a differences in the SLW.  Data from these experiments indicate that differences in soybean CAP values were associated with differences in SLW.