PHOTOINHIBITION OF PHOTOSYNTHESIS IN CHILLED POTATO LEAVES IS NOT CORRELATED WITH A LOSS OF PHOTOSYSTEM-II ACTIVITY - PREFERENTIAL INACTIVATION OF PHOTOSYSTEM-I

When 23 degrees C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 degrees C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 degrees C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function, A detailed functional analysis of the photosynthetic apparatus, using invivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P-700) of PS I and the efficiency of PS I to oxidize P-700 were dramatically reduced. Loss of P-700 activity was shown to be essentially dependent on atmospheric O-2 and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O-2 and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O-2 exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O-2 toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O-2-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 degrees C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O-2 toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.