Drought Mecha

8/7/2019 Drought Mecha

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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 asDrought Tolerance.

i. Drought Evading Plants

These plants remain under dormant / perennation to avoid stress period byseeds and shoots. Such plants complete their life-cycle in few weeks within therainy 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.

ii. Succulents

These (CAM) plants store enough water in their tissues. Their stomata open atnight. They have thick leaves and possess modifications (such as phyllodesand 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.

iii. Non-succulents

These plants endure drought with the following adaptive features:

1. Smaller leaves with thick cuticle2. Sunken stomata with hair (pubescence) eg. Nerium3. Shedding their leaves during summer to avoid excess water loss4. Dehydration of protoplasm5. Reducing enzyme activity6. Favouring the syntheses of ABA (stress hormone) and Ethylene

(senescence hormone)7. Closing stomata due to increase ABA concentration, thereby reducing

water loss

Thus, because of these special features, succulents and non-succulents growwell 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 donot 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, theseplants can be grown in drought facing / arid-zone areas.

1. Higher rate of photosynthesis because of efficient carboxylating systems (increased activities of RuBPCase,PEPCase, Malic Enzyme etc.)2. Store much water for proper hydration of protoplasm3. Fix carbon by C4 pathway rather than usual C34. Producing ³Aquaporins´ ± 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 istermed as drought resistance

Types of drought resistance

1. Drought escape: The ability of a plant to complete the lifecycle before serious soil and plant water deficitsdevelop.

2. Drought tolerance with high tissue water potential: The ability of the plant to endure periods of droughtwhilst maintaining a high plant water stress. This is also referred to as drought avoidance (Levitt, 1972).

3. Drought tolerance with low tissue water potential: The ability of the plant to endure periods withoutsignificant rainfall and to endure low tissue water potential.

I. Drought Escape

Two features of desert ephemerals that are important in drought resistance are

1. Rapid phonological development2. Developmental plasticity.

1. Rapid phonological development

Ability to produce flowers with a minimum of vegetative structure enables themto 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 ephemeralsto 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 lifecycle, thereby allowing the crops to escape drought. Therefore, there is a strong consistent negative correlationbetween 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 oneseven at the same intensity of drought. However, under adequate water supply, yield is often positively correlated withmaturity 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 toseed-filling to the grain during the seed filling stage. It was also suggested that when water supply is adequate only asmall proportion of grain dry weight comes from the store of prior assimilate in the stems and roots, but when stressoccurs 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 survivalmechanism in that it enables the large amounts of seed produced in wet years to carry the species through prolongeddrought 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 earlinessis 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 alsocalled as drought avoidance.



To maintain a high water status during a period of high evaporative demand / or increasing soil water deficit, the planthas 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

Leaf pubescence 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 beachieved by

1. Maintenance of Turgor

2. Desiccation Tolerance

Desiccation Tolerance

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 withstanddehydration 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 homoiohydricplants cannot withstand low water potential without injury. Dehydration caused mechanical injury to the protoplast byphysical tearing and destruction during water extraction and shrinkage. Small cells with no vacuoles and also thecells that lose their vacuoles and also the cells that lose their vacuoles during dehydration can withstand the mostsevere desiccation without mechanical injury. The changes in viscosity o the protoplasm and permeability of themembrane play a role in desiccation tolerance. It was also observed that cytoplasmic proteins are more stable todenaturation, coagulation or hydrolysis in desiccation resistant plants and that enzymes are less susceptible toinactivation by stress. RNA-DNA complex through which enzymes are manufactured is generally susceptible todesiccation and sugars play a role in protecting this mechanism in desiccation resistant species and varieties. Sugarsmay also provide protection against desiccation.





Water transpired by crops (season / plant):

Maize : 200 litresSunhemp : 27 litresCotton : 8 ± 10 litres / dayCitrus : 100 ± 200 litres / dayTrees (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 higherdifference lies with the maturity period and nutritive value of the crop. Cotton grows for six to seven months while sorghum grow

Drought reduces the yield by 0 ± 100% depending upon the severity. Prolonged drought can drastically reduce the yield to zero ± 15 days at early or late stage is common under rainfed conditions. Drought during the critical phenological phase like flowerindetrimental. 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 antitsaved. 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 Atrazine2. 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

Cycocel (CCC)

II. Reflectant Types

1. Kaoline2. China Clay3. Calcium bicarbonate4. Lime water

III. Thin-forming chemicals

1. Hexadecanol (Higher alcohols)2. Cetyl alcohol3. Methanol4. Paclobutrazol5. Brassinolide6. Resorcinol

IV. Polyethylene materials forming thick films

1. Mobileaf 2. Folicot3. Waxol4. S- 8005. Hico-110R(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 pro



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 pro26.2, 23.6 and 15.4 % over unsprayed control with the sprays of Hico-100 R, paclobutrazol and 8± Hydro Quinine respectively 9 irrigations and thus considerably saved irrigation water.

Thus, assured benefits of ATs to the crops can be summarized as below:

1. Optimized yield levels under infrequent rainfall situations2. Assured better crop growth and yield when no yields are expected using severe drought3. Getting normal sized grains4. Improved seed quality (so that produce can be used for seed purpose)5. Saving of crops with marginal crop productivity under drought6. Reducing irrigation especially in post-rainy long duration crops like cotton and pigeon pea7. Minimizing irrigation frequency and saving water through drip irrigation (eg. Cetyl alcohol and / or Hexadecan8. Monitoring crop loss with limited inputs9. Monitoring / managing drought10. Arresting fast receding soil moisture for better growth and yield of rabi crops11. Very useful for farmers with minimum irrigation facilities12. 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 rootdelayed 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 wPGRs 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 osenescence.

Cytokinins and Salicylic acid:

They delay the leaf senescence processes and also favour stem reserve utilization by the developing grains especially during t

Brassinolides:

These PGRs increase the photosynthetic activity of the plants

Ascorbic acid:

Ascorbic acid acts as an anti-oxidant agent for scavenging Reactive Oxygen Species (ROS) accumulating under stress and th

Pre-sowing Hardening of Seeds / Plants:

Hardening of seeds / plants to required temperature / chemicals enables the plants to overcome the specific stresses. This proc(by osmoregulation), which enables the seeds to absorb more water under favourable situations to maintain its viability under u

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 Adoption of the agrotechniques is the only solution for farmers of dryland and water stressed scenario to save millions of worldthe developing countries, like India.

Agro-techniques for mitigating Water Stress

1. Foliar spray of 2%c DAP + 1% KCl (MOP) during critical stages of flowering and grain formation2. 3% Kaoline spray at critical stages of moisture stress3. Foliar spray of 500 ppm Cycocel (1 ml of commercial product per litre of water)4. Mulching with 5 tones of sorghum / sugarcane trash, which saves 19-20% of irrigation water by reducing eva5. Split application of N and K fertilizers as in cotton at 45 and 60 DAS6. Use of biofertilizers viz ., Azospirillum or phosphobacteria @ 10 packets / ha along with 25 kg of soil or FYM7. Application of 12.5 kg / ha along with 37.5 kg of sand

8. Seed hardening with 1% KH2PO4 and other salts for 6 ± 8 hours (depending upon nature of seed coat) soak9. Spray of 40 ppm NAA (4 ml of Planofix in 4.5 litres of water)10. Seed treatment + soil application + foliar spray of Pink Pigmented Facultative Methnaotrops (PPFM) @ 106 11. As in cotton, nipping terminal portion f main stem beyond 15th (at 70 - 80 DAS) and at 20th node (at 90 DAS

respectively for arresting transpiratory loss of water)12. Foliar spray of 0.5% zinc sulphate + 0.3 % boric acid + 0.5 % Ferrous sulphate + 1% urea during critical stag

ource:

Dept. of Crop Physiology, TNAU, Coimbatore

Related links:

http://www.plantstress.com/Articles/drought_m/drought_m.htmhttp://www.plantstress.com/Articles/index.asphttp://www.drought.unl.edu/http://www.drought.noaa.gov/http://www.aces.edu/issues/drought//

University drought sites:

Alabama Cooperative Extension ServiceClemson University (South Carolina)Colorado State UniversityIowa State UniversityNorth Carolina State University

North Dakota State UniversityOhio State UniversityOklahoma State UniversityPennsylvania State UniversityPurdue UniversityTexas A&M UniversityUniversity of GeorgiaUniversity of KentuckyUniversity of MarylandUniversity of Nebraska



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