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)

Water deficit stress – insufficient plant water content for optimal physiological processes

Taiz and Zeiger 2006

Plant water status affects critical physiological processes3.14 Water potential of plants under various growing conditions

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)

Water deficit causes cell turgor pressure reduction/loss, water loss and volume reduction

Water-deficit stress reduces plant growth – drought stress reduces yield of crops to about 20% of the genetic potential

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

Cellular osmotic adjustment – ψp re-establishment in response to water deficit stress

PLANT BIOLOGY, Smith et al. Figure 3-66.

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

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

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

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

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

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

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

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

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

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

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

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

Plant transcription factor ZmNF-YB2 increases drought tolerance and yield stability of maize

Nelson et al. (2007) PNAS

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

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