Photosynthesis in horticultural plants in relation to light quality and CO2 concentration

Background: The use of light emitting diode (LED) technology within plant lighting applications provides possibilities for designing specific spectra for different purposes. In order to maximise the use of light, it has been suggested that the light spectrum should be composed with respect to the photosynthetic response of the green plant. In greenhouse applications, artificial light is often used in combination with CO2 injection to increase the concentration of CO2 in the greenhouse air to 600-900 mu mol mol(-1). Objectives: The objective of this study was to examine interactions between light quality and CO2 concentration with respect to photosynthesis in horticultural plants. Methods: Experiments using leaf-chamber photosynthesis measurements were performed in order to examine plant responses to different wavelengths in relation to CO2 concentration. Three different plant species were used in the experiments; Solanum lycopersicum (tomato), Cucumis sativus (cucumber) and Euphorbia pulcherrima (poinsettia). Photosynthesis was measured using four different narrow-band light qualities; 450 nm (blue), 530 nm (green), 620 nm (orange) and 660 nm (red). In addition, four different CO2 concentrations were used for measurements; 400, 600, 800 and 1,000 mu mol mol(-1). Results: The results revealed a clear difference in photosynthetic response to the four different wavelengths used, with the highest carbon assimilation obtained when the plants were subjected to 620 nm light, followed by 660, 530 and 450 nm. However, there was no interaction between light quality and CO2 concentration. Conclusions: In tomato and poinsettia, but not in cucumber, a multi-wavelength spectrum also containing green wavelengths results in higher photosynthetic activity than a narrow-band spectrum composed solely of blue and red wavelengths. However, increasing the CO2 concentration has a much larger effect than changing the spectral distribution, emphasising the importance of good control of CO2 concentration in the greenhouse.