A feedback control system for the precise and continuous regulation of photosynthetic photon flux density
A simple and inexpensive feedback control system that provides continuous and precise control of photosynthetic photon flux density (PPFD) in a whole plant cuvette is described. A ‘Plexiglass’ tank is interposed between a light source and cuvette and PPFD changed by varying the level of dyed liquid in the tank. The amount of liquid pumped into or drained from the tank is a function of the difference (error) between a defined set point value of PPFD and that measured in the cuvette. The set point can be varied as a function of time, can follow the output of a quantum sensor measuring ambient PPFD or can be driven by values of PPFD read from a data file. Within the 0.4 to 0.64 μm waveband, the dye acts as a neutral density filter so that there is no change in spectral distribution with PPFD. Photosynthetic photon flux density in the cuvette was controlled to better than 20 μmol m−2s−1 when the set point was varied from 200 to 1100 μmol m−2s−1 over 3 min. When the set point was held constant or changed less rapidly, errors did not exceed 5 μmol m−2s−1. Net photosynthesis of Western redcedar (Thuja plicata Donn.) seedlings held at 18 °C closely followed rapid changes in PPFD.