Water Balance Problems in Plants –Discussed!

In this article, we will discuss about the water balance problems in plants.

Primitive plants, not to speak of life itself, are believed to have originated in the seas where the control of such phenomena such as transpiration, wilting and drought did not arise at all.

In the aqueous medium, protoplast evolved to a high degree of complexity which adapted fully to a water-saturated condition.

Later, when plant life began to encroach upon the land, the most significant advance was represented by the translation from an aquatic to terrestrial habitat.

In this translation two main obstacles had to be surmounted:

Overcoming gravity and mechanical forces of air movement and overcoming drought. Both the obstacles were most successfully overcome by plants coming to the land from water which possessed or developed a vascular system and espe­cially those in which the soma was divided into a root for water absorption and an aerial shoot with stem and leaves.

The problem of keeping the water content of the protoplasm of the cells above a certain high minimum, necessary for its normal and continued exis­tence (in the terrestrial environment the risk of almost continual loss of water from the aerial parts to the atmosphere was great) was solved and the landward migration became possible with the gradual development of more and more efficient adaptations to cope with the entirely new conditions in which the migrated plants found themselves.

Adaptations permitting the maintenance of a satisfactory balance between the absorption and loss of water evolved mainly in two different directions. On the one hand, there were developed nearly water-impervious coverings of cutinised and suberised tissue which greatly helped in checking excessive water loss by transpiration.

On the other hand, roots and rhizoids, structures highly efficient in extracting moisture from the deeper layers of soil, were evolved. In the water habitat, absorption was no problem for water flows freely and abundantly to aquatic plants.

Cutin and suberin are, to some extent, effective agents in regarding the loss of water from a plant surface. However, at the same time they have the serious disadvantage of restricting the exchange of gases between the plant and the atmosphere -a function which is absolutely necessary for both photosynthesis and normal respiration.

This difficulty was surmounted by pulling apart of certain epidermal cells so as to leave inter­cellular gaps or pores (stomata) in the superficial layer which are encrusted with cutin or suberin.

Such a solution to the problem was at its best only an apparent compromise for even though all but a very small fraction of the total exposed surface is protected by water-proof coverings, the stomata and lenticels which constitute the small area, permit the loss of great quantities of water vapour.

Exchange of gases like CO₂ and O₂ and water vapour between the plant and the atmosphere takes place by diffusion that is always from higher concentration or pressure to lower concentration or pressure.

Furthermore air outside the leaf contains only 0.03% of CO₂ and 21% of O₂, while the internal atmosphere of the mesophyll of the leaves is nearly always saturated with water vapour under conditions of normal absorption by roots. Thus transpiration is much more rapid than inward diffusion of either CO₂ or O₂.

Owing to the fact that the average vascular plant absorbs water through its roots at the same time that it loses water vapour to the air from the aerial parts, the ratio between these two physiological processes determines the amount of water contained in the tissues.

The ratio between the water gain and water loss is called the water balance of the plant. The true significance of water to plants cannot safely be understood in terms either of water intake or of water loss but only as water balance.

In as much as absorption and transpiration are partly controlled by the environ­ment and partly by structural organisation of the plant there are evidently both internal and external aspects of water balance.

The external aspects consist of:

(1) The amount of soil water which is available to the root system and

(2) Intensity of transpiration- favouring factors.

The internal aspect of water balance depends largely upon those structural and accompanying functional characteristics which tend to minimise or increase the natural limitations of the habitat.

In many terrestrial plants, absorbing and conducting systems are relatively in­efficient in supplying water to meet the demands of high rates of transpiration parti­cularly in summer, even at times when available water is plentiful.

As a result of this inefficient conduction of water from the absorbing to the trans­piring sin-face, most plants, even if growing in moist soil, lose more water during day­light hours than can be fully compensated and replaced by absorption.

At night, however, transpiration rates decline sometimes to no more than a small fraction of the diurnal rate and this permits a reverse in the direction of water balance trend, for absorption by roots at night is more or less unaffected, so that the deficit due to transpiration during daylight hours is gradually changed to a surplus.

It has been truly said that if just once, night failed to follow the day, many of the plants on earth would perish.

Susceptibility to an unfavourable water balance varies continuously throughout the life cycle of the average plant. Some seeds and spores are often capable of enduring extreme drought. The seedlings of cereals retain a high degree of drought resistance until several days old when the first leaves unfold when they become very sensitive to desiccation.

In comparison with plants grown under optimal moisture conditions, those grown under an unfavourable water balance have all the xerophytic physiological characteris­tics. By xerophytic here, is meant non-inherited structural characteristics, which clearly results from an exposure of plants during tissue differentiation to an unfavourable water balance.