Species specific chlorophyll a fluorescence-temperature profile at high temperatures in the leaves.

Chlorophyll a (Chl a) fluorescence-temperature profile in the region of 20-80°C was recorded for fourteen different plant species. In all the species studied, there was a rise in the fluorescence intensity in the region of 45-50°C and around 55°C the fluorescence intensity started to decline. In four of the species (Acacia melanoxylon, Ervatamia montana, Eucalyptus tertecornius and Azardicta indica) tested, there was a secondary rise in the fluorescence intensity around 65-70°C whereas in all other species a sharp decline in the fluorescence intensity was observed at this point. These changes in the fluorescence intensity at high temperatures (65-70°C) appear to be species specific and cannot be explained either in terms of changes in the stoichiometry between the two photosystems or in terms of Chl a fluorescence emission from photosystem I (PS I) at higher temperatures. This conclusion is supported by following observations: (1) there was no definite correlation between the Chl a/Chl b ratio and the pattern of fluorescence-temperature profile at high temperatures; (2) the sun and shade plants of the same species had a similar pattern of fluorescence-temperature profile; and (3) preferential excitation of PS I did not alter the fluorescence-temperature profile.