BIO203 Lecture 6 Water Relations, Water Loss & Ecophysiology of Photosynthesis PDF

BIO203: Lecture 6 Water relations 1 Instructor: Dr. Hind Emad Email: [email protected] Office hours: Wed. 10-11am, Wed. 2-3pm ZOOM 1  Overview: Plant cell water relations Water loss: Transpiration C4 and...

BIO203: Lecture 6 Water relations 1 Instructor: Dr. Hind Emad Email: [email protected] Office hours: Wed. 10-11am, Wed. 2-3pm ZOOM 1  Overview: Plant cell water relations Water loss: Transpiration C4 and CAM and water use efficiency 2  Water & plants: Why care? Lab 4! “Human civilisation owes everything to 6 inches of soil & the fact that it rains.” 3  Water Relations: Key Concepts Water relations Plant cell Water potential  Movement Whole plant Uptake: Root Transport: Xylem Loss: Stomata Plants grow where there is water 4  Why is water important to plants? Cell contents mostly water (80-95%) Hydrates cell contents and is a solvent Participates in biochemical reactions (photolysis, hydrolysis) Medium for transportation of solutes Maintains plant shape Needed for cell expansion & growth But, more than 99% of the water that enters a well watered plant through the roots is generally transpired directly. As little as 0.1% may be used in plant tissues 5 Transport & cooling  Water: Plant shape & growth If plant has less than the full amount of water in its cells, it leads to wilting (leaf). Water loss by transpiration also regulates temperature Cool grass on a hot day How to keep cool without losing too much water? e.g. grasslands plant have narrow leaves (relatively little leaf surface), thick cuticles, and sunken stomata. 6  Water: Cell elongation & growth Movement of water into cell and thence into vacuole drives cell expansion Cell wall loosening H2O 7 Water: Cell elongation & growth Saving resources! Vacuole = Mostly water. Cell only needs a small amount of additional cytoplasm during expansion. 8  Water: Calculating water content Most cells contain 90+% water Fresh cut wood contains over 65% water To determine water content of a tissue/organ/plant: FW (fresh weight) = water + cell wall + cell contents DW (dry weight) = cell wall + cell contents Oven ~110°C Water content = FW-DW 9  Water relations of a single cell Selective membrane permeability Osmosis = Movement of water across cell membrane according to relative concentrations of dissolved substances inside & outside cell Plant cells possess cell wall & vacuole Underlies use of water in maintaining plant shape and plant growth PLANT CELL WALL MAINTAINS 10 INTEGRITY  Water relations of a single cell Plant cells have two selectively (NOT Semi-) permeable membranes Plasma Membrane/ Plasmalemma Regulates movement of dissolved substances into cytoplasm Vacuolar Membrane/Tonoplast Regulates movement of dissolved substances into vacuole Vacuole Essential for cell expansion Storage 11  Selective Permeability Water passes freely through the membrane Water channels (aquaporins) aid faster movement Salts pass through ion channels Other solutes also moved selectively through channels & transporters Concentrations differ inside/outside cell & organelles Gradients. Water movement. Membrane potentials. Signals 12 Key Points: Water relations of a cell Membranes: selectively permeable (plasmalemma and tonoplast). NOT semi-permable Water ‘freely’ passes through membranes (aquaporins). Solutes pass through restricted channels only Relationship between the vacuole and the surrounding medium is important. Water moves down a water potential gradient Cell wall is not selectively permeable; it provides rigidity 13  Water Potential  What ‘drives’ movement of water in/out of cells/organelles? Can we predict movement & how? 14  Water Potential  Water moves down a water potential gradient….always from: HIGH to LOW water potential  is influenced by two main factors: Osmotic Potential: (solute concentration). Pressure Potential: The physical pressure exerted on the water (like turgor pressure in plant cells). 15  Osmotic potential  or s Pure water has a water potential () of zero Sugar/solute decrease Ψs becomes more -ve +ve Difference between the potential of pure water and 0 Pure water water plus solute is the  Add solute -ve (or s, solute potential)  Add more Remember phloem! Mass flow! solute16  Pressure potential p Pure water has a water potential of zero Apply pressure Water gains energy +ve p under pressure and 0 Pure water becomes more +ve -ve Difference between the potential of pure water and water under pressure is Water being ‘pulled’ up xylem the p -p 17  Osmometer The osmometer works on the principle of osmosis (movement of water from lower solute concentration (higher water potential) to higher solute concentration (lower water potential) across a semipermeable membrane. 18  The cell as an osmometer 1  of liquid (water tonoplast cell wall + sugars/salts) in (rigid) vacuole more –ve vacuole than surrounding (water/solutes) water solution Water flows in plasmalemma cytoplasm 19  The cell as an osmometer 2 As water enters cell, the p builds tonoplast up and applies cell wall pressure on the (rigid) rigid cell wall vacuole (water/solutes)  = p Dynamic water equilibrium, when water flow out = plasmalemma cytoplasm water flow in cell = 0 20  The cell as an osmometer 3 Water availability drops. If a cell has less than a full water content, the tonoplast p = 0 and there is lack cell wall of pressure on the cell walls vacuole (water/solutes) Plant appears to wilt water At the wilting point cell =  cytoplasm Because: plasmalemma cell =  + p but p=0 21  Putting a value on   expressed in MegaPascals (MPa) Most fully hydrated cells have a cell of - 0.05 to -0.2 MPa Compare with  of: Sea water -2.5 MPa 1M sucrose -2.7 MPa 1M NaCl - 4.4 MPa Water-stressed plants –1 MPa to –1.5 MPa 22 Water Loss 23  Water loss Cuticular transpiration Lenticular transpiration Loss in mg/h/g fw: cactus 0.1 pine 1.5 Impatiens 130 Most plants lose 90%+ of water via Stomatal Transpiration 24  Transpiration (evapo-transpiration) From leaf, due to vapor pressure difference (vpd) No control over water loss between the inside (e.g. algae & most of the leaf and the bryophytes) surrounding atmosphere Tortula ruralis Simulation: water Moss gametophytes have no true cuticle & loss from a dish Liverworts have pores that usually remain open 25  Transpiration (evapo-transpiration) From leaf, due to vapor pressure difference between the inside of the leaf and the surrounding atmosphere Evaporation Simulation: water loss prevented by from a dish impermeable lid ( = leaf cuticle) 26  Transpiration (evapo-transpiration) From leaf, due to vapor pressure difference between the inside of the leaf and the surrounding atmosphere Simulation: water loss from a dish Controlled water loss can be achieved through holes in the lid, which can open and close ( = stomata) 27  Stomata Dicot The size of the stomatal opening is controlled by guard cells on the leaf epidermis, which open and close the pore 28  Water loss from stomata Water is lost by evaporation from the substomatal space to the atmosphere via stomata The vapor pressure Substomatal difference (vpd) between spaces the substomatal space the surrounding atmosphere determines rate of evaporation 29 Water loss from stomata As water evaporates from the sub-stomatal atmosphere, water is drawn from the surrounding leaf cells As water is lost from the Sub-stomatal space, from where is it replaced? 30 Water loss from stomata The walls of the cells surrounding the transpiration substomatal space This causes a change in m of the cell walls as they dry out a little The  of the walls is now more –ve than  of the cell vacuoles Water is drawn from the cells surrounding the substomatal space 31 Water loss from stomata Now the p of the cells surrounding the substomatal space have a more –ve  than their surrounding cells They draw water from these surrounding cells and their  becomes more -ve A water potential gradient is set up across the leaf from the xylem to the outside All because water flows from a high to a low potential 32  Stomata opening and closing - Proton pumps move H⁺ out of guard cells Stomatal - K⁺ ions and Cl⁻ ions move into guard cells H₂O Opening - Water enters guard cells by osmosis, increasing turgor pressure; stomata open - K⁺ and Cl⁻ ions move out of guard cells into the surrounding epidermal cells Stomatal - Water follows by osmosis, decreasing turgor Closing pressure Guard cells become flaccid , lose turgor, stomata close H₂O 33  K+ distribution in guard cells Open Closed 34  Factors affecting stomatal activity Opening: Light stimulates opening in many species Lowering of CO2 due to photosynthesis Overheating (cool) Closing: Water loss increases abscisic acid (ABA), which promotes K+ efflux from guard cells Root to shoot signaling (e.g. ABA) may precede any water stress in the shoot 35  Factors that affect transpiration Air movement Temperature - Increase in leaf temperature increases vpd between inside and outside - Evaporation cools the leaf Pollutants SO2 can cause stomata to remain open Small dust particles can block stomatal apertures 36  BIO203 Lecture 6b. Photosynthesis 2: Ecophysiology 37  Photosynthesis II Calvin (C3) cycle Ribulose 1,5-Bisphosphate Carboxylase Oxygenase (RuBisCO) C3 fixation, 1st Step. C from CO2 added to 5C RuBP Yields 2 X 3C (3-Phosphoglycerate) molecules Hence C3 Cycle Principal output G3P (Glyceraldehyde 3 phosphate) 38 Calvin (C3) cycle 1C 5C 3PGA RuBisCO 2 X 3C Glyceraldehyde 3-Phosphate 39 RuBisCO is an Oxygenase too.. 40  RUBISCO Most abundant protein on Earth (~40%!) Because photosynthesis is so ubiquitous & vital? Yes,.. & Because RuBisCO is: Slow Poor Substrate specificity + trade-off between the two 41  Photorespiration costs RuBisCO forms 3-PGA most O2 efficiently when there is low O2 & RuBP high CO2 Carboxylase reaction. But our atmosphere has high O2 (~22%) & low CO2 (~400ppm) Increased oxygenase activity Reduces the photosynthetic efficiency C3 plants. Up to 25% of fixed C lost at ~20C (~50% at higher temps). Connection between photosynthetic efficiency & water use efficiency… 42  RuBisCO & water-use efficiency To take in CO2 for photosynthesis, plants must open their stomata, which leads to loss of water by transpiration. Heat, low water availability Stomata close to reduce water loss Photosynthesis in mesophyll cells lowers CO2 & increases O2 Photorespiration increases… What physiological and morphological approaches have evolved to combat this? C4 & CAM metabolism Associated anatomy Associated responses 43 transpiration C3 plants CO2 is fixed in the mesophyll chloroplasts by RuBisCO How can the carboxylase reaction of RuBisCO be favored to increase photosynthetic efficiency & reduce 44 water loss?  C4 plants Light reactions and the Calvin cycle physically separated. Light-dependent reactions occurring in the mesophyll Calvin cycle occurring in bundle-sheath cells. CO2 is fixed in the mesophyll cells to form, Malate (4C) by a non-rubisco enzyme, Phosphoenolpyruvate (PEP) carboxylase. Malate is then transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing CO2. CO2 is then fixed via the Calvin cycle, exactly as in C3. 45  Chloroplast structure in C4 plants Mesophyll cell chloroplasts lack Thylakoids RuBisCO. No Grana Bundle sheath cells Deficient in PSII contain chloroplasts Low O2 with RuBisCO 46  C4 Plants often have a distinctive leaf anatomy http://sydney.edu.au C3 Panicum miliaceum (French Millet) Kranz (wreath) anatomy A chloroplast-rich bundle sheath surrounding the C4 vascular bundles. Chloroplasts lack grana Posses RuBisCO Starch rich Mesophyll cells surround bundle sheath 47 Chloroplasts lack RuBisCO Cornell BIOG 1445  C4 Costs & Trade-offs Higher energetic demand (C fixation x2) C3 = 18ATP/glucose (assuming no PR losses) C4 = 30+ATP/glucose (regenerating PEP etc.) Ecological tradeoffs Temperature CO2 solubility Oxygenase activity Break-even point ~20-25C Water availability Wang et al. 2012 PLoS 48 ONE  C4: Adaptation to environment A higher percentage of C4 in tropics No C3 – C4 boundary at 20-25C C3 common in the tropics Other aspects of physiology & development important C4 well represented % C4 grass species in grasslands amongst grasses C3 crops: Rice, wheat, barley C4 crops: Maize, sorghum, sugarcane & millet 49 Forseth, I. N. (2010) The Ecology of Photosynthetic Pathways. Nature Education Knowledge 3(10):4 C4 water use efficiency C3 C3 C4 50 Photosynthetic adaptations to drought & arid environments Major problem for plants in a arid/desert If the stomata open at environments: Water night, how can the CO2 loss be used for How to avoid opening photosynthesis? stomata for CO2 in the heat of the day? Convert the CO2 into Open them at night… something else at night, and then use it as a C source during the day when there is light! 51  Crassulacean Acid Metabolism (CAM) CAM plants import CO2 at night & fix it into malate (stomata open) Non-photosynthetic (dark) C-fixation The malate is stored in the vacuole Released during day yielding CO2 for Calvin Cycle CAM = separated in time C4 = separated in space 52  CAM: Costs & Benefits Dramatically reduced water loss