FACTORS AFFECTING RATE OF AEROBIC RESPIRATION 523 



in causing reduction in the rate of respiration with rise in temperature is 

 unknown. One of the seemingly more probable explanations is that this 

 effect is due to a progressively more pronounced inactivation of enzymes with 

 increase in temperature. Other possibilities are: (0 oxygen may not gain 

 access to the cells fast enough at higher temperatures to permit maintenance 

 of the respiration rate, (2) carbon dioxide may accumulate in the cells in 

 such concentrations at higher temperatures as to check the rate of respiration, 

 and (3) the supply of oxidizable foods may be inadequate to maintain high 

 rates of respiration. 



As the temperature is decreased below 0° C. the rate of respiration 

 gradually diminishes until it becomes imperceptible. Measurable rates of 

 respiration have been recorded, however, in some plant tissues at temperatures 

 as low as —20° C. 



The temperature coefficient of respiration, within the temperature range 

 0° C. to 30° C, appears to be about 2.0 to 2.5. 



The temperature to which a plant organ is exposed sometimes has im- 

 portant indirect effects on the rate of respiration. When the temperature 

 of a potato tuber is lowered from a few degrees above to about 0° C. the 

 respiration rate increases. According to Hopkins (1924) this is due to the 

 effect of low temperatures in causing a shift in the starch-sugar equilibrium 

 towards the sugar side (Chap. XXII). Increase in the quantity of respiratory 

 substrate in plant cells results in an increase in the rate of respiration when- 

 ever it is the limiting factor, a condition which apparently obtains in potato 

 tubers under these conditions. Similar indirect effects of temperature upon 

 the rate of respiration are probably of frequent occurrence in other plant 

 tissues. 



3. Food. — As a general rule increase in the soluble food content of plant 

 cells results in an increase in the respiration rate up to a certain point at 

 which some other factor becomes limiting. One example of the effect of the 

 concentration of foods in cells upon the rate of respiration has just been 

 described in the previous section. The effect of this factor on respiration 

 rates can also be demonstrated in etiolated leaves. For example, Palladin 

 (1893) found that 100 g. of carbohydrate deficient etiolated bean leaves re- 

 leased an average of 89.6 mg. of carbon dioxide per hour at room temperature. 

 After floating the same leaves upon a sucrose solution for two days, during 

 which considerable absorption of sugar occurred, the average rate of carbon 

 dioxide release was increased to 148.8 mg. per hour. 



4. Oxygen Concentration of the Atmosphere. — The effects of various 

 oxygen concentrations on the rate of respiration in young wheat seedlings is 

 shown in Table 53. 



