I came across a series of three journal articles recently that explored the effects of temperature, PPFD, and CO2 on cannabis (see bottom of post for links to the articles). They’re hardly the last word on these topics and I doubt that experienced growers will be surprised by the results, but I think the findings are worth sharing because they come from well-controlled experimentation.
Chandra et al. (2008) studied the effects of different PPFDs, temperatures, and CO2 concentrations on cannabis photosynthesis. They tested 20 “high-yielding Mexican variety” plants that were four months old. The relative humidity was 55% and CO2 was 350 ppm, which was the atmospheric level at the time at their location. Main results:
The figure above shows that photosynthesis for the strain they tested increased with PPFD at the lower temperatures (20-25° C; 68-77° F). At 30° C (86° F), photosynthesis increased only up to 1500 μmol/m2/s and decreased at the higher PPFD. Photosynthesis was maximized at 1500 PPFD and 30° C / 86° F.
Next, they tested the effect of CO2 concentration at 1500 PPFD and 30° C. Results:
The figure above shows no apparent benefit until CO2 reached 550 ppm (i.e., 200 ppm above the atmospheric level). The curve accelerates and, although extrapolation is risky, we might expect that 850 ppm would have doubled photosynthesis relative to the atmospheric 350-ppm level.
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Chandra et al. (2011a) evaluated the effect of 390, 545, and 700 ppm of CO2 on photosynthesis in four high-THC cannabis strains. The plants were female photoperiod plants, growing on an 18/6 light cycle at 25° C / 77° F, 55% relative humidity, and 700 PPFD. Results:
The figure above shows that 545 ppm (i.e., 155 ppm above the atmospheric CO2 level) produced small increases in photosynthesis for all four strains. Increasing CO2 to 700 ppm (i.e., 310 ppm above atmospheric) increased photosynthesis by an average of 44%.
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Chandra et al. (2011b) tested the effect of temperature on photosynthesis in three high-THC cannabis strains. The plants were female photoperiod plants, growing on an 18/6 light cycle at 55% relative humidity and 700 PPFD. Results:
The figure above shows that the effect of temperature differed for each strain. Strains A and C plateaued at 77-86 and 86-95° F, respectively. Strain B exhibited a maximum at 77° F.
Summary
- The PPFD that maximizes photosynthesis is temperature-dependent and probably varies across strains.
- Increasing CO2 has little effect until the concentration reaches 300 ppm above the atmospheric level.
- The optimal growing temperature varies across strains.
- Some strains grow equally well at temperature ranges that span 9° F; others need a narrower range.
References