CHLOROPHYLL FORMATION 



415 



maximum at about 55 ft-c, and thereafter slowly decreases as the light 

 intensity increases. The results derived from Myers's data are shown in 

 Fig. 7-15. 



The accumulation of chlorophyll is a local effect. Only the illumi- 

 nated portion of an etiolated leaf becomes green. According to Strain 

 (1949): "If this partially green seedUng is now returned to the dark for 

 several days, the green portion fades very much. Upon exposure of the 

 entire seedling to light, chlorophyll forms faster in the previously unex- 

 posed portion than in the exposed and 

 faded portion." 



Dependence of Chlorophyll Formation 

 on Intermittent Light. It has been 

 known since the time of Mikosch and 

 Stohr (1880) that the same quantity 

 of irradiation produces more chloro- 

 phyll when administered intermittently 

 than when administered continuously. 

 These workers found that barley plants 

 at 20°C accumulated the same quantity 

 of chlorophyll with 4.5 hr of illumina- 

 tion, part of which was intermittent, as 

 with 6.5 hr of continuous illumination 

 (cf. Biebel, 1942). 



Experiments of Mikosch and Stohr 

 (1880) also indicate that the trans- 

 formation of protochlorophyll to chlorophyll is more efficient in inter- 

 mittent than in continuous illumination. Barley and oat seedlings pro- 

 duced more chlorophyll during 5 min of intermittent lighting (1-sec 

 periods) than during 2.5 min of continuous lighting. Also cress seedlings 

 produced a detectable quantity of chlorophyll in 12.5 min of intermittent 

 illumination but none in 6.25 min of continuous illumination. This type 

 of experiment will bear repetition. 



The authors have determined the effect of intermittent illumination 

 on the formation of chlorophylls a and b. Their results are graphed in 

 Fig. 7-16. In this experiment the plants were exposed 5 min out of every 

 hour. It may be seen that less of each chlorophyll is accumulated dur- 

 ing the same period of elapsed time for intermittent than for continuous 

 illumination, but that, on the basis of total hght energy, the amount of 

 chlorophyll a accumulated is more than double for the intermittent 

 illumination and the amount of chlorophyll b is somewhat less than double. 

 Because the accumulation of chlorophyll is greater for a given amount of 

 radiation administered intermittently rather than continuously, the for- 

 mation of chlorophyll is undoubtedly a combination of slow thermo- 

 chemical and rapid photochemical processes. The product of the thermo- 



100 200 300 400 



LIGHT INTENSITY, ft-C 



Fig. 7-15. Relative amounts of 

 chlorophyll produced daily by Chlo- 

 rella pyrenoidosa at different light 

 intensities. [Derived from Myers's 

 data {Myers, 1946).] 



