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Plant and Cell Physiology, 1977, Vol. 18, No. 5 1047-1055
© 1977

Article

Aging of the photosynthetic apparatus VI. Changes in pH dependence of {Delta}pH, thylakoid internal pH and proton uptake and relationships to electron transport

Paul-André Siegenthaler and Françoise Depéry

Laboratoire de Physiologie Végétale et Biochimie, Université de Neuchâtel 20 rue de Chantemerle 2000 Neuchâtel Switzerland

The pH difference generated across the chloroplast membrane upon illumination ({Delta}pH) and the internal pH (pHi) were analyzed in aged spinach chloroplasts and in fresh chloroplasts supplemented with linolenate. In electron-flow conditions where both photosystems or either photosystem alone were functional, the {Delta}pH dropped and their optima shifted toward more acidic external pH (pHo) with a simultaneous increase in pHi. Upon aging or addition of linolenate, a decrease of pHo was therefore required to maintain the pHi in the range of 5–5.5 for maximum electron-flow activity. Moreover, aging like linolenate, diminished the proton pump activity and shifted its optimum (pH 6.7 in the controls) toward higher pHo. Although {Delta}pH and pHi changes were similar in all electron-flow conditions, the sensitivity of {Delta}pH toward aging and linolenate was eventually higher under photosystem II than photosystem I conditions.

In conclusion, the electron-flow activity seems to be delicately controlled by the proton pump, {Delta}pH, pHi and pHo. Unsaturated fatty acids which are released during chloroplast aging damage the membrane integrity in such a way that the subtle equilibrium between these factors is disturbed.

(Received April 19, 1977; )
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