520 RESPIRATION 



soon as the seed coat is ruptured, thus permitting free access of oxygen to 

 the developing embryo, anaerobic respiration ceases. 



4. Occurrence of Other Processes Resulting in the Release or Consump- 

 tion of Oxygen. — The respiratory ratio is measured in terms of the rate of 

 absorption of Oo and the rate of evolution of COo. Respiration is not, how^- 

 ever, the only process occurring in plant cells which results in the utilization 

 or liberation of oxygen. The simultaneous occurrence in plant cells of other 

 oxygen releasing or consuming processes and respiration will influence the 

 apparent respiratory ratio, and there is no critical method of distinguishing 

 between such apparent ratios and the true ratio. 



For example, as seeds which store fat mature, simple carbohydrates are 

 converted into fats. This process involves the elimination of oxygen, since 

 the molecules of fats contain much less oxygen in proportion to the carbon 

 and hydrogen present than do the molecules of sugars. Such a release of 

 oxygen during fat synthesis furnishes an internal supply of oxygen which 

 can be used in the process of respiration. Hence the volume of oxygen 

 absorbed by the seeds from the exterior atmosphere during this period will 

 be less than the volume of carbon dioxide released, and the apparent respiratory 

 ratio would theoretically be greater than unity. This expectation has been 

 confirmed for the seeds of a number of species which store fats. For example, 

 it has been found that the apparent respiratory ratio of maturing flax seeds 

 is about 1.22. 



Essentially the opposite situation prevails during the geiTnination of fatty 

 seeds in which very low respiratory quotients sometimes prevail after several 

 days. Murlin (1934), for example, found values for the respiratory ratio 

 of germinating castor bean seeds as low as about 0.3 and explained this find- 

 ing on the assumption that transformation of fat to sugar — an oxygen-con- 

 suming process — is proceeding much more rapidly than oxidation of either fat 

 or sugar. 



In species of the Crassulaceae some of the sugar present is often incom- 

 pletely oxidized to malic acid : 



2 CsHi.Og + 3 O2 ^ 3 C4H6O5 + 3 H2O + 386 kg.-cal. 



Other organic acids are formed in succulent species as a result of similar 

 incomplete oxidations. The synthesis of such compounds requires absorption 

 of oxygen for which there is no corresponding evolution of carbon dioxide. 

 Such metabolic conditions will obviously result in an apparent respiratory 

 ratio of less than one. 



In addition to the various internal conditions which influence the value 

 of the respiratory quotient, its magnitude may also vary with certain factors 



