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Nonsteady-State Photosynthesis following an Increase in Photon Flux Density (PFD) 1: Effects of Magnitude and Duration of Initial PFD

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  • Metabolism And Enzymology
  • Physics


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 (τ) 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), τ 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. τ 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 τ 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.

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