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With the expeditious development of optoelectronics, the light-emitting diode (LED) technology as supplementary light has shown great advancement in protected cultivation. One of the greatest challenges for the LED as alternative light source for greenhouses and closed environments is the diversity of the way experiments are conducted that often makes results difficult to compare. In this review, we aim to give an overview of the impacts of light spectra on plant physiology and on secondary metabolism in relation to greenhouse production. We indicate the possibility of a targeted use of LEDs to shape plants morphologically, increase the amount of protective metabolites to enhance food quality and taste, and potentially trigger defense mechanisms of plants. The outcome shows a direct transfer of knowledge obtained in controlled environments to greenhouses to be difficult, as the natural light will reduce the effects of specific spectra with species or cultivar-specific differences. To use the existing high-efficiency LED units in greenhouses might be both energy saving and beneficial to plants as they contain higher blue light portion than traditional high-pressure sodium (HPS) lamps, but the design of light modules for closed environment might need to be developed in terms of dynamic light level and spectral composition during the day to secure plants with desired quality with respect to growth, postharvest performance, and specific metabolites.

A daylight climate chamber was designed with the aim of testing new greenhouse climate control strategies on a small scale. Precise control and measure ment of the chamber climate and long-term measurement of canopy carbon dioxide (CO₂) exchan ge was possible. The software was capable of simulating a climate computer used in a full-scale greenhouse. The parameters controlled were air temperature, CO₂ concentration, irradiance, air flow, and irrigation. The chamber was equipped with a range of sensors measuring the climate in the air of the chamber and in the plant canopy. A chamber perfor mance experiment with chrysanthemum (Chrysanthemum grandiflorum `Coral Charm') plants grown in perlite was carried out over the course of 3 weeks. Five air temperature treatments at a day length of 13 hours were carried out, all with the same 24-hour mean temperature of 20 °C, but different day temperatures (18.0 to 25.1 °C) and night temperatures (14.0 to 22.4 °C). Rate of canopy CO₂ exchange in the chambers was calculated. In the range of day temperatures used, rates of canopy photosynthesis were almost equal. The results showed that leaf area and plant dry weight after 3 weeks were not significantly different among temperature treatments, which is promising for further investigations of how climate control can be used to decrease energy consumption in greenhouse production.

There is increasing use of electricity for supplemental lighting in the northern European greenhouse industry. One reason for this may be to secure a high growth rate during low-light periods by an attempt to increase net photosynthesis. We wanted to clarify which period of the day resulted in the best use of a 5-h supplemental light period for photosynthesis and growth. The periods tested were supplemental light during the night, day, morning, and evening. The experiments were carried out in daylight climate chambers measuring canopy gas exchange. The air temperature was 25 °C and the CO₂ level ≈900 ppm. Vegetative chrysanthemum was used, because this species responds quickly to change in light level. The leaf areas of the plant canopies were nondestructively measured each week during the 4-week experimental period. The fact that the quantum yield of photosynthesis is greater at low than at high light intensities favors the use of supplemental light during the dark period, but growth measured as dry weight of the treated plants at the end of the experiments was not significantly different given identical light integrals of the treatments. However, one experiment indicated that increased time with dark hours during day and night (24 h) might decrease net photosynthesis. The assimilation per unit leaf area was approximately the same during times of sunlight through a diffusing screen at 100 μmol·m⁻²·s⁻¹ of photosynthetic photon flux (PPF) as during times of supplemental (direct) light application at PPF of 200 μmol·m⁻²·s⁻¹ by high-pressure sodium lamps. We conclude that during the winter and periods of low light intensities, the daily carbon gain does not depend on the time of supplemental light application, but is linked to the total light integral. However, extended time with dark hours during day and night (24 h) might be a disadvantage because of longer periods with dark respiration and subsequent loss of carbon. Our results indicate that during times of low light conditions, it is not necessary to include factors such as the timing of supplemental lighting application to achieve higher net photosynthesis in climate control strategies.