816 FLUORESCENCE OF PIGMENTS IN VIVO CHAP. 24 



carotenoids) at 436 mju, does not excite the fluorescence phycoerythrin or 

 phycocyanin (cui-ve a). Light absorbed mainly by phycoerythrin, on the 

 other hand, excites the fluorescence not only of both phycobilins, but also 

 of chlorophyll curves a, h, c. The hump on the red side of the chlorophyll 

 a band probably is due to chlorophyll d {cf. below). 



Duysens' (1951) fluorescence spectra of Porphyra lacineata (fig. 24.5) 

 indicate that after excitation with 420 ra/x (absorbed by chlorophyll a and 

 the carotenoids) over 90% of excitation energy is transferred to an "un- 

 known pigment" (probably chlorophyll d); less than 10% of total fluo- 

 rescence is emitted by chlorophyll a, and only a negligible proportion by 

 phycocyanin. Excitation with 546 mju (light absorbed mainly by phyco- 

 erythrin) causes strong fluorescence of both phycocyanin and chlorophyll 

 a, and a comparatively weak fluorescence of "chlorophyll d." Duysens 

 interpreted these results as indicating the existence, in red algae, of two 

 kinds of pigment complexes: the largest part of chlorophyll a he suggested, 

 must be coupled with chlorophyll d, and transfer practically all excitation 

 energy to the latter pigment, although it is present in such a small amount 

 as to be hardly noticeable in the absorption spectrum at all {cf. p. 812). 

 A small part of chlorophyll a, not coupled with chlorophyll d, appears to 

 be associated with the phycobilins, and serves as ultimate recipient of the 

 major part of quanta absorbed by them. French's results indicate that 

 the extent of energy "leak" into chlorophyll d must vary Avidely from 

 species to species. In chapter 29, this hypothesis will be tied up with the 

 results of quantum yield determinations by Haxo and Blinks, who noted 

 a low photosynthetic efficiency of light absorbed by chlorophyll in some 

 red algae. In chapter 32, we shall explore whether the paradoxical fact 

 that fluorescence of chlorophyll a can be excited more strongly by light 

 absorbed by phycobilins than by light absorbed by chlorophyll a itself, 

 could be explained without Duysens' assumption of tAvo different pigment 

 complexes in red algae. 



The general rule, indicated by the above-described fluorescence experi- 

 ments, is that plant cells contain one (sometimes, perhaps, two) pigment 

 complex in Avhich light energy absorbed by any one component tends to 

 flow into the component with the lowest excitation level, and is therefore 

 remitted mainly as fluorescence of the latter — even if it is present in a very 

 low relative concentration. 



This picture is supported by Duysens' observations (p. 810) that all 

 energy absorbed, in purple bacteria, by some carotenoids or by different 

 forms of bacteriochlorophyll, flows into the form of bacteriochlorophyll 

 that has the lowest excited level. 



On page 801, we mentioned the increase in the intensity of the phyco- 

 bilin fluorescence observed by Arnold and Oppenheimer (1950) upon 



