BOTANY. BLACKHEART AND THE AERATION OF F. C. STEWART AND A. J. MIX. SUMMARY. Present knowledge of blackheart in potatoes rests upon investigations made by Bartholomew who proved that it may result from exposure of the tubers to a temperature of 38-45° C. for 14-48 hours and is due to changes in the tissues caused by a derangement of the process of respiration. The accidental discovery that by excluding the air from potatoes blackheart may be produced at temperatures much lower than those employed by Bartholomew led the writers to make experiments with tubers in sealed jars. From such experiments it was learned that potatoes cannot long endure close confinement. Within a certain length of time, which varies with the temperature and quantity of air available, tubers confined in hermetically sealed jars become moist over a part or the whole of their surface. If the tubers are then exposed to the air the moist surface areas turn brown and the color of the flesh changes first from white to pink then to black (blackheart). With a volume of air equal to the volume of the tubers a confinement of ten or twelve days is sufficient to produce the symptoms described provided the temperature is around 70° F. At a temperature of 55-60° F. about twenty days are required; and at 40° F. a still longer time somewhere between twenty-three and forty days. Tubers in half-full and quarter-full jars behave similarly to those in full jars except that the symptoms are slower in making their appearance. Tubers confined in sealed jars with less than about ten times their volume of air are unable to do more than barely start sprouts. For normal sprouting about nineteen volumes of air per volume of * *Reprint of Bulletin No. 436, June, 1917. tubers are required. Blackheart may be expected to appear whenever the volume of air available to the tubers is less than that required for normal sprouting. Different tubers of the same lot exhibit marked differences in susceptibility, both to blackheart and to surface discoloration. The cause of this has not been determined. It appears doubtful if the size of the tubers is an important factor. Upon learning of the injurious effects of insufficient aeration the writers made other experiments with tubers in deep tanks to determine how deeply potatoes may be piled with safety. The data obtained are insufficient for the formulation of definite rules, but it appears that six feet should be considered the maximum depth of piling when potatoes are to be stored for several months at temperatures below 45° F. At temperatures above 50° F. the depth limit should be three feet if the potatoes are to be stored longer than three or four weeks. Tubers suffering from insufficient aeration thru deep piling behave, in a general way, like tubers in sealed jars. They sprout feebly or not at all, become moist on the surface, discolor externally upon exposure to the air, and are often affected with blackheart internally. The principal difference is in the occurrence of rotten spots caused by fungi and bacteria. This form of injury is more common with potatoes in deep piles than with potatoes in sealed jars. Blackheart sometimes occurs in potatoes stored out-of-doors in pits and is due, undoubtedly, to insufficient aeration. However, the experiments so far made indicate that the aeration of potatoes in unventilated pits is better than might be supposed and that the ventilation of small pits is unnecessary. The injury resulting from insufficient aeration is due to the lack of oxygen rather than to the accumulation of carbon dioxid. Tubers affected with blackheart produced by exposure to high temperature usually appear normal externally, while those affected with blackheart produced by exclusion of the air usually show more or less surface discoloration. Insufficient aeration during storage does not cause spindlingsprout. It may retard sprouting temporarily, but when, subsequently, the tubers are supplied with air they sprout normally if at all. Tubers severely affected with blackheart are unfit for seed purposes, but slightly affected tubers may be planted. If tubers are sound and normal in appearance it is unlikely that they have been injured for seed purposes by any storage conditions to which they may have been subjected. The prevention of blackheart is a shipping problem as well as a storage problem. Blackheart often results from the overheating of potatoes during shipment in stove-heated cars. BARTHOLOMEW'S WORK ON BLACKHEART. Blackheart is the name proposed by Bartholomew for an abnormal condition of potato tubers in which the central portion is dark brown or black. Affected tubers may appear normal externally, but when cut open they show an area of dead, black tissue, occupying a large portion of the interior. Frequently, the trouble takes the form of a cavity lined with a thick layer of tough, dead, black tissue. 1 Our knowledge of the cause and nature of blackheart rests upon the investigations of Bartholomew who has published two papers on the subject. On page 618 of his second paper he says: "That the production of black heart in potatoes is due to changes in the tissues caused by overheating in an atmosphere devoid of sufficient oxygen to meet the demands of the rapidly respiring potatoes can not be questioned." He found blackheart occurring among potatoes shipped long distances in stove-heated refrigerator cars and came to the conclusion that the trouble resulted from overheating in transit. While considering this a common cause he recognized that there are, also, other causes. He says: "No doubt this malady may occur in potatoes at other times than during shipment. Experiments have shown that the only conditions necessary are excessive temperatures and a lack of sufficient oxygen. It is entirely possible and even probable that such conditions may exist where the potatoes have been stored in close warm cellars or in pits covered with manure." Proof of his statement that overheating is a cause of blackheart is found in the fact that he was able to produce blackheart at will by subjecting tubers to temperatures of 38-45° C. (100-113° F.) for 14-48 hours. 2 1 Bartholomew, E. T. Black heart of potatoes. Phytopathology, 3:180-182. 1913. A pathological and physiological study of the black heart of potato tubers. Centbl. Bakt, etc. II, 43:609-638. 1915. 2 Loc. cit., p. 623. Altho Bartholomew recognized that both the temperature and the oxygen supply are factors in the production of blackheart, and notwithstanding his statement 3 that, "it may be prevented in shipping, or otherwise, by proper ventilation and by keeping the potatoes in a temperature which does not exceed 35° C.", it seems he did not realize that blackheart may occur at temperatures considerably lower than 35° C. provided the supply of oxygen is scant. There is nothing in either of his papers to indicate that he succeeded in producing blackheart at a temperature lower than 38° C. ORIGIN OF THIS INVESTIGATION. In the spring of 1914 it was observed that some tubers enclosed for about two weeks in an air-tight glass jar had failed to sprout, and when cut they were found to be affected with blackheart. Since the temperature to which the tubers had been exposed could not have been at any time higher than about 20° C. the blackheart must have come about in a manner somewhat different from that described by Bartholomew. A few experiments demonstrated that the air supply has an intimate relation to the occurrence of blackheart, and that a considerable volume of air is required for the well-being of potatoes. This led to further experimentation to determine how large a quantity of air is required, and what is the effect of storing potatoes in deep piles in cellars and bins and in unventilated pits and piles out-ofdoors. METHODS AND APPARATUS. Most of the experiments were made with tubers in wide-mouthed glass museum jars having a capacity of 3500-3700 cubic centimeters (about 3.5 quarts) and provided with tight-fitting, ground-glass stoppers. A few experiments were made in smaller jars of the same kind. The jars were made air-tight by running melted paraffin or sealing-wax around the stoppers. Using tubers weighing from two to four ounces it required about four pounds of potatoes to fill one of the larger jars. In full jars the volume of the air enclosed with the tubers was equal, approximately, to the volume of the tubers. It varied from about 28 to 31 cubic centimeters for each ounce of tubers. By filling the jars only partly full other ratios were obtained. 3 Loc. cit., p. 637 |