« PreviousContinue »
For the effect of storing potatoes in deep piles use was made of tall, galvanized-iron cylinders which were open at the top but airtight on the sides and bottom. The diameter of these cylinders was nine inches. Their hight varied from 80 to 126 inches.
One experiment was made out-of-doors with piles of potatoes which were protected from freezing by covering them with oat-straw and soil according to the usual custom.
Unfortunately, it was impossible to control the temperature except within rather wide limits,- 5 to 10° Fahr. The highest and lowest temperatures employed were about 75 and 35° F. (24 and 2° C.) respectively.
The experiments were conducted during April and May, 1914; from January to May, 1915; and from October, 1915, to May, 1916. A single variety of potato, Sir Walter Raleigh, was used thruout. In all cases, excepting the out-door pit experiment, the tubers were thoroly washed. Only perfectly sound tubers were used and they were dry when put into the jars or tanks.
BEHAVIOR OF TUBERS IN FULL, SEALED JARS AT 70° F.
When jars completely filled with tubers are hermetically sealed and exposed to a temperature of about 70° F. the following results may be expected:
No sprouts will start; and any sprouts which may have been on the tubers at the time of sealing the jars will blacken and die. Otherwise, the appearance of the tubers will be normal until about the tenth day. Then the tubers will commence to show moist areas (“sweat”). This will appear earlier on some tubers than on others. By the fifteenth day the tubers will be moist over their entire surface. If removed from the jar before the appearance of the moist areas the tubers will, usually, show no discoloration externally and, if given a chance, they will sprout normally. If removed after they have begun to sweat the tubers quickly discolor upon exposure to the air. The discoloration is a shallow, brown one which appears over welldefined areas of various extent. It may cover any part or even the whole of the surface. The bud end of the tuber appears to be particularly susceptible. The stem end portion is least susceptible. (Plate IX.) In a general way the discolored areas coincide with the moist areas, but there are numerous exceptions to the rule.
After a few days' exposure to the air the discolored areas become slightly depressed and drops of a brown, sticky exudate appear here and there. Tubers having less than about half of their total surface discolored resist decay for a long time if kept dry, and may even produce sprouts; but tubers more severely affected soon decay. Besides the large discolored areas above described there often occur, also, circular, dead, brown, depressed spots two to five millimeters in diameter. These resemble considerably the spots produced by fumigation with formaldehyde gas as described in Bulletin No. 369 of this Station.
When tubers are left in the sealed jars until their entire surface becomes moist they are usually attacked by anaerobic bacteria which soften the tissue in spots under the skin. In some cases large clusters of gas bubbles appear. The odor of tubers so affected is extremely disagreeable. When exposed to the air they quickly decay. Occasionally, the tubers become moldy in the jars, but, as a rule, bacteria gain the supremacy if the jars are well filled and the temperature at 70° F. or higher. If tubers affected with this bacterial decay are permitted to remain long in a sealed jar pale-straw-colored liquid rapidly collects in the bottom until the jar is about one-half filled with it.
Blackheart may be expected to appear in any tuber which has been long enough in a sealed jar to show moist areas on the surface. Occasionally, it is found also in tubers which show no moisture while in the jar or external discoloration after exposure to the air. In general, the more severe the sweating and external discoloration the greater the severity of the blackheart.
The flesh of tubers cut immediately after removal from a sealed jar is of normal color, but after exposure to the air for a short time the central portion of the cut surface first becomes pink then gradually blackens. Tubers exposed to the air twenty-four hours or longer before cutting show blackheart immediately, if ever.
The tissue affected with blackheart is that which occupies the medullary region. In the main, it lies in the central portion of the tuber, but it is often very irregular in outline. (Plate X.) It is almost always elongated in the direction of the long axis of the tuber and, frequently, branches extend to the buds. (Plate XI.) Occasionally, blackheart appears in the form of large black spots scattered irregularly thru the flesh of the tuber. Sometimes it is accompanied by blackening in the vascular region. The color of affected tissue varies from gray black to coal black.
Tubers may be quite severely affected with blackheart yet sprout normally and produce normal plants. Special attention has been given to the character of the sprouts of tubers affected with blackheart. No tendency to produce spindling sprouts has been observed. If any sprouts appear they are of normal diameter.
When tubers affected with blackheart are exposed for several days to air of low humidity (e. g., that of an ordinary living room) cavities develop in them. Such cavities are often of considerable size and lined with a layer of tough, black tissue resembling rubber. Evidently, these cavities result from shrinkage of the affected tissue thru loss of water as described by Bartholomew. (Plate X.)
Smaller cavities of another kind and of a different origin are found frequently in tubers which have been enclosed in sealed jars for some time but not long enough to produce blackheart. These are usually lens-shaped, three to ten millimeters in diameter, and destitute of a black lining or flesh discoloration of any kind. Altho commonly found near the center of the tuber they may occur near one end or in the vicinity of a bud. Occasionally, two or more cavities are found in a single tuber. They occur much more frequently in small tubers than in large ones. How these cavities originate is not known to the writers, but the fact that they appear only in tubers from which air has been excluded indicates that lack of oxygen is, in some way, responsible for them. Diligent search has been made for such cavities in tubers well supplied with air, but none have ever been found. That they are not caused by shrinkage of the tissue thru loss of water is shown by the fact that they are formed while the tubers are yet in the jar and surrounded by an atmosphere heavily charged with moisture. They are found in tubers cut
. immediately after removal from sealed jars.
These cavities differ from those known as hollow-heart in being smaller, more regular in shape, free from discoloration of the surrounding tissue, and by occurring in small tubers rather than in large ones. They are of little economic importance.
BLACKHEART PRODUCED AT LOW TEMPERATURE.
The following experiment demonstrates that blackheart may be produced at temperatures below 43° F.:
On December 16, 1915, three glass jars filled with clean, dry, sound tubers were hermetically. sealed and placed in the Station potato cellar. One of the jars was opened at the end of twenty-three days. Eight of the twelve tubers which it contained were moist on the surface. Upon exposure to the air they became discolored all over. The other four, which were dry when removed from the jar, did not discolor; but they showed a number of small pits. Eight of the tubers had small areas of blackheart.
A second jar was opened at the end of forty-one days. None of the tubers were moist, but four showed large areas of discoloration while yet in the jar and the others were somewhat pitted. All of the twelve tubers were cut immediately. Eleven of them were found to be severely affected with blackheart. One of these had, also, a cavity. The remaining tuber was normal within.
The third jar was opened at the end of fifty-eight days. All of the tubers were dark-colored externally and wet all over, but none were moldy or affected with bacterial decay. They had an odor of alcohol. All were affected with blackheart and three had, also, good-sized cavities.
The temperature of the air in the cellar during the period covered by this experiment (December 16, 1915, to March 14, 1916) is given in Table IV. The highest temperature was 42° F. and the lowest 34° F.
QUANTITY OF AIR REQUIRED FOR THE MAINTENANCE
OF LIFE AND HEALTH IN POTATO TUBERS.
It having been demonstrated that tubers in full, sealed jars soon die and decay the question arose as to how large a volume of air is required for the maintenance of life and health in potato tubers.
It is a well-known fact of plant physiology that potato tubers require a certain amount of air. So long as they live they must respire; that is, they consume oxygen and give off carbon dioxid and water. Their source of oxygen is the air. Hence, they must have a constant supply of air. It is known, also, that, within certain limits, the rapidity of respiration increases with rise in temperature.
At a temperature of 70° F. potato tubers respire more rapidly, consume more oxygen and hence require more air than at a temperature of 50° F. Likewise, more air is required at 50° F. than at 35° F.4
Lacking means of controlling the temperature accurately the writers have been unable to secure experimental data which would enable them to give a wholly satisfactory answer to the question here raised. However, it seems worth while to record some of the results obtained in the experiments.
From the results of a few experiments with half-full and quarterfull jars it appears that with these larger quantities of air the tubers behave in practically the same way as tubers in full jars except that the symptoms of blackheart and decay are slower in making their appearance.
AN EXPERIMENT WITH A HALF-FULL TANK AND A HALF-FULL JAR.
In February, 1916, the following experiment was made: A cylindrical, galvanized-iron tank nine inches in diameter and eighty inches deep was half filled with potato tubers (244 tubers weighing 56.5 pounds) and the tank hermetically sealed by means of a tightfitting cover overrun with melted paraffin. The potatoes were taken from a cellar where they had been in storage at a temperature of 36-42° F. They were washed and thoroly dried before being placed in the tank. Only perfectly sound tubers were used. No sprouts had started on any of them.
At the same time a glass jar (capacity 3.5 quarts) was half filled with some of the same lot of tubers and sealed. This was to be used as an indicator to determine the proper time for opening the tank. A second glass jar of the same kind was completely filled with tubers and sealed. The tank and pair of glass jars were placed in a partially darkened room the temperature of which was recorded once a day. Owing to the temperature being rather low, 13-16° C.,
4 Ziegenbein (Jahrb. Wiss. Bot. [Pringsheim] 25:594. 1893.) has determined the rapidity of respiration of potato tubers at different temperatures by measuring the quantity of carbon dioxid produced by a given weight of tubers in a given length of time. His results are stated in milligrams of carbon dioxid produced by 100 grams of tubers in one hour. They are as follows: Temp. CO2 Temp. CO2 Temp.