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Plant Water Deficit ResponsesHORT 301 – Plant Physiology
November 11, 2009Taiz and Zeiger, Chapter 26 (p. 671-682), Web Topic 26.1
Abiotic stress – environmental factor deficiency or excess that limits growth and developmentWater deficit, temperature extremes, salinity, flooding (low or no O2)
Stress tolerance results from plant adaptation and responses to the environment (acclimation)
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Water deficit stress – insufficient plant water content for optimal physiological processes
Taiz and Zeiger 2006
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Plant water status affects critical physiological processes3.14 Water potential of plants under various growing conditions
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Water status of plants is defined by the cellular ψw and RWC
∆ ψw (water potential gradient) - drives water movement into or out of cells, water moves toward a more negative ψw
Low soil moisture causes more negative apoplastic water potential resulting in reduced turgor pressure and cellular water loss
3.9 Five examples illustrating the concept of water potential and its components (Part 3)
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Water deficit causes cell turgor pressure reduction/loss, water loss and volume reduction
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Water-deficit stress reduces plant growth – drought stress reduces yield of crops to about 20% of the genetic potential
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Turgor Pressure (MPa)
∆w gradient results in turgor pressure and facilitates water uptake for cell volume increase/expansion (fresh weight gain)
Growth rate is dependent on ψp and water uptakeDecrease in ψp reduces the growth rate until ψp falls below the yield threshold (Y)m (extensibility) and Y – regulated by complicated physiological processes that are not well definedIncreased ψp is due to osmotic adjustment
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Cellular osmotic adjustment – ψp re-establishment in response to water deficit stress
PLANT BIOLOGY, Smith et al. Figure 3-66.
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26.1 Dependence of leaf expansion on leaf turgor
Water deficit acclimation establishes ψw equilibrium or ∆ ψw
Reduced growth rate due to higher m and Y, ψp is required for growth, yield drag
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Water deficit stress-mediated leaf abscission – ethylene-dependent abscission to reduce leaf area (i.e., transpirational loss)
26.2 Leaves of young cotton (Gossypium hirsutum) plants abscise in response to water stress
-0.5 -1.2 -2.4 MPa
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Water deficit stress-enhanced relative root elongationCoordination of root and shoot growthEnsures that transpiration does not exceed the capacity of roots to “supply” water to the shoot Facilitates the capacity of roots to sense water (hydrotropism) and “mine” water in soils
5.6 Fibrous root systems of wheat (a monocot)
(A) Irrigated soil (B) Dry soil
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ABA coordinates water deficit stress responses of shoots and rootsABA promotes root growth relative to leaf cell expansion under water deficitWater deficit → ABA → shoot growth inhibition
23.6(A) Comparison of growth of the shoots of normal vs. ABA-deficient maize plants (Part 1)
(ABA deficient)
(ABA deficient)
Wild type and vp maize, high water potential – 0.03 MPa, low water potential – 0.3 MPa
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Root growth under water deficit is promoted by ABAWild type → water deficit → ABA → enhanced root growth
23.6 Comparison of the growth of the roots of normal vs. ABA-deficient maize plants (B, C) (Part 2)
B. High water potential – 0.03 MPa, low water potential – 1.6 MPa, wild type and vp maize
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Photosynthesis is less affected by water deficit than leaf expansion
26.4 Effects of water stress on photosynthesis and leaf expansion of sunflower
Photosynthate is partitioned to the root for growth, water acquisition
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23.4 Changes in three variables in maize in response to water stress
Stomatal closure is a water deficit-induced plant response that is regulated by ABA
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ABA accumulates in guard cells in response to water deficit and causes stomatal closureSoil water content (ψw) decrease - water deficit → ABA → stomatal closure (reduced stomatal conductance)
ABA is synthesized in roots and transported to leavesABA is available to guard cells due to water-deficit-induced alkalization of the leaf apoplastABA is synthesized in mesophyll chloroplasts and released to guard cells
23.5 Redistribution of ABA in the leaf from alkalinization of xylem sap during water stress
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23.14 Simplified model for ABA signaling in stomatal guard cells
ABA mediated stomatal closure mechanisms:Water deficit → ABA → stomatal closure
ABA → ROS → Ca2+↑ → Cl- efflux/membrane potential depolarization → K+ efflux/K+ influx is blocked → ψp decrease/water loss → volume reduction → stomatal closure
ABA → NO/S1P → cADP ribose/IP3 → Ca2+ → pm H+-ATPase inhibited → H+ gradient dissipation (pH) → K+ efflux
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Stomatal opening:K+ accumulation of K+ in guard cells causes a more negative cellular solute/osmotic potential (ψs)Increase in turgor pressure (ψp), water uptake and cell volume increase
Stomatal opening - K+ uptake → more negative guard cell ψs → increased ψp
/water uptake → cell volume increase → stomatal opening
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Water deficit stress induces gene expression – plant defensive response that results in induction or repression of gene expression ABA dependent and independent pathways
26.9 Signal transduction pathways for osmotic stress in plant cells
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B
A
DEBB2A over-expression can increase drought tolerance without a yield reduction in the absence of stress
Sakuma et al. (2006) Plant Cell
Watersufficient
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Plant transcription factor ZmNF-YB2 increases drought tolerance and yield stability of maize
Nelson et al. (2007) PNAS
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Late embryogenesis abundant (LEA) proteins – function in membrane protection under stress conditions, conserved in all plants
Abscisic acid biosynthesis:NCED (9-cis-epoxycarotenoid dioxygenase) – gene encoding the enzyme is upregulated by drought stress
23.2 ABA biosynthesis and metabolism
NCED
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Facultative CAM (crassulacean acid metabolism) transition – ice plant, Mesembryanthemum crystallinum
CO2 fixation occurs in the dark, requires phosphoenolpyruvate carboxylase activity
Transition from C3 to CAM is induced by severe NaCl stress (500 mM)/water deficit