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Outbreaks of the Southern rice black-streaked dwarf virus (SRBSDV) have caused significant losses in many rice-growing areas in Vietnam, especially in both North and Central Vietnam in recent years. To detect the virus, traditional reverse transcription polymerase chain reaction (RT-PCR) methodology and immunoassays are currently employed. RT-PCR is accurate but requires expensive chemicals and instruments, as well as complex procedures that limit its applicability for field tests. To develop a cheaper, simpler, and reliable SRBSDV diagnosis assay based on the dot-enzyme-linked immunosorbent assay (dot-ELISA) method, anti-SRBSDV polyclonal antibodies were produced by using the antigens derived from the P10 coat protein of SRBSDV, which was achieved from a previous study. The IgG antibody purified from the antiserum of recombinant P10-immunized mice by protein A-agarose affinity chromatography could specifically detect both the target protein and SRBSDV at a dilution of 1:100000. In the trial test of SRBSDV diagnosis, the dot-ELISA assay using the obtained anti-SRBSDV antibody showed an accuracy rate of 90.9% in comparison with the standard RT-PCR assay. These results are important premises for the large-scale application of dot-ELISA assay for SRBSDV diagnosis in order to protect rice crops against viral disease damage.

The reaction-diffusion equations including the Fisher-Kolmogorov-Petrovsky-Piskunov (Fisher-KPP) and Fisher-Stefan model have been used to study biological invasion. In this thesis, we first consider the standing waves and travelling waves of the Fisher- KPP equation and the behaviour of the dynamical system near the equilibrium points. Next, we examine the existence of solutions to the Fisher-KPP model by constructing a trapping region in the plane for the wave speed 𝑐 ≠ 0. Then, we study stability of standing waves and travelling waves of this model by looking at the point spectrum, the asymptotic operator and the essential spectrum. However, a limitation of the Fisher-KPP equation is that it cannot be used to model the extinction of invasive populations in practical situations. Hence, the Fisher-Stefan equation is introduced to simulate both biological invasion for 𝑐 > 0 and recession for 𝑐 < 0. This model is reformulated from the Fisher-KPP equation and includes a moving boundary, which gives rise to a spreading-vanishing dichotomy. Moreover, we look at the existence and uniqueness of solutions to the Fisher-Stefan equation. By analysing the phase portrait, numerical results and the perturbation solutions, we can construct approximate solutions of the Fisher-Stefan equation. This phase portrait of this model also suggests that the solutions of the Fisher-KPP model exist for 𝑐 < 2 but they are unstable. Therefore, these solutions are disregarded when 𝑐 < 2 and considered as not exist. This unstable solution has value in describing solution of Fisher- Stefan problem. Thus, the stable solutions exist in the Fisher-Stefan problem. Last, we consider the spectral stability of the Fisher-Stefan equation by analysing its asymptotic operator, continuous spectrum and point spectrum. The results indicate that there is no point spectrum of the Fisher-Stefan asymptotic solution for the eigenvalue parameter 𝜆 > 1 at 𝑐 ≥ 0.

Rice breeding was conducted for a long time during historical times and is an important job in Vietnam because rice is the major food for domestic consumption and export. In this review, we have provided a comprehensive insight into the importance of promising rice germplasm resources, breeding achievements, and breeding approaches as well as discussed challenges and perspectives of rice breeding in this country. With rice germplasm and wild rice relative resources with rich and various genetic diversity, their useful genes and traits have been exploited and integrated into commercial varieties as the final outputs of rice breeding programs. New achievements of the modern genetics era have been approached and effectively contributed to breeding activities in this country. Genome sequences, molecular breeding, and mutation are powerful tools and playing vital roles in developing new varieties with characteristics of interest that should be followed by the current market demands. In the last decades, there has been a plethora of newly generated varieties by Vietnamese scientists and rice breeders and approved by the state authorities. However, very few domestic mega varieties have prevailed over the imported varieties. Therefore, rice breeding in this country is faced with big challenges, including limitations of backgrounds, budgets, and even talents in basic research to compete with other rice-producing countries. The target goals and long-term approaches for rice breeding should be paid explicitly in priority to ensure national food security and the advantage and development of rice breeding in this country.

Objective: The aim of this study was to develop and optimize the formulation of tablets containing Aquilaria crassna extract using the direct compression method. Methods: D-optimal design based on three independent variables was applied to evaluate the cause-effect relations and optimize the A. crassna tablet formulation. The weight variation (Y 1 ), disintegration time (Y 2 ), hardness (Y 3 ) and friability (Y 4 ) were investigated with respect to three independent variables including % dicalcium phosphate anhydrous (DCPA) in filler (X 1 ), % filler (X 2 ) and % croscarmellose sodium (CCNa) (X 3 ). The dissolution study of the optimized A. crassna tablets were investigated in simulated gastric fluid (SGF) (pH 1.2) using a validated high-performance liquid chromatography (HPLC) method for mangiferin analysis. Results: All investigation factors were found to have significant effects on the physical properties of A. crassna tablet. The tablet hardness and the disintegration time increased in positive relations with the ratios of DCPA. The results exhibited the negative relations between disintegration time and the percentages of CCNa. The optimized A. crassna tablet formulation which included 35 % (w/w) DCPA in filler, 60 % (w/w) filler and 7% (w/w) CCNa possessed the weight variation of 1.38 % (w/w), the disintegration time of 6.29 min, the hardness of 85.63 N and the friability of 0.41 % (w/w). The optimized A. crassna tablet formulation was experimentally examined which demonstrated a good agreement between the experimental and predicted values. Mangiferin was found to release completely from the optimized A. crassna tablets within 30 min. Conclusion: The cause-effect relations and optimization of A. crassna tablet formulation were investigated and reported for the first time. The A. crassna spray-dried extract could be formulated into tablet by direct compression method with good mechanical properties and acceptable release profile.