The concept of Osmotic adjustment in plants under drought stress | How to impose a Drought Stress | How Plants deal with Stress conditions |

 Osmotic adjustment in plants under drought stress.

Plants take water from the soil with the help of their roots. Water is taken by roots and released back into the environment through the stomata of leaves (transpiration process). It's a natural process and very important for the completion of the water cycle. Sometimes plants faced a deficiency of water and this may disturb the whole mechanism of plants because plants have to be closed their stomata to conserve water. Plants start many processes inside them under drought-stress conditions. In this article, the osmotic adjustment of plants under drought stress is discussed in three parts. The first part discusses only drought stress, and the second part is all about osmotic adjustment in plants. The third and last part discusses and relates both drought and osmotic adjustment in plants and also the technique to impose drought in cotton plants and a general view of plants after drought stress. 

Drought Stress in Plants.

A drought normally means a shortage of water. It’s a temporary condition mostly faced by plants. It’s a condition when the requirement of water (for any purpose like water for plants, soil, animals, etc) is high but there is a very low or shortage of supply of water. 

For example, suppose a limited piece of land has plants on it. Water is the life for plants because the process of making food (i.e. photosynthesis) requires water and sunlight. On a hot day, the plant has enough light for making their food but there is not enough water for the plant because of evaporation, transpiration, and many other factors like that. Now plant faces some kind of stress which is normally termed drought stress because of a deficiency of water for the plant. But when there is rain or a normal environment, the same plant has enough light as well as enough water for the process of photosynthesis. So, drought stress is a short-term stress, 

if it goes long then it is said as aridity (can also be said as permanent drought ). Aridity belongs to climate conditions and a climate can be defined as the average long-term weather conditions of a particular region (more than 30 years). In this sense, we can say aridity is a permanent shortage of water while drought is a temporary shortage of water.

Drought has a diversity of concepts. Like when there is no rain (seasonal rains) in any area for one or two years then this area has some drought stress. Water is very essential for agricultural purposes and if the agricultural regions faced a shortage of water then this is also drought stress. In the same way, if reservoirs of water like rivers, streams, etc faced this stress (low water levels) then it may be a serious problem for that region and it is also a drought stress. If the drought stress goes longer then plants can die because of it. But the region like deserts have drought stress for a longer period (aridity) and they faced a deficiency of water year after year. Only plants with special adaptations (xerophytes) can survive in this region.

Plants are the main producers and they are very dependent on water to live their lives and also for food synthesis. Drought stress causes serious impacts on plant growth and its production of food. A drought is generally a short supply of water for plants but sometimes there is a healthy amount of water present in the soil but the plants are unable to take it and this kind of drought is called physiological drought. This kind of drought can be caused by low water temperature (or frozen water), overuse of fertilizers, or high ionic concentration in water. The misselection of crops in any agricultural area or trying to grow the wrong crop at the wrong time and place also produced drought stress in plants.

To study the drought effect on plants, we can introduce it to different plants by ourselves. This will help us to understand how plant adjust their osmotic conditions in the case of drought stress. But before we move forward, it's important to understand the concept of osmotic adjustment. 

Osmotic adjustment in Plants.

The concept of osmotic adjustment is based on the concepts of water potential, solute potential, osmosis, etc.

Water potential is the free inbuild energy present between water molecules. This is the free kinetic energy of water molecules that helps in the random motion of water molecules. Water potential is negative when anything decreases this free molecular kinetic energy of water molecules while water potential is positive when anything increases the kinetic energy of water molecules.

Water potential depends on many factors like solute potential, pressure potential, gravitational force, etc. Normally it is written as:

Water potential = Solute potential + Pressure potential

The water potential of pure water is zero (in an open beaker) because there are no solute particles present in pure water and the pressure acting on it is atmospheric pressure which is neglected or taken as zero. So it means water potential is dependent on solute potential when only atmospheric pressure is acting on it.

Water potential = Solute potential

When any solute particles are added to water, it decreases the random motion of water molecules which results in the decrease of water potential. As every time, the addition of solute decreases the random motion of water molecules so every time solute potential is taken as negative. It means the more solute is added to the water, the lower the solute potential is (the more solute, the lower it's solute potential) because it results in the decrease of random motion of water molecules.

Water moves from higher water potential to lower water potential (means high water level to low water level). But when the solute is added, then water moves from low solute concentration to high solute concentration (which means solute particles attract water molecules towards themselves). Water shows such movement because it moves from high to low water potential. And the region where solute particles are high the water potential must be low.

Osmosis is the movement of water molecules because of solute particles across a membrane. Usually, a cell has a semipermeable or selectively permeable membrane so, we will mainly be focused on these two types of membrane to understand the concept of osmotic adjustment.

We can impose drought stress on plants by two methods i.e. dehydration and withholding of water. The withholding water technique is very frequently practiced in recent years. So we will discuss this technique. It can be used to discuss the osmotic adjustment of (cotton leaves and roots) under water-stressed conditions. For this purpose, we grow cotton plants in sand pots and then placed them in growing chambers where a controlled environment is provided to the cotton plant for research and studies.

Imposing drought stress in cotton plants.

• Take 0.35L of the aluminum container of sand and placed it in a controlled environment.
• Germinates seeds under darkness with constant temperature (i.e 30°C), until the hypocotyl stem{stem of newly germinating seed present between leaves and roots of very young plant i.e below cotyledons (leaves) and above radicle (root)}is formed from this cotton seed.
• Then transferred this newly born plant or seedling into the growth chamber.
• The growth chamber has controlled conditions like 40% humidity, and about 14 hours of photoperiod, source of light is fluorescence tubes and incandescent bulbs.
• For the measurement of photosynthetic photon flux density (PPFD) use Licor LI-190S (i.e. a quantum sensor specially designed for measuring PPFD in both natural and artificial conditions of light).
• Provide half a nutritional solution of food (food + water) to this young plant.
• When the cotton plant is about 30 days of age, then pass this plant from water-stressed conditions by withholding water from the pot. It results in the wilting of leaves. 
• Retain these stressed conditions for 3-4 days.
• These water-stressed conditions were studied with the help of a thermocouple psychrometer, by measuring relative water content (RWC), stomal conductance can be measured by Licor LI-1600 (for the high-speed stomal conductance).

The apparatus and procedure are set up to study the things like:

1. To study osmotic adjustment after the immediate relief of water stress.
2. To study the persistence of osmotic adjustment in cotton plants.
3. To study the effect of varying numbers of stress cycles.

 (Research paper by Derrick M. Oosterhuis, Stan D. Wullschleger )

Results and Observations:

• The components of the leaf of a cotton plant are measured with an end-window thermocouple psychrometer while the root components were measured with a screen-caged thermocouple psychrometer.
• With every increase in the stress cycle, the osmotic adjustment also increased because the plant tries to maintain its capillary action. As in the first stress cycle osmotic adjustment is about 22% while in the third stress cycle, the value of osmotic adjustment increases up to 40%.
• The cotton plants growing in fields introduced more osmotic adjustment than the plants growing in growth chambers. As in the open field, there are many kinds of stresses that the plant has to be faced and there is some kind of competition also in the open field while in the case of the growth chamber, we provide just water stress, and thus plant adapted itself according to that conditions.
• The osmotic adjustment of leaves of the cotton plant was 0.41 megapascal while the osmotic adjustment in roots was 0.19 megapascal. The term osmotic adjustment is normally referred to the osmosis. And osmosis is the movement of water across a barrier or membrane. So in short words, we are talking about water potential and the unit of water potential is Pascal. However, the water potential is very dependent on the solute potential if the pressure acting on it is atmospheric pressure.
• Osmotic adjustment in cotton plants is also affected by the duration of the stress cycle. If the stress duration goes longer, longer the osmotic adjustment can be seen in plants but within limits. The results of the experiment show that plants are very adaptive to their environmental conditions and they try their best to maintain their capillary action to hold the water pressure in any kind of stress conditions.

Drought stress is an abiotic stress because it is caused by the deficiency of water. Drought stress can cause serious adverse effects on a plant’s growth and development. The process of photosynthesis can be turned into the process of photorespiration under drought conditions. Plant metabolism and growth are affected by drought stress.

Drought stress can be induced (in plants) inside a sand pot or can occur naturally when there is actual stress faced by plants or the whole crops. The deficiency of water can cause many processes that inhibit the growth of plants. But all of these processes or adaptations can be said to life saving processes because with the help of these (growth inhibiting processes) plants can survive in stressful conditions. Plants release many hormones under stressful conditions that help plants to survive in unfavorable conditions. Some of the examples of these hormones are:

Abscisic acid is a well-known growth inhibitor hormone. Its competitive hormone is gibberellins (gibberellic acid) which are responsible for growth and metabolism. The interactions of both these two hormones can be understood by the question:

Why the embryo of the seed did not grow when enough amount of food is present in the seed?

The reason is because of abscisic acid is present in the seed in a very high quantity which resists the embryo to get food from the endosperm of the seed. However, a continuous but very small amount of food is supplied to the embryo but it is not enough for proper growth. 

When a favorable environment like water, soil, temperature, etc is provided to the seed, the gibberellins (also present in the seed) start to dominate over the abscisic acid and a proper amount of food like starch is started to provide to the embryo of seed. This starch is broken into maltose by amylase enzyme and then from maltose to glucose by maltase enzyme. Gibberellins activate the amylase and maltase enzymes which proceed with their functions.

 

So the abscisic acid cause the dormancy of the seed. It also caused the shedding off the leaves by plants by inhibiting the formation of mRNA. This process is also called Senescence (old aging). An abscission layer is formed on the stem which results in to shed off of the leaves (mostly seen in the autumn season). The abscission layer was first time seen in cotton where a layer is formed on the fruit of cotton.

Plants release this hormone in stress conditions (especially drought stress conditions). that’s why it is also called the stress hormone. In drought conditions, the main problem is water deficiency and plants lose their water through stomata (i.e. transpiration process). Abscisic acid close the stomata to prevent water loss and in this way, it can also be called an antitranspirant hormone because it inhibits transpiration. This is the life-saving adjustment that plants have during drought-stress conditions.

Abscisic acid can be provided to the crops to inhibit water loss by transpiration.   

Ethylene is another hormone that causes absence and senescence. It decreases the rate of photosynthesis and increases the rate of respiration and in this way, it causes ripening of fruits. Shedding the leaves off is suitable for plants because in stressful conditions it decreases their usage of energy.

 

Photorespiration.

Plants also have a process of photorespiration which they perform under drought conditions. In normal conditions (i.e. when water is available) plants perform photosynthesis. The process of photosynthesis is subdivided into two phases, i.e. light reaction and dark reak reaction. 

In light reactions, water molecules break and release electrons (at photosystem 2). This process is called hydrolysis of water molecules. The end products of the light reaction of photosynthesis are ATP, NADPH, and FADH2. 

The dark reaction of photosynthesis (i.e. also called the light-independent process) takes the products of the light reaction and forms sugar molecules. Rubisco (a five-carbon molecule) runs a cyclic process called the Calvin cycle. The rubisco (5C) binds carbon dioxide molecule (1C) and formed two 3-carbon molecules (total carbon number is 6). One of the six carbon molecules is used for making carbohydrates while the remaining five-carbon molecules regenerate rubisco. This is the process that takes place in plants under normal conditions.

When water is deficient, the plant closes its stomata to save water by avoiding transpiration. When stomata is closed the supply of carbon dioxide is very low but its demand is going high because carbon molecules are continuously fixed by rubisco in the Calvin cycle. The concentration of oxygen also rises because the stomata is closed and it is continuously produced during light reaction.

In this case, rubisco binds oxygen with it because of its oxygenase ability and starts the process of photorespiration which take place in three organelles (i.e. chloroplast, peroxisome, mitochondria). it’s a long process than the Calvin cycle because the Calvin cycle takes place only in one organelle I.e chloroplast. The process of photorespiration is commonly seen in C3 plants. It may be a long and waste process but it helps plant to survive in drought stress conditions.