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Water Stress Prof. Samih Tamimi Plant Physiology 751 1 Plant Water Stress: What is it? Tissue Water Potential: Mild: Ψcell ~-0.5 MPa Moderate: Ψcell ~-0.5 to -1.5 MPa Severe: Ψcell ~<-1.5 MPa Relative Water Content: Mild: ~90% Moderate: 80-75% Severe: <75% Plant Physiology 751 2 Water stress is due to water shortage: • Water stress is induced when transpiration rate (T) is higher than absorption rate (A) • High T– low air humidity, high temperature, high irradiance, strong wind • Low A – low soil moisture, high concentration of salts, low soil temperature Plant Physiology 751 3 Fig. 3.12 Responses to deal with stress Plant Physiology 751 4 Decreased Leaf Area • An early response. • As water content of the plant falls, cells shrink and cell walls relax. • Solutes in the cell become more concentrated. • cell membrane may become more compressed due to lower surface area. Plant Physiology 751 5 Turgor and Cell Growth Cell growth peaks at night when turgor higher. • Leaf expansion is largely governed by cell expansion. • leaf area transpires less - conserves soil water. Plant Physiology 751 6 Abscisic Acid • ABA is a common plant response to stress. • In water-stressed plants ABA is an early response to leaf . • ABA stimulates stomatal closure. Plant Physiology 751 7 Plant Physiology 751 8 Stomatal Closure- the solute loss of guard cells can be triggered by decreasing water status in the rest of the leaf; probably mediated by 1) ABA (synthesized in mesophyll) when the mesophyll becomes dehydrated the plant moves some of the ABA from CP to the transpiration stream to the guard cells; and 2) a net increase in rate of production Plant Physiology 751 9 Abscisic Acid • ABA is a common plant response to stress. • In water-stressed plants ABA is an early response to leaf . • ABA stimulates stomatal closure. • Normally, ABA accumulates in mesophyll chloroplasts. • Accumulation depends on the relative pH of the stroma and cell cytosol and the weak acidity of ABA. Plant Physiology 751 10 ABA Release from Mesophyll Cells • As leaf cells lose turgor stromal pH falls. • ABA(H)moves passively into cytosol and apoplast. • Transpiration stream carries ABA to stomatal guard cells. • ABA stimulates stomatal closure. Plant Physiology 751 11 ABA mediated stomatal closure mechanisms: Water deficit → ABA → stomatal closure ABA → Ca2+↑ → Cl- efflux/membrane potential depolarization → K+ efflux/K+ influx is blocked → ψp decrease/water loss → volume reduction → stomatal closure Plant Physiology 751 12 12 Plant Physiology 751 13 Effects of Turgor Loss on Leaf Metabolism • PS rates are not directly affected by decreased cell turgor, however: • a fall in leaf turgor can lead to closure of stomates and consequent limitation of CO2 concentration within the leaf tissues, leading to decreased PS rates. • Other metabolic functions and individual enzymes show altered activity in response to mild water stress e.g. protein synthesis, nitrate reductase activity and protochlorophyll formation. Plant Physiology 751 14 T&Z Figure 25.4 Plant Physiology 751 15 Effects of Turgor Loss on Root System Development • When water uptake drops so does leaf expansion, reducing consumption of the organic products of PS. • These assimilates can be distributed to the roots to support an expansion of growth. Plant Physiology 751 16 of growth ABA promotes root growth and inhibits shoot growth at low water potentials Wild-type and ABA-deficient maize mutant seedlings grown under high and low water conditions Plant Physiology 751 17 Phloem translocation seems to be less sensitive to water stress than photosynthesis. Plant Physiology 751 18 Water stress and protein synthesis 1) Inhibition of synthesis of some proteins 2) Stimulation of synthesis of other proteins 3) Synthesis of specific stress proteins • A) proteins taking part in signal transduction and gene expression, e.g. transcription factors (MYC, MYB), protein kinases (MAPK), enzymes of phospholipid metabolism (phospholipase C, D) • B) proteins participating in stress tolerance, e.g. membrane proteins, proteins of water and ion channels, protection factors (chaperones, LEA proteins), syntases of osmoprotectants, stress proteins localized in chloroplasts, specific inhibitors of proteolytic activity, antioxidants, antioxidative enzymes, proteins taking part in recovery after stress Plant Physiology 751 19 Plant Adaptations – Moisture Stress • Water stress escapees • Water stress avoiders • Water stress tolerators ” Plant Physiology 751 20 2. DROUGHTescapers: Ephemerals (“annuals”) Grow only when water is available Life span of weeks to months Rapid photosynthetic and growth rates Cooled via transpiration (can’t tolerate drought) Plant Physiology 751 21 Method of saving water Structures adapted for this purpose Slowing/reducing transpiration rate Waxy cuticle on leaves Fewer stomata Sunken stomata & curled leaves which provide a moist micro-environment Stomata that close in daytime & open at night Fine hairs on the surface of plant to trap moisture Small leaves/spines (Cactus) that reduce surface area for water loss. Leaf loss during times of dryness Storing water for times of storage Fleshy, succulent leaves w/ flexible surface Fleshy, thick stems that can store water (Baobab tree) Fleshy underground tubers Increased water uptake Shallow, widespread roots to absorb maximum surface water Very long deep roots to reach underground water Some plants have a dual root system, which combines a deep tap root with shallow Plant Physiology 751 22 radial roots for maximum water uptake. Osmotic stress changes gene expression - Accumulation of solutes due to water stress - Several genes coding for enzymes associated with osmotic adjustment are upregulated by water stress. - LEA proteins (LATE EMBRYOGENESIS ABUNDANT) discovered by examination of naturally desiccating embryos during seed maturation - play role in cellular membrane protection, although function is not well understood - accumulate in vegetative tissue during water stress - hydrophilic proteins, strongly binding water (protective role may be associated with ability to retain water and to prevent protein denaturation during desiccation) Plant Physiology 751 23 LEA proteins are regulated by osmotic stress Function: Presumably have a role in membrane protection Plant Physiology 751 24 LEA proteins are regulated by osmotic stress Plant Physiology 751 25 Osmotic adjustment – Tolerance to drough • Osmotic adjustment • a biochemical mechanism that helps plants acclimate to dry conditions • Many drought-tolerant plants can regulate their solute potentials to compensate for transient or extended periods of water stress by making osmotic adjustments, which results in a net increase in the number of solute particles present in the plant cell. Plant Physiology 751 26 Osmotic adjustment ΨP = +0.5 MPa ΨS = -2.0 MPa ΨW = -1.5 MPa ΨP = 0 MPa ΨS = -1.2 MPa ΨW = -1.2 MPa Water deficit ΨP = pressure potential (hydrostatic pressure of solution) ΨS = solute (osmotic) potential ΨW = water potential Soil ΨW = -1.2 MPa Osmotic adjustment Plant Physiology 751 No osmotic adjustment 27 Osmotic adjustment • The cell actively accumulates solutes and as a result the solute potential (s) drops, promoting the flow of water into the cell. • Osmotic adjustments are believed to play a critical role in helping plants acclimate to drought or saline conditions. Plant Physiology 751 28 Osmotic stress changes gene expression - Accumulation of solutes due to water deficit or salinity stress causes osmotic stress - Several genes coding for enzymes associated with osmotic adjustment are upregulated by water stress - pyrroline-5-carboxylate synthase, a key enzyme in the proline biosynthetic pathway - betaine aldehyde dehydrogenase, an enzyme involved in glycine betain accumulation - myo-Inositol 6-O-methyltransferase, a rate-limiting enzyme in the accumulation of the cyclic sugar alcohol pinitol Plant Physiology 751 29 Synthesis and degration of proline L-Glu - L-glutamate, GSA - glutamate semialdehyde, P5C - -pyrroline-5carboxylate, P5CS - P5C syntetase, P5CR - P5C reductase, ProDH - proline dehydrogenase, L-Pro - proline Plant Physiology 751 30 Plant Environment: Water • Water deficiency in plants – Slight water stress causes stomates to close; photosynthesis reduced • Reduction in growth • Smaller leaves • Shorter internodes • Smaller plants – Slight water stress can effectively prevent fast growth Plant Physiology 751 31 Responses to water stress Osmotic adjustment Stomatal closure •hydropassive - guard cell dehydration •hydroactive - guard cell metabolism; ABA, solutes, etc. Leaf abscision and reduced leaf growth •reduces surface area for water loss •Smaller leaves lose more heat via convective heat loss Increased root growth •with reduced leaf expansion, more C translocated to roots •increases water supply Increased wax deposition on leaf surface •reduces cuticular transpiration, increases reflection Induction of CAM in facultative CAM plants •in response to water or osmotic stress Plant Physiology 751 32 What does water stress do to plant cells and plants? Loss of turgor Plasmolyzed vacuole Ψp = 0 Plant Physiology 751 33 What does water stress do to plant cells and plants? 2. Reduction in Leaf Expansion Smaller leaves Less extensive canopies Less light reception Less photosynthesis Internal turgor ‘powers’ cell expansion Plant Physiology 751 34 Many plants acclimate to drought conditions by osmotic adjustment: s of cell sap is reduced through accumulation of organic and inorganic solutes in the cytoplasm and vacuole. The solutes in the cytosol usually have low physiological activity. Examples: sorbitol (a sugar alcohol) or proline (an amino acid). Plant Physiology 751 35 Water uptake from the soil happens when soil potential is higher than plant water potential Osmotic adjustment helps plants cope with water stress. 1. W = S + P A decrease in S helps maintain turgor, P, even as total water potential decreases. Osmotic adjustment is a net increase in solute content per cell. Many solutes contribute to osmotic adjustment. K+, sugars, organic acids, amino acids Osmotic adjustment may occur over a period days. Costs of osmotic adjustment: synthesis of organic solutes, maintenance of solute gradients, and “opportunity costs”, Plant Physiology 751 36 energy the could be used for other functions Main pathway of proline synthesis in higher plants Plant Physiology 751 37 Avoidance mechanisms: root systems Root/Shoot ratio: Temperate forest: ~0.25 Dry Savanna woodland ~0.3 – 0.4 Prairie & deserts ~0.6-0.9 Root growth is plastic and responds to Local conditions (water, soil, etc.) Plant Physiology 751 38 mycorrhizal fungi extend root systems Avoidance mechanisms: leaf modification Leaf pubescence In oaks Blade of grasses “leads” water to base Water tanks in epiphytic bromeliads Leaf rolling in water stressed cornPlant Physiology 751 39 Leaf orientation in Eucalypts Avoidance mechanisms: leaf modifications SLA (specific leaf area) Leaf area / dry weight Deserts (xeric): 0.02 - 0.12 Dry forests: 0.36 - 0.70 Mesic forests: 1.4 -1.6 Lower number means smaller, thicker more dissected leaves Dissected leaves in Palo Verde Plant Physiology 751 40 Avoidance mechanisms: osmoregulation Lower leaf water potential by synthesizing solutes (amino acids, sugars, ions, etc.) What will this do? Plant Physiology 751 41 Avoidance Mechanisms: C4 and CAM Plants CAM plant - pineapple C4 grass - sugarcane Plant Physiology 751 42 The Transpiration Ratio Measures the Relationship between Water Loss and Carbon Gain • Transpiration ratio = the effectiveness of plants in moderating water loss while allowing sufficient CO2 uptake for photosynthesis • Transpiration ratio = Amount of water lost by transpiration Amount of CO2 fixed by photosynthesis • Also termed Water Use Efficiency (WUE) = inverse of transpiration ratio (e.g. plant with transpiration ratio of 500 has a WUE of 1/500 = 0.002) • Generally, H2O efflux is more than CO2 influx; this is due to: – Concentration gradient driving water loss is ~ 50 times larger than driving the influx of CO2 (due to low concentration of CO2 in air) and the relatively higher water vapor in leaf – CO2 diffuses ~ 1.6 times more slowly through air than water does (CO2 molecule is larger than H2O and has a smaller diffusion coefficient) – CO2 uptake must cross the plasma membrane, cytoplasm, chloroplast envelope before it is assimilated in the chloroplast → Plant Physiology 751 43 these membranes add to the resistance of CO2 diffusion pathway