ASSIMILATION OF NITROGEN BY PLANTS 227 



By means of condensation reactions, amino acids further give 

 rise to polypeptides and finally to proteins. 



All this scheme is to a considerable degree of a hypothetical 

 character. But it has been established that during the daytime, 

 leaves accumulate not only the direct products of photosynthesis, 

 the carbohydrates, but likewise protein substances (Sapozh- 

 nikov, 1890), which therefore may be considered the result of 

 photosynthetic activity. Frequently, no less than 25 per cent of 

 the total carbon assimilated is combined in the form of protein 

 substances. The accumulation of asparagine in leaves has also 

 been quite clearly demonstrated. However, the leaves of plants 

 with a very acid cell sap (sorrel, rhubarb, etc.) accumulate, 

 instead of asparagine, the ammonium salts of organic acids, which 

 are just as harmless to cells of these plants as asparagine (Ruhland 

 and Wentzel, 1929). During the night, a translocation of 

 nitrogenous substances from the leaves, chiefly in the form 

 of amino acids, may be observed, similar to the translocation of 

 carbohydrates. Thus, Hke starch, the primary protein sub- 

 stances represent in the leaf temporary compounds that are 

 transformed and move to the points of utilization. 



The decomposition of the protein substances formed in the 

 leaves is brought about by means of proteolytic enzymes, belong- 

 ing to the group of papainases (see Art. 14). According to 

 Mothes (1933), this decomposition depends to a great extent 

 on the oxidation-reduction potential. This is explained by the 

 fact that enzymes of the type of papainases act upon proteins 

 only in case they are activated by means of compounds contain- 

 ing the sulphydryl group (SH) , such as cysteine or glutathione. In 

 the presence of an excess of oxygen in general with a high oxida- 

 tion-reduction potential, this group is oxidized, the enzymes lose 

 their activator, and the decomposition of protein substances is 

 inhibited. Conversely, with deficiency of oxygen, the enzymes 

 are activated, and the processes of decomposition are accelerated. 

 This is probably the reason why intensive decomposition of 

 proteins occurs at night when stomata are closed, and photo- 

 synthesis and the liberation of oxygen, which increases the 

 oxidation-reduction potential, are absent. In the daytime, on 

 the contrary, synthetic processes prevail. 



Besides the primary synthesis of protein substances in plants 

 from niorganic compounds of nitrogen and nitrogen-free organic 



