An LED illumination system is operable to irradiate plant materials with photosynthetically active radiation. A lighting assembly includes a plurality of different LED types. Each LED lamp has a different spectral power matched to an absorption peak of the plant materials. All of the LED lamps of each different lamp type are driven by a different dedicated power source. Each power source can be independently modulated to vary the collective spectral power output of the LED illumination system. The lighting assembly includes fluid conduits disposed proximate to the LED lamps and a cooling fluid is flowed through the fluid conduits to removed thermal energy from the LED lamps.