The Physiology of Virus Diseases in Plants: II. Further Studies on the movement of mosaic in the Tomato Plant1
Authors: Caldwell John 
Published: February 2008

Summary.

In this paper the results of some experiments with aucuba mosaic in tomato are discussed. These results support the general thesis that the agent does not normally travel in the xylem stream. The movement of tobacco mosaic in tobacco and in tomato was found to be similar to that of aucuba mosaic. The majority of the experiments were carried out with aucuba mosaic in tomato.

It was found that filtered virus juice from virus‐infected plants was readily absorbed at the cut end of a petiole and thence travelled into the xylem of the main stem. The removal of the treated petiole within 48 hours prevented infection taking place. On the other hand, when the petiole was left attached the experimental plant developed symptoms in the usual manner. This type of experiment was repeated with the exception that, after the removal of the treated petiole, the leaves above were crushed. Infection of the plant followed this treatment. This experiment was combined with the earlier experiments in which the living tissue of an internode was killed by steam. The agent was found to be carried mechanically in the xylem across the dead tissue.

As a consequence of this observation the experiments with plants with “steamed” internodes were repeated. It was found that in no caw did crushing of the leaves induce symptoms on the upper part of the plant when inoculation had been made on the lower side of the “steamed” internode.

It was concluded that the virus agent did not normally enter the water stream, and when it was introduced experimentally into it. though it was carried round, there was no mechanism by which it could leave the vessels.

The absence of the agent from the hydathode exude was demonstrated.

Apparently the agent cannot enter an unbroken cell, nor can it move through or out of dead cells. It has been found that great care must be taken to ensure the absence of traces of toxic substances from inocula to be tested, otherwise infection may not take place even in the presence of the agent itself.

The rates of movement of the virus agent in the tomato are practically the same upward or downward. The slightly greater rate of upward movement appears to be associated with the greater metabolic activity which occurs in the upper portion of the plant. The movement of the virus agent along the protoplasmic strands has been examined by inoculating plants with infective juice after the removal of large portions of the vascular tissue. This treatment does not appear to delay the movement of the agent up the stem.

In a final group of experiments, darkness did not appear to have any effect on the multiplication of the virus in the tissues. Too prolonged periods in the dark, however, caused the permanent wilting of both diseased and healthy plants. This “wilting” is considered us In‐mir due to the respiration of carbohydrates, etc., and the earlier collapse of the diseased plants as being due to a smaller carbohydrate supply in them.

This work was carried out under the auspices of the Empire Marketing Board.