200 RADIATION BIOLOGY 



As indicated by the chemical equation, one molecule of carbohydrate is 

 formed in the growing plant while one molecule of carbon dioxide dis- 

 appears from the surrounding gas and one molecule of oxygen is formed. 

 However, plant material is not composed entirely of carbohydrates. Fats 

 and proteins are produced as well, and the conditions can be changed 

 so as to vary greatly the ratios of these basic organic materials (Spoehr 

 and Milner, 1949). Only in the production of carbohydrates will the 

 ratio of the molecules of carbon dioxide consumed to those of oxygen 

 evolved, defined as 7, be exactly —1. Sometimes the carbon dioxide 

 may not be carried all the way down to carbohydrates in the reduction 

 by the hydrogen released photochemically from water in the presence of 

 chlorophyll. If acids or other partially reduced products of carbon 

 dioxide are formed, the value of 7 is not unity and the amount of oxygen 

 released is not equal to the carbon dioxide consumed. Methods of 

 measuring the efficiency of photosynthesis which depend on adherence 

 to the equivalence of oxygen and carbon dioxide (i.e., to the relation 

 7 = — 1) may be subject to error. They require constant checking. 



The photosynthetic efficiency depends on many factors. In order to 

 determine the maximum efficiency, it is necessary that light energy be 

 the limiting factor. To be sure that this is the case, the light intensity 

 must be kept low, the carbon dioxide concentration high, the temper- 

 ature high but not too high, and the supply of water, carbon dioxide, 

 chlorophyll, and inorganic nutrients ample. These relations are funda- 

 mental in any study of photosynthesis. They are described in terms of 

 the Blackman curve, according to which, at low fight intensities, the 

 amount of photosynthesis is directly proportional to the intensity of the 

 light, or, in other words, the amount of photosynthesis per unit of fight 

 absorbed is a constant. At high fight intensities a "saturation" is 

 reached such that a further increase in the light intensity does not lead 

 to an increase in the rate of photosynthesis. The thermal and biological 

 reactions that follow the primary photostep cannot keep up with it. 

 Significant measurements of energy efficiencies should be made under 

 conditions below saturation, where the amount of photosynthesis doubles 

 when the intensity of fight is doubled. The complexity of photosynthesis 

 is indicated by the classic research of Emerson and Arnold (1932), in 

 which it was shown that increased fight efficiency can be obtained with 

 flashing fight, the rest period enabling the thermal reactions to accumu- 

 late intermediate products. 



2. PRINCIPLES OF PHOTOCHEMISTRY 



According to the well-established principle of Grotthus, only that fight 

 which is absorbed can be chemically active. That part of a fight beam 

 which passes through a substance unabsorbed is without chemical effect. 



