Hoagland — 172 — Plant Nutrition 



wheat plants there may still have been enough potas- 

 sium available to accomplish a function in photo- 

 synthesis, so that for this purpose potassium was not 

 a limiting factor. Perhaps some of the most direct 

 evidence that potassium may have a relatively imme- 

 diate role in photosynthesis comes from experiments by 

 PiRSON (1939) on Chlorella cells. These cells when in 

 a low potassium status, responded quickly in increased 

 photosynthesis to the application of potassium (also 

 of rubidium). The results of other types of experi- 

 ments on carbon assimilation performed on leaves of 

 higher plants in recent years also have been inter- 

 preted as evidence of a direct function of potassium 

 in this process. Even so the entire photosynthetic 

 system is exceedingly complex and involves various 

 types of metabolism. 



In this problem, as in nearly all others of plant 

 nutrition, we do not deal with single limiting factors. 

 One factor interacts with another. The climatic en- 

 vironment influences the requirement of the plant for 

 potassium, or its ability to absorb potassium. I recall 

 an investigation made many years ago in our labora- 

 tory on the effects of potassium concentration in a 

 nutrient solution on the growth of the tomato plant 

 (Johnston and Hoagland, 1929). Some plants were 

 grown in solutions of deficient supplying power for 

 potassium, some in solutions of high potassium supply- 

 ing power. Under each potassium condition, part of 

 the plants were grown in the full light of the green- 

 house and part under the shade of a cheese-cloth cover- 

 ing. In a sense two limiting factors could operate at 

 once. Considering the low potassium plants, plant 

 growth could be increased in the shade by increasing 

 the potassium supply and at the same level of potas- 

 sium, yield could be increased by improving the light 

 condition.* The Rothamsted experiments on barley 



♦Doubtless one effect may have been related to this indirect 

 effect of light on root activities and their potassium absorption. 



