Response of the in vivo chlorophyll fluorescence spectrum to environmental factors and laser excitation wavelength

The laser-induced chlorophyll (Chl) fluorescence spectrum in vivo at the steady state was investigated as a function of the leaf temperature between 25 and 4 degrees C at low light intensity (150 mu mol m(-2) s(-1)) The ratio between the red and far-red fluorescence bands, F685/F730, was seen to decrease with decreasing leaf temperature, while the fluorescence intensity at both the 685 and 730 nm peaks increased going from 25 to 4 degrees C. This behaviour appears to be the same in chilling-sensitive plants (Lycopersicon esculentum and Phaseolus vulgaris) as well as in chilling-tolerant species (Pisum sativum and Vicia faba). Under high light intensity (2000 mu mol m(-2) s-(1)), at controlled temperature, F685/F730 as well as the total fluorescence intensity was seen to decrease with irradiation time. Since photosystem I (PSI) contributes to the F730 emission band only, while photosystem II (PSII) contributes to both F685 and F730, the above results can be explained as being due to the change with light intensity and temperature of quenching processes that affect PSII more than PSI. Changing the light intensity impinging on the leaf, the total Chl fluorescence increases with increasing light intensity to a maximum reached at about 300 mu mol m(-2) s(-1), then it decreases to values close to the dark level of fluorescence (F(0)). The Chl fluorescence spectrum was seen to vary in shape by changing the excitation wavelength. The well known process of Chl fluorescence reabsorption is responsible for the decrease in the F685/F730 going from less (440 nm) to more (635 nm) penetrating excitation light. With UV excitation at 337 nm, an unexpected lower value for F685/F730 with respect to blue excitation was found.