The experiment was conducted in Yaounde in the Centre region, Mfoundi administrative Division, more precisely in the locality of Simbock (Latitude: 3˚49'13.76'' Longitude 11˚28'13.52'', Altitude: 694 m. The average annual rainfall varies between 1500 and 2000 mm/year. The average annual temperature is between 23˚C and 27˚C, and the relative humidity and average humidity are above 80%.

For plant material, we used: Avocado seeds consisting of an accession of the species Persea Americana obtained from vendors in the town of Mbouda in western Cameroon.

Two types of fertiliser were used in the study: 1) organic fertilizer and 2) biological fertilizer. The Aburcular Mycorrhizae Fungi (AMF) species used in this study are Gigaspora margarita and Acaulospora tuberculata.

In a germinator: The experimental set-up chosen was the randomised complete bloc design with two repetitions. The studied factor is the substrate with three modalities (soil, white wood sawdust and sand), each consisting of 100 seeds, with a total of 600 seeds (Figure 1).

In the nursery: The experimental design used is the split-plot factorial design with two factors and three repetitions including transplanting date (TransD/ main factor) and biological fertilisation (Fert/secondary factor). Transplanting date included: Trans40 (40 days after sowing), Trans 65 (65 days after sowing) and Trans75 (75 days after sowing) and fertilisation Fert1: control (soil + sand + fertilizer treatment); Fert2 (T1 + Gigaspora margarita); Fert3 (T1 + Acaulospora tuberculate and Fert4 (T1 + Gigaspora margarita and Acaulospora tuberculate, that is Fert2 + Fert3). Three repetitions of 10 plants/repetition, giving (3 × 4 × 10) × 3 plants; that is 360 plants in total (Figure 2).

The substrate used for the germplasm experiment in each compartment consisted of three basic elements that were used to prepare the substrates. These were: the S1 substrate made up of fine sand; the second substrate S2 consisted of black humus soil. The third, S3 substrate was white wood sawdust. The soil was taken from the locality of Etoudi in the city of Yaounde to a depth of 15 cm and then sieved. The wood sawdust was taken from a saw mill and the sand from a sand deposit and then sieved to eliminate residues. The different tubs of the germinator were filled with each of the substrates.

After removing the seeds from the fruits, they were cleaned to remove the fruit tissue and then washed with tap water before disinfection by soaking in a solution containing a mixture of insecticide (Cigogne 360 EC, pyrethroid insecticide; 50 ml/15L of water); and fungicide (Mancomax bleu 800 WP 80 g/15L of water) for 15 minutes.

The germinator was established in a sunny place, sheltered from the wind. The maintenance of the germinator consisted in watering every day in the evening or very early in the morning, with a watering can to maintain moisture in the substrate; and removing weeds that compete with the plants. Once the seeds germinated, they were transplanted.

The collected data were subjected to analysis using the SPSS version 20 software and the means were separated with the Least Significant Difference (LSD) at 5% threshold. The graphical representations of the data were made with Microsoft Excel 2016. The model used is a univariate general linear model whose dependent factors are the treatment and the date of transplanting of the seedlings. The relationship between growth parameters and chlorophyll index was established using Pearson correlation coefficient at 5%.

The development of research on the establishment of a technical itinerary is of primary interest in agronomy. The results obtained from this work have made it possible to appreciate the effect of different substrates, transplanting dates and biofertilisers on the nursery growth of avocado plants. Different substrates used for the germination of P. americana show different behaviours, as a function of

^aR Squared = 0.67 (Adjusted R Squared = 0.29); ^bR Squared = 0.90 (Adjusted R Squared = 0.79); ^cR Squared = 0.80 (Adjusted R Squared = 0.56); ^dR Squared = 0.59 (Adjusted R Squared = 0.12).

**Correlation is significant at the 0.01 level (2-tailed).

their varying structures and compositions. A higher germination rate (100%) was obtained in the wood sawdust growth substrate, followed by soil (98%) and finally sand substrate (92%).

The 65-day mean germination rate was higher in the wood sawdust (1.79), followed by soil (1.72) and finally sand (1.28). This difference in germinative behaviour of avocado seeds shows that wood sawdust by its ability to retain moisture and its mineral composition, favours the development of avocado seeds. As regards the soil and sand substrates, these results corroborate those of Sounon et al. [22] who worked on the best substrate for nurseries and showed that the ferralitic substrate presented the best results on the growth of Artemisia annua plants compared to the sand substrate. The wood sawdust contains moisture and mineral elements [23] while sand is siliceous in nature, with granules that are hard and heavy (http://umc.edu.dz, 2015); it is therefore assumed that these characteristics can cause dehydration of the seeds by preventing the passage of oxygen and water to the seeds. Similarly, the relatively low root volume (5.20) and higher in wood sawdust (7.77) which is the best substrate, could explain the high germination rate in wood sawdust, as a large root volume would would favour the development of the plants and therefore their germination thanks to the wood sawdust with available water and mineral elements. In an unconstrained environment, a few roots can be sufficient to meet the plant’s water and nutrient requirements [24].

The variation in transplanting dates of avocado seedlings significantly influenced the plant’s gain in height, diameter, leaf area of the seedlings and the chlorophyll index. At the end of our experiment, we observed that the TransD40 transplanting stage had the best results in terms of leaf area and chlorophyll index than TransD65 and TransD75, which had better results in terms of plant height and diameter gain; this could be due to the fact that the late transplanted plants required a longer adaptation phase due to the trauma suffered by the roots. It is noted that the white sawdust substrate used in the germinator is rich in mineral elements with a high humidity and oxygenation rate, which are favourable to root development. These results corroborate those of Satapathy et al. [25], Tahir et al. [26] and Goita et al. [16] who found that paddy yield decreased with delaying the transplanting time.

Plenchette and Morel [27] stated that whether plants are mycorrhized or not, they all feed from the same phosphorus (P) pool since the ions dissolve in the soil solution; in other words, the P released from the NPs can be used by mycorrhised or non-mycorrhised plants. On the other hand, the control treatment effect shows a significant difference in leaf area, with results significantly higher than those of the mycorrhizal treatments in the case of leaf area. This confirms the work of He and Cui [28] who observed no significant difference in avocado biomass when applying AMF on sterile and non-sterile soil. Indeed, the AMF should perform their function through the strongly branched outer hyphae which increase the plant’s absorption capacity [29]; this absorption capacity which is conditioned by the type of crop grown and the nature of the soil can influence the AMF [30]. In our case, the nursery study in bags could reduce the exploration zone of the AMF and therefore their action. This could explain why the mycorrhizal fertilisation effect did not show any significant difference on the growth parameters. Viera et al. [31] working on native mycorrhizae of the rhizosphere, established a correlation between the growth of avocado seedlings and the amount of accumulated phosphorus. The only significant difference for the fertilisation effect was noted only for the leaf area parameter, knowing that nitrogen plays a major role in the multiplication of chloroplasts (green foliage) and also in the increase of the leaf surface and that the Fert1 treatment composed of fowl droppings, very rich in nitrogen and phosphorus, increases the protein synthesis and the phosphorylated compounds in the plants, decreases the content of soluble sugars in the roots, and consequently the rate of mycorrhizal colonisation [32]. Phosphorus input in the soil decreases the mycorrhization rate of the host plant [27]. The organic amendment applied to the Fert1 non-mycorrhizal treatment possibly favoured the development of endogenous soil fungi and their symbiosis with avocado roots. Our results corroborate with those of Okur et al. [33] who stated that soil amendment with organic fertilizers increases the activity of the soil microbial biomass, hence competition when a mycorrhizal amendment is added such as mycorrhizal fertilizer.