Plant Physiology Absorption of water in plants prof

Plant Physiology • Absorption of water in plants prof samih tamimi

Plant Absorption of water in plants prof samih tamimi

Entry of water into plants prof samih tamimi

Adaptations of roots for sufficient water uptake • Hydrotropic growth – slower growth on the side where is higher soil moisture and more rapid growth on the opposite side. • Root distribution – species specific (shallow, deep, or multilayer root system) • Anatomical adaptations – endodermis, exodermis, etc. • Physiological adaptations – e. g. osmotic adjustment • • Soil water potential is mostly less negative than root water potential, but under special conditions it might be more negative (transport of water from plant to soil). Plant - lift for water prof samih tamimi

Root morphology and anatomy • Most rapid absorption is in the apical part of the root (5 - 10 cm) where is the highest occurrence of root hairs. • Number of root hairs is huge, their longevity short (max. several weeks). They increase absorption area considerably. • Water absorption by older (suberized) root parts is much slower. Nevertheless, due to large area the amount of water might be about 30 %. • Arbuscular mycorrhiza or ectomycorrhiza • Great variability in root anatomy prof samih tamimi

prof samih tamimi

Absorption of water • Root is the major absorption organ except hydrophytes • The zone of maximum water absorption (about 20 -folds) is located about 20 -100 mm from the root tip • This region is full of root hairs • Length of root hairs is 0. 1 mm to 10 mm • Density of root hairs is 2500 hairs per cm 2 • Root hairs are unicellular while hairs of stem or leaf are multicellular • But the rhizome carries adventitious roots without hairs • Plants growing in solution culture and aquatic plants lack root hairs • The older parts of roots are suberized and no longer are absorbing prof samih tamimi

Radial water transport in root Three pathways: apoplast (cell walls, intercellular spaces) symplast (cell protoplasm connected by plasmodesmas) across cells In water transport (Jr), both the gradient of pressure potential ( p) and the gradient of osmotic potential ( s) take part, their importance is dependent on transport pathway and transpiration rate. • Jr = Lr w = Lr ( p + s) • Lr - root conductance, - reflection coefficient • • • prof samih tamimi

prof samih tamimi

Pathways for water movement • • • There are three pathways for water movement 1. Apoplastic pathway 2. Trans-membrane pathway 3. Symplastic pathway 1. Apoplastic pathway – it is nonliving, discontinuous and separated into two regions – a) Outside the casparian strip (deposition of suberin and waxes) – b) Initially water moves through intercellular spaces but “casparian strips” in endodermis is main barrier in water flow so water moves symplastically here and again moves it can move apoplastically through intercellular spaces – This pathway is discontinuous pathways due to casparian strips barrier prof samih tamimi

Continuo---- • 2. Cellular (Trans-membrane pathway): There are number of membranes in cytoplasm i. e. membranous system of different cellular organelles • 3. Symplastic pathway: – a) continuous- because connected from cell to cell via plasmodesmata. – b) symplast consists of total protoplasts of cells excluding vacuole. – c) movement of water through symplast is slow due to high viscosity of cytoplasm • Whether the pathway is taken, water has to move through cytoplasm (cellularly) because of casparian strips impregnated with subrin (barrier to water) prof samih tamimi

• vascular cylinder (stele) consists of: 1. endodermis – regulates movement of materials into center of root (xylem) • each cell has special bandlike region – Casparian strip – casparian strips contain suberin (fatty, waterproof material) prof samih tamimi

Driving forces for symplastic pathway • Active absorption or Osmotic movement of water occurs in slowly transpiring plant – Roots as osmometer-solutes (ions) become highly in roots because roots absorb ions from the dilute soil solution and transport them into the xylem due to which solute potential of xylem decreases thereby decreasing xylem water potential – Water enters the roots due to positive pressure – guttation or exudation – Osmotic potential -0. 1 to -0. 2 MPa – main factor – Turgor potential 0. 1 to 0. 15 MPa but up to 0. 5 MPa • Passive absorption (mass flow) in rapidly transpiring plants – Forces originate at evaporating surface i. e. leaves in the shoots and transmitted to the roots – Roots act as absorbing organs prof samih tamimi

Water would enter root hair cell by osmosis. Root hair cell High conc. Lower of sugar and water salts potential prof samih tamimi Soil Dilute Higher solution of water mineral salt. potential

Absorption of water by roots Water would pass from root hair cell to the inner cell by osmosis B Lower water potential A Higher water potential prof samih tamimi

Water would pass from one cell to the next cell in the cortex until the water enters the xylem tube Cortex – cells inprofbetween epidermis and xylem samih tamimi

Absorption of mineral salts by roots • Ions enter mainly by active transport • Energy from cellular respiration Lower conc. of ions (K, Higher Na) conc. of ions prof samih tamimi

Absorption of mineral salts by roots • Ions also by diffusion (not common) Lower conc. of ions prof samih tamimi Higher conc. of mineral ions

Moving water up the xylem vessels prof samih tamimi

Moving water up the xylem vessels • Via 3 processes: – Root pressure – Capillary action – Transpiration pull the main process prof samih tamimi

Movement of water from the root hair cells to the xylem vessels would create an upward force to push the water up the xylem vessels. This is known as root pressure. prof samih tamimi

How is root pressure generated? 1) Ions pumped from cortex into xylem Angular in shape Cell in coretx xylem 2) Water enters by osmosis prof samih tamimi

A demonstration to show root pressure What would happen to the water level after 5 hrs? Glass tubing Connector Cut stem with roots prof samih tamimi

Capillary action • Movement of water up a narrow tube The narrower the tube, the higher level the water level prof samih tamimi

Capillary action 1 Force of attraction between water molecules and wall of vessel ‘pulls’ water molecules into a vessel. (adhesion force) prof samih tamimi 2 Force of attraction between water molecules ‘pull’ the next molecule along. (cohesion force)

3 Adhesion and cohesion forces move the stream of water molecules up a xylem vessel. prof samih tamimi

Transpiration What is transpiration? – The evaporation loss of water vapour through the stomata of the leaves is known as transpiration. prof samih tamimi

Drinking ribena Sucking of water up the straw Movement of water up straw Transpiration pull Transpiration of water from the leaves Movement of water up a xylem tube Transpiration pull is a suction force caused by transpiration, that pulls the water up the xylem tube. It is the strongest force that moves water up a xylem tube. prof samih tamimi

Do not be confuse between…. . • Transpiration evaporation of water • Transpiration pull suction force • Transpiration stream the stream of water up the xylem vessels prof samih tamimi

How is the suction force being created at the leaves by transpiration? prof samih tamimi

Transpiration occurs here prof samih tamimi

1) Water continuously moves out of mesophyll cells to form a thin layer of moisture. 2)water evaporates from the thin film of moisture and moves into the intercellular air spaces as water vapour. prof samih tamimi

Water evaporates from the thin film of moisture to give water vapour. Lower conc. of water vapour Water vapour would move down the concentration gradient into the atmosphere. prof samih tamimi

XYLEM B Water moves out of mesophyll cells to replace thin film of moisture surrounding prof samih tamimi the mesophyll cell. Mesophyll cell A

Transpiration set up an suction force to pull water up the xylem tube prof samih tamimi

Think about it! • Usually, only 99% of the water absorbed is lost through transpiration. • Where is the remaining 1% of water? prof samih tamimi

Importance of transpiration • To pull water and mineral salts from roots to leaves. • Evaporation of water cools the leaves. • Water used for photosynthesis; to keep cell frim and turgid. prof samih tamimi

Factors affecting the rate of transpiration • • Humidity Wind or air movement Temperature Light prof samih tamimi

Humidity • What is humidity? - >Refers to the amount of water vapour that the air can hold. prof samih tamimi

Higher conc. of water vapour (100% saturated) Lower conc. of water vapour The drier the air, the steeper the concentration gradient, the faster the rate of transpiration. prof samih tamimi

Wilting Rate of transpiration > rate of absorption of water What happens to the leaves when they wilt? How does wilting benefit the plant? prof samih tamimi