THE banana fruit of commerce is always gathered iminature and

Transcription

1 [170] THE RESPIRATION OF BANANAS IN PRESENCE OF ETHYLENE BY R. GANE, M.SC, PH.D. Low Temperature Research Station, Cambridge (With 7 figures in the text) THE banana fruit of commerce is always gathered iminature and green and ripened after transport. There are not yet available the results of any complete study of the temperature-respiratory activity-time relationships of immaturely gathered fruit. Such results as have been obtained with post-transport fruit (1936) suggest that the onset of ripening is accompanied by a sharp rise in respiratory activity similar in general character to that occurring in the apple 8OT Days Fig. I. Rate of production of carbon dioxide by bananas at 15 C. in air and in air containing i part per million of ethylene. Bananas II in fresh air throughout. At A, bananas I ventilated with air containing i p.p.m. of ethylene. and pear as described by Kidd & West and termed by them the chmacteric. In the case of the apple these authors have shown that ethylene administered before the climacteric stimulates its immediate onset and that administration after the climacteric has no effect. A series of experiments has been carried out in which post-shipment bananas have been treated with ethylene in the pre-climacteric phase of low respiratory activity. In all cases the action of ethylene has been the stimulation of the immediate onset of the rise in respiratory activity associated with ripening. Fig. I presents the results of one such experiment. Comparable samples of fruit were obtained by using the two halves of a hand, one

2 Respiration of Bananas in Presence of Ethylene 171 of which was ventilated with air and the other with the gas mixture. The gas mixture was made by introducing 2-1 c.c. of ethylene in a partially evacuated steel cylinder and then adding 75 cu. ft. of air from a similar cyhnder containing 150 cu. ft. of air under pressure. Fig. 2 presents the result of an experiment in which ethylene was administered in the post-climacteric phase. At this stage it is without effect on respiratory activity. An interesting extension of this type of experiment is one in which the apphcation of ethylene is made after the upward tendency in respiratory activity has already begun. The results obtained in this case are shown in Fig. 3. It will be seen that the rate of rise is accelerated. Fig. 2. Rate of production of carbon dioxide by bananas at 15 C, in air and in air containing ethylene. At A, i c.c. of ethylene was added to the air stream. Now in the case of apples the present author (1935) has shown that with the onset of the climacteric rise in respiratory activity ethylene is produced by the fruit itself, while Kidd & West have shown that the volatiles from post-climacteric apples stimulate the climacteric in unripe fruit and also that the same is true in the case of bananas. Confirmation of their observation with regard to bananas has been obtained. Three samples of bananas were obtained from one hand of fruit, sample I was ventilated with air from a cylinder, sample II was similarly ventilated but o-i c.c. of ethylene added to the air stream, while sample III was ventilated with air that had previously been used to ventilate ripe bananas and then passed over moist soda lime to remove carbon dioxide.

3 172 R. GANE The course of respiratory activity of all three samples is shown in Eig. 4. Readings of the respiratory activity were obtained every so- Days Fig. 3. Rate of production of carbon dioxide by bananas at 15 C, in air and in air containing i part per million of ethylene. Bananas I in air throughout. At A, bananas II ventilated with air containing i p.p.m. of ethylene. Days Fig. 4. Effect of volatile products from ripe bananas on the production of carbon dioxide by unripe bananas (at 15 C). Banana I in fresh air throughout. At A, air from a ripe banana was passed over banana III. At B, banana III was returned to fresh air. At C, O'l c.c. of ethylene was added to air stream over banana II. 4 hours, and it will be seen that in samples II and III the respiration started to increase after 8 hours from the time dosing started.^ A germinating-seed test for the production of traces of ethylene by bananas has been applied. Peas germinating in a closed vessel in ' In this particular series, the peak of respiratory activity was reached at almost the same time by all three samples, and although the respiration was increased by the treatment it was observed that ripening in this experiment did not appear to be greatly accelerated by the ethylene treatment.

4 Respiration of Bananas in Presence of Ethylene 173 the presence of ripe bananas at 25 C. with the minimum ventilation required to keep the carbon dioxide below 10 per cent exhibited the abnormalities in growth which are described as typical of the effects of ethylene (Crocker, 1929). Such effects, however, were only produced by restricting ventilation and at a high temperature. Similar effects are produced by apples with ventilation rapid enough to keep the carbon dioxide below 0-5 per cent and at lower temperatures. When unripe green bananas are used these effects are not produced. Eig. 5. Production of carbon dioxide by bananas in air at 15 C. with continuous and discontinuous ventilation. Bananas II, continuous ventilation throughout. Bananas I, no ventilation at periods A, B, C and D. In parallel with these germinating-seed tests for the presence of ethylene, the effects upon the subsequent respiratory activity of bananas were studied after periods in which ventilation was restricted to allow the accumulation of 10 per cent carbon dioxide. The data shown in Fig. 5 refer to two samples from the same hand of fruit, one sample was ventilated continuously and the other discontinuously.^ There is no sign of the action of any stimulating substance in the pre-climacteric phase, but as soon as the fruit passes into the chmacteric phase, a period of restricted ventilation accelerates the completion of the climacteric. There is a strong probability therefore that bananas behave like apples and begin to produce ethylene with the onset of the climacteric. ^ There is, of course, an initial high value due to the accumulated carbon dioxide, but this is soon lost.

5 174 R- GANE The fact that treatment of the fruit with ethylene (i part per million) or a temporary restriction of ventilation accelerates the climacteric rise is interesting. In their work on apples and pears Kidd & West (1933) have shown on the one hand that restricted ventilation or ethylene treatment will stimulate a climacteric change even in very immature fruit, and on the other hand that if single fruits are used (pears in this case), gathered sufficiently immature and ventilated sufficiently rapidly, no climacteric ever occurred. They argue that (i) ethylene or a substance with similar action is being produced prior to the climacteric; (2) there must be a threshold value of ethylene concentration in the cells necessary for stimulation; (3) probably the rate of production of ethylene rises with age while the threshold value for stimulation falls. The results of the experiments described above suggest that the cells of the fruit do not all reach the point of auto-stimulation simultaneously. If so, it should be expected that at the stage when some have reached the critical point and others are near it, an artificial raising of the concentration of ethylene would cause an accelerated rise in the respiratory activity of the whole fruit. The active substance from ripe bananas is destroyed, or inactivated, by ozone. A stream of air was passed over ripe bananas, and then divided into two halves, each of which was led over a similar sample of green bananas; into one stream a small oxonizer introduced ozone at a concentration of from 70 to 90 parts per million. The bananas exposed to this gas ripened at a much later date than the other sample. In this experiment the lenticels of the fruit exposed to ozone became discoloured and dark green unfiltrated areas were formed round them, which later became brown, and finally black. The experiment was repeated in a modified form in which the comparison instituted was between fruit ventilated with fresh air in the normal way and fruit ventilated with air passed first over ripe bananas, then through an ozonizer, and finally through a deozonizer.i The behaviour of the two samples was identical. The ozone had clearly removed stimulating substances while the deozonizer had eliminated the damaging effects of ozone. With a view to surveying the sensitivity of the banana fruit to ozone comparable samples of bananas were stored at 15 C. and ventilated with air and with air containing a low concentration of ozone. The ozone was produced by a small ozonizer built into the ' A glass tube filled with scrap rubber.

6 Respiration of Bananas in Presence of Ethylene 175 inlet tube of the fruit container. The quantity of ozone produced could be varied by altering the resistance in the mains leads of the high tension transformer. The concentration of ozone is approximately constant over a range of air speeds. Any ozone in the air stream issuing from the fruit container was removed by passing through a glass tube containing scrap rubber. Even the lowest concentration of ozone used, 1-5 parts per million, caused injury to the peel, A new observation was also made, namely, that one of the effects of ozone is a retardation of ripening. This retardation is probably due to a plugging of the stomata of the fruit by the decomposition products formed by the action of ozone. The data are plotted in Fig. 6. A similar retardation of ripening is shown by fruit treated with hydrogen peroxide, dilute aqueous iodine and coated with vaseline, and the conclusion has been drawn that the effect is due to a blocking of the channels through which the gaseous exchange takes place and consequently to an increased carbon dioxide and diminished oxygen tension within the fruit. Certain suggestions of possible value in the handling of fruit arise from the facts established above. The first is that the problem of "ships ripes" in banana-carrying vessels may find its partial solution in the combined use of ozonizers and deozonizers for the treatment of the air circulated through the holds and over the coolers by fans. The second is that great care should be taken to avoid storing ripe fruit or fruit that has been treated with ethylene, as is often the case with early gathered oranges, in the same air with unripe fruit. The effect upon bananas of the volatile products of a ripe apple (Bramley Seedling) is shown in Fig. 7. Sufficient ethylene escapes from one apple ventilated at the rate of 25 c.c. per min, to start the ripening processes of bananas immediately. In view of the work of Kidd & West (1932) on the ripening of apples, it is probable that premature ripening of bananas also can be induced by the volatile products from pears, peaches and tomatoes, but not by fully coloured oranges or grapes. The opinion is held in the banana-carrying trade that oranges and citrus fruits in general are potential sources of danger and liable to cause the bananas to ripen. Laboratory experiments do not confirm this, though it should be remembered that, when citrus fruits have been treated with ethylene, this gas would dissolve in the tissues of the fruit and escape from solution again after the fruit was removed from the conditioning rooms. There is no evidence as to the time required for all the ethylene to be lost from these tissues.

7 176 R. GANE 2O 40 plp.m. OZONE I p.p.m. ^40 ab I 2O 2 ri rt O I-5 p.p.m 4O Days Fig. 6. The effect of ozone on the production of carbon dioxide by bananas at 15 C.

8 Respiration of Bananas in Presence of Ethylene 177 Laboratory tests suggest that no ethylene-like substance is produced by rotting banana stems, rotting bananas, yeast growing in a culture solution, or by onions. None of these produce any substance which acclerates the ripening of bananas. Using a different indicator, either tomato shoots or potato shoots, Denny, Miller, Crocker, Days Fig. 7. Effect of the volatile products from an apple on the production of carbon dioxide by bananas at 15 C. Bananas II in fresh air throughout. At A, air was passed over an apple (Bramley Seedling) and then over bananas I. At B the apple was removed from the air stream. Hitchcock & Zimmerman (1935) have shown that ethylene is produced by a variety of tissues including parts of flowers, e.g. petals, anthers; immature fruits; leaves; roots and tubers; but data is not available to show whether the quantity produced is sufficient to affect bananas. SUMMARY Evidence is given to show that ripe bananas produce ethylene. The effect on seedlings of Pisum sativum is paralleled by low concentrations of ethylene. An acceleration of ripening of unripe bananas by ethylene at a concentration of i part per million is similar to that produced by

9 178 R. GANE the products of metabolism of ripe bananas. It would appear that ethylene is a normal product of metabolism during the climacteric when it acts as an autocatalyst. Ethylene can be removed from air by ozone. Ozone causes a retardation of normal ripening, probably by gumming up the stomata. Similar retarding effects are shown by fruit treated with hydrogen peroxide, iodine and vaseline, REFERENCES CROCKER, W. (1929), A delicate method of detecting illuminating gas in a greenhouse. Prof. Pap. Boyce Thompson Inst. 1, No, 11, p. 8i. CROCKER, W,, HITCHCOCK, A. E. & ZIMMERMAN, P. W. (i935)- Similarity in the effects of ethylene and the plant auxins. Contr. Boyce Thompson Inst. 7, 231- DENNY, F. E, (1935). Testing plant tissue for emanations causing leaf epinasty. Contr. Boyce Thompson Inst. 7, 341. DENNY, F, E. & MILLER, L. P. (1935). Production of ethylene by plant tissue as indicated by the epinastic response of leaves. Contr. Boyce Thompson Inst. 7, 97, GANE, R. (1935). The formation of ethylene by plant tissues, and its significance in the ripening of fruits. /. Pomol. 13, 351, (1936). A study of the respiration of bananas. New Phytol. 35, 383, KIDD, F. & WEST, C. (1932). Effects of ethylene and of apple vapours on the ripening of fruits. Rep. Food Invest. Bd., Lond., p. 55. (1933)- The influence of the composition of the atmosphere upon the incidence of the climacteric in apples. Rep. Food Invest. Bd., Lond., p. 51,

10