Photosynthetic changes in Cymbidium orchids grown under different intensities of night interruption lighting

• LNI induced net CO2 assimilation rate (An) and reduced dark respiration.
• An during NI was significantly higher under HNI than under LNI.
• An and photosystem II activities reduced during daytime in the plants under HNI.
• Midday depression of An with decreased chlorophyll can be caused by nitrogen.

Gas exchanges and photosystem II (PSII) activities were investigated in Cymbidium ‘Red Fire’ and ‘Yokihi’ to find out the influence of light intensity during night interruption (NI) on photosynthesis. The plants were grown under a 9 h truncated natural daylength (control). Some plants were night-interrupted by artificial lighting at a photosynthetic photon flux (PPF) of 3–7 μmol m−2 s−1 (LNI) or 120 μmol m−2 s−1 (HNI) from 22:00 to 02:00 h. Photosynthesis was induced during NI irrespective of light intensity in both cultivars. Net CO2 assimilation rate (An) during NI was significantly higher under HNI than under LNI in both cultivars. However, the plants grown under HNI condition exhibited lower An during the daytime as compared to those under control and LNI conditions in both cultivars. Maximum quantum yield (Fv/Fm), actual quantum yield and electron transport rate were also lower in the plants grown under HNI condition than under control and LNI conditions in both cultivars, except Fv/Fm in ‘Yokihi’. The plants grown under HNI condition had lower chlorophyll and nitrogen contents than those under control and LNI conditions. These results indicate that NI at a relatively high PPF can significantly increase An during NI in Cymbidium ‘Red Fire’ and ‘Yokihi’, but the An and PSII activities can be reduced during daytime due to the reduction of chlorophyll content, probably caused by nitrogen deficiency. Consequently, NI at a relatively high PPF in Cymbidium ‘Red Fire’ and ‘Yokihi’ must be accompanied by additional fertilization to prevent depression of An during daytime and to advance flowering and improve flower quality.