BIOCHEMISTRY OF PLANTS 



succinic acid has been established as a component of the tissues of 

 several plant species whose content of organic acids has been examined. 

 Frequently all the organic acid content of the plant is not accounted 

 for, and unknown organic acids require identification and quantitative 

 estimation. But it seems that all the organic acids postulated as com- 

 ponents of organic acid cycles may be present in plant tissues. 



A general theory of protein metabolism based on experiments 

 with seedlings and detached leaves has been evolved which assigns 

 significant roles to the amides, asparagine and glutamine, and to 

 various keto acids. As already noted, the mechanisms postulated draw 

 heavily on the explanations advanced to account for transformations 

 of nitrogen compounds in animal tissues; but caution is needed in 

 applying mechanisms based on the study of animal tissues to plant 

 processes, without adequate confirmatory evidence. Efforts have been 

 made, however, to integrate available data on changes in the organic 

 composition of excised plant leaves under experimental conditions, as 

 well as data on the changes that occur in the intact growing plant, into 

 schemes of protein synthesis and breakdown correlated with catalytic 

 cycles. The possibilities of applying to this field of study in the plant 

 the new tool of iso topic nitrogen have been opened by Vickery and his 

 collaborators in a preliminary experiment with the tobacco plant, 

 following the well-knowm research of the Schoenheimer group on animal 

 metabolism. 



Another aspect of the problem ol nitrogen metabolism is con- 

 cerned with the symbiotic fixation of nitrogen by leguminous plants. 

 Much more will have to be learned about protein metabolism before 

 the biochemical reactions of the nodule organism can be properly 

 understood. Some progress has been made — compare the review by 

 Wilson (19) — but the theories and evaluation of evidence now 

 available are apparently subject to controversy. The extraordinary 

 practical importance of nitrogen fixation and its scientific interest in- 

 vites further efforts in research by biochemists. 



The brilliant study of oxidation systems in living organisms rests 

 primarily on the experiments and insight of those who have been 

 concerned with muscle and othci animal tissues, or with yeast. No 

 comparable achievements by investigators of highrr pl.-mis come to 

 mind. Some investigators of higher plants, howexcr, h;i\e sought to 

 apply fundamental knowledge gained by studies on other organisms 



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