FACTORS AFFECTING RATE OF AEROBIC RESPIRATION 525 





carbon dioxide release or oxygen absorption. The decreasing effect on the rate 

 of carbon dioxide release was more marked than on the rate of absorption 

 of oxygen. Hence the higher the carbon dioxide concentration of the atmos- 

 phere, the lower the respiratory ratio. 



The rate of respiration of some plant tissues, on the contrary, is increased 

 when they are exposed to relatively high concentrations of carbon dioxide. 

 For example, Thornton (1933) studied the rate of respiration of potato 

 tubers at 25° C. in various concentra- 

 tions of carbon dioxide. The initial 

 concentration of oxygen in the atmos- 

 phere was 20 per cent in all determina- 

 tions. Exposure of the tubers to con- 

 centrations of carbon dioxide in excess 

 of about 20 per cent for periods of 

 greater than 20-24 hours resulted in a °^ 

 marked increase in respiration rate as 2 

 measured in terms of oxygen consump- i 

 tion. In 60 per cent carbon dioxide the ^ 



a 



rate of respiration sometimes exceeded ^ 



UJ2 



that of the controls by niore than 200 s 

 per cent. Shorter periods of exposure § 

 resulted in a decreasing rather than in- ' 

 creasing effect upon respiration. High 

 concentrations of carbon dioxide simi- 

 larly resulted in an increase in the rate 

 of respiration of a number of other stor- 

 age tissues. With asparagus shoots and 



shelled lima beans, on the other hand, high concentrations of carbon dioxide 

 resulted in a reduction in respiration rate as found by Kidd for germinatmg 

 mustard seeds. Evidently the effect of carbon dioxide upon the rate of 

 respiration is influenced not only by its concentration, but also depends upon 

 the kind of tissue and the period of exposure. 



6. Hydration of the Tissues. — As is true of a number of other processes 

 the effect of the hydration of the tissues upon the rate of respiration can best 

 be observed in germinating seeds. The influence of the moisture content of 

 wheat seeds upon their rate of respiration is indicated graphically in Fig. 115, 

 which is self-explanatory. 



Similar results have been obtained for tissues of some of the more extreme 

 xerophytes, which can be desiccated to an air dr^' condition without destroying 

 their viabilitv. As the water content of such tissues is increased, often no 



14 IS le 



PERCENTAGE OF MOISTURE 



Fig. 115. Relation between water 

 content of wheat seeds and rate of respi- 

 ration. Data of Bailey and Gurjar 



(1918)- 



