Submitted to: Frontiers in Plant Nutrion
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 2, 2013
Publication Date: July 23, 2013
Citation: Bellaloui, N., Hu, Y., Mengistu, A., Kassem, A., Abel, C.A. 2013. Effects of foliar boron application on seed composition, cell wall boron, and seed delta 15N and delta 13C isotopes in soybean are influenced by water stress. Frontiers in Plant Nutrion. 4:1-12. Interpretive Summary: Boron is an essential mineral for crop growth, production, and quality. The effect of foliar boron fertilizer on seed yield is still controversial, and literature on the effects of boron on seed composition (protein, oil, fatty acids, and sugars) is almost non-existent. The source of controversy could be that plant response to boron effect depends on environmental factors, especially drought. A greenhouse experiment was conducted where half of the soybean plants were exposed to water stress and the other half were well watered. Seeds of watered plants that were fertilized with boron had more protein (11 % increase), oleic acid (27% increase), sucrose (up to 40 % increase), glucose, and fructose than watered plants without boron. The same trend was observed in water stressed plants with and without boron fertilizer. Seed oil concentrations decreased in water stressed plants compared with watered plants, and this was due to smaller seed size. The increase in protein, oleic acid, and sugars (sucrose, glucose, and fructose) was accompanied by a decrease in oil, linoleic and linolenic acids, and stachyose sugar. Water stress altered the physiology and chemistry of nitrogen and carbon fixation. This research demonstrated that foliar boron fertilizer had an effect on seed composition and the physiology and chemistry of nitrogen and carbon fixation. The increase in protein and oleic acid is desirable because high protein contributes to soymeal quality, and high oleic acid contributes to oil stability. The increase of sucrose, glucose, and fructose by foliar boron fertilizer is desirable because these sugars improve taste and flavor of tofu and soymilk. Also, the changes in nitrogen and carbon physiology under water stress can be used as a tool in breeding to select for drought tolerant soybean lines.
Technical Abstract: Although the effect of foliar boron (B) application on yield and quality is well established for crops, limited information and controversial results still exist on the effects of foliar B application on soybean seed composition (seed protein, oil, fatty acids, and sugars). The objective of this research was to investigate the effects of foliar B on seed protein, oil, fatty acids, and sugars. Since the concentrations of seed composition constituents, especially protein and oil, may be affected by nitrogen and carbon fixation under water stress, seed delta 15N and delta 13C values were also evaluated. A repeated greenhouse experiment was conducted where half of the soybean plants had been exposed to water stress (WS) (soil water potential ranged between -90 and -100 kPa) and the other half had been well watered (soil water potential ranged between -15 and -20 kPa) . Foliar boron in the form of boric acid was applied twice at a rate of 1.1 kg ha-1 during flowering stage (R1-R2) and during seed-fill stage (R5-R6). Treatments were water stressed plants with no FB (WS-B); water stressed plants with FB (WS+B); watered plants without FB (W-B), and watered plants with FB (W+B). The treatment W-B was used as a control. Comparing with WS-B plants, the results showed that B concentration was the highest in leaves and seed of W+B plants (84% increase in leaves and 73% in seed). Seeds of W+B plants had higher protein (11 % increase), oleic acid (27% increase), sucrose (up to 40 % increase), glucose, and fructose comparing with W-B. However, stachyose concentrations increased by 43% in WS-B plants seed compared with W-B. Cell wall (structural) B concentration in leaves was higher in all plants under water stress, especially in WS-B plants where the percentage of structural B reached up to 90 %. Water stress changed seed delta 15N and delta 13C values in both B applied and non-B applied plants. This research demonstrated that foliar B can alter seed protein, fatty acids, and sugars, especially under water stress conditions. Also, water stress can alter natural abundance of delta 15N and delta 13C in seed, suggesting altered nitrogen and carbon metabolism pathways. A repeated field experiment should be conducted under dryland conditions before definitive conclusions are made.