GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 639 



enzyme systems of the citric-acid cycle, and it can activate most of the 

 enzymes of the glycolytic cycle (McElroy and Nason, 1954). Photo- 

 synthesis is believed to be inhibited in the absence of manganese. One 

 would expect, therefore, to find positive correlations between man- 

 ganese effects on host growth and virus synthesis. However, this is not 

 the case with tobacco-mosaic virus in tobacco. \\'elkie and Pound 

 ( 1958 ) showed that, regardless of the assay method used, virus concen- 

 tration was greatest in manganese-deficient plants. This suggests either 

 that manganese triggers some mechanism inhibitory to virus synthesis 

 or that virus synthesis occurs independently of physiological processes 

 which are related to growth and controlled by manganese. If virus syn- 

 thesis is independent of some of these manganese-controlled metabolic 

 reactions of the host, multiplication might continue when host growth 

 would be suppressed by the absence of manganese. Further studies in 

 this area are needed to determine whether manganese affects the syn- 

 thesis of other viruses as it does tobacco mosaic virus. 



Iron is another element of major importance in host growth. Its 

 total role in host metabolism is not certain, but the major metabolic 

 processes are influenced by its presence or absence. Both respiration 

 and photosynthesis have been correlated with the degree of chlorosis 

 or amount of chlorophyll present in plants supplied with low levels of 

 iron. Perhaps the most important function of iron is its catalytic action 

 when it occurs as an iron-porphyrin enzyme. These are constituents of 

 cytochromes which function as an electron-transport system of the cell. 

 Moderate iron deficiency of tobacco does not influence the synthesis of 

 tobacco-mosaic virus in eiiher growing plants or floating leaf discs 

 ( Pound and \\'elkie, 1958 ) . E.xtreme iron deficiency does, however, re- 

 duce virus concentration. Only at a critically low level does iron defi- 

 ciency exert a greater limitation on virus multiplication than it does on 

 host growth. Iron has been shown (Loring and Waritz, 1957) to be 

 tightly bound to highly purified preparations of tobacco-mosaic virus. 

 Whether it is an integral part of the virus protein is not certain, but in 

 any event, the amount is so small that it does not seem likely that the 

 level of deficiency studied would limit the availability of iron for 

 direct incorporation into virus. 



The effect of boron on virus synthesis is also apparently an in- 

 direct effect ( Shepherd and Pound, 1960a ) , but virus concentrations do 

 not directly parallel host growth. In boron-deficient tobacco plants the 

 concentration of tobacco-mosaic virus in expressed saps or from dried 

 tissue is reduced during the first 7 to 14 days following inoculation, but 

 in later assays the concentration equals that in normal plants. In inocu- 

 lated leaves of deficient plants the virus increases at a rate equal to that 

 of normal plants and after a few days even exceeds the concentration 

 of the latter. How does boron affect virus synthesis? It has been shown 



