Location: Crop Genetics ResearchTitle: Role of boron nutrient in nodules growth and nitrogen fixation rates in soybean genotypes under water stress conditions Author
Submitted to: Intech
Publication Type: Book / Chapter
Publication Acceptance Date: 8/21/2013
Publication Date: 1/29/2014
Citation: Bellaloui, N., Mengistu, A., Kassem, A., Abel, C.A., Zobiole, L.H. 2014. Role of boron nutrient in nodules growth and nitrogen fixation rates in soybean genotypes under water stress conditions. Intech . 237-258. Interpretive Summary: Boron is an essential nutrient for crop growth, development, yield, and seed quality. Its deficiency in soil results in yield loss and poor seed quality. Under drought or high heat conditions, boron deficiency can occur even when the soil contains adequate amount of boron. This is because boron level in the crop is determined by boron absorption by the crop from soil and boron movement within the crop. Absorption and movement of boron within the crop is interrupted by drought and heat, negatively impacting nodules (root bacteria formation essential for nitrogen fixation) and nitrogen fixation. A greenhouse experiment using four cultivars under three water stress level and either receiving foliar boron application or no boron resulted in higher sugars, especially glucose and fructose (simple sugars), and sucrose in watered plants. Under severe water stress, boron was the lowest in leave and seeds, and application of boron did not increase boron in leaves or seeds. This research showed that foliar boron resulted in higher glucose, fructose, and sucrose, and these sugars are desirable for soybean seeds because they improve taste and flavor of soymeal based products such as tofu, soymilk, and natto.
Technical Abstract: Although boron has a stimulatory effect on nodule growth and nitrogen fixation, mechanisms of how boron affects nodules growth and nitrogen fixation, especially under water stress, are still unknown. The stimulatory effect of boron (B) on nodules and nitrogen fixation (NF) is influenced by biotic (such as genotype) and abiotic (such as drought/water stress) factors. Therefore, the objective of this research chapter was to investigate the effects of B on nodule growth and NF using different soybean genotypes under well watered and water stressed plants. To avoid the confounding effects of multi-environmental factors in the field on boron application effects, a repeated experiment was conducted on three soybean genotypes under greenhouse conditions. Plants were divided into well watered (soil water potential between –15 to –20 kPa) (this was considered field capacity for the control plants), water stressed (soil water potential between -90 and -100 kPa), and severely water stressed (soil water potential between –150 to –200 kPa). Foliar B (FB) as boric acid was applied at a rate of 1.1 kg ha-1 once at flowering stage (R-R2) and once at seed-fill stage (R5-R6). Combined treatments were well watered plants with no B (W-B), well watered plants with B (W+B); water stressed plants with no B (WS-B); water stressed plants with B (WS+B); severely water stressed plants with no B (SWS-B); severely water stressed plants with B (SWS+B). Well watered plants with no B (W-B) were used as control for comparison. The results showed that FB increased B concentrations in leaves, including the severely water stressed plants. Well watered plants with B had the highest boron concentration in leaves and seed, and the lowest concentration was in the severely water stressed plants with no B. Boron concentration in seed showed that B movement from leaves to seed was the lowest in SWS-B and SWS+B, indicating lack of B movement from leaves (source) to seed (sink) under water stress. Nodule number and mass were higher in both well watered plants with or without FB application, and the lowest in water stressed plants. Nitrogen fixation rates were highest in well watered plants with B, and FB application to water stressed plants induced NF rate, indicating possible B movement through the phloem to the root system and nodules. A significant correlation was found between seed sucrose, glucose, and fructose and boron concentrations in seed, indicating a possible role of these sugars in B movement within the plants. Using natural abundance of delta 15N and delta 13C isotopes, WS+B or WS-B plants showed a significant change in seed delta 15N and delta 13C values, indicating a possible shift mechanism in nitrogen and carbon fixation pathways. In addition to the current research findings, the present chapter will also highlight the previous and current major research findings in boron nutrition and the role of B in nodule growth and symbiotic nitrogen fixation in soybean.