Phénologie reproductive chez Dacryodes edulis ((G. Don) H.J. Lam) et variation de quelques métabolites primaires associés

Abstract

The world population has been growing and passed the 7.8 billion mark in 2020, resulting in increased poverty and food insecurity. In Cameroon, 40 % of the population lives below poverty, especially farmers, in a context where local fruit trees present numerous advantages. This is the case of Dacryodes edulis, the fruits of which have major nutritional values and their sale is an important source of income. Yet, several constraints prevent farmers from taking full advantage of this tree. For example, its fruits, commonly known as safou or African plum, are highly perishable when fresh. Due to the lack of appropriate techniques for long-term storage, farmers generally process them for oil production. Today, its exploitation must take into account the environmental constraints in order to earn maximum benefit from it. Thus, in order to facilitate decision-making in the field, agricultural stakeholders currently benefit from several tools, including bioclimatic models, allowing the prediction of biological processes. The general objective of this work is to improve the management of the safou sector through the control of its reproductive phenology in association with variations of some primary metabolites. Sixteen safou trees were selected in four major safou production localities in Cameroon, corresponding to three of the five main agro-ecological zones (AEZ) of the country (Foumban for AEZ III, Njombe for AEZ IV, Makenene and Yaounde for AEZ V). The temporal changes in the physical parameters of safou fruits were observed in order to identify their development and ripening phases, as well as their reproductive phenophases. Then, biochemical analyses were performed to describe the chronology of lipid, carbohydrate and water accumulation in safou mesocarp compared to their development and ripening stages. Finally, thanks to the climatic data (precipitation, temperature and relative humidity) of the different study localities, the dates of occurrence of the development and ripening phases of safou as well as the content of quantified biochemical elements were correlated to the spatio-climatic heterogeneity of the growth environments. This work was conducted during two consecutive production years (2012 and 2013). Four developmental and ripening stages of safou and their respective durations were determined as follows: cell division (CD) (7-14 days); fruit elongation (70-98 days); preripening (14-35 days) and ripening (21-35 days) phases. Accumulation of total soluble sugars (TSS), starch, and lipids begins at CD and evolves differentially during fruit maturity. TSS content is highest at the CD stage (11.62±7 (2012) and 11.47±9.5 (2013) µg/mg FW) and is not significantly different between the two study years for each developmental stage. During he whole development and ripening of the fruit, the starch content is low (˂1 µg/mg FW) and constant in the safou pulp. Total saturated fatty acid content was highest at the cell division stage (71.85 (2012) and 95.5 (2013) %) with palmitic and stearic acids as the majority fatty acids, respectively (contents of both ˃ 65% of the total saturated fatty acid content). On the other hand, it is at the end of pre-ripening stage that the maximum content of total unsaturated fatty acids is reached (54.33 (2012) and 37.69 (2013) %), with oleic, linoleic, linolenic (contents of the three ˃ 45 % of the total unsaturated fatty acid content) as the majority fatty acids respectively. The water content of safou pulp is highest at the CD stage and lowest at the end of ripening stage. During the development of D. edulis fruits, there was a concomitance between the time of decreasing water content and increasing lipid content (100.63±14.66 (2012) and 97.42±13.17 days (2013) from fruit set). There was no significant difference (for the sites and both study years) between the period from fruit set to maximum lipid content (133.88±18.41 days in 2012 and 128.92±12.11 days in 2013, corresponding to the ripening stage). For the same period, the cumulative temperature was significantly lower in 2013 (2914.11±396.58 °C) compared to 2012 (3291.29±472.63 °C). Nevertheless, there was no significant difference between the cumulative temperatures of different sites in the same year. The low average and cumulative temperatures associated with the delay in the dates of flower bud appearance, anthesis, and fruit set in 2013 are probably the cause of the significant increase in the annual average lipid content (33.09±2.77 (2012) and 39.36±2.65 (2013) %). The late pre-ripening stage (141.17±9.16-143.94±20.6 days from flower buds) provides the best fatty acid profile with less saturated fatty acids and more essential fatty acids. Thermal time (cumulative temperature required from flower bud emergence to full safou ripening) can be determined by temperature data governing certain phenophases, such as the average temperature from flower bud emergence to fruit set. The results of this study is an important contribution to set up a harvesting schedule to avoid losses, and determine the right time to extract higher quality oil.