406 



RADIATION BIOLOGY 



workers who have obtained very discordant results. The inconsistency- 

 may be due to errors arising from partial reabsorption of fluorescent light. 

 The results are tabulated in Table 7-2. 



The fluorescence band reported by Rothemund appears to be too far 

 to the red when compared with the other values. A similar discrepancy 

 may be noted for the fluorescence bands of chlorophyll measured in the 

 same laboratory (cf. Rabinowitch, 1951, p. 744). The position of the 

 fluorescence maximum in the corn leaf is independent of the wave length 



of the exciting light and is unaffected by killing the leaf through hot- 

 water immersion (V. M. K. Young, unpubHshed). 



Optical Activity. The porphyrin nucleus of protochlorophyll is opti- 

 cally inactive. This places the seat of optical activity in chlorophyll a 

 at the 7 or 8 position (Fischer, 1940). 



3-3. PHYSIOLOGICAL PROPERTIES OF PROTOCHLOROPHYLL 



The formation and transformation of protochlorophyll in the angio- 

 sperms will be discussed first. 



Formation of Protochlorophyll in Dark-grown Seedlings. Since most 

 seeds contain no protochlorophyll, this pigment must result from the 

 germination process (Liro, 1908, pp. 21 tf.). The pigment is detectable 

 after the appearance of the root. The production of protochlorophyll 

 during the growth of dark-grown barley seedlings has been followed by 

 Smith (1950-1951). 



No. of days from planting 3 4 



Protochlorophyll, Mg per 10 plants ... . 23 . 85 



When plotted, these values give a typical sigmoid-shaped curve. As 

 development of the seedlings proceeds, the protochlorophyll content 

 increases slowly at first, then at an accelerated rate, and finally slows 

 down as it approaches a limiting value. These results also confirm the 

 observations of others that little if any protochlorophyll is present in the 

 initial stages of germination. 



The quantity of protochlorophyll formed in dark-grown seedlings is 



5 



2.28 



6 

 3 



17 



7 

 4.09 



8 

 4.61 



10 

 5.33 



