Exposure of leaves of Cucumis sativus L. To low temperatures in the light causes uncoupling of thylakoids II. Non-destructive measurements with intact leaves

To examine the effects of chilling of leaves of cucumber (Cucumis sativus L.) in moderate light on the coupling state of thylakoids in situ, changes in fluorescence, changes in light scattering and flash-induced changes in absorbance at 518 nm were examined in intact leaves. After chilling of leaves at 5°C in the light for 5 h, the non-photochemical quenching of fluorescence, a measure of energisation of thylakoids, was largely suppressed. The treatment also caused a suppression of light-induced changes in the light scattering by leaves, which depends on the formation of a pH gradient across thylakoid membranes. When thylakoids were prepared by very gentle methods from the leaves chilled in the light, through a step of preparation of intact chloro-plasts, the transport of electrons from H₂O to ferricyanide was uncoupled, being insensitive to an uncoupler, methylamine.These data provide consistent evidence that the thylakoids are uncoupled in situ by the chilling of leaves in the light and, as a consequence of the uncoupling, the energisation of the membranes is suppressed. However, the decay of the flash-induced change in absorbance at 518 nm in leaves was not markedly accelerated by the treatment. The thylakoids isolated from leaves chilled in the light, which were in the uncoupled state, also did not show a rapid decay, unless an efficient uncoupler such as gramicidin was added. These results suggest that even a considerable uncoupling of thylakoids, brought about by chilling of leaves in the light, is not sufficient to cause a marked acceleration of the decay of the flash-induced change in absorbance at 518 nm. Therefore, analysis at 518 nm is not always a sensitive method for assessing the coupling state of thylakoids.