Maize (Zea mays L.) is a tropical annual plant of the grass family. It is cultivated as a cereal for its starch-rich seeds, but also as a forage plant. The consumed part is the soft seed part, at the milky-doughy stage. The seed has a high energy nutritional value. The seed is  also  rich in proteins and vitamins. It is the most cultivated cereal in the world (König, 2017). The economy of the Central African Republic (CAR) is largely based on the primary sector (agriculture, fishing, hunting, forestry). For three decades, the agriculture and livestock sub-sectors, which employ nearly 70% of the country's workforce and account for nearly 50% of gross domestic product (GDP), have experienced low average annual growth of around 2% (FAO, 2016).

In tropical agriculture, the management of natural soil fertility is one of the major challenges that agricultural operators must face on a daily basis in order to move this agriculture, which remains empirical and based on subsistence, towards an economic, more rational and income-generating agriculture. Currently, the living conditions of agricultural operators as a whole leave much to be desired, given that the yields they obtain from their crops remain at a low level and do not allow them to take up the challenge of food insecurity, which in principle is a very important factor if we want to get out of the poverty that the population of the CAR is facing on a regular and permanent basis. One of the main constraints to agriculture in Sub-Saharan Africa is the constant decline in the level of soil fertility, mainly nitrogen and phosphorus, resulting in a continuous decline in crop yields, including maize (Saïdou et al., 2012).

However, it is well established that maize is capable of forming a symbiosis with arbuscular mycorrhizal fungi (AMF) (Tchiegang-Megueni et al., 2000; Jemo et al., 2007; Benjelloun et al., 2014). In addition, the improvement of mineral element uptake (phosphorus and nitrogen), nutrition and growth of plants associated with mycorrhizae has been widely studied (Diatta et al., 2013; Haougui et al., 2013; Haro et al., 2015; Manga et al., 2017; Haro et al., 2020). Also, it has been shown that plant growth and production are enhanced by mycorrhizal symbiosis even when these plants are growing on relatively mineral-poor soils (Haro, 2016; Haro and Sanon, 2016; Haro et al., 2016a; Haro et al., 2017; Haro and Sanon, 2020). In CAR, no research has been conducted to date on the effect of AMF on crop growth and development to our knowledge. The objective of  this study was to evaluate the effects of AMF on the growth and development of maize under controlled conditions.

Study area description

The trial was conducted in a greenhouse for three months in the experimental field of the liaison office of the Higher Institute of Rural Development Rural (HIRD) of Mbaïki located in the Faculty of Health Sciences (FACSS) of the University of Bangui (altitude: 436 m, latitude North: 4° 22'38'' and longitude East: 18°33'37''). It consisted of growing maize in 10 L pots containing 7000 g of sterilized soil from Boukoko (Table 1). The plants were grown under the following conditions: average temperature 30/ 25.9 ± 2°C and relative humidity above 50%.

The method used for the measurement of above-ground, root and total biomass was that described by Haro et al. (2020) modified. At 60 days after transplanting, each seedling was carefully stripped to recover the aerial part and all roots of the seedlings. All these parts were oven-dried at 70°C for 72 h until a constant weight was obtained for measurement of above-ground, root, and total biomass. After biomass measurement, the roots were used for the study of mycorrhizal colonization.

Statistical analysis

All data obtained were analyzed by analysis of variance (ANOVA) using R software for Windows. Tukey's test was used to determine any significant difference between treatments at the P < 0.05 threshold. All experiments were performed in four replicates.

Effect of mycorrhizal fungi inoculation on growth of corn plants

Corn plant height varied among treatments with different strains of mycorrhizal fungi used (Figure 1). Statistical analyses showed significant P= 0.0001) differences in height between inoculated and non-inoculated treatments. The average crown diameter of plants under the effects of the fungal strains studied was 13 mm. However, the G4 strain had higher (15.5 mm) diameter, followed by the G2 strain which had a diameter of 14.77 mm and finally came the other strains of AMF. The ANOVA performed shows a significant difference (P < 0.05) between inoculated  and  non-inoculated  treatments  between  the diameters at the neck of the plants (Figure 2).

Inoculation had no significant (P = 0.0001) effect on average leaf number. Treatment G4, G2 and as well as the control treatment have an equal average of 8 leaves, during the whole study period in terms of number of leaves, followed by G1 and G3 plants with an equal number of leaves (Figure 3).

Evaluation of above-ground, root and total biomass

Results indicated satisfactory vegetative development throughout the crop. No signs of necrosis or leaf loss were observed. For each plant, the weight of the aerial and root parts varied depending on the strain inoculated. The aerial, root and total biomass parts showed variability according to the different treatments (Table 2). The results showed significant differences between the different treatments (P= 0.0001). The highest values were obtained with the G4 mix strain (322.95 g for total biomass at 60 DAS and 535.44 g for total biomass at 90 DAS). It is noted that the G1 strain presented a low total biomass (200.34 and 252.06 g), respectively, 60 and 90 DAS compared to the other treatments (Table 2).

The main purpose of this work is to evaluate the comparative effects of AMF on the growth and development  of  the maize crop. Results on the effects of treatments on plant height growth, plant crown diameter, leaf number, and above-ground and root biomass in the vegetative phase of maize plants revealed that AMF had a beneficial effect.

As observed in maize, the supply of mycorrhizal inoculum results in the formation of mycorrhizal structures (De Alwis and Abeynayake, 1980; Lodge, 1987; Newbery et al., 1988; Béreau and Garbaye, 1994). All plants  tested  responded  significantly  to  mycorrhizal inoculation. The most significant results obtained with G3 and the G4 (G2+G3) composite on height growth of most plants were related to differential effectiveness of the strains for the plants tested. These results are similar to those of Caris et al. (1998) on groundnut (Arachis hypogeae) and sorghum (Sorghum bicolor) and Sulochana et al. (1989) on sesame (Sesamum indicum L.). The most significant results obtained with G1 and G2 on   height   growth  and  crown  diameter  on  maize  are related to a differential effectiveness of the strains for the tested varieties. These results are similar to the work of Haro et al. (2012) on the response to mycorrhizal inoculation of four cowpea [Vigna unguiculata (L.) Walp.] varieties grown in Burkina Faso and Senegal.

The results of this study on the average number of leaves obtained were not significantly different with the tested strains and the control. These results are not similar to the work of Parvathi et al. (1985) and Daft and El-Giahmi (2006) who reported that the inoculation of plants with AMF leads to an improvement in nutrition and a more significant increase in the number of leaves. The application of AMF resulted in a significant increase in root biomass, above-ground biomass and total biomass in inoculated plants, compared to non-inoculated plants after 90 days of sowing. These results are in agreement with those obtained by Diop et al. (2013) who showed that biomass production of Vigna is enhanced by mycorrhizal inoculation. Similarly similar results were found by Haro et al. (2017) who showed that mycorrhizal inoculation of cowpea with native inocula improves biomass production and seed production yield of this plant.

In tropical agriculture, the management of natural soil fertility is one of the major challenges that agricultural operators must face on a daily basis in order to move from an empirical and subsistence-based agriculture to an economic, more rational and income-generating agriculture. The results of the vegetative growth and especially the dry and fresh biomass tests show that the AMF strains (Gigaspora rosae, Acaulospora laevis, Funneliformis mossae) have a significant (positive) effect than the control treatment. Although variability is observed between the different growth parameters, the four AMF strains show interesting results adapted to the pedoclimatic conditions of the study site. Thus, microbial biofertilizers based on AMF could be recommended to producers  on  soils  poor  in  mineral  elements  such  as phosphorus.

The authors have not declared any conflict of interests.