There are orders of magnitude power differences between "glow" and "illuminate". First, your eye has a logarithmic response to light intensity, which is great,

There are orders of magnitude power differences between "glow" and "illuminate". First, your eye has a logarithmic response to light intensity, which is great, it gives you nice dynamic range. But if you are judging photons/second, you are going to do a terrible job. It doesn't take many photons for your to perceive dim light, particularly when you are dark adapted. Looking from the other direction, say that plant has about a 10cm x 10cm total leaf area. It gets the equivalent of 4 hours a day of sunlight at 1000 watts/sq meter (typical of earth). So 40 watt hours of energy per day hits it. Wikipedia says 3-6% efficient for photosynthesis, that gets us down to 2 watt hours per day. That gives it 100mW to live on. If it put all of that into glowing and were as efficient at making light as an LED it would make about the amount of light of an indicator LED that says something is on, but isn't silly bright. I think that is still orders of magnitude high. The plant still needs to use energy on growing, and the biological light making process is probably not as efficient as an LED.