NONSTEADY-STATE PHOTOSYNTHESIS FOLLOWING AN INCREASE IN PHOTON FLUX-DENSITY (PFD) - EFFECTS OF MAGNITUDE AND DURATION OF INITIAL PFD

The response of photosynthesis to an increase in photon flux density (PFD) from low to higher PFD was investigated using spinach (Spinacia oleracea L.). The time-course for this response was qualitatively similar to that observed for a dark-to-high-PFD transition, showing an initial, rapid increase in photosynthesis over the first minute or so, followed by a slower increase lasting 5 to 10 minutes. This slow increase was approximately exponential and could be linearized using a semilogarithmic plot. The relaxation time (tau) for this slow phase was found to be a function of the starting PFD value. At starting PFD values below approximately 135 micromoles per square meter per second (including darkness), tau for the slow phase was approximately twice that observed for starting PFD values above 135 micromoles per square meter per second. This indicates a slower approach to steady state for leaves starting at PFD values below this threshold and a greater loss of potential photosynthesis. tau was relatively insensitive to starting PFD values below or above this transition value. The contribution of the slow phase to the total increase in photosynthesis following a low-to-high-PFD transition increased approximately exponentially with time at the lower PFD. The tau for the increase in the contribution of slow phase was determined to be 10.1 minutes. The implications of these data for activation and deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase and for the functioning of the leaf in a fluctuating light environment are discussed.