An analysis of the effects of tobacco mosaic virus on growth and the changes in the free amino compounds in young tomato plants

Tomato plants at the four-leaf stage were inoculated on the first leaf with TMV in a growth room and the effects studied in systemically-mfected leaves with reference to growth, virus multiplication and changes in water, nitrogen, nitrate and chlorophyll contents. Parallel changes in the free amino compounds were also studied in the growth room (incident radiation 152 mwh cm-2 day-1) and in two experiments in a glasshouse (352 and 226 mwh cm-2 day-1). Dry matter accumulation and leaf expansion in leaves 3, 4 and 5 were checked by TMV 5–7 days after inoculation but not in leaf 2. In the period 7–25 days after inoculation the relative growth rates of whole plant and leaves 3 and 6 and net assimilation rate were not affected by TMV. Stem height and dry weight were not affected by TMV but ‘root’ dry weight was reduced from days 5–15. Virus was present in the stem and in leaves 2 and 6 by days 3, 5 and 15 respectively so that infection per se did not always check growth. Chlorophyll content of systemically-infected leaves was reduced 10 days after inoculation. Total N and ammonia contents were not affected by TMV but infected leaves contained less nitrate. At the two lower levels of incident radiation the initial effect of TMV was to reduce the content of total free protein amino acids and amides, which were minimal 5–7 days after inoculation. In the glasshouse experiment a reduction could be measured only 1 day after inoculation or before virus was present in the younger leaves. With high incident radiation there was no initial reduction but an increase at day 13 when mottling symptoms were visible. Total non-protein amino acids, of which λ amino butyric acid was the major constituent, were increased by TMV in all three experiments for up to 13 days after inoculation. It is suggested that inoculation of a leaf with TMV temporarily interferes with export of photosynthates and import of root-synthesised amino acids and that the results reported above can be interpreted in this context. Evidence in support of this is adduced from an experiment in which 13 foliar sprays of gibberellic acid (2·5 ppm) were combined with TMV inoculation and the changes in free amino compounds followed. It is concluded that analyses of the changes in individual free amino compounds are unlikely to provide useful information concerning the sources of virus coat protein.