Radiosensibilité de quelques variétés de pastèque (Citrullus lanatus) à la mutagenèse induite aux rayons gamma

Abstract

Watermelon (Citrullus lanatus Thunb.) is a vegetable fruit native to Central Africa and the Kalahari Desert. It is adapted to tropical and subtropical regions and arid deserts where its fruit is reputed to be rich in water, nutrients and phytochemicals with therapeutic properties and is cherished by the populations. Despite these advantages, watermelon is characterized by extreme sensitivity to parasites and climatic hazards. In Cameroon, watermelon is subject to an intensive production system requiring much chemical inputs, thus threatening the sustainability of its production system. A desire for improvement is therefore imposed on plant breeders and genetic improvement by induced mutagenesis appears to be an alternative method. The objective of this thesis was to set the basis of genetic improvement of watermelon by means of induced mutagenesis with gamma rays. Initially, ten agromorphological parameters were chosen to evaluate five watermelon varieties selected on the Cameroonian market in order to determine their agromorphological performances. Then the radiosensitivity to gamma rays of two of the most performant varieties was evaluated in the greenhouse, for this purpose the seeds were irradiated at several irradiation doses (0, 100,200, 300, 400 and 600 Gy) and the LD50 of each variety was evaluaded using a linear regression analysis based on the growth parameters. Finally, the efficiency of LD50 on the first generation of M1 mutants was studied in the field using agromorphological and biochemical markers. The results of the agromorphological characterization showed that only four of the quantitative characteristics evaluated (Fruit Length, Diameter of fruits, Weight of the mature fruit and Weight of 100 seeds) allowed complete distinction between the five varieties, while the other parameters only allowed partial distinction. The result on seed shape gives for each variety a Length/Height (L/H) ratio of less than one, which allows to say that the five varieties evaluated in this study have elongated seeds. On the other hand, with regard to the maturation time, the Kaolack variety was early with a delay of 80 days after sowing (DAS) and the Sugar baby variety was late with a delay of 110 DAS. All varieties showed a relatively high emergence rate (61.11 % to 90.71 %). However, Kaolack and Crimson sweet had the highest rates of 90.71 % and 89.81 % respectively. The evaluation of the parameters showed that these Kaolack and Crimson sweet varieties are elite varieties as they showed the best agromorphological performance (average number of fruits per plant 3.55 and 3.32 higher than the others). A positive and highly significant correlation was noted respectively between fruit length and weight of the fruit (r = 0.957**), and between flowering time and ripening time (r =0.960**). The study of radiosensitivity showed that the two elite varieties Kaolack and Crimson sweet are perfectly sensitive to mutagenic treatment by means of gamma rays. It allowed the determination of LD50 of 225.40 Gy for the Kaolack variety and for the Crimson variety a LD50 of 221.56 Gy. For these two varieties, the LD50 is between 200 and 250 Gy. The analysis of the efficiency of the LD50 of 200 Gy on the first generation of M1 mutants by means of agromorphological and biochemical markers revealed a significant effect on these parameters. Parameters such as seed emergence, emergence rate, number of leaves per plant and number of flowers per plant significantly decreased compared to the control, while flowering time and fruit ripening time increased significantly. The Kaolack variety showed a significant reduction in the number of fruits per plant with an average of 3.88 for the control and 2.99 for the treated lot. The Crimson sweet variety flowered but the lot that received the irradiation treatment with the LD50 of 200 Gy did not produce any fruit. The total chlorophyll (a+b) content of the leaves of the Kaolack variety was significantly higher in the irradiated lot (40.4 μg/ml) compared to the control (39 μg/ml) while the Crimson sweet variety showed a significant decrease in total chlorophyll content in the irradiated lot (34.1μg/ml) compared to the control (39 μg/ml). The results obtained show a variability in the parameters evaluated following the induction of mutagenesis. These results thus allow to lay the strategic basis for the improvement of watermelon (C. lanatus) faced with biotic and abiotic constraints thanks to radio-mutagenesis in Cameroon.