Chlorophyll a (Chl a) florescence induction kinetics from the minimum yield F, to the maximum yield F, provide information on the filling up of the plastoquinone pool with reducing equivalents. In this paper, we have examined the effect of high temperature (above 40°C) on Chl a fluorescence rise kinetics starting from 40 μs (to 1 s) in pea leaves (Pisum sativum). The variable Chl a fluorescence is strongly quenched after heat treatment. With increasing temperature or the duration of heat treatment atypical O-J-I-P transient (Strasser et al. (1995) Photochem. Photobiol., 61, 32-42) is transformed into an O-K-J-I-P transient, with an additional rapid step called K detected in the 200-300 μs range. After prolonged heat treatment, the K-step becomes a dominant peak in the Chl a fluorescence transient followed by a large dip. We have investigated the origin and the possible interpretation of these changes by using NH2OH which acts as an electron donor to PS II, and DCMU which is known to block the PS II electron transport chain by displacing Q,. From the present data we propose that the appearance of this K-step is due to two effects: (1) inhibition of the water splitting system that leads to a much slowed turn over of the reduction of Q(A); (2) changes in the architecture of the antenna of PS II which affect the energy migration properties within the photosynthetic unit. The K-step can thus be used as an indicator of the heterogeneity of photosynthetic units and as an indicator for the physiological state of the photosynthetic sample.
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