632 PLANT GROWTH AND PLANT COMMUNITIES 



tended to be lower than in healthy tissue, and that the respiratory quo- 

 tients did not differ significantly. Although evidence indicates that in 

 many instances plant viruses cause an increased respiration, it is not 

 evident that increased respiration always accompanies viral infection, 

 and where it does accompany infection it does not necessarily mean a 

 stimulated rate of respiration. It may be due, as Whitehead ( 1934 ) 

 suggested, to an increase in the available substrate. 



Owen, in a series of papers ( 1955, 1956), has shown where some of 

 the confusion lies. Respiration rates of tobacco-mosaic leaves (as meas- 

 ured by CO2 liberation) may be higher, lower, or identical with those 

 of healthy leaves, depending upon the time that has elapsed since inoc- 

 ulation, the physiological condition of the plants, the environmental 

 conditions of host growth, and how the results are expressed. In care- 

 fully controlled measurements over a 20-hour period, Owen showed 

 that the rate of CO2 production per gram of dry matter in young leaves 

 was 10 per cent less in diseased than in healthy leaves but that in older 

 leaves there was no difference. Older leaves of infected plants had an 

 initial water content less than that of healthy leaves, and both older 

 and younger infected leaves absorbed less water over the 20-hour 

 period than did healthy leaves. This difference in initial water content 

 was great enough to conceal a decrease in respiration due to infection, 

 and differences in the water uptake of detached leaves were sufficient 

 to reverse such an effect when respiration was expressed in terms of 

 wet weights. Thus the only safe way to express results is on the basis 

 of dry weights. Owen also showed that whether results were obtained 

 in winter or summer made a difference, respiration being increased in 

 detached infected leaves in winter but not in summer. In winter, in- 

 creasing light intensity prior to inoculation decreased respiration rates 

 after infection. Photoperiod had no effect. Respiration rates were 

 changed within one hour after inoculation, which suggested that such 

 change was unlikely to be associated with the formation of new virus, 

 since there is no evidence for the formation of infective virus prior to 

 six to eight hours ( Kassanis, 1959 ) . Within the first hour the introduced 

 virus is likely to be still confined to epidermal cells, and in relatively 

 few of them. Thus it appears that this almost immediate effect on 

 respiration is due to entry of the virus in the cell and is the product 

 of abnormal metabolism. In contrast with the effect on Nicotiana 

 tabacum, this virus does not effect an increased respiration of N. gluti- 

 nosa until symptoms appear (Owen, 1958). The tobacco-etch virus 

 shows a similar picture, in that it does not cause an increase in respira- 

 tion rates in tobacco until leaves show external symptoms, when in- 

 creases as high as 40 per cent may be obtained (Owen, 1957a). Also, the 

 increased respiration is maintained for long periods, and the decline 

 that has been reported for tobacco-mosaic virus was not detected. Lo- 



