Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/6/1995
Publication Date: N/A
Citation: Interpretive Summary: Photosynthesis is the process by which green plants use sunlight to convert carbon dioxide in the air into carbohydrates. The yield of any green crop plant is ultimately dependent on this process and increasing photosynthesis will increase the yield potential. Increasing the density of soybean leaves has been suggested as a way of increasing photosynthesis. In this research we were successful in developing experimental soybean lines that had consistent differences in leaf density. When we measured photosynthesis rate on these lines we discovered that the more dense leaves generally had lower rates of photosynthesis and lower seed yields, which was the opposite of what we had hypothesized. However, we also found that as leaf density increased the roughness or rugosity of the leaf surface increased. There was one experimental line with high leaf density and a smooth leaf surface. This line did have the highest rates of photosynthesis and produced a yield that was similar to the best commercial variety included in the test. We also found that when we doubled the concentration of carbon dioxide available for the leaf that the more dense leaves had greater rates of increase than the less dense leaves. These findings indicate that high density in soybean leaves may be beneficial for photosynthesis, that highly wrinkled leaves may be detrimental to photosynthesis and that the two traits are often found together. These results may help to explain the inconsistent effects of high leaf density on yield and photosynthesis in previous research. If future research can identify experimental lines with high leaf density and a smooth leaf surface, it may be possible to develop new varieties with higher rates of photosynthesis that can ultimately improve soybean yield.
Technical Abstract: Increasing leaf apparent photosynthesis (AP) may improve seed yield in soybean [Glycine max (L.) Merr.]. Selection for high specific leaf weight (SLW), the dry mass of tissue per unit leaf area, has been proposed as a method of increasing AP. The objectives of this study were i) to incorporate extremes in SLW from exotic germplasm into adapted soybean lines with high agronomic potential, ii) to compare the time course of SLW in lines known to vary in SLW, and iii) to evaluate the relationships among SLW, apparent photosynthesis (AP), and seed yield under field conditions. In 1991 and 1992, sixteen genotypes representing extremes in SLW were grown at Urbana, IL and West Lafayette, IN. SLW and AP were measured on the most recently expanded terminal leaflet at the R2, R4, and R5 growth stages. Leaflets were sampled in full sunlight on clear days. At the R5 growth stage in 1992, AP was also measured at approximately twice ambient CO2 concentrations. A subset of lines was sampled weekly for SLW from R1 until R7. The high-SLW lines displayed a greater rate of increase in SLW than the low SLW lines beginning after R2 and continuing until approximately R6. Significant class mean differences (P<0.01) in SLW were observed on all sample dates across locations. In addition, class mean differences in AP at both ambient and twice ambient CO2 concentrations were observed at Urbana, although ranking of SLW classes was inconsistent across years. Leaf rugosity was negatively correlated with AP and seed yield. LG89-1917, a high SLW line with low rugosity, expressed high AP and produced yields very similar to the adapted cultivars Chapman and Elgin 87, suggesting that lines with high SLW and low leaf rugosity may hold the greatest potential for high AP and yield improvement.