EFFECTS OF HEAT AND HUMIDITY 817 



breaking Croococcus cells under water, and their interpretation of this effect 

 as a consequence of suppression of energy transfer from excited phycobilin 

 molecules to chlorophyll molecules, caused by dilution. 



If we assume that all chloroph^dl in the cells is present in the same form, 

 then a fluorescence yield ip means the shortening of the normal life-time of 

 the excited state, to, to r = ^ro. In the case of chlorophyll in ChloreUa, 

 assuming ro = 8 X 10~^ sec. (page 634), we obtain: 



(24.1) T = (1.5 to 3 X 10-3) X 8 X IQ-^ = (1.2 to 2.4) X lO-" sec. 



and in the case of bacteriochlorophyll in Chromatiuni, assuming the same 

 value of To : 



(24.2) r = 7 X 10-3 X 8 X 10-8 = 0.6 X iq-io sec. 



Two factors may determine the life-time of excited chlorophyll mole- 

 cules in live cells, and thus account for the above-calculated small values of 

 t: "normal" energy dissipation in the pigment-protein-lipide complex 

 (chloroplastin) and quenching (or stimulation) of fluorescence by metabolic 

 processes. The latter phenomena may themselves be of two kinds : direct 

 "photochemical quenching" by competition between sensitized photo- 

 chemical reaction and fluorescence, and indirect quenching (or stimulation) 

 of fluorescence due to the metabolic formation of substances that diminish 

 (or enhance) the fluorescence of chlorophyll. Observations described in 

 section 3 can be interpreted as revealing changes in the general structure 

 of the chloroplastin complex, while in section 4 — and, in more detail, in 

 chapters 27, 28 and 33 — we will discuss variations in intensity of fluores- 

 cence closely associated with participation of chlorophyll in photosynthesis. 



3. Effects of Heat and Humidity on Chlorophyll Fluorescence in Vivo 



It has been found that, when chloroplast sediments (Noack 1927) or 

 live leaves (Seybold and Egle 1940) are placed in hot water, their fluorescence 

 vanishes almost immediately; at the same time, the red absorption band 

 is shifted toward the shorter waves. This transformation occurs at a tem- 

 perature of 64-72° C. If the leaves are kept in hot water for several 

 minutes, fluorescence reappears, but the absorption band remains in the 

 siiifted position. Metzner (1937) probablj^ dealt with the same phenom- 

 (4ion when he described a "burst" of fluorescence caused by heating the 

 chloroplasts under the fluorescence microscope. 



According to Seybold and Egle (1940), drying extinguishes the fluores- 

 cence of fresh leaves, but not that of leaves killed l)y boiling. The fluores- 



