Changes in the Stoichiometry of Photosystem II Components as an Adaptive Response to High-Light and Low-Light Conditions during Growth

The effect of different growth light intensities (60 W • m -2, 6 W • m -2) on the performance of the photosynthetic apparatus of mustard plants (Sinapis alba L.) was studied. A distinct decrease in photosystem II content per chlorophyll under low-light conditions compared to high-light condi­ tions was found. For P-680 as well as for QA and Q B protein the molar ratio between high-light and low-light plants was 1.4 whereas the respective concentrations per chlorophyll showed some variations for P-680 and 0 A on the one and Q B protein on the other hand. In addition to the study of photosystem II components, the concentrations of PQ, Cyt / , and P-700 were measured. The light regime during growth had no effect on the amount of P-700 per chlorophyll but there were large differences with respect to PQ and Cyt / . The molar ratio for C y t/a n d PQ between highand low-light leaves was 2.2 and 1.9, respectively. Two models are proposed, showing the functional organization of the pigment system and the electron transport chain in thylakoids of high-light and low-light leaves of mustard plants.