Effects of Shading on the N2‐Fixation, Yield, and Plant Composition of Field‐Grown Soybeans1

Abstract

AbstractSoybeans (Glycine max L. Merr.) are becoming important in cropping systems that increase competition for light among crops. One of the products of N2‐fixation is protein production. A field study was conducted to determine the effects of various degrees of shading (0,20,47, 63, 80, and 93%) on soybean No‐fixation, grain yield, and plant composition. The soil was a Flanagan series, a fine, montmorillonitic, mesic Aquic Argiudoll. Ambient illumination was 124.2 klux. Various degrees of shading were obtained by using different screens. Acetylene‐ethylene assay was used to determine N2‐fixation.Shading accelerated the rate of loss of total nodule N2‐fixing activity (TNA) as plants developed. The average TNA and dry weight of plant tops were highest at 20% shade (99.3 Klux) and decreased curvilinearly as shading increased, while specific nodule activity decreased linearly. Grain yields under 20, 47, 63, 80, and 93% shade were 90, 75, 48, 18, and 2% of unshaded plants. Seed percent protein and oil content of the seed were virtually unaffected between 20 and 80% shade. The highest percent protein and lowest percent oil occurred at 93% shade. Number of pods per plant percent leaf N, total stem N, TNA, and grain yield were highly correlated with shading and may be good selection indices for soybean shade tolerance. Soybean N2‐fixation was positively correlated with seed protein content (r = 0.77) but negatively correlated with seed protein (r = −0.44). Cropping practices should allow at least 80% ambient illumination measured at the height of 50 cm for substantial soybean No‐fixation.