Fluorescence induction studies in isolated chloroplasts I. Number of components involved in the reaction and quantum yields

• 1.

  1. A quantitative analysis of the fluorescence induction in isolated chloroplasts is presented. The rise of the fluorescence yield with time is ascribed to the photoreduction of a primary oxidant “Q” of photosystem II. The analysis yields a method of determining the amount of light actually utilized in the process. By comparing results obtained in the absence or presence of Hill oxidants, the quantum yield of results obtained in the absence or presence of Hill oxidants, the quantum yield of the primary photoreduction and the concentrations of all internal oxidants were estimated. The average value of the yield was 0.5 equiv/Einstein (for green light, 510–640 mμ). The total oxidant concentration was 1 equiv per 35 moles chlorophyll.

• 2.

  2. The shape of the rise curve as well as its intensity and temperature dependence suggest that, besides Q, a second electron carrier, P, is involved, reacting in a consecutive dark step:

  Q and P are present in a ratio 1:1, the concentration of each being 1:70 chlorophyll.

• 3.

  3. The restoration of the fluorecsence induction, i.e. the reoxidation of Q and P in the dark, which is accelerated by far-red light (700–740 mμ), was also subjected to a quantitative analysis. The quantum yield of the far-red reaction proved to be close to 1. The maximum (saturation) rate of the far-red effect was about I/50 of the Hill-reaction saturation rate.

  The results are discussed in terms of two photosystems and electron carriers arranged between them.