Physiological Investigation of Black Heart of Potato Tuber

Abstract

1. This paper presents the results of a physiological study of black heart of the potato tuber. 2. Observations were made with the purpose of discovering the nature of the physiological changes which must precede or accompany the profound changes that take place in the interior region which suffers the breakdown and undergoes the color changes known as black heart. 3. The disease was produced in the laboratory at a temperature of 45⚬ C., in a carbon dioxide free atmosphere in which abundant oxygen was available. The following determinations were made on the affected tissues over a period of about 16 hours: ratio of carbon dioxide to oxygen in the intercellular spaces, conductivity of the tissues, catalase activity, and H-ion concentration changes. 4. During the time preceding the incidence of black heart, carbon dioxide accumulates rapidly in the internal atmosphere and oxygen is rapidly depleted, until the intercellular gases contain more than 50 per cent of carbon dioxide and less than 4 per cent of oxygen. 5. This change in the internal environment of the cells is the first change detected, and is followed by increasing permeability of the protoplasm, together with the other changes involved in the development of black heart. At the temperatures used, black heart is apparently the result of high respiratory activity and the failure of the gas exchange to keep pace with respiration rate, but the possible effects of temperature and other factors must not be overlooked. 6. The progress of injury to the protoplasm was followed by electrical conductivity methods, which enable one to judge the degree and rate of injury in any given set of conditions. 7. By these methods it has been shown that there is first a rise of resistance in the tissues, extending over a period which varies with the individual tuber at 45⚬ C. This increased resistance is followed by a continuous fall, beginning with the death of the tissues. 8. These changes are more pronounced with the inner tissues of the potatoes. The cortical regions of the tubers show less change in resistance, and are also much slower to become affected with black heart. 9. The catalase activity of the affected tissues does not seem to be correlated with respiratory activity. 10. During the development of black heart there is a slight increase of H-ion concentration, but this change is not localized. The change as revealed by the indicator method is small and its significance not clear. 11. Temperature may exert a direct effect above 38⚬ C., as some evidence has been obtained that this temperature becomes critical for the maintenance of the normal water relations of the cells. 12. Color changes similar to, or identical with those produced in black heart, may be induced in various ways. Probably anything which causes the death of the tissues without destruction of the enzyme involved, or alteration of the nature of its substrate, may cause the same changes of color in the tissues killed.