High temperature chlorophyll fluorescence rise within 61–67 °C. Spectroscopic study with intermittent light grown barley leaves

Abstract

The chlorophyll a (Chl a) fluorescence rise detected during linear heating of intermittent light (IL) grown barley leaves at a temperature range of 61–67 °C was studied. The pronounced fluorescence increase appeared at preferential Chl a excitation, whereas it was strongly suppressed if auxiliary pigments (Chl b and carotenoids) were preferentially excited. However, the excitation dependence of this rise did not correspond with selective degradation of the auxiliary pigments. To describe this high temperature phenomenon, 77 K excitation and emission fluorescence spectra of IL leaves linearly preheated up to 61, 64 and 67 °C were measured. No significant uncoupling of free chlorophylls from pigment-protein complexes was detected. With increasing preheating temperature (i) a relative decrease of fluorescence excitation at 460–540 nm for both F685 and F735 emissions, (ii) a pronounced relative increase of F685 emission, and (iii) a relative increase of red excitation band at 77 K with maximum at 670 nm were found. Analysis of the results revealed that two processes influencing the fluorescence emission went parallel in opposite directions in thylakoids when being heated within this temperature range. The first of those is the relative increase of F685 Chl a emission band, which we expect to originate in the internal antennae of photosystem II even at these high temperatures. The second one is an impairment of the excitation transfer from Chl b and carotenoids to fluorescing Chl a. The relative height of the high temperature fluorescence rise is discussed with respect to the different extent of these two processes.