638 PLANT GROWTH AND PLANT COMMUNITIES 



marked effects on host growth than did nitrogen, and host growth dif- 

 ferences at the higher phosphorus levels were not great. 



Potassium effects on host growth are much less marked than the 

 effects of nitrogen and phosphorus. Correspondingly, it has less effect 

 on virus synthesis. Increases in plant growth, however, are generally 

 accompanied by increases in virus concentration and vice versa ( Cheo 

 et al, 1952; Pound and Weathers, 1953b; Weathers and Pound, 1954 ) . 

 It is thus indicated that potassium is not directly involved in virus 

 synthesis. 



Varying the osmotic concentration of balanced nutrient solutions 

 also results in varying growth patterns. Such variations in the solution 

 concentration also result in variations in virus concentration, and in all 

 cases a positive correlation in host growth and virus concentration 

 occurs ( Cheo et al., 1952; Pound and Weathers, 1953b; \\'eathers and 

 Pound, 1954). 



The minor element nutrition of tobacco in relation to the synthesis 

 of tobacco-mosaic virus has been studied in some detail. In the case of 

 zinc, the virus concentration in growing tobacco plants or excised leaf 

 discs increases with increased zinc up to a level toxic for growth, and 

 thereafter the virus concentration decreases (Helms and Pound, 1955b). 

 As the virus multiplies in zinc-deficient plants, symptoms of zinc de- 

 ficiency are markedly and rapidly increased. No direct relationship be- 

 tween virus multiplication and host utilization of zinc has been demon- 

 strated, and it is probable that the primary effect of zinc on virus syn- 

 thesis has to do with the host's growth. How zinc indirectly affects 

 virus synthesis can only be surmised. It is known that zinc deficiency in 

 some plants results in auxin deficiency ( Skoog, 1940 ) . It has also been 

 suggested ( see page 633 ) that virus synthesis in plants results in a re- 

 duction of auxin in the host, and it may be that virus synthesis is asso- 

 ciated with utilization or destruction of auxin. However, Helms and 

 Pound showed that the virus content of zinc-deficient plants and of 

 plants growing at optimal levels of zinc was not only not increased but 

 was actually decreased when indole-3-acetic acid was supplied through 

 the roots to plants growing in nutrient solutions. Furthermore, the ad- 

 dition of zinc to deficient plants did not immediately result in increased 

 virus concentration, even though it probably increased the auxin con- 

 tent of the plants. It is known that zinc deficiency produces marked 

 changes in the concentration of certain plant enzymes and affects min- 

 eral uptake in plants ( Brown and Steinberg, 1953; Nason ct al., 1952 ) . 

 Thus it is also plausible that the zinc effects on virus synthesis are due 

 to association with enzymes involved in protein metabolism. 



Manganese is very important in plant nutrition, being essential for 

 a number of metabolic processes that affect growth. It functions as a 

 co-factor in enzymatic reactions, especially in the activation of the 



