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Influence of Supplemental Light on Apparent Photosynthesis, Yield, and Yield Components of Soybeans (Glycine max L.)¹

Experiments were conducted to determine the effect on soybean seed yields and yield components of increasing the amount of light available in the soybean leaf canopy and to evaluate the photosynthetic contribution of leaves at different levels in the canopy.

Wide spectrum fluorescent lamps were placed at three levels in the canopies of ‘Amsoy’ and ‘Wayne’ varieties planted in rows 50-cm and 100-cm apart. White reflective polyethylene strips were placed between the rows. At harvest each plant was divided into top, middle, and bottom thirds corresponding to the levels of artificial lamps in the canopy.

Measurements of apparent photosynthesis (AP) and respiration of individual soybean leaves were made with a plexiglass single leaf photosynthesis chamber. AP and respiration rates (rag CO₂/dm²/hr) were determined by recording the change in CO₂ concentration with time in a closed system.

Adding light increased the yields of bottom, middle, and top canopy positions of plants 30, 20, and 2%, respectively. Light-rich plants had more seeds, nodes, pods, branches, pods per node, seeds per pod, and a higher oil content than normal plants. Protein content and seed size were decreased by adding light.

The rates of AP of bottom and middle soybean leaves were 13 and 60% of the 20.2 mg CO₂/dm²/hr rate of top leaves under natural canopy conditions. When exposed to full sunlight, bottom and middle leaves fixed 258 and 50% more CO₂, respectively, than when naturally shaded.

Respiration rates decreased with depth in the canopy. Top, middle, and bottom leaves had respiration rates of 6.8, 4.3, and 2.8, mg CORespiration rates decreased with depth in the canopy. Top, middle, and bottom leaves had respiration rates of 6.8, 4.3, and 2.8, mg CO₂/dm²/hr, respectively.

Key Words: Respiration • Leaf canopy • Single leaf chamber

² Assistant Professor of Crop Science, Michigan State University; Professor of Agronomy, University of Illinois; Soil Scientist, SWCRD, ARS, USDA; and Assistant Professor of Crop Extension, University of Minnesota. First and fourth authors were formerly Research Assistants, Department of Agronomy, University of Illinois.

Received for publication February 10, 1969.

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