1020 BIOLOGICAL EFFECTS OF RADIATION 



formation occurs at a definite light intensity, that this varies with the 

 species and age of plant, and that either above or below this intensity 

 chlorophyll accumulation may decrease. Thus, Shirley (106) found that 

 the chlorophyll concentration per unit leaf weight and unit leaf area of a 

 variety of plants showed an increase with decreasing light intensities. 

 Further decrease in light intensity resulted in a decrease in chlorophyll 

 content. Leaves of different species vary widely in chlorophyll content, 

 and sunflower, which has a relatively low content, showed relatively 

 slight variations with light intensity, while hog peanut [Amphicar'pa 

 monoica (L.) Ell.], with a high content, showed large variations with 

 different concentrations. 



The effect of increasing the duration of illumination, by supplying 

 supplementary electric light, is demonstrated by the experiments of 

 Guthrie (36) and of Sjoberg (108). In Guthrie's experiments an increase 

 in illumination (12 hr. daylight and 12 hr. electric light) resulted in a 

 decrease in chlorophyll content. Reducing the light intensity to 12 per 

 cent of normal sunlight resulted in an increase in chlorophyll. Provided 

 the intensity was kept the same, elimination of blue light (by means of 

 glass filters) resulted in a slight decrease in chlorophyll, while elimination 

 of the red light caused a very slight increase, though this is not certain. 

 Shirley's (106) results indicate that with light intensities of 10 per cent 

 of full sunlight all of the five spectral regions used by him gave about the 

 same chlorophyll concentrations, though the spectral region X5290 to 



o 



7200 A usually gave lower values. 



The experiments of Sjoberg demonstrate that during February and 

 March the natural light intensity was not sufficient for maximal chloro- 

 phyll formation and even with supplementary illumination this value 

 was not reached. As the natural illumination increased during April, 

 supplementary illumination produced maximal chlorophyll content, until 

 later in the year the artificial illumination had no effect. Sjoberg's 

 experiments also demonstrate that a longer period of relatively low 

 intensity illumination is more advantageous for pigment formation than 

 a shorter period of high intensity. Thus Brassica rapa, using artificial 

 illumination during periods of 4, 6, 9^4:> 13H> ^^^ 24 hr. with equal total 

 intensities for all the periods, showed maximal pigment formation when 

 illuminated for 9 to 13 hr. each day. 



Owing to inherent complications in the methods of separation and 

 analysis, it is extremely difficult to arrive at thoroughly valid conclusions 

 concerning the possible changes in the two chlorophyll components under 

 different environmental conditions. Guthrie (36) has obtained some 

 evidence that the a:b ratio is lowered by the elimination of blue light. 

 This was also found to be the case in tomato plants which were exposed to 

 continuous artificial illumination. The conclusions of Wlodek (146) 

 that the intensity of the illumination causes a change in the a.b coefficient 



