why does the simultaneous illumination with 600nm and 700nm light support a higher photosynthetic rate in higher plants than the sum of the photosynthetic rates supported by illumination with each wavelenght separately ?

 Robert Emerson in 1950's demonstrated that the photosynthetic rate was much higher under simultaneous illumination by shorter wavelength red light bearing the wavelength of 680nm and long wavelength red light bearing wavelength of 700 nm. This is due to the fact that  PS-II absorbs shorter wavelength red light (680 nm) whereas PS-I absorbs long wavelength red light (700 nm) and both must be photoactivated to make the ATP and NADPH needed by the dark reactions. The above experiment was also known by the name of the Emerson effect.

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