Univ. of Wisconsin Agri. Exp. Sta. Research Bull. No. 22, pp. 87-181. March 1912.
Metabolic Water: its production and role in vital phenomena
S. M. Babcock

Bulbs And Tubers

All bulbs and tubers undergo transformations similar to the ones which seeds undergo; they constantly respire, absorbing oxygen, evolving CO2, and producing water within the tissues. Respiration occurs chiefly at the growing centers, as in the central bud of an onion, or in the eyes of a potato. The enzymes, located at or near these centers act upon the stored food products, in the bulb or tuber, converting them into soluble and diffusible substances, which in turn are oxidized in the developing cells, thereby raising their water content and increasing their turgidity to a point where growth is induced.

If a bulb like an onion be immersed in water which has been boiled to expel air, it does not sprout, nor does it appear to absorb water. If, however, the bulb be exposed to warm, moist air, it soon sprouts and grows to a considerable extent upon the stored nutrients which the bulb contains. In this case the sprouts contain a higher proportion of water than was originally present in the bulb. The excess of water in the sprouts is chiefly due to metabolic water formed from the organic nutrients by respiration. The fleshy tissues of a bulb serve not only to supply nutrients to the developing bud, but also to protect it from a too abundant supply of oxygen, until conditions are favorable for growth.

A similar provision is found in fruits, in which the pulpy material protects the seeds from free access of oxygen and reduces respiration of the seeds to a point where germination cannot occur until the easily oxidized constituents which surround them are destroyed. In most cases, this maintains the seed in a nearly dormant condition until the following spring, when germination occurs quickly and the plant has a whole season to develop and mature its woody tissues and buds before winter. Were it not for this protection, many seeds would germinate in late summer and the immature growth would be killed by freezing. The influence of the pulpy substance of fruit, in delaying germination of seeds is seen in melons and similar fruits the seeds of which do not germinate so long as the fruit is unbroken, although temperature and moisture conditions may be ideal for growth, but when seeds are removed from the fruit and placed in moist soil, or between wet filters, in contact with air, they germinate quickly. Even when the fruit is broken so that air has free access to the seeds, some seeds germinate in a short time, if molds are suppressed, showing that the one condition, essential to growth, that is absent, is a supply of oxygen. Germination of fresh seeds from pulpy fruits like melons is facilitated by thoroughly washing the seeds to remove the adhering slimy material; unless this is done molds quickly appear in abundance and prevent germination. The best results have been obtained when the washed seeds were placed between filter papers that were moistened with a 3 per cent solution of hydrogen peroxide, which supplies oxygen in abundance and keeps molds from gaining the ascendency, until after the sprouts start. Treated in this way the fresh melon seeds germinate nearly as quickly and as well as dried seeds.

Viability Of Immature Seeds

Seeds seldom germinate while they are attached to the succulent living tissues of the parent plant, although temperature and moisture conditions are usually favorable at this time. The failure to grow may be due to exclusion of free oxygen by the seed envelope, and it may be that organic nutrients, in a form suitable for the growing sprout, are absent at this time. Whatever the direct cause may be, the adverse conditions disappear after the immature seed is exposed to air for a period that depends upon its variety and maturity.

Radish seed, taken from green seed pods, all failed to germinate, when transferred directly from the pod to wet filters, but seed from the same lot, after being kept ten days in warm dry air, all germinated within forty-eight hours, under the same conditions.

Corn that was apparently mature, but picked while the husks were still green, behaved in a similar manner to the radish seed. Not a single kernel sprouted when tested immediately after picking. All grew after a preliminary ten days' exposure to warm, dry air. The same result was obtained with sweet corn, picked in an edible condition while the kernels were still soft and milky.

In the case of corn, this change in viability is not due to preliminary drying, since soft kernels from the same ears, when immersed in hydrogen peroxide, all germinated within two weeks, without having been dried at any time. This indicates that direct respiration is the chief factor in bringing about those changes in the seed, that are essential to germination. Further confirmation of this view is supplied by failure of such seeds to germinate after being kept, for a similar period, in carbon dioxide or in boiled water, where no free oxygen was available.

Development Of Hydrolytic Ferments In Seeds

Kernels of corn, from the same ears described above, were tested for diastatic ferments, when first picked, and again when germination tests were afterwards made. For this purpose, the crushed kernels were digested for a few hours in water to which a little toluol or chloroform was added to inhibit fermentation; to the clear filtrate from this mixture, sufficient starch, in solution, was added to give a faint but distinct blue color with iodine; the solution was kept warm and tested with iodine, at frequent intervals. A control solution in water only, containing the same amount of starch, was exposed and tested in the same manner; if no difference in the intensity of the reaction, with these solutions, was observed after two hours, it was assumed that diastatic ferments were absent from the seed. In all tests made with corn, after it had been picked sufficiently long to germinate well, the starch reaction disappeared in a short time, indicating the presence of a starch inverting enzyme. In only one test of soft corn, that of an ear with the husks dry when picked, was any indication of a diastatic ferment found.

A summary of a number of these tests is given in Table XII. The water content of samples of corn tested is also given to show the relative maturity of the seed.

TABLE XII. GERMINATION OF MATURE AND IMMATURE SEEDS
Influence of maturity and exposure to air upon germination and upon the presence of a diastatic ferment in the seed.

Variety of seeds State of maturity Per cent
water
Percent germination Diastatic ferment
In wet filters In hydrogen peroxide
Yellow dent corn Ripe, one year old 8.5 100 in 48 hours 100 in 48 hours Present in abundance
Yellow dent corn Ripe but soft 40.62 0 100 after 14 days Small amount
Yellow dent corn Ripe but soft 54.22 0 100 after 14 days Doubtful
Stowell’s Evergreen sweet corn In edible condition 74.71 0 -- None
Radish From green pods -- 0 -- --
Radish From same green pods as above after 10 days -- 100 in 48 hours -- --

After being exposed to warm dry air for ten days, seed from each of the ears tested as shown in Table XII germinated within forty-eight hours in wet filters, and also when immersed in a solution of hydrogen peroxide.

These tests indicate that the presence of certain specific enzymes is essential to the germination of seeds, and also that the production of such enzymes occurs only under conditions which admit of direct respiration. The great increase in respiration, as well as in the production of such enzymes at the time of germination gives support to this view.

The course of metabolism in an immature seed differs widely from that in a germinating mature seed. In the former case, soluble nutrients are being converted into insoluble reserve materials, while in the latter case the reactions are reversed, since the sprouting embryo must receive its nutrients in a highly hydrated and soluble form. It is not strange, therefore, that hydrolytic enzymes are wholly absent from immature seeds, so long as a surplus of suitable organic nutrients is supplied through the circulatory system of the parent plant, and that they are formed only after direct and independent respiration is established.

Unbroken seeds never germinate in the digestive tract of an animal, because free oxygen to support direct respiration is not available. In this case, temperature and moisture conditions are ideal for growth and when voided, in the excrement, and exposed to air, such seeds in general germinate quickly.

The absorption of water by seeds, the conversion of starch into dextrose by diastase, the distribution of dextrose by osmosis and diffusion throughout the water of the seed, and all other phenomena that precede germination take place with equal facility in dead and living seeds. In fact nearly all parts of a viable kernel of corn, or other seed, are dead and may be removed from the embryo without affecting the development of new tissue, provided proper nutrients are supplied from other sources.

The very slow action of diastase in converting starch into dextrose, under conditions prevailing in an air dried seed, and its greatly increased activity when an abundance of water is supplied are shown by the following experiment.

Five grams of coarsely pulverized corn meal were boiled in 100 c.c. of water to destroy the activity of the diastase contained in the meal. To another similar portion of the same meal was added 100 c.c. of cold water. A few drops of toluol were added to each to prevent fermentation, and both portions were left at room temperature for twenty-four hours. The filtrate from the cold extract, when boiled with Fehlings solution, gave a copious precipitate of cuprous oxide, while scarcely a trace appeared with that from the boiled meal. The boiled extract contained all of the dextrose that had accumulated during several months in the air dried seed, while the cold extract contained in addition, the amount of dextrose formed in only twenty-four hours, by the same quantity of diastase, in the presence of sufficient water to insure its maximum activity.