Abstract
Far-red (FR) light elicits two distinct processes in plants. First, a shade avoidance response which is triggered when the ratio of red to FR (R: FR) declines. Second, it interacts synergistically with higher frequency wavelengths of light (e.g. red or white) which improves the efficiency of photosynthesis. We investigated whether we could harness these phenomena in medicinal Cannabis to improve yields so that the duration of the photoperiod could be reduced to 10 h ("10L") whilst returning similar or improved yields compared to a 12 h photoperiod ("12L"). The THC concentrations were elevated in both high THC varieties by the different FR treatments. In Hindu Kush the concentration of THC was elevated by the addition of 4 h of total FR ("10L_2_2D"), and in Northern Lights total cannabinoid yields were increased by nearly 70% over the 12 L control (0.43 versus 0.25 g Plant(- 1)) by the addition of 2 h of FR in darkness after 10 h of light ("10L_2D"). Our results show a strong yield and quality advantage in high THC lines treated with end-of-day FR treatments. Furthermore, a lighting schedule of 10L_2D instead of 12 L would result in a saving of 5.5% in power usage and resultant emissions.