Life Movements in Plants Vol 1/Chapter 13

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XIII.—EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH

By

Sir J. C. Bose.

The movements of leaves of sensitive plants are caused by variation of turgor in the pulvinus induced by stimulus. The down movement or negative response of Mimosa is caused by a diminution or negative variation of turgor, while the erection or positive response is brought about by an increase, or positive variation of turgor.

We shall now investigate the change induced in a growing organ in the rate of growth by variation of turgor. Turgor may be increased by enhancing the rate of ascent of sap or by an artificial increase of internal hydrostatic pressure. A diminution of turgor may, on the other hand, be produced by withdrawal of water through plasmolysis. In order to maintain a constant terminology I shall designate an increase, as the positive, and a diminution, the negative variation of turgor.

RESPONSE TO POSITIVE VARIATION OF TURGOR.

In experimenting with Mimosa the plant was subjected to the condition of drought, water being withheld for a day. On supplying water, the leaf, after a short period, exhibited a positive or erectile movement (Expt. 12). The delay was evidently due to the time taken by the water absorbed by root to reach the responding organ.

Method of Irrigation: Experiment 70.—In order to investigate the effect of enhanced turgor on growth, I took a specimen of Kysoor which had been dug up with an attached quantity of soil; this latter was enclosed in a small bag. The plant was then securely clamped and fixed on a stand. This precaution was taken to prevent upward displacement by the swelling of the soil in flower pot of the plant under irrigation. The specimen was then subjected to a condition of drought, water being withheld for a day. The depressed rate of growth is seen in record (Fig. 70). Ordinary cold water was now applied at the root, the effect of which is seen in record C. Finally the record (H) was obtained after irrigation with tepid water. It will be seen that the spaces between successive dots, representing magnified growth at intervals of ten seconds, are very different. While a given elongation took place under drought in 19 $\times$ 10 seconds, a similar lengthening took place, after irrigation with cold water, in 13 $\times$ 10 seconds, and after irrigation with warm water in 3 $\times$ 10 seconds. Irrigation with warm water is thus seen to increase the rate of growth more than six times.

Fig. 70.—Effect of irrigation: D, record of growth under drought; C. acceleration after irrigation with cold water; H, enhanced acceleration on irrigation with warm water. (S. Kysoor.)

The enhancement of the rate of growth on irrigation with cold water took place after seventy seconds. The interval will obviously depend on the distance between the root by which the water is absorbed and the region of growth. It will further depend on the activity of the process of the ascent of sap. The time interval is greatly reduced when this activity is in any way increased. Thus the responsive growth elongation after application of warm water was very much quicker; in the case described it was less than 20 seconds. With regard to application of warm water, the variation of temperature should not be too sudden; it should commence with tepid, and end with warm water. Sudden application of hot water brings about certain complications due to excitatory effect. As regards the persistence of after-effect of a single application of warm water, it should be remembered that the absorbed water gradually cools down. In an experiment with a peduncle of Zephyranthes the growth under partial drought was found to be 0.04 μ per second; application of warm water increased the growth rate to 0.20 μ per second. After 15 minutes the growth rate fell to 0.13 μ per second; and after an hour to 0.08 μ per second. It will be noted that even then the rate was twice the initial rate before irrigation.

TABLE XII.—EFFECT OF IRRIGATION.

Specimen.	Condition of Experiment.	Rate of growth.
Kysoor … … … … … … … … … … … … … … … … … … … … … … … … … … … …	Dry soil … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.21 μ per second.
	Irrigation with cold water … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.30 μ per„ second.„
	Irrigation with warm water … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …	1.33 μ per„ second.„
Peduncle of Zephyranthes.	Dry soil … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.04 μ per„ second.„
Peduncle of Zephyranthes.	Irrigation with warm water … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.20 μ per„ second.„

EFFECT OF ARTIFICIAL INCREASE OF INTERNAL HYDROSTATIC PRESSURE.

Increased turgor was, next, artificially induced by increase of internal hydrostatic pressure.

Experiment 71.—The plant was mounted water-tight in the short limb of an U-tube, and subjected to increased hydrostatic pressure by increasing the height of the water in the longer limb. Table XIII shows how increasing pressure enhances the rate of growth till a critical point is reached, beyond which there is a depression. This critical point varies in different plants.

TABLE XIII.—EFFECT OF INCREASED INTERNAL HYDROSTATIC PRESSURE (Kysoor).

Specimen.	Hydrostatic pressure.	Rate of growth.
No. I … … … … … … … … … … … … … … … … … … … … … … … … … … … …	Normal … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.18 μ per second.
	2 cm. pressure … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.20 μ per„ second.„
	4 cm. pressure„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.11 μ per„ second.„
No. II … … … … … … … … … … … … … … … … … … … … … … … … … … … …	Normal … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.13 μ per second.
	1 cm. pressure … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.20 μ per„ second.„
	3 cm. pressure„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.18 μ per„ second.„
	4 cm. pressure„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.15 μ per„ second.„

RESPONSE TO NEGATIVE VARIATION OF TURGOR.

I shall now describe the influence of induced diminution of turgor on the rate of growth.

Method of plasmolysis: Experiment 72.—Being desirous of demonstrating the responsive growth variations of opposite signs in an identical specimen under alternate increase and diminution of turgor, I continued the experiment with the same peduncle of Zephyranthes in which the growth acceleration was induced by irrigation with warm water. In that experiment the growth rate of 0.04 μ per second was enhanced to 0.20 μ per second after irrigation. A strong solution of KNO₃ was now applied at the root; and the growth-rate fell almost immediately to 0.03 μ per second, or nearly to one-third the previous rate, the depression induced being thus greater than under condition of drought (Fig. 71).

Fig. 71.—Effect of alternate increase and diminution of turgor on the same specimen: N, normal rate under drought; H, enhanced rate under irrigation with warm water; N′, normal permanent rate after irrigation; P, diminished rate after plasmolysis (Zephyranthes).

TABLE XIV.—EFFECT OF ALTERNATE VARIATION OF TURGOR ON GROWTH (Zephyranthes).

Condition of Experiment.	Rate of growth.
Dry soil … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.04 μ per second.
Application of warm water … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.20 μ per„ second.„
Steady growth after 1 hour … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.08 μ per„ second.„
Application of KNO₃ solution … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.03 μ per„ second.„

From the series of results that have been given above, it will be seen that employing very different methods of turgor variation, the rate of growth, within limits, is enhanced by an increase of turgor. A diminution or negative variation of turgor, on the other hand, brings about a retardation or negative variation in the rate of growth. We should, in this connection, bear in mind the fact that, growth is dependent on protoplasmic activity, and the variation of turgor itself is also determined by that activity.

RESPONSE OF MOTILE AND GROWING ORGANS TO VARIATION OF TURGOR.

I have already described (p. 40) the effects of variation of turgor on the motile pulvinus of Mimosa. There is a strict correspondence between the responsive movement of the leaf of Mimosa and the movement due to growth, which is summarized as follows:—

(1) An increase or positive variation of turgor induces an erection or positive response of the leaf of Mimosa, and a positive variation or enhancement of the rate of growth.

(2) A diminution or negative variation of turgor induces a fall or negative response of the leaf of Mimosa, and a negative variation or retardation of the rate of growth.

EFFECT OF EXTERNAL TENSION.

Experiment 73.—The recording levers are at first so balanced that very little tension is exerted on the plant. Record of normal growth is taken of a specimen of Crinum. The tension is gradually increased from one gram to ten grams. The table given above shows how growth-rate increases with the tension till a limit is reached, after which there is a retardation.

TABLE XV.—EFFECT OF TENSION ON GROWTH.

Tension.	Rate of growth.
100 (Normal) … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.41 μ per second.
104 grams … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.44 μ per„ second.„
106 grams„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.48 μ per„ second.„
108 grams„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.52 μ per„ second.„
10 grams„ … … … … … … … … … … … … … … … … … … … … … … … … … … … …	0.40 μ per„ second.„

SUMMARY.

Increase of turgor induced by irrigation enhances the rate of growth. Irrigation with warm water induces a further augmentation of the rate of growth.

The latent period for enhancement of growth depends on the distance of glowing region from the root. The latent period is reduced when the plant is irrigated with warm water.

Artificial increase of internal hydrostatic pressure, up to a critical degree, enhances the rate of growth.

A diminution or negative variation of turgor depresses the rate of growth.

Theme is a strict correspondence between the responsive movement of the leaf of Mimosa, and the movement due to growth. An increase or positive variation of turgor induces an erection of positive response of the leaf of Mimosa, and a positive variation or enhancement of the rate of growth. A diminution or negative variation of turgor induces a fall or negative response of the leaf of Mimosa, and a negative variation or retardation of the rate of growth.

External tension within limits, enhances the rate of growth.