A FEEDBACK-CONTROL SYSTEM FOR THE PRECISE AND CONTINUOUS REGULATION OF PHOTOSYNTHETIC PHOTON FLUX-DENSITY

A simple and inexpensive feedback control system that provides continu ous and precise control of photosynthetic photon flux density (PPFD) i n a whole plant cuvette is described. A 'Plexiglass' tank is interpose d between a light source and cuvette and PPFD changed by varying the l evel of dyed liquid in the tank. The amount of liquid pumped into or d rained 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 mu m waveband, t he 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 mu mol m(-2)s(-1) when t he set point was varied from 200 to 1100 mu mol m(-2)s(-1) over 3 min. When the set point was held constant or changed less rapidly, errors did not exceed 5 mu mol m(-2)s(-1). Net photosynthesis of Western redc edar (Thuja plicata Donn.) seedlings held at 18 degrees C closely foll owed rapid changes in PPFD.