INORGANIC ELEMENTS AND IONS 335 



sugar solutions: photosj^thesis was found to decrease by 30% when the 

 concentration of sucrose reached 0.35 m./l.; from there on, plasmolysis 

 set in and the rate of photosynthesis decreased rapidly until a complete 

 inhibition was reached in an 0.5 molar sucrose solution. Returned into 

 pure water, the plants regained their capacity for photosynthesis only 

 very slowly, even though no permanent effects of plasmolysis were 

 visible. Greenfield's curve (Fig. 34, Curve 4) also shows the beginning 

 of inhibition at 0.35 m./l. of sucrose, but a complete inhibition only at 

 0.9-1.0 m./l. Walter suggested that the osmotic inhibition of photo- 

 synthesis is the consequence of the shrinking of the protoplasm. Photo- 

 synthesis seems to be more sensitive to changes in the colloidal state of 

 the protoplasm than are all other metabolic processes (e. g., respiration). 



ChrelashviU (1940) found that the removal of a certain proportion of 

 water by osmotic methods may have an effect on photosynthesis which 

 differs not only in magnitude, but sometimes even in sign, from that 

 caused by an equivalent direct desiccation. 



Greenfield (1941, 1942) noticed that osmotic effects can be observed 

 only in strong light. This indicates that desiccation affects the efficiency 

 of enzymatic reactions, and not the primary photochemical process. 

 Danilov (1935, 1936, 1937, 1940) asserted that dehydration effects are 

 different in light of different colors. 



The effect of heavy water was discussed in chapter 11. We may 

 repeat here that the carbon dioxide assimilitation in pure heavy water is 

 (in strong light) from 2-2.5 times slower than in ordinary water. This 

 can be attributed to the slower rate with which deuterium oxide is 

 transformed by photosynthesis, and is thus not an inhibition effect in the 

 proper sense of this word. In mixtures of heavy and ordinary water, 

 the rate of oxidation of H2O is not affected by the presence of D2O. 



E. Inorganic Elements and Ions * 



This is an extensive field which includes problems of plant nutrition 

 and fertilization which cannot be discussed here. We are interested 

 primarily in the direct effects of certain inorganic ions on the rate of 

 photosynthesis. However, we must also mention some indirect effects, 

 caused by ions whose deficiency affects the formation of chlorophyll (or 

 other components of the photosynthetic mechanism). In contrast to di- 

 rect (positive or negative) "catalytic" effects, indirect ''deficiency effects" 

 cannot be reheved instantaneously by a supply of the deficient element. 



In addition to distinguishing between direct and indirect effects, we 

 may classify the ions according to the order of magnitude of concentrations 

 which are required to produce a marked change in photosynthesis. 

 Ions which inhibit photosynthesis in concentrations of 0.01 m./l. or less 



* Bibliography, page 349. 



