Lecture 2 — 37 — Micronutrient Elements 



terested in having me trace definitely the function of 

 zinc in living organisms. Unfortunately that cannot 

 as yet be done, but I should like to report a few 

 physiological and biochemical experiments that pos- 

 sibly have some value as starting points for further 

 research. The difficulty confronting us is to fix the 

 role of zinc in any of the chains of processes that 

 result in plant growth as an integrated effect. A 

 missing link anywhere in any one of the chains will 

 bring growth failure or deranged metabolism. That 

 difficulty of course is not peculiar to the study of zinc. 



The quantitative requirement of a plant for zinc 

 is not simply determined. It is in part governed by 

 climatic factors. We have made experiments in a 

 greenhouse at various seasons of the year with corn 

 plants growing in solutions containing graduated 

 amounts of zinc. In the winter some retardation of 

 growth, but only slight leaf symptoms of zinc de- 

 ficiency appeared, even on plants in the highly purified 

 solutions containing only extremely small amounts of 

 zinc. In similar solutions the plants would die in 

 summer during periods of high illumination (STOUT 

 and HOAGLAND, unpublished). These seasonal effects 

 were manifested also in experiments with a zinc de- 

 ficient soil. The symptoms of zinc deficiency shown 

 by plants grown in summer could be decreased in 

 severity by shading the plants with cheese cloth. 

 Likewise under conditions in the field climatic effects 

 seem to be important. Generally the zinc deficiency 

 diseases are found in regions of high summer light 

 and temperatures ; in California, not often in the foggy 

 coastal belt. With citrus the observation has been 

 made by several investigators that the sunny side of 

 the tree is usually much more mottled than the shady 

 side, if zinc deficiency prevails in the soil. (See plate 

 Ha and h). 



Some further information on this question was 

 secured incidental to a study of the distribution of 

 zinc in different parts of the plant by P. R. Stout. 



