About Abiotic stress
The occurrence of unfavorable environmental factors such as moisture deficit / excess, high radiation, low and high temperature, salinity of water and soil, nutrient deficiency or toxicity and pollution of atmosphere, soil and water are likely to affect the crop growth in terms of morphology (plant size, architecture, malformation of plant organs, growth (height, volume, weight), physiological and metabolic processes and yield of crop plants.
Stress and strain
Any environmental factor potentially unfavorable to plant is termed as stress. The effect of stress on plant condition is called strain. According to Newton's law of motion, a force is always accompanied by a counterforce, for an action there is always equal and opposite reaction. Stress is the action and whereas strain is the reaction. A body of a plant subjected to stress is in a state of strain.
Strain may be elastic or plastic strain.
Up to a point, a strain may be completely reversible and when the stress is relieved, the plant becomes normal.
Beyond the point of elastic strain, the strain may be partially reversible or partially irreversible, which is called plastic strain or permanent set.
Biological stress and strain
Biological stresses differ from mechanical stresses. The plants are able to erect barriers between the body and environmental stress by expending energy. Biological stresses always cause certain amount of injury which is irreversible and is a plastic strain. Hence, a biological stress is defined as any environmental factor capable of inducing a potentially injurious strain in living organisms. The living organism may show physical strain e.g. cessation of cytoplasmic streaming or a chemical strain e.g. a shift in metabolism. If the strain is severe, the organism may suffer a permanent set i.e. injury or death.
Ability of the plants to survive under adverse environmental condition is termed as stress resistance. The stress resistance of biological organisms is of two main types.
Ability of the organism to prevent reversible or elastic strain (physical or chemical change) when exposed to a specific stress.
Plastic resistance is the ability of an organism to prevent irreversible or plastic strain. The term resistance to environmental stress has been used for plastic resistance and elastic resistance is not clearly recognized.
This is the level of exposure of plants to environmental factors that leads to neither injury nor reduction in growth and yield of crops.
The availability of water to the plants depends on the periodicity, degree of rainfall, sub-soil water status, water slope etc. Water stress may occur either due to water shortage or due to excess of water. The former is more common in India and in arid and semi-arid regions and causes Drought, while the latter is caused by the Flood and Water logging resulting in O2 insufficiency to the roots.
What is drought?
Physiologically, it is explained as the reduction in leaf water potential which occurs due to excess transpiration than the water absorption. That is, when water absorption lag behind transpiration, water deficit develops. It is also defined as, “deficiency or dearth of water severe enough to check the plant growth”.
Other factors causing drought:
Besides the deficiency of soil moisture, high temperature, low relative humidity, fast wind etc. aggravate the adverse effect of drought.
Types of drought:
The term drought has been classified in two broads categories.
01. Soil Drought:
- It often leads to atmospheric drought; mainly resulting due to soil
- If both combine, it becomes disastrous
- Otherwise, the struggle for the plants soil drought is more serious/ difficult than that for the atmospheric drought.
More over, the water deficiency in the soil may be physical or physiological in nature.
a. Physical Soil Drought
In this case, there is an actual shortage of water due to limited or non-availability of water from various sources like rainfall and irrigation.
b. Physiological Soil Drought
In this case, water is available in plenty in the soil but the plants growing in such environment can not be able to avail or absorb the water due to the physiological reasons such as presence of excessive salts, pH alterations etc.
02. Atmospheric Drought
This drought occurs due to low atmospheric humidity, high wind velocity and high temperature which cause a plant to lose most of its water by transpiration, thus resulting in water deficit situations.
Categories of Drought Stress
Under normal and stress-free situations, the plant will exist in a soil moisture potential range between - 0.01 and -1.5 MPa. However, at permanent wilting point, the soil water potential will be between – 2.0 and – 4.0 MPa. At this point, leaf water potential will be still low than the soil water potential.
Hsiao (1973) categorised the drought stress based on the water potential as given below:
|Sl. No.||Category of Stress||Soil Water Potential||Reduction in Leaf |
Relative Water Content (%)
|01.||Mild Stress||- 0.1||8 - 10|
|02.||Moderate Stress||- 1.2 to – 1.5||> 10 – 20|
|03.||Severe Stress||> 20|
Effect of Water Stress on Crops
In the mesophytes, salinity and sometimes high / low temperature may also induce water stress, even though the soil is not dry. Normally, when water absorption lag behind he water absorption, water deficit often develops in the plant. Some morphological, physiological and biochemical effects water stress on mesophytes are discussed below:
A. Morphological changes due to Water Stress:
Water stress in relation to Ontogeny:
The amount of injury caused by the water stress (WS) depends on the stage of plant development at which it occurs. Generally, the life-cycle of the annual crops is conveniently divided into three distinct phases as:
- Seed germination and seedling development
- Vegetative growth
- Reproductive growth
i. Seed germination and Seedling growth:
Under field conditions, seed germination and seedling growth are inhibited by the WS in the soil resulting in the poor stands of the crop.
ii. Vegetative growth:
Vegetative growth, in general, and leaf expansion, in particular is severely inhibited by the soil water deficit. Visible injury of WS is seen in the form of wilting. Paleness and dryness of leaves is seen in the drought. Leaf abscission is often noticed due to the accumulation of ABA under drought. Reduced growth is also due to reduction in cell volume and water potential.
iii. Reproductive growth:
Reproductive phase of the crop is highly sensitive to water stress
a. Effect on Cereals:
Water deficit during initiation of floral primordial and anthesis is injurious to cereals like rice and wheat. Loss of flowering synchrony is detrimental to the crop yield. Stress during ripening stage results in reduction in the test weight in cereals.
b. Effects on Pulses:
In pulses, WS during flower induction shortens the flowering period and cause flower abortion. But, stress during pod filling reduces the seed number and its weight as in soybean crop.
Example: In Soyabean,
c. Effect on Cotton:
Cotton crop is highly sensitive to water stress between 45 and 60 days after sowing of coinciding with the square and boll formation respectively.
d. Other crops:
In fruit crops, Water Stress often causes shedding fruits; the “June Drop” of apples and citrus are the best examples.
However, Alvim (1970) reported that coffee tree must be subjected to Water Stress before flowering. This results in profuse flowering and also increases the yield of berries.
Earliness in flowering:
Extended period of drought causes premature flowering (i.e. earliness in flowering) which results in reduction in yield due to reduced size of pods, seeds, fruits etc. Moisture-sensitive stages (critical periods) of water stress in major crops are listed out the following table:
Table. Moisture sensitive (critical) periods of major crops
|Rice||Panicle initiation, flag leaf and milky|
|Sorghum||Booting and flowering|
|Maize||Silking and tasseling|
|Pearlmillet||Booting and flowering|
|Groundnut||Peg penetration and pod development|
|Sunflower||Head formation and early grain filling|
|Soybean||Flowering and pod filling|
|Blackgram and Greengram||Flowering and early pod development|
|Cotton||Square formation and boll formation and development|
|Sugarcane||Cane formation (Upto 120 days after sowing)|
|Banana||All stages especially shooting stage|
|Tomato||Flowering and fruit development|
|Onion||Blub formation and development|
|Flower crops||Bud formation and development|
Beneficial effects of water stress in crops:
Water stress is not always injurious to the crops. It sometimes improves the quality of the crop produce. Some of the examples are discussed below:
- Rubber content is increased significantly
- Desirable aromatic properties of Turkish tobacco is increased
- Alkaloid content of belladonna, datura, digitalis etc. is increased
- Oil content of mint, olive and also in soybean is increased
- Moderate Water Stress improved the quality of the fruits of apples, plum, cherry, peaches, prunes etc. by increasing the contents of soluble sugars and by developing fascinating colours of the fruits
- An increase in protein content of wheat is often noticed
- Under air pollution situations, water-stressed plants are injured lesser than the normal and well-watered plants because of the hindrance in the entry of polluting gases / particles as the stomata are closed due to Water Stress
- Moderate degree of Water Stress is often encouraged before lifting the seedlings and also after transplanting in the main field for better root development. This will also reduce the duration of “Transplanting shock” and also aids in quicker establishment in the main field.
Response of Plants to Drought Stress
Plants growing in drought stress may have the ability to control / avoid stress by escaping (Enduring Drought) or tolerating stress (by developing succulent or Non-succulent habit). These two capabilities are collectively termed as Drought Tolerance.
These plants remain under dormant / perennation to avoid stress period by seeds and shoots. Such plants complete their life-cycle in few weeks within the rainy season (eg. CO 16 variety of sorghum). They are called as Ephemerals. These plants also prolong their life cycle for some time based on the necessity. They reduce water loss by certain mechanism.
These (CAM) plants store enough water in their tissues. Their stomata open at night. They have thick leaves and possess modifications (such as phyllodes and phylloclades) under water stress conditions. They fix carbon during day time with the help of malic acid and CO2, which is released internally during respiration.
- Smaller leaves with thick cuticle
- Sunken stomata with hair (pubescence) eg. Nerium
- Shedding their leaves during summer to avoid excess water loss
- Dehydration of protoplasm
- Reducing enzyme activity
- Favouring the syntheses of ABA (stress hormone) and Ethylene (senescence hormone)
- Closing stomata due to increase ABA concentration, thereby reducing water loss
Thus, because of these special features, succulents and non-succulents grow well under drought conditions. They are not or least affected by stress. Similarly, the arid zone plants also develop mechanisms to tolerate water stress, hence they are not adversely affected in terms of growth and yield. But, the non-arid zone plants suffer heavy loss in growth and yield because they do not have above said mechanisms to tolerate the stress.
iv. Drought Resistant Plants
These plants resist the water stress situations due to the following adaptive features / mechanisms. Therefore, these plants can be grown in drought facing / arid-zone areas.
- Higher rate of photosynthesis because of efficient carboxylating systems (increased activities of RuBPCase, PEPCase, Malic Enzyme etc.)
- Store much water for proper hydration of protoplasm
- Fix carbon by C4 pathway rather than usual C3
- Producing “Aquaporins” – an intrinsic membrane protein in water-stressed plants, which enhance the water flow by 10 – 20 folds (Chrispeels and Maurel, 1994).
Drought Resistance Mechanism
The ability of a crop species or variety to grow and yield satisfactorily in areas subjected to periodic water deficits is termed as drought resistance
Types of drought resistance
- Drought escape: The ability of a plant to complete the lifecycle before serious soil and plant water deficits develop.
- Drought tolerance with high tissue water potential: The ability of the plant to endure periods of drought whilst maintaining a high plant water stress. This is also referred to as drought avoidance (Levitt, 1972).
- Drought tolerance with low tissue water potential: The ability of the plant to endure periods without significant rainfall and to endure low tissue water potential.
I. Drought Escape
Two features of desert ephemerals that are important in drought resistance are
- Rapid phonological development
- Developmental plasticity.
1. Rapid phonological development
Ability to produce flowers with a minimum of vegetative structure enables them to produce seeds on a limited water supply.
2. Developmental plasticity
This feature enable the plants to produce an abundance of vegetative growth, flowers and seeds in seasons of abundant rain, enables the desert ephemerals to both escape drought and survive long periods without rain.
In crop plants, the greatest advance in breeding for water limited environment is achieved by a shortening of life cycle, thereby allowing the crops to escape drought. Therefore, there is a strong consistent negative correlation between grain yield and days to first ear emergence and 40-90% variation in wheat yield under drought condition was accounted for by earliness. In wheat it was observed that drought resistance is greater in early lines than late ones even at the same intensity of drought. However, under adequate water supply, yield is often positively correlated with maturity date in determinate annual crops such as maize, sorghum and sunflower.
An important aspect of developmental plasticity is the ability of plants to transfer assimilates accumulated prior to seed-filling to the grain during the seed filling stage. It was also suggested that when water supply is adequate only a small proportion of grain dry weight comes from the store of prior assimilate in the stems and roots, but when stress occurs in the seed filling stage, an increased proportion of the prior assimilate is transferred to the seed.
To achieve the developmental plasticity, plants frequently have an indeterminate habit. This is an important survival mechanism in that it enables the large amounts of seed produced in wet years to carry the species through prolonged drought periods.
Selection of rapid phonological development is the most rewarding approach in breeding for drought resistance in crops. In cereals, drought resistance varieties of wheat and barley flowered early than the others. However earliness is often negatively correlated with yield in year of adequate rainfall.
II. Drought Tolerance at High Tissue Water Potential
Ability of the plant to endure periods of drought by maintaining high tissue water potential. This mechanism is also called as drought avoidance.
To maintain a high water status during a period of high evaporative demand / or increasing soil water deficit, the plant has two options. It must either reduce the water loss or maintain its supply of water.
A. Reducing Water Loss
i) Increased pubescence and ii) Increased leaf waxiness
B. Maintenance of water uptake
i) Deeper root system
ii) Hydraulic conductance of plants (increasing either the diameter of xylem vessels or their numbers).
III. Drought Tolerance at low tissue water potential
It is the ability of the plant to endure periods of drought and endure low tissue water potentials. This tolerance can be achieved by
Based on the desiccation tolerance of the protoplasm, plants can be classified as poikilohydric or homohydric plants.
1) Poikilohydric (resurrection plants)
The protoplasm of poikilohydric plants can withstand almost complete dehydration and can also withstand dehydration and rehydration in concert with available water without damage.
2) Homoiohydric plants
Majority of the plants are homoiohydric plants. During growth and development, the protoplasm of homoiohydric plants cannot withstand low water potential without injury. Dehydration caused mechanical injury to the protoplast by physical tearing and destruction during water extraction and shrinkage. Small cells with no vacuoles and also the cells that lose their vacuoles and also the cells that lose their vacuoles during dehydration can withstand the most severe desiccation without mechanical injury. The changes in viscosity o the protoplasm and permeability of the membrane play a role in desiccation tolerance. It was also observed that cytoplasmic proteins are more stable to denaturation, coagulation or hydrolysis in desiccation resistant plants and that enzymes are less susceptible to inactivation by stress. RNA-DNA complex through which enzymes are manufactured is generally susceptible to desiccation and sugars play a role in protecting this mechanism in desiccation resistant species and varieties. Sugars may also provide protection against desiccation.
Biochemical effect of drought tolerance
1) Accumulation of Proline, Glycinebetaines etc.
2) Synthisis of Abscisic acid (ABA) etc.
Mitigation of Water Stress
The adverse effects of water stress on crop growth can be mitigated by the application of chemicals such as nutrients, anti-transpirants and Plant Growth Regulators (PGRs), which induce the plants to become adaptive to water stress situations for a specified period and the water requirement for such periods can be minimized or saved.
1. Nutritional Management
Among the major nutrients, potassium and magnesium are found to be highly deficient due to water deficit conditions. Therefore, application of potassium enhances the water uptake as well as the water relations in the plant tissues by osmoregulation processes, by acting as a potent osmoregulator (osmolyte), thereby the solute potential is reduced. Besides, potassium nutrition also helps in the favourable stomatal regulatory mechanisms, which regulate the water balance of the plants. This has also resulted in the increased WUE of the plants. Similarly, magnesium is component of chlorophyll, its content and uptake is drastically reduced due to the water stress effect. This is most prominent in Mg-loving crops like cotton.
Besides macronutrients, deficiencies of micronutrients also appear under water deficit situations due to the following reasons:
- Depletion due to erosion and leaching. In India, annual soil loss is estimated to be about 6000 Metric tons and obviously due to loss through run off water and soils
- Continuous use of micronutrients free NPK fertilizers in dryland agriculture and diminishing the use of organic matter, FYM, compost and green / green leaf manures.
- Use of high-yielding varieties (HYVs), adoption of intensive systems of farming and cropping and use of heavy doses of fertilizers, increased proportionately the mining of micronutrients from the soil
- Since increased crop production arising from the heavy demand of the nutrients in rapid depletion of macro and micro-nutrients unless regularly replenished. Consequently, the deficiencies of micro-nutrients in general and that of Zn, Fe and B in particular are widely spread under stress conditions.
Therefore, foliar application of the following nutrients depending upon the occurrence of their efficiencies will mitigate the water-stress induced nutritional imbalance in crops.
- 2 % DAP
- 0.5 to 1 % potassium chloride (KCl)
- 0.5 % Zinc sulphate
- 0.5 – 1.0 % Ferrous sulphate + 1 % urea
- 0.3 % Boric acid
Use of Antitranspirants
In India, about 90% of the land is under rainfed farming; therefore, it is very essential to manage every drop of water received through rains. Though various measures are adopted to conserve he rain water, yet rainfed farming is often subjected to drought. Transpiration is said to be unavoidable evil but it has several functions to attend in the crop cycle. For producing one tone of food, the crop plant requires varied amount of water as furnished below:
Cereals and legumes: 400 – 500 litres of water / kg of grains
Fruits and vegetable: 1000 litres / kg of food
Water transpired by crops (season / plant):
Maize : 200 litres
Sunhemp : 27 litres
Cotton : 8 – 10 litres / day
Citrus : 100 – 200 litres / day
Trees (9 – 10 m height) : 300 – 800litres / day
Forest trees of 400 – 600 trees: 20,000 barrels / day (1 barrel = 500 litres)
Similarly, the WUE of crops is also different and ranges from 0.24 to 1.75 kg / mm of water / ha. The WUE of sorghum is higher but that of cotton is the lowest. This difference lies with the maturity period and nutritive value of the crop. Cotton grows for six to seven months while sorghum grows for four moths.
Drought reduces the yield by 0 – 100% depending upon the severity. Prolonged drought can drastically reduce the yield to zero level. But, intermittent drought for 10 – 15 days at early or late stage is common under rainfed conditions. Drought during the critical phenological phase like flowering and grain development is highly detrimental. However, the crop productivity is dependant on how fast a plant can recover after a stress of 6-10 days.
The severity of intermittent drought of 6-10 days during critical stages of the crop can reasonably be avoided by the use of antitranspirants and thus crops can be saved. Antitranspirants can effectively be used to the crop under water stress with adverse rainfall.
Classification of Antitiranspirants (ATs) and field responses
The ATs are categorically classified on mode of action in the following four types:
I. Materials causing stomatal closure
1. Herbicides like 2, 4 – D, Phosphon D and Atrazine
2. Fungicides like Phenyl Mercuric Acetate (PMA)
3. Metabolic inhibitors like hydroxy sulfonates, potassium metabisulphite etc.
4. Growth hormones like ABA, Ethrel, TIBA, succinic acid, ascorbic acid and
II. Reflectant Types
2. China Clay
3. Calcium bicarbonate
4. Lime water
III. Thin-forming chemicals
1. Hexadecanol (Higher alcohols)
2. Cetyl alcohol
IV. Polyethylene materials forming thick films
4. S- 800
(All the above chemicals are trade names given by the companies)
The purpose of ATs is to maintain the growth and productivity under stress conditions and it is never recommended for high productivity / unit area. It saves the crop and helps to get marginal yield when the expectations are zero.
Role of ATs in Irrigation Water Saving
Some of the ATs can also be used through drip (as Fertigation) to save the frequency of irrigation. In this context, the crop productivity could also be increased by 26.2, 23.6 and 15.4 % over unsprayed control with the sprays of Hico-100 R, paclobutrazol and 8– Hydro Quinine respectively under 6 limited irrigations as against 9 irrigations and thus considerably saved irrigation water.
Thus, assured benefits of ATs to the crops can be summarized as below:
- Optimized yield levels under infrequent rainfall situations
- Assured better crop growth and yield when no yields are expected using severe drought
- Getting normal sized grains
- Improved seed quality (so that produce can be used for seed purpose)
- Saving of crops with marginal crop productivity under drought
- Reducing irrigation especially in post-rainy long duration crops like cotton and pigeon pea
- Minimizing irrigation frequency and saving water through drip irrigation (eg. Cetyl alcohol and / or Hexadecanol)
- Monitoring crop loss with limited inputs
- Monitoring / managing drought
- Arresting fast receding soil moisture for better growth and yield of rabi crops
- Very useful for farmers with minimum irrigation facilities
- Saving large nurseries when water is scarce in summer months
USE OF PLANT GROWTH REGULATORS (PGRS)
The plants possessing moderate canopy development (moderate values for LAI), less reduction in photosynthesis, deeper root system, higher root / shoot ratio and delayed senescence will perform better under water stress conditions.
Toward this, application of some of the PGRs will prove beneficial for better crop growth and development when grown under water deficit situations. Some of the PGRs and their effects on crops in order to suit to the water stress conditions are:
Cycocel & Mepiquat chloride:
For promoting root growth (for more water absorption) and suppressing leaf area development (for reducing transpiration loss of water) and delaying on set of leaf senescence.
Cytokinins and Salicylic acid:
They delay the leaf senescence processes and also favour stem reserve utilization by the developing grains especially during the water defict situations.
These PGRs increase the photosynthetic activity of the plants
Ascorbic acid acts as an anti-oxidant agent for scavenging Reactive Oxygen Species (ROS) accumulating under stress and thus avoiding membrane damage.
Pre-sowing Hardening of Seeds / Plants:
Hardening of seeds / plants to required temperature / chemicals enables the plants to overcome the specific stresses. This process actually hardens the protoplasm (by osmoregulation), which enables the seeds to absorb more water under favourable situations to maintain its viability under unfavourable conditions.
Chemicals used for seed hardening process especially under rainfed conditions:
1. 1% KCl 2. 1% KH2PO4 3. 100 ppm Succinic acid 4. 0.5% NaCl 5. 100 ppm ZnSO4
6. 100 MnSO4 7. 100 ppm Ascorbic acid 8. 250 ppm Cycocel 9. 0.5% MgSO4
Thus, these chemicals / PGRs could serve as boon to the frustrated farmers of rainfed areas, if rightly adopted with perspective vision to have food security. Adoption of the agrotechniques is the only solution for farmers of dryland and water stressed scenario to save millions of world population in millennium especially in the developing countries, like India.
Agro-techniques for mitigating Water Stress
- Foliar spray of 2%c DAP + 1% KCl (MOP) during critical stages of flowering and grain formation
- 3% Kaoline spray at critical stages of moisture stress
- Foliar spray of 500 ppm Cycocel (1 ml of commercial product per litre of water)
- Mulching with 5 tones of sorghum / sugarcane trash, which saves 19-20% of irrigation water by reducing evaporation loss of water
- Split application of N and K fertilizers as in cotton at 45 and 60 DAS
- Use of biofertilizers viz., Azospirillum or phosphobacteria @ 10 packets / ha along with 25 kg of soil or FYM
- Application of 12.5 kg / ha along with 37.5 kg of sand
- Seed hardening with 1% KH2PO4 and other salts for 6 – 8 hours (depending upon nature of seed coat) soaked in equal volume of water
- Spray of 40 ppm NAA (4 ml of Planofix in 4.5 litres of water)
- Seed treatment + soil application + foliar spray of Pink Pigmented Facultative Methnaotrops (PPFM) @ 106 as a source of cytokinins.
- As in cotton, nipping terminal portion f main stem beyond 15th (at 70 - 80 DAS) and at 20th node (at 90 DAS) in the case of hybrids and varieties respectively for arresting transpiratory loss of water)
- Foliar spray of 0.5% zinc sulphate + 0.3 % boric acid + 0.5 % Ferrous sulphate + 1% urea during critical stages of moisture stress