Root Pressure in Plants (With Experiment)

Root Pressure in Plants (With Experiment)!

If a well-aerated plant growing vigorously in spring is cut off slightly above the ground, water is seen to exude from the cut end of the stump through the xylem.

This positive pressure occurs in the xylem either in deciduous trees when the leaves are shed in early winter or when the plant is in a saturated or near saturated condition.

This exudation is particularly marked and prompt in conditions where transpiration from the leaf surface is very low, and the conditions favour rapid absorption of water from the soil. A pressure is thereby developed in the xylem.

The exudation of the xylem fluid under these conditions, although slow, can take place against considerable pressure. The magnitude of this pressure can be measured by attaching a closed manometer to the cut end of the stump.

This pressure, which is a push from below arid not due to any tension developed in the xylem vessels due to transpiration and which seems to originate in roots has been termed root pressure. The quantities of water moving upwards through the xylem by root pressure are, however, very small compared to the amounts moved upwards during active transpiration.

Under soil conditions favouring rapid water absorption combined with slow transpiration, water is also excreted from the leaves in Nasturtium, Colocasia, grasses, etc. in liquid form, particularly early in the morning when the soil is warm and moist and the atmosphere nearly saturated.

This is sometimes referred to as guttation or bleeding. Guttation or bleeding and root pressure are now considered to be merely different aspects of the same phenomenon.

The development of this root pressure in the dilute sap of the xylem vessels originat­ing certainly in the root cells is not quite fully understood as yet. Some investigators believe that this is due to a development of higher osmotic concentrations in the xylem vessels than in the external soil solution.

The magnitude of the hydrostatic pressure developed due to this difference, is a measure of root pressure. Osmotic movement takes place from the soil solution to the xylem through a multicellular semipermeable mem­brane- —the intervening cortical cells of the root being fully turgid with a consistently decreasing gradient of water potential, allow flow of soil solution through them passively.

This osmotic explanation of root pressure can, at best, be only partial for the rate of exudation from a cut stump is usually too rapid to be explained only in terms of simple osmosis. It is more than probable that other mechanisms, as yet not fully understood, are involved.

The magnitude of the root pressure rarely exceeds 2 atmospheres (pressure as high as 7 atm. has been reported in tomato roots), and is generally much less and thus the magnitude of the pressure developed is seldom sufficient to force water to the top of any but small herbaceous plants.

The fundamental objection to the idea that root pressure plays any prominent part in the ascent of sap in plants lies in the fact that root pressure is usually negligible in plants of temperate regions during the summer where, in general, conditions of the soil rarely favours excessive absorption for any considerable length of time.

During the periods of rapid transpiration particularly in midsummer, it has some­times been observed that the cut end of a stump absorbs water, instead of exuding, if water is supplied at the cut surface. This has sometimes been referred to, very inappro­priately, negative root pressure. This should really be called root suction.

It must be understood here that root pressure is a hydrostatic pressure developed in the sap of the xylem vessels. But how is this root pressure maintained in the non­-living xylem vessels? The non-living xylem cells certainly do not possess any semiperme­able membrane like the living cells of the plant body.

Active absorption of solutes from the soil solution and secretion of solutes by the vessels from the adjacent living cells of the root must accompany rapid absorption of water in order to maintain root pressure in the xylem vessels.

The leakage of the solutes from the vessels into the neighbouring living cells and ulti­mately out of the roots into the soil is presumably prevented by the narrow, usually one-row thick, hollow cylinder strip of cells— the endodermis—with its cell walls peculiarly thickened with fatty substances—the casparian strip.

Soil conditions favouring rapid water absorption by root such as low concentration of the solutes, higher temperature, soil aeration, etc., accompanied simultaneously by conditions which greatly reduce transpiration rates from the leaves make the development of the root pressure possible.

Experiment on the Development of Root Pressure in Plants:

Soil Formed Cut across the stem of a vigorously growing healthy potted plant, a few inches above the ground level, preferably in the mor­ning in spring. Fix by means of strong, thick rubber tubing, a mercury manometer to the decapitated stump as shown in Fig. 672.

Make all connections air tight with vaseline or paraffin and freely water the soil. The level of mercury in the vertical arm of the manometer is noted.

After a few hours, the level of mercury in the manometer is seen to rise and difference in the level is a quantitative measure of the magnitude of the root pressure develo­ped in the xylem pushing out water from the cut end of the xylem vessels into the tube.