Answer
You would conclude that red and far red were being absorbed by one photosystem.
Work Step by Step
The increase in photosynthesis, beyond the simple sum of activity with red and far red illumination, shows that there are two photosystems with different peak absorption of light. If the increase in photosynthesis were just the sum of the results with the two kinds of light, it would show that they were going through one-and-the-same photosystem for absorbing light.
