1. Plant Water Deficit Responses HORT 301 – Plant Physiology
November 15, 2010
Taiz and Zeiger, Chapter 26, Web Topics 26.1 and 26.4
Abiotic stress – climatic or edaphic factor deficiency or excess that limits growth and development
Water deficit, temperature extremes, salinity, flooding (low or no O2)
Adaptation and acclimation (phenotypic plasticity)

2. Primary and secondary effects of abiotic stresses

3. Primary and secondary effects of abiotic stresses (continued)

4. Water deficit stress - insufficient plant water content for optimal physiology
Soil moisture content and size of the transpirational flux (relative humidity) determine the plant water status
Soil moisture content
Transpirational flux
Taiz and Zeiger 2006

5. Plant water deficit affects critical physiological processes

6. Water status of plants is defined by the cellular ψw and RWC
∆ ψw (water potential gradient) drives water movement into or out of cells (regulates expansion)
Water moves toward a more negative ψw

7. Water deficit causes cell turgor pressure reduction
Cells lose water and decrease in volume

8. Water-deficit stress reduces plant growth
Drought stress and crop yields

9. Growth rate is dependent on ψp and water uptake
Plants acclimate to water deficit by decreasing (more negative) cellular w
∆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 uptake
Increased ψp is due to osmotic adjustment
Turgor Pressure (MPa)

10. Osmotic adjustment is due to more negative ψs
Cellular osmotic adjustment re-establishes ψp in response to water deficit stress
Osmotic adjustment is due to more negative ψs
PLANT BIOLOGY, Smith et al. Figure 3-66.

11. Water deficit acclimation establishes ψw equilibrium or ∆ ψw
Higher ψp is required for growth

12. Water deficit stress and morphological responses
Ethylene-dependent abscission reduces leaf surface area (i.e., transpirational loss)
-0.5
-1.2
-2.4 MPa

13. Water deficit stress enhanced relative root elongation
Coordination of root and shoot growth
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
(A) Irrigated soil
(B) Dry soil

14. Soybean leaf movement in response to water deficit

15. Maize stay-green phenotype and drought tolerance

16. Reduction in programmed cell death increases drought tolerance

17. ABA coordinates plant water deficit stress responses
Shoot and root growth due to carbon partitioning
Water deficit → ABA → shoot growth inhibition
(ABA deficient)
(ABA deficient)
Wild type and vp maize, high water potential – 0.03 MPa, low water potential – 0.3 MPa

18. Shoot and root growth coordination (continued)
Water deficit → ABA → enhanced root growth

19. Photosynthesis is less affected by water deficit than leaf expansion
Photosynthate is partitioned to the root for growth, water acquisition

20. Stomatal closure is a water deficit-induced plant response that is regulated by ABA
ABA deficient mutants have reduced capacity for stomatal closure

21. ABA accumulates in guard cells in response to water deficit and causes stomatal closure
Soil water content (ψw) decrease - water deficit → ABA → stomatal closure (reduced stomatal conductance)

22. 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

23. ABA, Ca2+ and stomatal closure

24. 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 – H+-ATPase activation → K+ uptake → more negative guard cell ψs → increased ψp /water uptake → cell volume increase → stomatal opening

25. Water deficit stress induces gene expression
Induction or repression of gene expression
ABA dependent and independent pathways

26. DEBB2A over-expression can increase drought tolerance without a yield reduction in the absence of stress B A
Water
sufficient
Sakuma et al. (2006) Plant Cell
Sakuma et al. (2006) Plant Cell

27. Plant transcription factor ZmNF-YB2 increases drought tolerance and yield stability of maize
Nelson et al. (2007) PNAS
Nelson et al. (2007) PNAS

28. Osmotic solute biosynthesis (osmotic adjustment) in response to water deficit

29. Abscisic acid biosynthesis:
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
NCED