Photosynthetic Light Responses of Cucumber Leaves Adapted to Illumination From Different Light Sources

We evaluated the photosynthetic light responses of cucumber (Cucumis sativus) leaves adapted to illumination from 3-band fluorescent lamps (FL) with high red:far-red (R:FR) light or metal-halide lamps (ML) that provided a spectrum similar to that of natural light. The seedlings were grown under FL (R:FR = 7.0) or ML (R:FR = 1.2) in growth chambers at a photosynthetic photon flux density (PPFD) ranging from 100 to 500 µmol·m^–2·s^–1. We also grew the seedlings under natural light in a greenhouse. The average PPFD in the greenhouse, valued in 12 h photoperiod, was 420 µmol·m^–2·s^–1. The gross photosynthetic rate (P_g), quantum yield of photosystem II (Φ_PSII), and photosynthetic electron transfer rate (ETR) of the first foliage leaves were evaluated at PPFDs ranging from 0 to 1800 µmol·m^–2·s^-1 using a photosynthesis and fluorescence measuring system when first foliage leaf expanded. Illumination was supplied by red and blue LEDs in a combination ratio of 9:1 for the measurements. The photosynthetic light response curve of seedlings adapted to FL illumination was similar to those of sun leaves, and the responses of seedlings adapted to ML illumination were similar to those of shade leaves. The response curve of seedlings adapted to FL illumination at PPFD of 100 µmol·m^–2·s^-1 was almost same as that of seedlings adapted to ML illumination at PPFD of 300 µmol·m^–2·s^-1. The maximum P_g, Φ_PSII and ETR of seedlings adapted to FL illumination was 1.3 – 1.4 times those of seedlings adapted to ML illumination regardless of the growing PPFD level. The seedlings adapted to FL illumination had greater mass and chlorophyll content per leaf area than those adapted to ML. The greater photosynthetic ability of seedlings adapted to FL illumination was probably due to the improved ETR resulting from physiological and morphological changes resulting from high R:FR illumination. The photosynthetic light response of seedlings adapted to natural light seemed to be similar to that of seedlings adapted to ML illumination at the same growing PPFD level.