1040 BIOLOGICAL EFFECTS OF RADIATION 



WAVE-LENGTH LIMITS OF PHOTOSYNTHESIS 



The wave-length Umits for photosynthesis have not been definitely 

 established. In fact, it is very probable that they will differ for different 

 species of plants. Burns (12a) found the limits in Pinus Strobus and 



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Picea excelsa were from about 7400 A to 4500 A. The long-wave limit 

 agrees fairly well with that found by other investigators. The short- 

 wave limit seems to be too far in the visible. It is probable that the 

 light source used by Burns was too weak in violet rays to give adequate 

 illumination. Ursprung (123) found that the limits for starch formation 



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under ordinary illumination lie between X7600 and 3300 A. In a 40-hr. 

 exposure, however, through an ebonite plate which removed all visible 

 radiation a small amount of starch was formed. Meier (78) irradiated 

 plate cultures of the alga Chlorella vulgaris with ultra-violet lines from the 



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mercury arc. All wave-lengths below 3022 A were injurious. 



THE MECHANISM OF THE PHOTOSYNTHETIC REACTION 



It was the great contribution of F. F. Blackman (5) to describe the 

 relation of the photosynthetic reaction to the various environmental 

 factors which affect it. Starting from this point Willstatter and Stoll 

 (143) defined the quantitative relationships between the rates of photo- 

 synthesis and certain internal factors. They were concerned primarily 

 with the function of chlorophyll. But through their quantitative studies 

 they also brought to light the fact that another internal factor, associated 

 with the colorless portion of the cell protoplasm was operative, a concept 

 which in a qualitative way had been expressed by a number of earlier 

 investigators. Warburg (133) then demonstrated that, with high light 

 intensity and high CO2 concentrations, the temperature coefficient of 

 photosynthesis varies decidedly with the temperature. Under these 

 conditions the temperature coefficient decreases with increasing temper- 

 atures: between 5° and 32° the temperature coefficient decreases from 

 4.3 to 1.6. Thus, with high light intensity a rise of 10°, from 15° to 25°, 

 cau.ses approximately a doubling in the rate of photosynthesis. Here the 

 rate of the total reaction is evidently determined by a chemical dark 

 reaction. At low light intensity the rate of photosynthesis is independent 

 of the temperature between 15° and 25°. Under these conditions a 

 photochemical reaction is determining the rate of the total reaction. 



These facts indicate that at least two fundamentally different reac- 

 tions are involved in the photosynthetic process. The dark reaction 

 (Warburg, 138, 150) has been designated as the "Blackman reaction." 

 This reaction is sensitive to changes of temperature and is also decidedly 

 affected by low concentrations of hydrocyanic acid. Its characteristics 

 become evident when rates of photosynthesis are measured at high light 

 intensities. Emerson (23) has made careful measurements of these 



