THE PHOTOSYNTHETIC-RESPIRATORY RATIO 527 



effect of such substances we will consider the influence of chloroform on the 

 rate of respiration in the leaves of the cherry laurel {Prunus laurocerasus) 

 as shown by the work of Irving (igii). Small doses caused an increase in 

 the rate of respiration which persisted as long as the leaf was maintained in 

 contact with the chloroform. Somewhat larger doses resulted in a temporary 

 increase in respiration rate, which was followed by a decrease to much below 

 the initial rate. The greater the concentration of the chloroform, the more 

 rapid this decrease. Strong doses of this reagent resulted in a rapid decrease 

 in the rate of respiration without the occurrence of any preliminary increase. 

 In general the effect of other compounds of this type upon respiration is very 

 similar to the effect of chloroform. It is noteworthy that the rate of photo- 

 synthesis is markedly reduced by concentrations of chloroform so minute that 

 they have no detectable effect upon the rate of respiration. 



The Photosynthetic-Respiratory Ratio. — No plant can survive indefi- 

 nitely under conditions which permit maintenance of a rate of respiration in 

 excess of the rate of photosynthesis. Although plants subjected to such 

 conditions can "coast" for a while at the expense of previously stored foods, 

 eventually they will starve to death. In other words, if a plant is to survive 

 indefinitely its photosynthetic-respiratory ratio (P/R ratio) must exceed unity. 

 The magnitude of the P/R ratio in any plant will be affected by any factor 

 which influences either or both of the processes of photosynthesis and respira- 

 tion. Of the factors which affect this ratio, temperature is one of the more 

 important and the following discussion will be restricted to a consideration 

 of its effects. 



In many and perhaps all species of plants the optimum temperature for 

 respiration is higher than the optimum for photosynthesis. For example, in 

 the potato plant the optimum temperature for photosynthesis, under conditions 

 of full daylight and normal atmospheric concentration of carbon dioxide, is 

 about 20° C. The optimum temperature for respiration of the leaves of the 

 potato plant, on the other hand, is apparently about 35° C. (Lundegardh, 

 1930- With increase in temperature above 20° C. there is little further 

 rise in the rate of photosynthesis in this species or there may be an actual de- 

 crease. The rate of respiration, on the contrary, continues to rise with 

 increase in temperature up to about 35° C. Hence the higher the temperature 

 above the photosynthetic optimum the greater the proportion of the photos^'n- 

 thate which will be consumed in respiration and the smaller the proportion 

 which can be utilized in assimilation or which can be accumulated. When 

 tuberization occurs foods are utilized in the construction of the cellular frame- 

 work of the tubers and accumulate within the cells. If the excess of food re- 

 maining after consumption of that portion of the photosynthate utilized in 



