1764 CHEMISTRY OF CHLOROPLAST PIGMENTS CHAP. 37B 



Koski, French and Smith (1951) using a powerful monochromator with 

 normal and albino corn. The albino mutant {cf. Koski and Smith 1951) 

 formed, in the dark, more protochlorophyll than the normal strain ; it con- 

 verted it into chlorophyll in hght, but this chlorophyll was rapidly de- 

 stroyed by further illumination, indicating that albinism was due to lack of 

 protection of chlorophyll against photoxidation. Other, "virescent," 

 mutants, described by Smith and Koski 1948, produced only very little 

 protochlorophyll in the dark, but kept forming chlorophyll slowly upon 

 prolonged illumination, and once formed, did not lose it easily. 



The albino seedlings had the great experimental advantage of contain- 

 ing almost no carotenoids. 



The action spectra for chlorophyll synthesis of the two strains are 

 shown in fig. 37. B3. The ordinate is inversely proportional to the number 

 of quanta needed to transform 20% of the total available protochlorophyll 

 into chlorophyll; the peak at 650 m^ corresponds to this transformation 

 being completed in 18.4 sec, in a flux of 80 erg/(cm.2 X sec). The absorp- 

 tion peaks are situated, in both cases, at 445 and 650 m^. The ratio of the 

 ordinates in the two peaks is 1.89 in albino and 0.66 in normal plants, 

 clearly indicating the relative (or complete) inefficiency of the carotenoids. 

 (Frank, 1946, had found a ratio of 1.47. This result may indicate either a 

 lower content, or a higher efficiency of carotenoids in oat seedlings. A more 

 trivial explanation also is possible — a smearing-out of the action spectrum, 

 caused by the use of light filters.) 



The red action peak in fig. 37. B3 is shifted by 21 mju, and the violet 

 peak, by 11 m/z, toward the longer waves, compared to the absorption peaks 

 of protochlorophyll in methanol solution; both figures correspond to a 

 shift of about 500 cm.~^ on the wave-number scale. It seems plausible 

 that the absorption bands of protochlorophyll in vivo are shifted by that 

 amount toward the red from their position in vitro. A transmission mini- 

 mum was in fact noted in the spectrum of squash seeds at 650 m/z {cf. 

 Chapter 37C, section 2e). 



When plants in which one-half of protochlorophyll had been converted 

 to chlorophyll, were illuminated with a monochromatic band centered 

 at 680 m/x (a wave length only slightly absorbed by protochlorophyll, but 

 strongly absorbed by chlorophyll a in vivo) , very little additional chlorophyll 

 formation was observed, indicating the practical absence of "auto-photo- 

 catalysis." 



Smith (1951, 1954) used Franck and Pringsheim's phosphorescence 

 quenching method (Vol. 11,1, p. 851) to determine whether any oxygen was 

 produced during the photochemical reduction of protochlorophyll to chloro- 

 phyll. The conversion was not inhibited by an atmosphere of pure hydro- 

 gen (<10~" per cent O2) needed for the application of this method. The 



