The three-way lockstep
Together
Light, temperature, and CO2 must all rise for the plant to use the extra gas well
Dim room
Under about 1,150 fc (12,330 lux), raising CO2 past 400 ppm does not raise photosynthesis until you add light
Brighter step
At about 4,600 fc (49,310 lux) the rate still climbs past 400 ppm, more slowly past 600, and levels near 1,200 ppm
Very bright
At 4,500-5,500 fc (about 48,240 lux) plants can use 1,200-1,300 ppm
Top of the book
Few gardens exceed 7,500 fc (80,400 lux); at that intensity plants can use up to 1,500 ppm
Response curve
Biggest jump
The steepest gain is from 0 to 200 ppm
Below 100 ppm
No net photosynthesis. Respiration spends more sugar than the leaf makes
At 100 ppm
Photosynthesis and respiration balance. No net gain or loss
To 400 ppm
Photosynthesis rises quickly
400 to 800 ppm
Still up, but more moderate
800 to 1,200 ppm
Still significant, then the curve flattens
Outdoor and greenhouse boost
Summer sun
Outdoor plants in bright light grow heavier and faster when CO2 is raised toward 1,500 ppm, a bit more than four times outdoor air
Tunnels
Enclosed outdoor spaces deplete fast and respond well to enrichment
Hose on the stem
Outdoors, run a line up the plant so gas washes the canopy in still or gentle air
Lights-on only
Night enrichment is wasted
Safety at garden rates
Nonflammable
CO2 does not burn
Garden band
Levels growers use are treated as non-toxic in the book
Human danger
Risk starts at extreme concentrations above 50,000 ppm, more than 30 times the highest useful plant level
Do not chase that number
Stay in the 1,000-1,500 ppm working band