636 PLANT GROWTH AND PLANT COMMUNITIES 



Other factor is limiting. Certain complicating situations may make this 

 untrue. For example, under xery low light intensities, stomata may be 

 closed and the CO2 uptake reduced to the point of retarding photosyn- 

 thesis. Also, under very high light intensities, transpiration may be in- 

 creased to a point where photosynthesis will be reduced because of 

 limited water content of the cells (Meyer and Anderson, 1952). Sub- 

 optimal light intensity will reduce the photosynthetic rate by inhibit- 

 ing the photochemical reaction of photosynthesis. Since the tempera- 

 ture coefficient of the photochemical reaction in photosynthesis is ap- 

 proximately 1, temperature would not aflFect photosynthesis under low 

 light intensities. Under high light intensities, however, and in a suitable 

 atmosphere of CO2, temperature does markedly aflFect the photosyn- 

 thetic rate by aflFecting the enzymatic reaction of this process ( Meyer 

 and Anderson, 1952 ) . Thus, if virus synthesis is directly correlated with 

 host growth, the rate of synthesis should be expected to increase, 

 within limits, as the temperature of the host environment increases, 

 and it should directly parallel the eflFects of light and light-temperature 

 interactions on growth. As we shall point out below, such direct rela- 

 tionships do not always exist. 



Mineral nutrition. Spencer (1939) reported that the concentration 

 of tobacco-mosaic virus in tobacco plants was directly correlated with 

 the amount of nitrogen supplied to the host. He detected no correla- 

 tion between the amount of host growth and virus concentration. 

 Bawden and Kassanis ( 1949 ) were unable to confirm Spencer's work 

 and concluded that nitrogen eflFects on virus concentration were cor- 

 related with the eflFects on host growth. It should be pointed out that 

 Bawden and Kassanis were, for the most part, using plants grown in 

 soil, and therefore did not have adequate control of the mineral 

 nutrition. 



Other workers have shown that the nitrogen eflFects on virus con- 

 centration in expressed sap extracts, in some host-virus combinations, 

 are directly related to the amount of nitrogen supplied the host, as 

 Spencer reported (Weathers and Pound, 1954; Helms and Pound, 

 1955b ) . The concentration of tobacco-mosaic virus in tobacco increases 

 as available nitrogen increases, and at 1,050 ppm. of nitrogen it is 

 higher than that at 630 p.p.m., even though, at the highest nitrogen 

 level, growth (wet weight) is markedly reduced in comparison with 

 that at 630 p.p.m. (see Figure 6). Also, the concentration of potato X 

 virus in Nicotiana glutinosa and of tobacco-ringspot virus in cucumber 

 increase with a rise in nitrogen, even though marked suppression of 

 host growth occurs at the highest nitrogen levels (Helms and Pound, 

 1955a). In other cases, direct correlations do not occur between the nitro- 

 gen effects on host growth and virus concentration ( Cheo et al., 1952; 

 Pound and Weathers, 1953). With the cucumber-mosaic virus in 



