Azospirillum brasilense and Zinc Rates Effect on Fungal Root Colonization and Yield of Wheat-Maize in Tropical Savannah Conditions

Abstract

A successful microbial inoculant can increase root colonization and establish a positive interaction with native microorganisms to promote growth and productivity of cereal crops. Zinc (Zn) is an intensively reported deficient nutrient for maize and wheat production in Brazilian Cerrado. It can be sustainably managed by inoculation with plant growth-promoting bacteria and their symbiotic association with other microorganisms such as arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE). The objective of this study was to evaluate the effect of Azospirillum brasilense inoculation and residual Zn rates on root colonization and grain yield of maize and wheat in succession under the tropical conditions of Brazil. These experiments were conducted in a randomized block design with four replications and arranged in a 5 × 2 factorial scheme. The treatments consisted of five Zn rates (0, 2, 4, 6 and 8 kg ha^-1) applied from zinc sulfate in maize and residual on wheat and without and with seed inoculation of A. brasilense. The results indicated that root colonization by AMF and DSE in maize-wheat cropping system were significantly increased with interaction of Zn rates and inoculation treatments. Inoculation with A. brasilense at residual Zn rates of 4 kg ha^-1 increased root colonization by AMF under maize cultivation. Similarly, inoculation with A. brasilense at residual Zn rates of 2 and 4 kg ha^-1 reduced root colonization by DSE under wheat in succession. The leaf chlorophyll index and leaf Zn concentration were increased with inoculation of the A. brasilense and residual Zn rates. The inoculation did not influence AMF spore production and CO₂-C in both crops. The grain yield and yield components of maize-wheat were increased with the inoculation of A. brasilense under residual Zn rates of 3 to 4 kg ha^-1 in tropical savannah conditions. Inoculation with A. brasilense under residual Zn rates up to 4 kg ha^-1 promoted root colonization by AMF and DSE in the maize cropping season. While the inoculation with A. brasilense under 2 and 4 kg ha^-1 residual Zn rates reduced root colonization by AMF and DSE in the wheat cropping season. Therefore, inoculation with A. brasilense in combination with Zn fertilization could consider a sustainable approach to increase the yield and performance of the maize-wheat cropping system in the tropical savannah conditions of Brazil.

Keywords: Arbuscular mycorrhiza; Triticum aestivum L.; Zea mays L.; dark septate endophytes; microbiomes; zinc fertilization.

Figure 1

Root colonization of arbuscular mycorrhizal—COL AM (A), dark septate—COL D (B) as a function of Zn rates and Azospirillum brasilense interaction while number of spores (NSP) of arbuscular mycorrhizal fungi and released carbon from CO₂ (CO₂-C) in maize roots system as a function of residual Zn rates (C,E) and inoculation or not with Azospirillum brasilense (D,F). The letters correspond to a significant difference at 5% probability level (p ≤ 0.05). ** and ns = significant at p < 0.01 and not significant, respectively. Error bars indicate standard deviation of means (n = 4). L.S.D. (5%) = Least significant difference.

Figure 2

Leaf chlorophyll index and leaf zinc (Zn) concentration of maize as a function of single effect of inoculation or not with Azospirillum brasilense (A,C) and Zn rates (B,D). The letters correspond to a significant difference at the 5% probability level (p ≤ 0.05). ** = significant at p < 0.01. Error bars indicate standard deviation of means (n = 4). L.S.D. (5%) = Least significant difference.

Figure 3

Number of grains cob⁻¹, 100-grains weight and grain yield of maize as a function of single effect of inoculation or not with Azospirillum brasilense (A,C,E) and Zn rates (B,D,F). The letters correspond to a significant difference at 5% probability level (p ≤ 0.05). * = significant at p < 0.05. Error bars indicate the standard deviation of the mean (n = 4). L.S.D. (5%) = Least significant difference.

Figure 4

Root colonization of arbuscular mycorrhizal—COL AM (A), dark septate—COL D (B) as a function of the interaction of Zn rates and Azospirillum brasilense inoculation, while a number of spores (NSP) of arbuscular mycorrhizal fungi released carbon from CO₂ (CO₂-C) in the wheat root system as a function of the single effect of the Zn rates (C,E) and inoculation or not with A. brasilense (D,F). The letters correspond to a significant difference at the 5% probability level (p ≤ 0.05). ** and * significant at p < 0.01 and significant at p < 0.05, respectively. Error bars indicate standard deviation of means (n = 4). L.S.D. (5%) = Least significant difference.

Figure 5

Leaf chlorophyll index and leaf zinc (Zn) concentration of wheat as a function of single effect of inoculation or not with Azospirillum brasilense (A,C) and the Zn rates (B,D). The letters correspond to a significant difference at the 5% probability level (p ≤ 0.05). * and ns = significant at p < 0.05 and not significant, respectively. Error bars indicate standard deviation of means (n = 4). L.S.D. (5%) = Least significant difference.

Figure 6

Number of grains spike⁻¹, 100-grain weight and grain yield of wheat as a function of single effect of inoculation or not with Azospirillum brasilense (A,C,E) and the Zn rates (B,D,F). The letters correspond to a significant difference at the 5% probability level (p ≤ 0.05). **, * and ns = significant at p < 0.01, p < 0.05 and not significant, respectively. Error bars indicate the standard deviation of the mean (n = 4). L.S.D. (5%) = Least significant difference.

Figure 7

Study area at Selvíria, State of Mato Grosso do Sul, Brazil (20°22′ S, 51°22′ W, the altitude of 335 m above sea level). Map created by using QGIS software and Google Earth program. QGIS Development Team (2019). QGIS Geographic Information System. Open Source Geospatial Foundation Project. http://qgis.osgeo.org (accessed on 10 January 2022). Image obtained in Google Earth program. Google company (2020).