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RADIATION BIOLOGY 



increases at considerably greater rates and with a shorter induction period 

 than at 4°C. 



Experiments carried out by Smith (1949a) gave similar results, shown 

 in Figs. 7-8, 12, and 13. These curves, besides showing the course of 

 chlorophyll accumulation, display the behavior of the total ether-soluble 

 magnesium fraction at different temperatures. The results may be inter- 

 preted in the following manner: At low temperature, 0°C, the limited 

 amount of chlorophyll precursor (protochlorophyll) present in the leaves 

 is transformed photochemically to chlorophyll, but no further precursor 



100 200 



TIME ILLUMINATED, hr 



300 



is a and b in 



TIME ILLUMINATED, fir 



Fig. 7-17. The effect of temperature on the accumulation of chlorophyll 

 dark-grown barley seedlings. 



is formed; at intermediate temperatures, e.g., 7°C, after the initial pre- 

 cursor has been transformed, time is required to form additional pre- 

 cursor. Following this induction period, precursor formation, and conse- 

 quently chlorophyll accumulation, is accelerated. This acceleration may 

 possibly result from accelerated photosynthesis. At favorable temper- 

 atures, e.g., 19°C, the induction period is shortened so as to be almost 

 indiscernible, and the acceleration of chlorophyll accumulation is compen- 

 sated by a deceleration, so that the increase of chlorophyll is almost 

 constant with time until the steady-state condition is nearly reached. 

 The photochemical formation of other organic magnesium compounds at 

 0°C and their persistence during the whole course of chlorophyll accumu- 

 lation at other temperatures are clearly evident. 



The effect of temperature is a local phenomenon. Two parts of an 

 etiolated leaf illuminated at two temperatures, one below and the other 

 above the* threshold temperature for greening, show a distinct line of 

 demarcation between them. The part kept at low temperature remains 

 yellow; the part kept at the higher temperature becomes green (ibid.). 



