Hoagland — 38 — Plant Nutrition 



Tomato plants were set out in a nutrient solution very 

 low in zinc and at the season when the experiments 

 were made the plants were subjected to good natural 

 illumination. Marked failure of the plants to con- 

 tinue growth was observed. The plants were then 

 brought to the laboratory with its less intense light. 

 Without any further addition of zinc the plants re- 

 sumed growth. 



It happened that this was an experiment in which 

 radioactive zinc had been supplied to the plants in 

 extremely small amounts. It was therefore possible 

 to show that the zinc originally present in the stems 

 was translocated to growing points under the low light 

 conditions of the laboratory. The explanation of this 

 response may be that a breakdown of zinc protein 

 compound in one part of the plant occurs under re- 

 duced light, with a release of zinc and its transporta- 

 tion to a region capable of more active growth. 



I wish to turn now to another series of experi- 

 ments in which a light factor associated with zinc 

 nutrition appears to be involved. Sometime ago we 

 conceived the idea, based on various observations un- 

 necessary to describe now, that there might exist 

 some sort of interrelation between zinc and an auxin 

 growth substance. At that time Dr. Skoog, recently 

 a member of your staff, came to Berkeley and thought 

 it worthwhile to pursue the suggestion further. I 

 shall have time to indicate only the general nature of 

 some of his results (Skoog, 1940). When tomato 

 plants were grown in zinc deficient solutions in full 

 greenhouse light the plants failed to elongate and 

 their auxin content was extremely low. Addition of 

 a small amount of zinc to the culture solution soon 

 caused a large increase of auxin in the plant and 

 subsequently elongation took place. When the plants 

 were placed under a red light filter of cellophane the 

 same zinc deficient solutions permitted considerable 

 elongation of the stem and its auxin content was 

 higher than that of the zinc deficient plants subjected 



