Transpiration in Plants

Transpiration

• Transpiration is water movement through a plant, evaporating from aerial parts like leaves, stems, and flowers.
• It's crucial for water transport and cooling in plants.
• Transpiration happens when stomata, small leaf openings, allow gas exchange for photosynthesis.
• Water evaporates from mesophyll cells inside the leaf, diffusing out through the stomata.
• Environmental factors like temperature, humidity, wind speed, and light intensity influence transpiration rate.
• Higher temperatures raise the rate of evaporation.
• High humidity lowers evaporation, as the air is saturated with water.
• Wind increases evaporation by removing humid air from the leaf surface.
• Light boosts stomatal opening, increasing transpiration.
• Plants regulate transpiration via guard cells, controlling stomata opening and closing.
• Stomata open when guard cells are turgid (swollen with water).
• Stomata close when guard cells are flaccid (lacking water).
• Abscisic acid (ABA), a plant hormone, triggers stomatal closure during water stress.
• Transpiration pull is the tension from transpiration, drawing water up from roots through the xylem.
• The cohesion-tension theory explains how water defies gravity in plants.
• Water molecules cohere and adhere to xylem walls.
• Transpiration drives water movement.
• Transpiration aids nutrient transport from roots to shoots.
• Excessive transpiration results in water stress, leading to wilting and reduced photosynthesis.
• Adaptations like thick cuticles, sunken stomata, and reduced leaf surface area help plants reduce transpiration.

Water Absorption

• Water absorption occurs as plants take up water from the soil through their roots.
• Root hairs, extensions of epidermal cells, greatly expand the surface area for water absorption.
• Water enters root cells via osmosis, following the water potential gradient.
• Water potential measures the relative tendency of water to move.
• Water moves from high (less negative) to low (more negative) water potential areas.
• Water moves through the root cortex via apoplast and symplast pathways.
• The apoplast pathway involves water movement through cell walls and intercellular spaces.
• The symplast pathway involves water movement through cell cytoplasm, connected by plasmodesmata.
• Water must enter the symplast at the endodermis because the Casparian strip blocks apoplastic movement.
• The Casparian strip, made of waterproof suberin, is in endodermal cell walls.
• The Casparian strip ensures water and nutrients pass through the cell membrane, granting the plant control.
• Water then enters the xylem, the vascular tissue transporting water throughout the plant.
• Root pressure, a positive xylem pressure, aids water movement, especially in smaller plants.
• Guttation, the exudation of water droplets from leaves, results from root pressure.
• Mycorrhizae, symbiotic root-fungi associations, boost water and nutrient absorption.
• Fungal hyphae increase absorption surface area and access soil nutrients.
• Soil properties like texture, structure, and water content affect water availability to plants.
• Sandy soils have large pores and low water retention, while clay soils have small pores and high water retention.

Support in Plants

• Plants need structural support to stand upright and endure environmental stresses like wind and gravity.
• Plant cells have cellulose cell walls for rigidity and support.
• Turgor pressure helps maintain cell shape and firmness.
• Collenchyma and sclerenchyma tissues are specialized for support.
• Collenchyma cells have unevenly thickened cell walls, providing flexible support in growing regions.
• Sclerenchyma cells have heavily thickened cell walls, offering rigid support in non-growing regions.
• Fibers and sclereids are the two types of sclerenchyma cells.
• Fibers are long, slender, providing tensile strength; sclereids are shorter, irregular, and provide hardness.
• Xylem offers structural support due to its lignified cell walls, in addition to transporting water.
• Lignin, a complex polymer, adds rigidity and strength to cell walls.
• Woody plants have plenty of lignified xylem, forming wood.
• Tendrils, thorns, and prop roots provide additional support.
• Tendrils are modified leaves or stems that coil around objects.
• Thorns are modified branches or leaves that protect against herbivores.
• Prop roots are aerial roots that support stems, common in plants like mangroves and corn.
• Buttress roots are large, wide roots for stability in tall trees in shallow soils.
• Plant architecture, branching patterns, and stem thickness influence overall support.
• Plants in windy areas adapt with flexible stems and deep roots.
• Vascular bundle distribution in stems affects support.
• Dicots have vascular bundles in a ring, while monocots have them scattered.
• Vascular bundle arrangement affects the stem's strength and flexibility.