Water Stress or Drought: Effects and Mechanisms

In this article we will discuss about Water stress or Drought. After reading this article you will learn about: 1. Effect of Water Stress on Growth and Metabolism 2. Mechanisms to Overcome Water Stress.

Effect of Water Stress on Growth and Metabolism:

i. Effects on Growth:

When a plant tissue suffers from water stress, there will be a reduction in turgor pressure. Since cell expansion is influenced by turgor potential 0Ft), developing cells will expand less and cell size will be smaller under this condition. However, the critical water potential for inhibition of cell expansion is different among species and among organs within plants.

ii. Effect of Cell Ultra-Structure:

Water stress affects the structure and function of membranes, which may lead to change in the ultra-structure of cells. Chloroplast and mitochondrial structure can be damaged by severe water stress. With the disruption of thylakoid structure, lipases along with fat droplets increase inside chloroplasts.

Associated with reduced water potential, plastoglobules derived from thylakoid membrane increase in number and size. In plants suffering from water stress, the PS II complexes may be disturbed which in turn may lead to separation of thylakoid membranes from cell membranes.

iii. Effects on Photosynthesis:

Water deficit has several effects on photosynthesis. However, the initial effect of water limitation is usually stomatal closure. Any of the factors, viz., a root signal like ABA, or low turgor pressure in guard cells or increasing vapour pressure gradient between the leaf and air may lead to stomatal closure. When stomata are closed, it is likely that there will be a depletion of CO₂ in the intercellular spaces.

This is termed stomatal inhibition of photosynthesis. Once CO₂ has decreased relative to O₂, diversion of carbon from photosynthesis to photorespiration will be stimulated.

If the light intensity is too high, photo-inhibition of photosynthesis may occur resulting in the formation of free radicals in the chloroplasts, which is caused by failure to utilize all the energy-rich products of electron transport chain. These free radicals, generally active oxygen species, generated inside the chloroplasts, are harmful to the protein environment in which the chloroplast reactions occur.

Photo-inhibition is the main non-stomatal inhibition of photosynthesis under water stress. In some cases, however, water stress directly inhibits the photosynthetic apparatus through reduced turgor, which results in a change in chloroplast pH and ion concentrations.

As a consequence, the activity of rubisco changes along with a few other enzymes of the Calvin cycle. Other events of non- stomatal impact of water stress are chlorophyll degradation and the concomitant decrease in light harvesting and electron transport associated with PS II.

iv. Effects on Dark Respiration and Carbohydrate Metabolism:

Dark respiration of whole plants or shoots or mature leaves subjected to moderate water stress either remain unchanged or increase slightly over unstressed material. With increasing stress severity and duration, however, the respiration rate has been found to decline.

Under such water limitation, photosynthesis decreases before respiration. It has been envisaged that decrease in the ratio of photosynthesis to respiration and increase of both photorespiration and dark respiration during water stress may also give rise to plant starvation stress.

As regards carbohydrate metabolism, loss of starch and increase in simple sugars are the common features linked with water limitation. Carbohydrate translocation also decreases during water stress.

The decrease in sucrose translocation is caused by change in source-sink relationships during water stress. A decrease in the gradient of sucrose between source leaves and photosynthetic sinks is caused by low CO₂ assimilation by leaves and increased respiration in mesophyll cells.

v. Effects on Nitrogen Metabolism:

The movement of nitrogen from roots to leaves slows down and consequently higher concentration of nitrate accumulates in water-stressed roots than in the roots of irrigated plants.

Under this condition, high nitrogen level in roots inhibits further uptake of nitrogen from the soil. A low level of both oxidized and reduced forms of nitrogen may be due to higher losses of foliar nitrogen in water-stressed plants than in well-watered plants.

Nitrate reductase activity has been shown to decline when plant water status is lowered whereas the activity of nitrite reductase appears to be relatively insensitive to water stress. Water stress is also associated with increase in protein hydrolysis and decrease in protein synthesis along with concomitant increase in free amino acids.

Mechanisms to Overcome Water Stress:

The following are the different aspects of plant adaptation to water deficit or drought:

i. Drought Escape:

Some plants, known as ephemerals, are short-lived desert plants. These ephemeral plants germinate, grow and flower very quickly after rains. Thus, they complete their life cycle before the supply of water in soil is depleted and form dormant seeds before the onset of dry season. These plants are known as drought escapers since they escape drought by rapid development.

ii. Drought Avoidance:

Some plants are able to endure periods of water deficit while they still maintain a high tissue water potential. So by avoiding tissue dehydration, they can avoid drought. Such drought avoidance can be associated with either maintenance of water uptake or a reduction of water loss.

In order to maintain water uptake, roots grow deeper into the soil with a concomitant increase in root density. Reduction of water loss can be achieved through stomatal control of transpiration. Succulents are the best examples of epidermal control of tissue water potential.

Another mechanism for reducing water loss is the reduction in the amount of radiation load on plants. This can be directly achieved by leaf movements whereby some leaves lie parallel to the incident radiation. Production of hairs, surface wax or salt on leaf surface increases reflectance of the leaves.

Water loss may also be reduced through reduction in the evaporative surface area. The shedding of large, thinner leaves and production of smaller, thicker leaves with the onset of dry season provides the plant with a smaller evaporative surface and aids in water economy.

iii. Drought Tolerance Through Solute Accumulation (Osmotic Adjustment):

When the plant water potential declines, the maintenance of turgor pressure is important for its growth and metabolic activities.

Turgor pressure can be maintained by an accumulation of solutes. Solute accumulation is an important aspect of plant adaptation to water deficit. In order to adapt to external low soil water potential, a low solute (osmotic) potential by solute accumulation must be achieved if volume and turgor are to be retained.

Ψ = Ψ_µ + Ψ_p + Ψ_m

where Ψ is total water potential, Ψ_µ is osmotic or solute potential. Ψ_p is turgor or pressure potential and Ψ_m is matric potential. Thus, the adjustment of Ψ or osmotic adjustment by solute accumulation is an adaptive response to osmotic stress.

The compounds, which accumulate during water deficit include amino acids such as proline, quaternary ammonium compounds such as glycinebetaine, polyhydroxy alcohols like glyoerol and sorbitol and sugars like sucrose. The accumulation of proline is usually the most outstanding change among the free amino acid pools. These compounds have protective effects against osmotic stress.

Protoplasmic Resistance:

Many species have protoplasm that survives periods of low water content. These desiccation- tolerant plants are called poikilohydricor resurrection plants. The loss of water from these plants causes dehydration of cytoplasm but the metabolic function can be regained following rehydration. The examples of such plants are lichens and mosses like Sphagnum.

Three criteria must be fulfilled by the tissues of desiccation-tolerant species. First, cellular metabolism should be so designed that highly concentrated cytoplasm caused by desiccation does not affect the basic cellular machinery.

Second, there must be relatively less frequent connections between the cells by plasmodesmata because cell shrinkage is likely to break the connections. Third, the cell wall must be able to withstand extensive dehydration without losing structural integrity.

(iv) Development of Embolisms: Xylem Cavitation:

Another mechanism by which plants compensate for water limitation is the development of embolisms in xylem cells. When a xylem cell has an embolism, cavitation occurs which prevents water movement through that cell.

As the number of cells developing cavitation increases in a stem, hydraulic conductance (i.e., water movement) decreases. Plants, which are devoid of embolisms will be able to continue transpiration even at low cellular water status than will species that are susceptible to embolism.

Xylem cavitation is a mechanism to reduce water loss during dry seasons. Under such a condition, a loss of continuity of water column in xylem pipelines will reduce water movement to the leaves, which in turn will induce stomatal closure and lower transpiration.

Root is the Sensor of Water Stress of Shoot:

Roots are able to sense the drying of the soil and this information is communicated from the root to the shoot. It is believed that dehydration of roots may generate a chemical response that moves to the shoot.

Such root signals are related to some hormones, which are exported to the shoot possibly in the transpiration stream. Two hormones like cytokinin, the concentration of which- is reduced by water stress and ABA increased by water stress may be held responsible in signal transduction between different plant organs.