88 Journal of Agricultural Research volvi.no. 2 



per cent in those of young grapefruit. It will be recalled that bacterial 

 growth occurs on artificial media rendered acid by hydrochloric or citric 

 acid when a sufficient amount of acid has been used to make the acidity 

 of the media 2 per cent. The acidity of the leaf tissue is therefore not 

 sufficient to inhibit the growth of the canker organism and is not regarded 

 as sufficient to account for the difference in susceptibility. No deter- 

 minations have been made of the kinds and relative amounts of the 

 several organic acids in the tissues of the two species. Until this is 

 known there still remains the possibility of a correlation between sus- 

 ceptibility to canker and acidity. 



CHEMICAL CHANGES IN CITRUS LEAVES BROUGHT ABOUT BY 

 CITRUS CANKER 



Little attention has been given by the biochemist to the chemical 

 transformations occurring in diseased plant tissues. Such studies would 

 no doubt throw a flood of light upon the intimate relationship of parasite 

 and host and would materially contribute to our knowledge of the nature 

 of parasitism. The literature dealing with the chemical changes induced 

 by plant pathogens is more or less fragmentary, mainly because of the 

 inexact state of our knowledge regarding the separation and quantitative 

 estimation of the various compounds occurring in plant tissues. An 

 historical resume of this literature has therefore been purposely omitted. 

 However, among the recent excellent papers along this line may be men- 

 tioned the work of Hawkins (7) upon the changes in peaches induced by 

 the brown-rot organism, Scleroiinia cinerea. He found in brown-rotted 

 tissues an increase in acid content, a decrease in certain alcohol-soluble 

 substances, a decrease in the total sugar content, and practically a dis- 

 appearance of the cane sugar. It was with the view of determining 

 something of the changes produced by Citrus canker that this portion of 

 the investigation was undertaken. 



Diseased and healthy leaves were taken from grapefruit trees affected 

 with Citrus canker. Circles of diseased tissue and tissue from healthy 

 leaves were excised with a cork borer. These leaf circles were then 

 triturated in a mortar until the material was finely divided, their wet 

 weight determined, 27.25 gm. in each case, and preserved in such volume 

 of 95 per cent alcohol that the alcohol concentration of the mixture was 

 85 per cent. This concentration could not be accurately made until it 

 had been determined that the moisture content of normal leaves was 

 61.69 per cent and that of diseased leaves 61.57 P^^" cent. The material 

 was then set aside for two weeks and was shaken occasionally to permit 

 the gradual extraction of the cold alcohol-soluble portions. The method 

 followed subsequently was based upon those devised by Koch (9, 10, 

 II, 12) for use in the quantitative chemical analysis of animal tissues. 



