The effect of infection with beet yellows and beet mosaic viruses on the carbohydrate content of sugar-beet leaves, and on translocation

The loss of total carbohydrate (sugars and starch) per cent of residual dry matter (dry matter less total carbohydrate) during a period of darkness from leaves of sugar‐beet plants infected with yellows virus was as great as that from the leaves of healthy plants. The conclusion of previous workers, based on the results of the Sachs iodine test for starch and the occurrence of ‘phloem gummosis’ in infected plants, that starch accumulates in infected leaves because translocation is prevented by blockage of the sieve‐tubes, is therefore incorrect.

Older leaves of infected plants had a higher content of reducing sugars and sucrose, and usually but not invariably of starch, both at the beginning and end of the dark period, than comparable leaves of healthy plants. By far the greater part of the increase was in reducing sugars. In leaves taken in late September from infected plants growing in the field, 20 % or more of the total dry matter was present as reducing sugars. The reducing sugars in both healthy and yellows‐infected leaves were shown by paper chromatography to be glucose and fructose in approximately equal amounts.

Accumulation of carbohydrate in infected leaves is probably not a passive consequence of differences in carbohydrate production, distribution and utilization, but is attributable to changes in the physiology of the cells of the leaf.

The carbohydrate content of sugar‐beet leaves was little affected by infection with beet mosaic virus.

Yellows‐infected leaves had a lower water content per cent of fresh weight than healthy leaves. This was accounted for by the higher carbohydrate content of infected leaves, for the ratio of water: residual dry matter was not affected by infection or was slightly reduced. This implies that hydration was independent of carbohydrate content.