This review presents recent developments in plant photobiology and lighting systems for horticultural crops, as well as potential applications for cannabis (Cannabis sativa and C. indica) plant production. The legal and commercial production of the cannabis plant is a relatively new, rapidly growing, and highly profitable industry in Europe and North America. However, more knowledge transfer from plant studies and horticultural communities to commercial cannabis plant growers is needed. Plant photosynthesis and photomorphogenesis are influenced by light wavelength, intensity, and photoperiod via plant photoreceptors that sense light and control plant growth. Further, light properties play a critical role in plant vegetative growth and reproductive (flowering) developmental stages, as well as in biomass, secondary metabolite synthesis, and accumulation. Advantages and disadvantages of widespread greenhouse lighting systems that use high pressure sodium lamps or light emitting diode (LED) lighting are known. Some artificial plant lighting practices will require improvements for cannabis production. By manipulating LED light spectra and stimulating specific plant photoreceptors, it may be possible to minimize operation costs while maximizing cannabis biomass and cannabinoid yield, including tetrahydrocannabinol (or Δ9-tetrahydrocannabinol) and cannabidiol for medicinal and recreational purposes. The basics of plant photobiology (photosynthesis and photomorphogenesis) and electrical lighting systems are discussed, with an emphasis on how the light spectrum and lighting strategies could influence cannabis production and secondary compound accumulation.

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An Update on Plant Photobiology and Implications for Cannabis Production

The work begins with an introductory part in which the basic aspects related to the photosynthetic radiation, the photobiology of plants and the technology of light-emitting diodes (LEDs) are overviewed. It is followed by a review of related research works that have been conducted during the last two decades, and by the main design issues of LED luminaires for plant growth. The following part of the work reports the experimental growth tests performed. The effects of the radiation emitted by spectrally tailored LED luminaires on plant growth have been investigated. A total of four growth tests using lettuce and radish cultivars were performed. Two basic approaches were used to investigate the effects and the future possibilities of the usage of solid-state lighting (SSL) in plant growth. The first approach evaluates the growth development of lettuce plants in real greenhouse conditions using LEDs as supplementary light sources to natural daylight. In the second approach the evaluation was carried out with a total absence of natural daylight by growing lettuce and radish plants in phytotron-chamber conditions. The effects of SSL treatments on the growth development and quality of crops were compared with reference lighting systems composed of conventional and well-established light-source technologies, such as fluorescent and high-pressure sodium lamps. During the process of the investigation, the need to coherently quantify and evaluate the spectral quality of the radiation in terms of its photosynthetic appetence arose. Different metrics are still been used indiscriminately to quantify radiation used by plants to perform photosynthesis. Therefore, the existing metrics are discussed and a new proposal for coherent systematization is presented. The proposed system is referred to phyllophotometric and it is developed using the average photosynthetic spectral quantum yield response curve of plants. The results of the growth tests showed that the usage of SSL in plant growth offers an unprecedented possibility to optimise the morphogenesis, the photosynthesis and the nutritional quality of crops. This can be done by controlling the quantity and the spectral composition of the radiation provided, areas where LED-based luminaires excel. These possibilities can contribute to respond to the increasing demand for high-quality horticultural products by the consumers and to the conservation of global natural environment and resources.

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Usage and control of solid-state lighting for plant growth

For more plant species to be suitable for plant factory production, their optimal light regimes need to be optimized. We evaluated the effects of light quality, light intensity, and photoperiod on the growth and yield of cherry radish grown under red plus blue LEDs in a controlled environment. Radish plants were cultivated under two light qualities with different red:blue ratios (1R:1B, 2R:1B) at three light intensities (180, 240, 300 μmol m−2 s−1) or two photoperiods (12 h/12 h, 16 h/8 h), respectively. The light quality 2R:1B increased root diameter, root volume, and the biomass of shoot and root compared to light quality 1R:1B under a light intensity of 240 and 300 μmol m−2 s−1, but the growth differences between 1R:1B and 2R:1B were significant when the light intensity was 240 μmol m−2 s−1. New leaf chlorophyll content, root growth indices and the root-shoot ratio increased with light intensity. Cherry radish only formed storage roots with commercial value when light intensity was equal to or over 240 μmol m−2 s−1. The root diameter, root volume, root-shoot ratio, and the biomass of shoot and root of plants grown in the 2R:1B treatment was significantly higher than those in the 1R:1B treatment under the 16 h/8 h photoperiod. However, no significant difference was observed in the 12 h/12 h photoperiod. These results indicated that light regime in combination with a light intensity between 240 and 300 μmol m−2 s−1, the light quality 2R:1B, and a 16 h/8 h photoperiod produced appropriate growth of cherry radish in plant factory settings using an LED light source. In conclusion, the production of commercial storage roots in cherry radish is primarily dependent on light intensity, followed by light quality and photoperiod. Furthermore, the effectiveness of light quality regulation of storage roots was highly depended on light intensity and photoperiod.

Effects of light quality, light intensity, and photoperiod on growth and yield of cherry radish grown under red plus blue LEDs

Specific distribution of gibberellins, cytokinins, indole-3-acetic acid, and abscisic acid in radish plants closely correlates with photomorphogenetic responses to blue or red light

BACKGROUND

Interest is increasing around both the use of plants as functional foods and the agronomic techniques which can increase nutrients and phytochemicals. Nevertheless, little research has focused on the effects of light on accumulation of active compounds in root storage organs. Red beet was treated with RED (red/far red ratio: 1.29; transmitted photosynthetically active radiation: 66.9%) and GREEN (red/far red ratio: 0.43; transmitted photosynthetically active radiation: 25.8%) photo-selective films and changes in nutrients and biomass accumulation were measured.



RESULTS

Plants subjected to GREEN treatment had less dry weight accumulation both in storage roots (68%) and leaves (42%); moreover, soluble and structural carbohydrate concentration in roots was increased, as were the K, Mg and Zn concentrations (40.08, 2.95 and 0.023 mg g−1 fresh weight, respectively). Conversely, GREEN lowered total phenolic concentration (0.33 vs. 0.47 mg g−1 fresh weight) and antioxidant activity (0.65 vs. 0.94 µm Trolox equivalents g−1 fresh weight) compared to CONTROL. Total pigment concentration was reduced by 20% and 48% with RED and GREEN treatments, respectively.



CONCLUSION

Red beet showed a strong plasticity in its adaptation to light availability. Some macronutrients (fiber, sugars, minerals) can be concentrated in roots by modifying the amount and quality of the light, principally with GREEN photo-selective films. © 2013 Society of Chemical Industry

Application of photo‐selective films to manipulate wavelength of transmitted radiation and photosynthate composition in red beet (Beta vulgaris var. conditiva Alef.)

A. A. Vasil’ev

Papers

Development of storage roots in radish (Raphanus sativus) plants as affected by light quality

Photosynthesis and Carbon Partitioning in Radish Plants Grown under Blue or Red Light