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UPSC MainsAGRICULTURE-PAPER-I201815 Marks
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Q21.

Discuss the effects of water stress on plant growth and development. Describe plant borne mechanisms to escape from drought and stress situations.

How to Approach

This question requires a comprehensive understanding of plant physiology and adaptation mechanisms. The approach should be to first define water stress and its impact on plant growth, followed by a detailed discussion of morphological, anatomical, and physiological adaptations plants employ to cope with drought. Structurally, the answer should be divided into two main sections: effects of water stress and plant-borne mechanisms. Examples of specific plant species demonstrating these adaptations should be included to showcase understanding.

Model Answer

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Introduction

Water stress, encompassing both drought and flooding, is a significant abiotic factor limiting plant growth and agricultural productivity globally. Drought, specifically, is characterized by a prolonged period of abnormally low rainfall, leading to soil moisture deficit. The Intergovernmental Panel on Climate Change (IPCC) projects increased frequency and intensity of droughts in many regions, highlighting the critical need to understand plant responses and develop drought-resilient crops. This response will explore the detrimental effects of water stress on plant physiology and detail the various strategies plants have evolved to mitigate these effects and survive in arid and semi-arid environments.

Effects of Water Stress on Plant Growth and Development

Water stress profoundly impacts plant growth and development at various levels. The effects are not solely limited to a reduction in water availability; they trigger a cascade of physiological and biochemical changes.

Physiological Impacts

  • Photosynthesis Reduction: Water stress leads to stomatal closure to prevent water loss. This reduces CO2 uptake, severely inhibiting photosynthesis. The efficiency of photosystem II (PSII) also decreases.
  • Transpiration Regulation: While stomatal closure reduces water loss, it also limits cooling, potentially leading to leaf overheating.
  • Nutrient Uptake: Water stress impairs nutrient uptake due to reduced transpiration flow and altered root metabolism. Essential nutrients like potassium and nitrogen are particularly affected.
  • Hormonal Imbalance: Abscisic acid (ABA) levels increase significantly under water stress, triggering stomatal closure and promoting stress-responsive gene expression. Auxin and cytokinin levels can also be affected, disrupting cell division and differentiation.
  • Growth Inhibition: Overall growth is stunted, with reduced leaf area, shoot elongation, and root development.

Developmental Impacts

  • Flowering and Fruiting: Water stress can delay or completely prevent flowering and fruit set. Fruit size and quality are also compromised.
  • Seed Germination: Low soil moisture inhibits seed germination.
  • Senescence: Accelerated leaf senescence (aging) occurs as plants prioritize resource allocation to survival.

Plant-Borne Mechanisms to Escape from Drought and Stress Situations

Plants have evolved remarkable mechanisms to tolerate or avoid water stress. These mechanisms can be broadly categorized as morphological, anatomical, and physiological.

Morphological Adaptations

  • Deep Root Systems: Plants like mesquite (Prosopis juliflora) develop exceptionally deep roots to access groundwater.
  • Reduced Leaf Area: Smaller leaves or spines (modified leaves) reduce the surface area for transpiration. Cacti are prime examples.
  • Leaf Rolling: Some grasses, like Digitaria species, roll their leaves inwards to reduce transpiration.
  • Deciduousness: Shedding leaves during the driest periods minimizes water loss.

Anatomical Adaptations

  • Thick Cuticle: A thick waxy cuticle on leaves reduces water evaporation.
  • Sunken Stomata: Stomata located in pits or depressions create a humid microenvironment, reducing water loss.
  • Trichomes: Hairs on leaf surfaces increase boundary layer thickness, reducing transpiration.
  • Xeromorphic Tissue: Development of specialized tissues with increased water storage capacity.

Physiological Adaptations

  • Osmotic Adjustment: Accumulation of osmolytes (e.g., proline, glycine betaine, sugars) in cells lowers the osmotic potential, allowing plants to maintain turgor pressure and continue water uptake.
  • Antioxidant Defense System: Water stress induces oxidative stress. Plants enhance antioxidant enzyme activity (e.g., superoxide dismutase, catalase) to scavenge reactive oxygen species (ROS).
  • ABA Signaling: ABA plays a crucial role in regulating stomatal closure and activating stress-responsive genes.
  • Water Use Efficiency (WUE): Plants improve WUE by maximizing CO2 assimilation per unit of water lost. C4 plants, like maize, generally have higher WUE than C3 plants.
  • Epiphytic Lifestyle: Some plants, like certain orchids, develop an epiphytic lifestyle to avoid water stress in the soil.
Adaptation Type Specific Adaptation Example Plant
Morphological Deep Root System Mesquite (Prosopis juliflora)
Anatomical Thick Cuticle Oleander (Nerium oleander)
Physiological Osmotic Adjustment Tamarind (Tamarindus indica)

The National Mission for Sustainable Agriculture (NMSA) under the National Action Plan on Climate Change (NAPCC) includes components aimed at promoting drought-prone area development and water use efficiency in agriculture. This includes promoting drought-resistant varieties and water conservation techniques.

Case Study: The Thar Desert Ecosystem The Thar Desert in India exemplifies a region heavily impacted by water stress. Plants in this ecosystem, such as Acacia senegal and Calligonum polygonoides, exhibit a combination of morphological (small leaves, spines), anatomical (thick cuticle), and physiological (osmotic adjustment) adaptations to survive. Local communities have traditionally employed water harvesting techniques like ‘tankas’ to supplement water availability. However, unsustainable practices have led to desertification in some areas, highlighting the need for integrated water management strategies.

Conclusion

In conclusion, water stress poses a significant challenge to plant growth and development, triggering a range of physiological and developmental disruptions. Plants have evolved diverse and sophisticated mechanisms—morphological, anatomical, and physiological—to mitigate these effects. Understanding these adaptations and integrating them into agricultural practices, alongside sustainable water management strategies, is crucial for ensuring food security in a world facing increasing climate variability and drought conditions. Continued research focusing on drought-resistant crop varieties and improved water use efficiency remains paramount.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Water Potential
The tendency of water to move from one area to another. It's a measure of the free energy of water and is expressed in units of pressure (e.g., megapascals). A more negative water potential indicates a greater tendency for water to move into that area.
Osmolytes
Small organic molecules (e.g., proline, glycine betaine, sugars) that accumulate in cells to lower the osmotic potential and maintain turgor pressure under water stress conditions.

Key Statistics

Approximately 40% of the world's agricultural land is affected by drought conditions (FAO, 2020).

Source: FAO (Food and Agriculture Organization of the United Nations)

ABA concentrations in leaves can increase by up to 100-fold during severe drought stress (Sharp, 1984).

Source: Sharp, R. E. (1984). Abscisic acid.

Examples

C4 Plants and Water Use Efficiency

C4 plants like maize (<i>Zea mays</i>) and sorghum (<i>Sorghum bicolor</i>) have a higher water use efficiency compared to C3 plants like wheat (<i>Triticum aestivum</i>) due to their specialized CO<sub>2</sub> fixation pathway, which minimizes photorespiration and reduces stomatal opening.

Frequently Asked Questions

▶What is the difference between drought tolerance and drought avoidance?

Drought tolerance refers to a plant's ability to survive under severe water deficit conditions, while drought avoidance refers to strategies that help plants avoid drought stress, such as deep roots or early flowering.

Topics Covered

BotanyAgriculturePlant PhysiologyDrought ToleranceStress Response
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