Introduction
The cultivation of fruit trees generally requires a nursery phase during which the young seedlings are monitored and treated to improve their ability to adapt to the environment. This leads to the production of seedlings that are used to create orchards. It consists of four essential steps or operations: 1) The germination phase of the seeds in germinators for the production of rootstocks; 2) Transplanting into pots or bags; 3) Fertilisation in order to obtain seedlings of a satisfactory vigour (stem diameter) ready for grafting. The nursery phase requires a good understanding and mastery of plant regeneration and fertilisation techniques.
In Cameroon, the demand for avocado (Persea americana) fruits is increasing, but the supply is not keeping up with this demand. After a summary monograph on the production practices of avocado seedlings in the Yaounde area, this work aims to optimise the aforementioned three steps in order to obtain seedlings of sufficient sizes for grafting. Three factors are considered in this study: 1) The substrate (Substrate), whose effects are evaluated by the germination rate (GR), the daily average germination (DAG) and the root volume of seedlings (RootV). 2) The transplanting date (TransD), determined by considering three dates including 40 (Trans40), 65 (Trans65) and 75 (Trans75) days after sowing, and 3) Fertilisation using biological fertilisers, evaluated by testing four fertilisation levels, Fert1 (10 gr of 20-10-10 plus 10 gr fowl droppings), Fert2 (Acaulospora tuberculata), Fert3 (Gigaspora margarita) and Fert4 (Mixed mycorrhizal strains of Gigaspora margarita and Acaulospora tuberculata). This third factor is evaluated by growth parameters including leaf area (LeafA), chlorophyll index (ChlorInd), gain in Plant height (GainPltH) and plant diameter (GainPltD).
Methodology
Experimental Setup
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.
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.
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).
Substrate Preparation
The substrate used for the germplasm experiment in each compartment consisted of three basic elements: fine sand (S1), black humus soil (S2), and white wood sawdust (S3). The soil was taken from the locality of Etoudi in the city of Yaounde, and the wood sawdust was taken from a sawmill.
Seed Preparation and Germinator Maintenance
After removing the seeds from the fruits, they were cleaned, washed, and disinfected. The germinator was established in a sunny place, sheltered from the wind. Maintenance consisted of daily watering and weed removal.
Nursery Setup
The substrate used for the nursery experiment was a mixture of black soil, fine sand, and fowl droppings in the proportions of 1/2, 1/4, and 1/4, respectively. The seedlings from the germinator were extracted and transplanted into the nursery bags, with the mycorrhizae applied to the roots before transplanting.
Measurements and Observations
Germinator Evaluation:
Three parameters were recorded for the evaluation of the germination in the germinator: the germination rate (GR), the daily average germination (DAG), and the root volume (RootV) of avocado seeds sown on the different substrates.
Nursery Evaluation:
Four variables were used to evaluate the different transplanting dates (TransD) and biological fertilisations (Fert): the gain in plant height (GainPltH) and diameter (GainPltD), the leaf area (LeafA), and the chlorophyll index (ChlorInd).
Root Colonisation Analysis
To verify the effectiveness of mycorrhizal colonisation, root samples were collected, stained, and observed under a microscope for the presence of arbuscules and vesicles.
Statistical Analysis
The collected data were subjected to analysis using SPSS software, and the means were separated with the Least Significant Difference (LSD) at a 5% threshold. Graphical representations were made with Microsoft Excel.
Results and Discussion
Substrate Effects on Germination
Seedling germination started on the 10th day after sowing and reached its maximum between the 40 – 50th day. The germination rate was lower in sand (83.64%) than on soil (98%) and wood sawdust (100%) after 65 days of germination.
The substrate had a significant effect on the daily average germination (DAG) and the root volume (RootV) of the seedlings. Seedlings germinated on wood sawdust or soil had a gain of 8 days (43 – 35) compared to those germinated on sand. This resulted in a gain of 49.42% of root volume on wood sawdust over sand, and 19.53% of wood sawdust over soil.
These results indicate that the wood sawdust substrate, with its ability to retain moisture and its mineral composition, favours the development of avocado seeds. In contrast, the sand substrate, being siliceous in nature with hard and heavy granules, can cause dehydration of the seeds by preventing the passage of oxygen and water.
Transplanting Date and Biological Fertilisation Effects on Seedling Growth
The transplanting date had a very significant influence on the studied growth parameters, including gain in plant height (GainPltH), gain in plant diameter (GainPltD), leaf area (LeafA), and chlorophyll index (ChlorInd). The 40-day transplanting date (TransD40) resulted in the most vigorous plants, suitable for grafting.
The biological fertilisation had a significant effect only on the leaf area (LeafA) parameter. The non-mycorrhizal treatment (Fert1) with organic amendment (fowl droppings) showed a higher leaf area compared to the mycorrhizal treatments (Fert2, Fert3, and Fert4). This could be due to the fact that the organic amendment favoured the development of endogenous soil fungi and their symbiosis with avocado roots, whereas the addition of mycorrhizal inoculants may have led to competition in the root zone.
Verification of Mycorrhizal Colonisation
The presence of hyphae and vesicles was observed in the plants of treatments Fert2, 3, and 4, confirming the establishment of the mycorrhizal symbiosis. In contrast, no mycorrhizal colonisation was observed in the roots of the plants from the Fert1 treatment.
Conclusion
This study aimed to optimise the production of avocado seedlings using different substrates, transplanting dates, and biological fertilisers. The key findings are:
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The wood sawdust substrate is the best for avocado seed germination, increasing the root volume by 49.42% compared to sand and 19.53% compared to soil.
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Early transplanting (40 days after sowing) results in the most vigorous seedlings, suitable for grafting.
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The non-mycorrhizal treatment with organic amendment (fowl droppings) showed a better response in terms of leaf area compared to the mycorrhizal treatments, possibly due to the development of endogenous soil fungi.
These results provide valuable insights for developing an optimised technical itinerary for the production of high-quality avocado seedlings, which is crucial for meeting the increasing demand for avocado fruits in Cameroon. The use of wood sawdust as a germination substrate, early transplanting, and the strategic application of organic amendments can contribute to the successful establishment of avocado orchards.
