Carbohydrate changes in Cymbidium 'Red Fire' in response to night interruption

- Carbohydrates in Cymbidium rapidly accumulated under night interruption condition.
- Pseudobulbs and roots in Cymbidium acted as both source and sink for flowering.
- Carbohydrate accumulation with night interruption was crucial to promote flowering.

The effects of night interruption (NI) on growth and soluble carbohydrate contents were investigated in Cymbidium 'Red Fire'. Plants were vegetatively propagated and grown under different light conditions for 2 years. NI was employed twice during experimental period, 16 weeks for each year: 9/15 h ambient light/dark (control), 9 h ambient light plus NI (22:00-02:00 h) with low light intensity at 3-7 μmol m−2 s−1 (LNI) and 9 h ambient light plus NI with high light intensity at 120 μmol m−2 s−1 (HNI). The light intensities of LNI and HNI at plant canopy were adjusted by placing pots at 4 and 1 m from the high-pressure sodium lamps. The pseudobulb diameter was larger under both LNI and HNI conditions than under control condition. Plants grown under LNI and HNI conditions had higher total dry mass than those grown under control condition. Soluble sugar contents were higher in the plants grown under LNI and HNI conditions than in those grown under control condition. Glucose was the most abundant soluble sugar in the pseudobulbs with flower initiation in the plants grown under LNI and HNI conditions. Sucrose contents were much higher in the leaves than in the pseudobulbs and roots after NI treatments. Starch contents in the leaves and roots were higher during vegetative growth, while those in the pseudobulbs were higher during reproductive growth. The pseudobulbs and roots of the plants acted as a strong sink during flowering, preferentially metabolizing carbohydrates, rather than storing them. These results suggest that the increases of starch in leaves during vegetative growth and soluble sugars in pseudobulbs and roots during reproductive growth of Cymbidium 'Red Fire' under NI are crucial for increasing plant size and thereby promoting flowering.