Inactive Photosystem II Complexes in Leaves : Turnover Rate and Quantitation

The flash-induced electrochromic shift, measured by the amplitude of the rapid absorbance increase at 518 nanometers (ΔA518), was used to determine the amount of charge separation within photosystems II and I in spinach (Spinacia oleracea L.) leaves. The recovery time of the reaction centers was determined by comparing the amplitudes of ΔA518 induced by two flashes separated by a variable time interval. The recovery of the ΔA518 on the second flash revealed that 20% of the reaction centers exhibited a recovery half-time of 1.7 ± 0.3 seconds, which is 1000 times slower than normally active reaction centers. Measurements using isolated thylakoid membranes showed that photosystem I constituted 38% of the total number of reaction centers, and that the photosystem I reaction centers were nearly fully active, indicating that the slowly turning over reaction centers were due solely to photosystem II. The results demonstrate that in spinach leaves approximately 32% of the photosystem II complexes are effectively inactive, in that their contribution to energy conversion is negligible. Additional evidence for inactive photosystem II complexes in spinach leaves was provided by fluorescence induction measurements, used to monitor the oxidation kinetics of the primary quinone acceptor of photosystem II, Q_A, after a short flash. The measurements showed that in a fraction of the photosystem II complexes the oxidation of Q_A⁻ was slow, displaying a half-time of 1.5 ± 0.3 seconds. The kinetics of Q_A⁻ oxidation were virtually identical to the kinetics of the recovery of photosystem II determined from the electrochromic shift. The key difference between active and inactive photosystem II centers is that in the inactive centers the oxidation rate of Q_A⁻ is slow compared to active centers. Measurements of the electrochromic shift in detached leaves from several different species of plants revealed a significant fraction of slowly turning over reaction centers, raising the possibility that reaction centers that are inefficient in energy conversion may be a common feature in plants.