26 BULLETIN 79, TJ, S. DEPARTMENT OF AGRICULTURE. 



decrease in the stems, no doubt due to loss of pentosans or related 

 hemicelluloses rather than to any change in true cellulose. 



CHANGES IN THE NONVOLATILE ORGANIC ACIDS. 



Tobacco leaves contain considerable quantities of oxalic, malic, 

 and citric acids, and the changes which these undergo during the 

 curing are interesting from the physiological point of view. That 

 these acids are of importance in plant metabolism is generally recog- 

 nized, but their role has not been fully determined. Because of their 

 close relationship to the amido acids, such as aspartic acid and 

 asparagin, it seems reasonable to associate them with protein syn- 

 thesis. It is generally believed, however, that oxalic, citric, and malic 

 acids have their origin in the incomplete oxidation of sugars when 

 respiration proceeds in the presence of a limited oxygen supply. It 

 appears that under certain conditions these acids may be converted 

 partially into sugars, on the one hand, and partially into carbon 

 dioxid and water, on the other hand. So far as we know, no dis- 

 tinction has been made heretofore as to the metabolic transforma- 

 tions which these three different acids undergo in the plant, but our 

 results with green and cured tobacco leaves show that they behave 

 quite differently during the curing process, which involves essen- 

 tially the phenomena of respiration and of translocation. 



It is evident that the content of oxalic acid is not changed during 

 the curing, and this is equally true in both methods of curing. There 

 is a decided loss of malic acid during the curing when the leaf is primed, 

 as well as when it is cured on the stalk. Citric acid, on the other hand, 

 undergoes no decrease during the curing by either method, and, in- 

 deed, there is a considerable increase in content of the acid, which is 

 surprising in view of the behavior of the malic acid. It is, of course, 

 conceivable that the malic acid is partially transformed into citric 

 acid, but a simpler view is that the citric acid resists oxidation to 

 lower products more than the malic acid, so that during the process 

 of partial oxidation of the sugars to acids there is a gradual accumu- 

 lation of citric acid, while the malic acid is more completely oxidized 

 to water and carbon dioxid. 



CHANGES IN THE NITROGENOUS CONSTITUENTS. 



Referring to Tables IV and V, it will be seen that the loss in protein 

 in curing is very large in all cases, in some instances exceeding 60 per 

 cent of the total. The marked decrease in protein content, like 

 the disappearance of starch, is a characteristic change involved in 

 curing, and the relative decrease is an index of the completeness of the 

 curing. 



The decrease in starch content and in protein content furnishes a 

 simple and reliable means of determining by chemical analysis the 

 progress and the completeness of barn curing. 



