DEPARTMENT OF BOTANICAL RESEARCH. 63 



PHOTOSYNTHESIS. RESPIRATION. AND NUTRITION. 



Rate of Respiration of Leaves in relatioji to Amino-Acid and Carbohydrate 

 Content, by H. A. Spoehr and J. M. McGee. 



In the earlier investigations on the rate of respiration of leaves it 

 was found that other factors besides the carbohydrate-content influence 

 the rate of carbon-dioxid emission. Of all the experiments to estab- 

 lish \yhat these other factors or conditions are, the most noteworthy 

 results have been obtained on the basis of analytical and experimental 

 data of the relation of the amino-acid and carbohydrate-content. By 

 means of the methods for sugar determination in very small amounts 

 of leaf material, previously worked out in this laboratory, the Van 

 Slyke apparatus for determination of amino-acids and precise methods 

 for establishing the rate of carbon-dioxid emission, it has been possible 

 to obtain a mass of data which is amenable to rational coordination. 

 For most of the experiments mature cut leaves of Helianthus and 

 Phaseolns were used. The principal conclusions may be summarized 

 briefly as follovrs: 



1. In the normal course of respiration of leaves in the dark at 24° the 

 rate of carbon-dioxid emission decreases at a diminishing rate with 

 time; the carboh3'drate content of the leaves decreases and the amino- 

 acid content increases. This increase of amino-acids in leaves kept in 

 the dark was observed whether the leaves were in nutrient solutions 

 containing nitrates or in distilled water. Under normal conditions the 

 difference in amino-acid content of leaves between early morning and 

 evening is not as great as between leaves taken at any time of the day 

 and then kept in the dark for 40 hours or longer. 



2. \Mien leaves are kept in the dark with the petioles in a solution 

 of d-glucose, the carbohydrate content remains the same or mcreases, 

 depending upon temperature and the concentration of the sugar 

 solution. The amino-acids increase. The rate of carbon-dioxid 

 emission exhibits some remarkable variations. Thus, with a leaf of 

 original high carbohydrate content and kept in a d-glucose solution, 

 the rate of respiiation diminishes during the first 35 hom'S and then 

 gradually rises again to about its original rate. If leaves are kept in 

 the dark for about 35 hours and then given d-glucose, there is an im- 

 mediate rise in the rate of carbon-dioxid emission. This apparent 

 secondary auto-stimulation is an important support of the theory of 

 respiration in leaves which is being formulated on the basis of these 

 experiments. 



3. WTien leaves are kept in the dark ^\'ith the petioles in a solution of 

 an amino-acid, e. g., gl3-cocoll, the rate of respiration decreases grad- 

 uall}^, the carbohydrate content decreases, and the amino-acid content 

 increases. \Mien given amino-acids, the rates of carbohydrate con- 

 sumption and of carbon-dioxid evolution in leaves are considerably 

 higher than without the amino-acids. WTien leaves are given both 



