What we just said about fusarium and its real or imagined propensity to transform sugars into alcohol will, in any case, be useful in understanding the tomato/TMV (tobacco mosaic virus) relationship.
Like all viruses, TMV consists of a nucleic acid and a protein envelope, two constituents which are very rich in N (160 mg per gram of dry material). To be able to grow, it has to find large quantities of nitrogen in its plant-host tissues. For example, it is estimated that in certain leaves affected by the virus, 80% of the assayed nitrogen is viral nitrogen.
This leads to two unavoidable consequences:
- N assays of diseased plants don’t mean much, from our point of view here, since the method used assays not only plant tissue nitrogen but also viral nitrogen;
- tissues affected by the virus can be expected to be richer in sugars than corresponding healthy tissues, due the fall in nitrogen content in plant tissues resulting, logically, in proliferation of the virus and thus explaining the hypoauxinia characterising diseased leaves. e.g.
In the control series, the concentration of sugars in the leaves increases with the age of the plant and N concentration falls. As usual, IAA slows and GA accelerates the process. The concentrations of sugar assayed at the end of the experiment in sick leaves were higher than those of the control leaves. N concentrations were apparently equivalent. Considering the presence of the virus, nitrogen concentrations corresponding to plant tissues alone are therefore doubtless lower than N concentrations in control leaves, which matches the expected results (see previous paragraph).