Introduction to Crop Physiology & Biochemistry (AGR-405)

Introduction to Crop Physiology & Biochemistry(AGR-405)Dr Saboohi RazaAssociate Professor

Mechanism of absorption and translocation of water

Importance of waterMorphology and anatomy of plantGood solvent Component of protoplasm Absorbance and translocation mineral, salts and dissolve substancesPhotosynthesis Seed germination, respiration transpiration, activation of enzymes, hydrolysis of ATPMaintain temperatureTurgidity of plant body

Soil waterGravitational water Capillary waterHygroscopic waterCrystalline waterRunning water

Water holding capacity=capillary +hygroscopic +crystalline water

Water absorbing parts of the plantsROOTS

Regions of RootsRoot Cap regionMeristematic regionRegion for elongationRegion for root hairMature region

Internal structure of root

Some term involved in water absorption and translocationOsmosisActive transport Passive transportATPHypertonic solutionHypotonic solutionIsotonic solution

Water potential ()is a measurement that combines the effects of solute concentration and pressuredetermines the direction of movement of waterWater flows from regions of higher water potential to regions of lower water potential (osmosis)MPa = unit of measurement (megapascal) = 0 MPa for pure water at sea level and room temperatureBoth pressure and solute concentration affect water potentialsolute potential (S) is proportional to the number of dissolved molecules also called osmotic potential

Pressure potential (P) is the physical pressure on a solution = s + p

Measuring Water PotentialConsider a U-shaped tube where the two arms are separated by a membrane permeable only to waterWater moves in the direction from higher water potential to lower water potential

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= 0.23 MPa

0.1 MsolutionPurewaterH2OP = 0S = 0P = 0S = 0.23 = 0 MPa

If no pressure is applied:The addition of solutes reduces water potential

12Figure 36.8a Water potential and water movement: an artificial model

PositivepressureH2OP = 0.23S = 0.23P = 0S = 0 = 0 MPa

= 0 MPa

Physical pressure increases water potential

13Figure 36.8b Water potential and water movement: an artificial model

P = 0.30 S = 0.23

IncreasedpositivepressureH2O = 0.07 MPa

P = 0S = 0 = 0 MPa

Increased positive pressure on the right causes the water to move to the left

14Figure 36.8c Water potential and water movement: an artificial model

Negativepressure(tension)H2OP = 0.30S = 0

P = 0S = 0.23 = 0.30 MPa

= 0.23 MPa

Negative pressure decreases water potential

15Figure 36.8d Water potential and water movement: an artificial model

(a) Initial conditions: cellular > environmental P = 0 S = 0.9

P = 0 S = 0.9

P = 0S = 0.7

= 0.9 MPa

= 0.9 MPa

= 0.7 MPa

0.4 M sucrose solution:

Plasmolyzed cell

Initial flaccid cell:60% H2OA cell placed in a high solute concentration it will losewater, plasmolyzingTurgor loss in plants causes wilting, which can be reversed when the plant is watered

16Figure 36.9 Water relations in plant cells

P = 0S = 0.7

Initial flaccid cell:Pure water:P = 0S = 0

= 0 MPa = 0.7 MPaP = 0.7S = 0.7

= 0 MPaTurgid cell(b) Initial conditions: cellular < environmental

100% H20If the same flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid

17Figure 36.9 Water relations in plant cells

Mechanism of water absorption

Path for translocation of waterRoot Hairs Epidermal cell Cortex CellEndodermal cellsCells of PericycleXylem cellsXylem DuctUpward

Non osmotic Active absorptionSometime water absorbance takes place against the osmotic gradientRequire ATP produced during respiration It requires oxygenCreate root pressure

Passive absorptionAccording to osmotic gradientDoes not require energyDoes not require oxygenRoot pressure not created

Factors affecting on the rate of water absorptionAvailable Soil waterSoil aeration : oxygen requiredConcentration of soil solutionSoil temperature :20-30CRoot system: hairy and well developed root system

Ascent of Sap The water and soluble minerals salts absorbed by the roots reached to the leaves through roots, stem and branches of plant. The phenomenon of ascending of absorbed water against gravitation through vessels and tracheids of xylem is called ascent of sap

Cohesion TheoryBy Dixon and Jolly (1894)The water molecule remain attracted by a force called cohesive force This force maintain the continuity of water column in the xylem vessels.Water evaporates from the leave due to transpiration that creates a transpiration pull.Ascent of sap and water is directly proportional to the rate of transpiration

Transpiration

The loss of excess water in form of vapours from various aerial parts of plants is called transpiration

Only 5% absorbed water is retained in plant body Remaining 95% lost through aerial parts

Difference between transpiration and evaporationTranspirationEvaporationTranspiration is biological phenomenonEvaporation is simple physical phenomenonIt is control by guard cellsIt is control by guard cellsIt take place through the surface of leavesIt take place through the surface of various open water bodiesIt take place due to osmotic pressure and suction pressureno suction and osmotic pressure is involvedIt take place in living cellsNo required living cellsThe temperature of the plant is maintained due to transpirationNot any relation with plant cell

Kinds of transpirationCuticle transpiration: from aerial parts of plants 5-15% water lossLenticular Transpiration: small pores presents below the bark of treesStomatal Transpiration: Through stomata of leaves80-90 % water loss

STOMATA

CuticleCuticleMesophyllStomataGuard cellsPrevents water lossSite of photosynthesisOpenings allow gases and water to move in and out of leafOpen and close the stomataStomatal transpiration

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Stomata movementSun rises- stomata start openingLight intensity increases- more opening of stomataHigh rate of transpiration decreases the turgidity of leave cells- water deficit-partial closing of stomataMore water absorption due to difference in water potential guard cells become turgid- stomata reopen-start transpiration As light intensity decreased transpiration rate decreasedAt sunset stomata become closed

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Regulating Stomatal Opening:-the potassium ion pump hypothesis

Guard cells flaccidStoma closed

K+K+K+K+K+K+K+K+K+K+K+K+K+ ions have the same concentration in guard cells and epidermal cellsLight activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells

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Regulating Stomatal Opening:-the potassium ion pump hypothesis

K+K+K+K+K+K+K+K+K+K+K+K+Increased concentration of K+ in guard cellsLowers the in the guard cellsWater moves in by osmosis, down gradient

H2OH2OH2OH2OH2O

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Stoma openGuard cells turgid

K+K+K+K+K+K+K+K+K+K+K+K+Increased concentration of K+ in guard cellsLowers the in the guard cellsWater moves in by osmosis, down gradientH2OH2OH2OH2OH2OH2O

35Adaptations to Reduce Water Loss in XerophytesThick waxy cuticle to reduce evaporation

Reduced leaf area e.g.needles

Hairy leaves:- the hairs trap a layer of saturated air

Sunken stomata:- the pits above the stomata become saturated

Rolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the rollIncreasing the water vapour around the stomata reduces the water potential gradient so slows water loss

Factors affecting TranspirationExternal or environmental FactorsHumidity in airTemperatureLightWind velocityWater content in soil

Factors affecting TranspirationInternal Factors for reduce transpirationMorphological featureAnatomical featuresPhysiological features

Morphological featuresPresence of dry, hard and cylindrical stemPresence of bark on stemReduction in number of branchesPresences of fleshy roots for storage of waterPresences of scaly and reduced leavesPresence of thick leaves covered by thick waxy layer or cuticle

Anatomical FeaturesPresence of multilayered epidermisPresence of sunken and less number of stomataPresence of excessive amount of Sclerenchyma

Physiological featuresPresence of high OP in cell-sap of leavesExcessive growth in roots due to which they become quite long and reach at the greater depth to absorb waterClosing stomata during adverse conditions of environment

Importance of transpirationThe water and minerals absorbed by the roots from the soil reached continuously in different parts of the plants through transpirationTranspiration maintains the concentration of mineral saltsMaintain the temperature of plants

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