The kinetics of P‐700⁺reduction in leaves: a novelin situprobe of thylakoid functioning

The half time (t 1/2 ) of the reduction of P‐700 + in the millisecond time frame is known to be limited by the reaction between plastoquinol and the cytochrome cytb 6 f complex. This is considered to be the rate limiting reaction of thylakoid electron transport and measurements of it provide a means of analysing how thylakoid election transport is regulated in vivo. The half time for the reduction of photochemically oxidized P‐700 has been measured in vivo using absorbance changes around 820 nm. The results showed that t 1/2 is independent of irradiance and decreases as photosynthetic induction progresses. Even with a constant t 1/2 the quantum efficiency of PSI declined as irradiance increased. The significance of the concept of photosynthetic control of electron transport is discussed in the light of these observations.