मराठी
Maharashtra State BoardHSC Science (General) 12th Standard Board Exam
प्रश्नपत्रिका३५१
पाठ्यपुस्तक उत्तरे१३१७३
MCQ ऑनलाइन सराव परीक्षा७३
महत्वाचे प्रश्न आणि उत्तरे१०१६१
संकल्पना स्पष्टीकरण आणि व्हिडिओ९७९
टाइम टेबल३३
अभ्यासक्रम

Overview of Plant Water Relation

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Maharashtra State Board: Class 12

Key Points: Water absorbing organ

  • Root is the main organ responsible for absorption of water and minerals from the soil in terrestrial plants.
  • Epiphytic plants like orchids absorb water vapour from air using special aerial roots with a tissue called velamen.
  • A typical root is divided into different regions, and the zone of absorption bears root hairs.
  • Root hairs are unicellular, tubular extensions of epiblema cells; they are colourless, delicate, short-lived and increase the surface area for absorption.
  • Each root hair has a thin, permeable cell wall and a selectively permeable plasma membrane with a large central vacuole, facilitating efficient water absorption.
 
Maharashtra State Board: Class 12

Definition: Gravitational Water

The water present in the soil that percolates downward due to gravity and is not available for plant absorption is called gravitational water.

Maharashtra State Board: Class 12

Definition: Hygroscopic Water

The water that is tightly adsorbed on the surface of fine soil particles and cannot be absorbed by plant roots is called hygroscopic water.

Maharashtra State Board: Class 12

Definition: Combined Water

The water that is chemically bound to soil minerals such as hydrated oxides of silicon and aluminum and is unavailable to plants is called combined water.

Maharashtra State Board: Class 12

Definition: Capillary Water

The water held in the small pores between soil particles due to capillary action and available for absorption by plant roots is called capillary water.

Maharashtra State Board: Class 12

Key Points: Water Potential

  1. Water Potential (Ψ):
    The chemical potential or free energy of water responsible for its movement is called water potential and is represented by the Greek letter psi (Ψ).
  2. Value and Units:
    Water potential is zero in pure water, becomes negative when solutes are added, and is measured in bars, pascals, or atmospheres.
  3. Components:
    Water potential consists of osmotic potential (negative) and pressure potential (always positive).
  4. Movement of Water:
    Water moves from a region of higher (less negative) water potential to lower (more negative) water potential through plasmodesmata.
Maharashtra State Board: Class 12

Key Points: Transport of mineral ions

  1. Source and Form
    Soil is the main source of minerals, and plants absorb mineral nutrients in the dissolved ionic form, mainly through roots.
  2. Mode of Transport
    Absorbed mineral ions are transported upward through xylem sap due to transpiration pull.
  3. Distribution in Plant Body
    Mineral ions are supplied to actively growing and storage regions such as young leaves, flowers, fruits, seeds, and storage organs, where they are actively absorbed by cells.
  4. Remobilization and Pathways
    Mineral ions can be remobilized from older to younger parts, and their transport occurs through both xylem and phloem.
Maharashtra State Board: Class 12

Key Points: Transport of food

  • Food is synthesized in green leaves (source) and utilized or stored in non-green parts (sink).
  • Movement of food from source to sink in plants is called translocation of food.
  • Food is transported mainly through phloem tissue, especially sieve tubes.
  • Food is always translocated in soluble form as sucrose.
  • Transport of food occurs in both vertical and lateral directions and is bidirectional.
  • Phloem sap contains water, sucrose, amino acids, and hormones.
  • The most accepted mechanism of food transport is Munch’s pressure flow (mass flow) theory, based on turgor pressure gradient.
Maharashtra State Board: Class 12

Key Points: Types of Transpiration in Plants

Type of Transpiration Structure Involved Site Percentage of Total Transpiration Important Features
Cuticular Transpiration Cuticle (cutin) Epidermis of leaves and stem 8–10% Occurs by diffusion, continues all day, inversely proportional to cuticle thickness
Lenticular Transpiration Lenticels Bark of old stems, woody roots and fruits 0.1–1.0% Very slow rate, occurs throughout the day, absent in leaves
Stomatal Transpiration Stomata (guard cells) Epidermis of leaves and young stem 90–93% Occurs mainly during daytime, regulated by stomata
Maharashtra State Board: Class 12

Key Points: Structure of stomatal apparatus

  1. Stomatal Apparatus
    A typical stomatal apparatus consists of two guard cells, a stoma (pore), and accessory (subsidiary) cells.
  2. Guard Cells
    Guard cells are modified epidermal parenchyma cells, kidney-shaped in dicots and dumbbell-shaped in monocots, with unevenly thick walls.
  3. Cell Structure
    Guard cells are living, nucleated, contain few chloroplasts, and have a thick inner wall and thin elastic outer wall.
  4. Accessory Cells
    Accessory cells are specialized epidermal cells surrounding guard cells and act as reservoirs of K⁺ ions.
  5. Opening and Closing Mechanism
    Opening and closing of stomata depend on turgor changes in guard cells due to endosmosis (opening) and exosmosis (closing).
  6. Theories of Stomatal Movement
    Stomatal movement is explained by starch–sugar interconversion theory and proton (K⁺) transport theory.
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