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.

Production in a dynamic photosynthesis optimized climate (DC) was compared to production in a traditional and more stable climate (TC). Production of a tropical plant species (Hibiscus rosa-sinensis L.) in a DC resulted in between 18% and 63% reduction in energy use, mainly due to lower temperatures and increased use of thermal screens. In high light periods, the average day temperatures (ADT) were virtually the same in the different treatments, while in low light periods both ADT and average night temperature (ANT) were lower in the DC. Differential use of the screens resulted in a higher cumulative light integral in the DC. The number of lateral breaks was either the same or higher in the DC. Dry weight at the end of the production period was not significantly different in six of the seven experiments, and in five out of seven replications, plants grown in the DC were shorter than plants in the TC. Production periods between 10 days shorter and 21 days longer, for the DC compared to the TC, could not be explained by temperature integration alone. In the DC, a high positive DIF (difference between ADT and ANT) does not seem to increase elongation growth. The study illustrates that it is possible to produce a heat-demanding plant and save energy using a DC.

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.

Circadian rhythms are believed to be of great importance to plant growth and performance under fluctuating climate conditions. However, it is unclear how plants with a functioning circadian clock will respond to irregular light environments that disturb circadian-regulated parameters related to growth. Chrysanthemum (Chrysanthemum morifolium ‘Coral Charm’) was exposed to supplemental light provided as irregular light breaks during the night, achieved by controlling the light based on forecasted solar irradiance and electricity prices. Growth, in terms of carbon gain, was linearly correlated to both daylength and daily light integral. This response was observed irrespective of the irregularity of the light breaks and despite circadian-regulated processes of carbohydrate metabolism, chlorophyll fluorescence, and leaf chlorophyll content being affected. Leaf expansion and stem elongation occurred at a faster rate in plants grown in short days with irregular light breaks during the night period compared with plants grown in a climate with a consecutive long light period, showing that low average light intensity promoted expansion of the photosynthetic area of the plants. These results are important to gain an understanding of the relationship between circadian-regulated processes and plant growth. These results will also contribute to increased energy savings in the use of supplemental light in greenhouse production.

Whole-plant CO2 exchange and root-shoot interactions during transition from vegetative to reproductive growth of `Coral Charm' chrysanthemum (Dendranthema ×grandiflorum Ramat.) were investigated over a range of P concentrations considered to be deficient (1 μM), adequate (100 μM), or high (5 mM). Transition from vegetative to reproductive growth resulted in reduced photosynthate production, root respiration, biomass accumulation, and starch accumulation in leaves. Root respiration was low in high-P plants regardless of growth stage. Reduced root respiration may indicate changes in source-sink relationships during the transition from vegetative to reproductive growth, making roots less competitive sinks than developing flowers. Plant responses to P deficiency included decreased CO2 assimilation and shoot biomass accumulation but increased root respiration, root:shoot ratio, specific leaf mass (SLM), and starch accumulation in leaves. Reduced root respiration activity in high-P plants was presumably due to differences in root architecture resulting in proportionately fewer root apices in high P. Daily CO2 assimilation, shoot biomass, SLM, and root:shoot ratio were similar in plants grown with adequate-P and high-P availability, although plant P accumulation increased with P availability. Our results suggest that the excessive P fertilization often used in ornamental production systems is detrimental to root activity.