Magnus, the problem here is that you are looking only at the Photosynthesis process, and are not paying attention to the other physiological processes regulated by Pyhtochrome (absorving mainly red/far red) and Crytochrome (absorving mainly blue and UVA - there is another photoreceptor for UVB, but I do not remember its name).
It was long long ago that I had my Plant Physiology lessons (and I must admit it was not my favorite field!), but I can remember something regarding photoperiodism, wich is actually activated by different light temperatures (more than per hour of light itself!) and being regulated by Phytochrome. There was a "very elegant" experiment from McAlister et al. which gave basis for the discovery of the Phytochrome some years later, in which they demonstrated that germination was controlled by red and blue light, and once inhibited it coud be desinhibited by exposing the seeds to the other light temperature (there was another guy who did something similar with flowering, but I do not remember his name, and McAlister's expermient was more important)
If I remember correctly, activated Phytochrome would stimulate germination and leaf production, but would inhibit flowering (and if I remember correctly, in some plants also internodal stem elongation)...
as I said, I am just trying to remember my Physiology lectures (that was back to the late 80s / early 90s
) I do not remember the details, but the story was more or less as described above
I hope this helps
P.S.- "Sometimes, the proofs given by popular experience are more important that the facts given by a scientist ion a lab! Keep your eyes open!" <-- That was a statement that my Biostatistic prof told us in the very first day of the Biostats Semester at the university.. Then he added: "Remember that many of the greatest experiments in history are based on observation of popular experience"