EFFECTS OF ALTERNATING LIGHT INTENSITY ON CO₂ ASSIMILATION OF FICUS ELASTIC AND DIEFFENBACHIA PICTA PLANTS, GROWN FOR INDOOR LANDSCAPING

Indoor landscaping increasingly uses plants on new hydroponic systems, including green walls, for greening private and public spaces, not always well or sufficiently lighted. Lamps are often essential to ensure a suitable light intensity for plant growth and for the production of a dense and brightly coloured canopy, even if many commonly used species originate in the lower layers of tropical ad subtropical forests and have a high ability to tolerate poor light levels. The photosynthetic response to light flecks is a well known characteristic of many plants native of the lower layers of forest and especially those living on the floor of dark and moist tropical forests. The effects on CO₂ assimilation of a constant or alternating light intensity were tested in Dieffenbachia picta ‘Camille’ and Ficus elastic ‘Decora’ plants, grown under 8/16 hours of day/night in a phytotron. The compared intensities were 20, 40 or 80 µmol m^-2 s^-1 for constant light and 20/40 or 40/80 µmol m^-2 s^-1 for alternating light, at 8 minute intervals. The CO₂ assimilation was measured throughout the 8 hours of lighting, by an infrared gas analyzer LI-6400XT, on completely expanded leaves. The light intensity regimes influenced the trend of CO₂ assimilation curves and the total daily assimilated CO₂ in both species. In Ficus the photosynthetic activity under constant 20 and 40 µmol m^-2 s^-1 was very low (0.7-0.9 µmol CO₂ m^-2 s^-1 on average during the 8 hour light period) and increased to 1.4 µmol CO₂ m^-2 s^-1 under 80 µmol m^-2 s^-1, with two well marked peaks after 0.5 and 5 hours from the start of lighting. The 40/80 alternated intensity resulted in a total daily assimilation of 54 mmol CO₂ m^-2, 33.2% higher than under 80 constant lighting. In Dieffenbachia the CO₂ assimilation was around 0.2-0.6 µmol CO₂ m^-2 s^-1, averaged for the 3 constant intensities, whereas at 20/40 alternating lighting the rate increased to 1.2 µmol CO₂ m^-2 s^-1, equivalent to +67% on total daily assimilation.