## 4 Discussion  
Carotenoid regulation in plants is a complex mechanism due to the high variation among the different developmental stages, species/cultivars, and plant tissues [17; 18; 19]. In this study, three commercial mango cultivars in Thailand with different colors were used as plant materials. To gain an understanding of the regulatory mechanism of carotenoid accumulation in mango, the carotenoid profiles, the expression of carotenoid metapolic genes, and relevant metabolic changes were investigated at different developmental stages in the peel and pulp of mango. Various physiological and biochemical processes progressively changed during fruit ripening, particularly plant respiration rate, ethylene production, and fruit pigmentation as well [20; 21; 22; 23]. In climacter fruits, the early changes during the ripening process were the onset of ethylene production and the rise in cellular respiration [24; 25]. In this study, the patterns of fruit respiration, ethylene production, and color development were examined at different stages of fruit development. It was found that mango substantially produced ethylene and increased respiration rate during fruit development and ripening. In the meantime, carotenoid content considerably increased, accompanied by a lower hue value  
Figure 8: The expression of genes involved in carotenoid metabolism in pulp of mango fruit cv. ’Nam Dok Mai Sithong’, ’Nam Dok Mai No. 4’ and ’Kaituk’ during fruit development (45–88 days) and full ripening stage (98 days). The vertical bars represent the means with SE for three replications. and higher a* and b* values, which enhanced the degradation of green pigment and the presence of yellow pigment in mango fruits. It might be concluded that those changes were particular to the typical climacter pattern of ripening. The results in carotenoid identification and quantification illustrated that six carotenoids were identified, of which \(\beta\)-carotene was largely accumulated in all developmental stages and accounted for almost 70% of the total carotenoid content in both peel and pulp. It might be proposed that the increase in \(\beta\)-carotene played a very huge part in elevating the level of total carotenoid content in mango. These results corresponded with the previous studies in several fruits, which were found in yellow/orange colored fruits such as carrot [26], loquat [27], melon [28], and persimmon [29]. Carotenoid accumulation was described as a dynamic process during fruit development and ripening [30]. In these results, it was found that carotenoid accumulation in mango fruits was directly influenced by the stage of fruit development from immature to full ripening. In the early developmental stage, carotenoid accumulation was maintained at a low level. Then, it increasingly accumulated until it reached the maximum in the full ripening stage, where 'Kaituk' contained the highest carotenoid content in all developmental stages, followed by 'Nam Dok Mai No.4' and 'Nam Dok Mai Sithong'. The present results were similar to previous studies in mango cv. 'Tainong', 'Hongyu', and 'Jinhuang'. It was reported that the differences in carotenoid content and the development of yellow coloration were associated with the masking of carotenoids by chlorophyll and unmasking by chlorophyll degradation during fruit development and ripening [5; 31; 32; 33]. It was also observed that the distinct level in carotenoid concentration influenced fruit color and color differences among cultivars and developmental stages. Fruits and vegetables triggered a deeper yellow/orange/red coloration by increasing carotenoid content [34; 35; 36]. This might suggest that the largest amount of carotenoids in 'Kaituk', particularly \(\beta\)-carotene, contributed to a more yellowish color in 'Kaituk' than in 'Nam Dok Mai No.4' and 'Nam Dok Mai Sithong' in all developmental stages, which was evidenced by a lower hue value and higher b* value evaluated in 'Kaituk' compared to those of the other two cultivars.