640 PLANT GROWTH AND PLANT COMMUNITIES 



that boron functions in sugar translocation ( Sisler et al., 1956 ) . Sugar- 

 borate complexes are believed to pass through cell membranes more 

 readily than non-borated sugars. It has also been suggested that boron 

 may be a constituent of cell membranes, where temporary union is 

 formed with sugar molecules, thereby enhancing passage through 

 membranes. As Shepherd and Pound have pointed out, the initial lag 

 in virus concentration in deficient plants and the increased concentra- 

 tion in the inoculated leaves of deficient plants may well be explained 

 on the basis of reduced virus movement and spread in the plants. If 

 tobacco-mosaic virus is spread in the plant in conjunction with sugars, 

 then translocation would be inhibited in deficient plants. It is well 

 known that spread of some plant viruses in the host is related to the 

 translocation of the products of photosynthesis. 



Magnesium has only minor eflFects on tobacco-mosaic virus syn- 

 thesis in tobacco (Shepherd and Pound, 1960b). Differences in virus 

 concentration of deficient and non-deficient plants are small, but virus 

 yields are consistently less from severely deficient plants. Magnesium 

 is a constituent of chlorophyll and also activates a number of enzymes, 

 several of which are active in protein and carbohydrate metabolism 

 ( McElroy and Nason, 1954 ) . Undoubtedly, one of the major functions 

 of magnesium in enzyme systems is in the adenosine triphosphate- 

 catalyzed reactions that control many biosynthetic processes. It is 

 surprising that the effect of magnesium on virus synthesis is as small as 

 it is, since the energy-providing mechanisms of the plant upon which 

 host growth and virus synthesis depend are undoubtedly impaired by 

 magnesium deficiency. Since host growth is affected by deficiency to a 

 greater degree than virus synthesis is, the virus-synthesis mechanism 

 must compete successfully for the components and energy necessary 

 for synthesis. 



Temperature. Two types of temperature effects on virus synthesis 

 occur in systemically-infected plants. In one, represented by turnip- 

 mosaic virus, the virus concentration in plants over a range of tempera- 

 tures shows a gradient pattern, and the concentration at one tempera- 

 ture remains relatively constant in relation to other temperatures. For 

 example, the cabbage A and cabbage black-ring strains of turnip-mosaic 

 virus always are in greater concentration in systemically-infected cab- 

 bage plants growing at 28° C. than in cabbage plants growing at 16° C. 

 Concentrations progressively decrease from 28° to 16° (Pound, 1952). 

 These reactions are reversible, in that if infected plants growing at 28°, 

 24°, 20°, 16° are transferred to 16°, 20°, 24°, 28°, respectively, a cor- 

 responding reversal in the virus concentration gradient follows. In 

 horseradish (also in the cruciferae), however, all strains of this virus 

 tested show a virus-concentration pattern in response to temperature 

 the reverse of that in cabbage (Pound, 1949). If the same viruses are 



