TRACE ELEMENTS IN PLANTS 111 



Reed and Dufrenoy conclude that the vacuolar sap contains 

 both oxidizable phenolic compounds and catechol oxidase cap- 

 able of catalysing the oxidation of these compounds. Normally 

 this oxidation is prevented owing to the presence of hydrogen 

 donators which may include the ascorbic-dehydroascorbic acid 

 system, dihydroxymaleic acid, cysteine and glutathione. During 

 the earlier part of the growing season the relatively high con- 

 centration of hydrogen donators in the cell protects the catechol 

 compounds from oxidation and they remain dispersed through- 

 out the vacuole. With the approach of senescence, or with 

 nutrient deficiency such as a shortage of zinc, the oxidation- 

 reduction equilibrium is disturbed and coacervation results, the 

 process, according to Reed and Dufrenoy, being a 'simple 

 consequence of a gradient in the distribution of cations and 

 correlative distribution of polyphenol oxidase'. They further 

 suppose that a difference in electrical potentials will exist 

 between the aggregations and the surrounding medium, since 

 the former are foci for catechol oxidase, and that there will 

 therefore be a tendency for cations to move into the coacervate, 

 and this in turn will greatly influence the intake of dissolved 

 material by the cell and consequent derangement of meta- 

 bolism. 



A somewhat more precise suggestion of the way in which zinc, 

 through its effect on oxidation-reduction systems, may affect 

 growth, has been put forward by Skoog (1940). Experiments 

 made and described by this worker on tomatoes and sunflower 

 grown in zinc-deficient culture solutions indicate a connexion 

 between zinc and the growth-promoting substance auxin. 

 Terminal buds and stems of such zinc-deficient plants appeared 

 to contain no auxin or only a trace of it. Appreciable amounts 

 were, however, found to be present in the leaves, although less 

 than in leaves of control plants provided with an adequate supply 

 of zinc. The visible symptoms of zinc deficiency only appear 

 after the decrease in auxin content, and if plants in an extreme 

 state of zinc deficiency are supplied with zinc, the auxin content 

 of these plants increases considerably in one to a few days, while 

 growth is resumed after the passage of several more days. These 

 observations suggest that zinc is necessary for the maintenance 

 of a normal auxin content. 



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