"Despite a broad array of spectra and PPFD applied by horticulturists, a significant knowledge gap exists on the effects of these factors in medical cannabis." Therefore a recent study by Wageningen University looked at the role of red and white light in optimizing growth and accumulation of plant specialized metabolites at two light intensities in medical cannabis. The researchers found an interaction between spectrum and PPFD on plant dry matter and inflorescence yield of medical cannabis. "Dual red peaks of 640 and 660 nm shows potential in improving light use efficiency and promoting plant dry matter production."
The study
The King Harmony variety from Perfect Plants was cultivated in two sequential growth cycles in climate-controlled chambers. The PPFD at canopy height was 600 and 1200 µmol m-2 s-1. Four spectra were applied at both PPFD: two low-white and two high spectra. The low-white spectra either contained a single peak wavelength at 660 nm or dual peak wavelengths at 640 and 660 nm. The high-white spectra differed in broadness of the white spectrum: narrowband spectrum, featuring peak wavelengths at 450 nm and 660 nm, and broadband spectrum, which displayed a more uniform light distribution across a wide range of wavelengths, approximately spanning 400-750 nm.
Results
Red peaks
The researchers found that white light with a dual red peak of 640 and 660 nm increased inflorescence weight and light use efficiency, compared to white light with a single red peak at 660 nm. "The inclusion of two maximum absorption peaks at 640 and 660 nm appeared to drive photosynthesis and plant dry matter production more effectively than a single maximum absorption peak at 660 nm. This effect may be attributed to the fact that, within the red waveband, chlorophyll b and chlorophyll a have their maximum absorption peaks around 642 nm and 663 nm, respectively. Chl b is specifically bound to light-harvesting complexes while Chl a is bound to both photosystem core- and light-harvesting complexes."
White light
According to the researchers, a larger fraction of white light did not affect inflorescence weight, but it did influence the proportions of blue, green, and red light. This shift likely contributed to the observed treatment effects. The experiments, conducted at low to average PPFD levels compared to conventional medical cannabis cultivation, indicated that leaf-level photosynthesis increased at PPFD levels above 1200 µmol m-2 s-1. High PPFD can cause stress by overexciting photosystems and generating reactive oxygen species (ROS). Increasing the white light fraction, particularly the green fraction, may mitigate this stress by enhancing light distribution within the leaf, known as the 'detour' effect.
Decreasing the white fraction increased plant height, leading to more open structures that enhance light distribution and potentially increase yield and metabolite production. However, the researchers found that spectrum and PPFD did not affect total cannabinoid concentrations, contrary to some previous studies suggesting that higher blue light fractions or specific spectra can increase cannabinoid levels. Differences in experimental conditions, such as PPFD and photoperiods, may account for these discrepancies.
The researchers also observed that low-white spectra at high PPFD could cause bleached inflorescences, possibly due to photoinhibition and ROS production. These bleached tissues had higher total cannabinoid concentrations, particularly CBD, suggesting that cannabinoids may accumulate as antioxidants to counteract oxidative stress.
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