FACTORS AFFECTING RATE OF AEROBIC RESPIRATION 521 



of external origin. Increase in temperature causes an increase in the respira- 

 tory quotient of apple seeds (Harrington, 1923). Internal conditions are 

 often such that the absorption of oxygen and liberation of carbon dioxide 

 are not equally affected by changes in temperature, hence the apparent 

 respiratory ratio may shift with a change in temperature. The magnitude 

 of the respiratory ratio is also sometimes influenced by other environmental 

 factors such as the oxygen or carbon dioxide concentration of the atmosphere 

 (Table 54). 



The principal justification for study of the respiratory quotients of plants 

 is that the results of such investigations afford important clues regarding the 

 nature of the respiratory process itself. Certain conclusions regarding the 

 type of substrate being oxidized, transformations in the foods present in cells, 

 and even the chemical mechanism of respiration can often be drawn from 

 the results of determinations of respiratory ratios. Some caution is always 

 necessary, however, in interpreting the results of such experiments. It is 

 probable that a number of different processes are often occurring simultaneously 

 in plant cells, each of which will influence the magnitude of the respiratory 

 ratio. Several types of substrates may be undergoing oxidation concurrently, 

 or other oxygen consuming or releasing processes may be in progress in addi- 

 tion to the direct oxidation of foods. If one of these several reactions is 

 strongly predominant, it will in the main determine the value of the respiratory 

 ratio, and the quotient will be a valid indicator of the nature of the respiratory 

 process. If, on the contrary, several different processes are proceeding at ap- 

 proximately equal rates, their net influence on the gaseous exchanges of the 

 tissue may be such that the apparent respiratory ratio will bear no relation 

 whatever to any one of the processes, and may even lead to entirely erroneous 

 inferences. 



Factors Affecting the Rate of Aerobic Respiration. — A number of fac- 

 tors, some internal, others external, are definitely known to influence the rate 

 of respiration of plant cells. These will be discussed under appropriate 

 headings. 



I. Protoplasmic Conditions. — Young, meristematic tissues, which are rela- 

 tively rich in protoplasm, usually have higher rates of respiration than older 

 tissues in which the proportion of cell walls is greater. Not only is the gross 

 amount of protoplasm present a factor, but various internal conditions in that 

 protoplasm also influence the respiration rate. In view of our incomplete 

 knowledge of the physico-chemical structure and dynamics of the protoplasm 

 the influence of only a few of these protoplasmic factors can be recognized 

 at the present time. One of these is the degree of hydration of the protoplasm. 

 The effect of this factor will be discussed later under a separate heading. 



