Changes in Blue-Green and Chlorophyll Fluorescence Emission and Fluorescence Ratios during Senescence of Tobacco Plants

Abstract

Blue-green fluorescence and the red/far-red chlorophyll (Chl) fluorescence emission spectra of green tobacco leaves and of aurea leaves in different stages of senescence were measured using excitation at 355 nm in order to test the suitability of the fluorescence ratios green/red and green/far-red together with other ratios as stress and pigment indicators. Young yellowish-green leaves of the aurea mutant tobacco showed a much higher blue-green fluorescence emission and lower chlorophyll fluorescence as compared to the green wild type tobacco. As a consequence, the fluorescence intensity ratios blue/green ( F440/ F520), blue/red ( F440/ F690), blue/far-red ( F440/ F740), green/red ( F520/ F690) and green/far-red ( F520/ F740), as determined from the measured fluorescence emission spectra of the adaxial and abaxial leaf sides, were quite different for both leaf types and were functions of the content of chlorophylls and carotenoids. As chlorophyll levels decreased in senescing leaves of the aurea tobacco plant, the Chl a/ b ratio remained high at 3.3 to 4.0, while the ratio of chlorophylls to carotenoids declined slightly. In contrast, the Chl fluorescence ratios ( R fd 690-values, F v / F m and F v / F 0) as physiological indicators continuously declined, indicating a successive loss of photosynthetic activity. Values of the chlorophyll fluorescence ratio F690/ F740 for adaxial and abaxial leaf-sides increased with yellowing, whereas F440/ F520 tended to decrease with the senescence-induced decline in Chl content. F440/ F690 and F520/ F690 were found to increase after an initial decrease, with leaf age and senescence. The same trend was found for F440/ F740 and F520/ F740, which exhibited maximum sensitivity with respect to Chl breakdown. Whereas the F690/ F740 ratio for abaxial versus adaxial leaf surfaces correlated linearly, with the abaxial leaf-side values always being higher than the adaxial leaf-side, values for both sides corresponded in a curvilinear fashion with leaf Chl content. Changes during early stages of leaf development were seen not only in the most commonly used red/far-red ratio, F690/ F740, but to some extent also in F440/ F520. The results indicate that changes in photosynthetic activity due to senescence and Chl breakdown can easily be detected and qualified via fluorescence ratios that are based on the UV-A excited fluorescence bands F440, F520, F690 and F740. These ratios provide a solid basis for the laser-induced remote sensing of the state of health and Chl content of vegetation. The fluorescence ratios green/red and green/far-red were found to be very suitable complementary indicators of early stress events in plants.