ENERGY-TRANSFER PROCESS DURING SENESCENCE - FLUORESCENCE AND PHOTOACOUSTIC STUDIES OF INTACT PEA LEAVES

In the present study, the time-course of changes in fluorescence and in in vivo photochemical activities, from full expansion through senescence, have been studied in third-pair leaflets of pea (Pisum sativum cv. Early Dwarf). The ratio between the fluorescence intensities observed at 680 nm for excitation wavelengths of 475 nm and 600 nm respectively, decreased around the 20th day, indicating a decreased energy transfer from cartenoids to chlorophylls in Photosystem II. Photoacoustically monitored photosynthetic oxygen evolution and photochemical energy storage decreased in parallel with the decrease in the fluorescence intensity ratio. In contrast, no decline was seen until the 25th day in the ratio between the intensity of fluorescence emission at 730 nm for this same excitation wavelength pair. This suggests that the energy transfer mechanism is affected first in Photosystem II and only later in Photosystem I. The decrease in the photothermal component preceded the massive destruction of photosynthetic pigments. The ratio of fluorescence emission at 685 nm to 730 nm with excitation at 450 nm increased after a massive decrease in pigment content of leaves. Furthermore, fluorescence emission at 600 nm, possibly due to carotenoids, and the ratio of fluorescence intensity at 600 nm to 685 nm, increased during the course of leaf senescence.