Abstract
The response of photosynthetic oxygen evolution and chlorophyll a fluorescence in isolated barley protoplasts to a change in light intensity (from approx. 1 10 saturating to saturating light levels) under saturating CO 2 concentrations was examined. Marked oscillations in the rate of O 2 evolution and chlorophyll fluorescence were observed, resembling those seen in leaves under similar conditions. The effect of antimycin A addition on O 2 evolution at various light intensities and at different times during photosynthetic induction was examined to determine the role of cyclic photophosphorylation during steady-state photosynthesis and during oscillatory behaviour. Antimycin (1 μM) addition reduced the photosynthetic rate by 50–75% but only at light intensities over 80–100 μE · m −2 · s −1. Below this threshold light level, antimycin had no inhibitory effect. Thus, antimycin had a progressively greater effect at higher light intensity. When antimycin was added at various points along the time course of photosynthetic induction, the rate of O 2 evolution was reduced to the same level regardless of the time of addition. Antimycin addition also had a marked effect on the oscillations in O 2 evolution observed after a transition in light intensity from 100 to 1000 μE · m −2 · s −1, increasing the frequency and reducing the damping of the oscillations. Measurements of adenylate levels indicated that antimycin prevented a rapid rise in ATP ADP ratio observed in the chloroplast compartment after the light transition. These results are discussed with reference to regulation of photosynthesis during a transition in light intensity and the limitation of photosynthesis by photophosphorylation under various environmental conditions.
Published Version
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