Short Term Acclimation of Spinach to High Temperatures

Using intact leaves of Spinacia oleracea (L.), reversible temperature-induced changes in chlorophyll fluorescence emitted at room temperature and at 77K were studied. Interpretation of fluorescence at 77K was largely facilitated by developing a new method to minimize reabsorption artifacts (;diluted leaf-powder'). Leaves of plants grown at 15 to 20 degrees C were exposed for several hours to different temperatures. Upon incubation at 35 degrees C in the dark or in the light, the following changes in 77K fluorescence occurred with a half-time of less than 1 hour: (a) the initial fluorescence (F(0)) of photosystem I increased by 15%, while that one of photosystem II somewhat decreased; (b) although variable fluorescence declined in both photosystems, the decrease in photosystem II (40%) was more severe; (c) the changes were less significant after 480-nanometer excitation light was replaced by 430-nanometer light. The data were interpreted in terms of a reversible, temperature-induced change in thylakoid structure and related change in the distribution of the absorbed energy in favor of photosystem I, at the expense of photosystem II excitation, probably accompanied by an increase in the rate of thermal deactivation of excited states. The considerable decrease in the variable part of room temperature fluorescence gives rise to the suggestion that this transition has lowered the reduction level of plastoquinone, i.e. has increased electron flow through photosystem I, relative to photosystem II. Possible physiological and mechanistic analogies between this temperature-induced state transition and the light-dependent state 1-state 2 regulation has been discussed.