THYLAKOID MEMBRANE STABILITY TO HEAT-STRESS STUDIED BY FLASH SPECTROSCOPIC MEASUREMENTS OF THE ELECTROCHROMIC SHIFT IN INTACT POTATO LEAVES- INFLUENCE OF THE XANTHOPHYLL CONTENT

A flash-induced transthylakoid electric field was measured at 515 nm a s an electrochromic absorbance shift in intact potato leaves using a d ouble flash differential spectrophotometer, The decay rate of the elec trochromic shift in dark-adapted samples was used to examine the condu ctance to ions of thylakoid membranes, Heat stress (39.5 degrees C for 15 min) was found to accelerate drastically the electric field decay, with the half decay time falling from more than 200 ms to less than 4 5 ms, Heat-induced acceleration of the electric field breakdown was in sensitive to the PSII electron donor Hydroxylamine and to the ATPase i nhibitor dicyclohexylcarbodiimide (DCCD), thus indicating that it refl ects an increase in thylakoid membrane permeability after heat stress, This phenomenon did not involve peroxidative damage of membrane lipid s, Acceleration of the electric field relaxation exhibited the same te mperature dependence as that of PSII deactivation, suggesting that the ionic permeability of thylakoid membranes is one of the most heat-sen sitive components of the photosynthetic apparatus, When potato leaves were infiltrated with 100 mol m(-3) ascorbate (in a buffer of pH 5), t here was massive conversion of the carotenoid violaxanthin to zeaxanth in, This change in carotenoid composition protected thylakoid membrane s against heat-induced changes in permeability, as revealed by the mai ntenance of a slow decay of the 515 nm absorbance change after heat st ress, No such effect was observed after treatments which did not induc e the violaxanthin-to-zeaxanthin conversion: leaf infiltration with 0 mol m(-3) ascorbate (at pH 5 or 8), 100 mol m(-3) ascorbate at pH 8 or 100 mol m(-3) ascorbate + 5 mol m(-3) dithiothreitol at pH 5, Increas ed stability of the permeability properties of thylakoid membranes was also observed after a mild heat treatment (2 h at 35 degrees C), The data presented suggest that de-epoxidized xanthophylls irt vivo stabil ize thylakoid membranes and protect thylakoids against heat-induced di sorganization.