The effects of potato virus X upon the carbohydrate and total nitrogen contents of potato leaves have been followed over diurnal and seasonal periods in the varieties President and Arran Victory.

The gross metabolism of carbohydrates has been found to be similar in both mosaic-diseased and normal leaves.

Three major trends of carbohydrate metabolism, each of which may be identified with a particular stage of leaf development, have been recognized and discussed.

Slight modifications of the fundamental metabolism of diseased leaves have been found in a reduced photosynthetic activity, disturbed starch-sugar relationships, and an impediment in translocation during the pre-senescent stage. All these have been ascribed to secondary effects arising out of the pathological change brought about as a result of virus infection.

Transient modifications in carbohydrate metabolism have been traced to prematurely retarded growth activities leading to a diminished demand for carbohydrate in support of growth in the diseased plants.

A significantly greater content of nitrogen has been found in diseased leaves at all stages of growth, and it has been suggested that the pathological symptoms and prematurely retarded growth activities which give rise to disturbances in carbohydrate metabolism are direct manifestations of a disorganized nitrogen metabolism.