Growth of noni fruits (Morinda citrifolia L.) and accumulation of phenolic compounds during fruit development

•We investigated the growth of noni fruits and established a fruit growth curve.•We examined the flowering and fruitlet development of noni fruit which is an interesting multiple fruit.•We assessed the accumulation of important phenolic compounds during fruit development.•The results indicated that it was a continuous development from flowering to fruit growing in noni fruit.•The optimal time for harvesting noni fruits is 126 days after inflorescence development in Taiwan with appropriate picking size and high content of scopoletin.

We investigated the growth of noni fruits (Morinda citrifolia L.) from 30 plants. The two investigating periods for noni fruits were from March to August, summer fruit, and from June to November, winter fruit. The initial investigation started once the visible inflorescences were 10 mm in diameter. Each single fruit consisted of 97–104 florets. The characteristics of fruit growth, including fruit fresh weight, dry weight, length, width, and volume, showed two stage patterns during the growth of noni fruits. The initial stage of the fruit growth, including inflorescence flowering, showed a sigmoid curve for all characteristics. Then a ‘lag phase’ (LP) of about 14 days comes after the initial stage. After the LP, fresh fruit weight, size, and volume showed a linear relationship, while only the fruitlets grew and developed. Before the bottom fruitlet setting, the levels of total phenolic compounds (TPCs) were extremely low and scopoletin could barely be detected. Rutin content per gram dry weight was significantly higher in the inflorescence flowering stage than in the fruitlet growth stage. Scopoletin accumulated as the fruit grew and the content was 2–3 mg per fruit dry weight. The accumulation patterns of TPCs, rutin, and scopoletin showed a sigmoid curve. Our study showed that a single fruit approached the maximum size of about 100 g, and the levels of biological components reached a peak at 70 g of fruit weight. We suggest the optimal harvesting time for using noni fruits for commercial purposes is 126 days after inflorescences development in both harvest seasons.