Plant water regime

1
Plant water regime

• Transport of liquid water
• Transport of water across membranes
• Absorption of water by roots
• Radial transport of water in roots
• Root pressure
• Absorption of water by shoots

2
Basic characteristics of water transport on
cellular level

• Transport of liquid water across membrane
• (plasmalemma, tonoplast, organelle membranes)
• Jw Lp ??w Lp (??p ???s)
• Jw - membrane water transport rate, Lp - membrane
  water permeability,
• ??w - water potential difference, ??p - pressure
  potential difference,
• ? - reflection coefficient, ??s - osmotic
  potential difference between two membrane sites

3
Coupling of water and solute flow

• Thermodynamic of irreversible processes, equation
  for water and single solute flow
• Jw Lw??w Lws??s
• Js Ls??s Lsw??w
• Jw - transport of water, Js - transport of
  solute, ??w - gradient of chemical potential of
  water, ??s - gradient of chemical potential of
  solute, Lw, Ls - membrane permeability for water
  and solute, respectively, Lws Lsw (Onsager
  reciprocity coefficients)

4
Interactions between water and solute during
membrane transport
5
Membranes, aquaporins

• Membrane phospholipid bilayer (transport of O2,
  CO2) transmembrane proteins (transport of
  water, ions including protons, some hydrophylic
  organic substances) diffusion and active
  transport (ATPase, carriers, antiport, symport)
• Aquaporins belongs into "major intrinsic
  proteins" (MIP) 25 - 30 kDa
• tonoplast intrinsic proteins (TIP), plasmalemma
  intrinsic proteins (PIP)
• passive transport in both directions, water
  permeability 10 to 100 times higher than that of
  phospholipid bilayer widespread occurrence, high
  heterogeneity inside plant and among plant
  species (Arabidopsis 35 TIP or PIP genes)

6
Aquaporins
7
Aquaporins
(Luu and Maurel 2005)
8
Aquaporins

• Regulation of water transport by changes in
  occurrence (gene expression) or by changes in
  permeability
• phosphorylation increases permeability,
  dephosphorylation decreases permeability
• Ca2
• Na, Cl-
• lack of O2, ROS
• water stres, ABA
• circadial rhythm
• different selectivity
• Importance for water redistribution inside cells
  (much more TIP than PIP), and rapid transport
  between neighbouring cells (stomata, motoric
  cells, elongation growth, under decreased
  apoplast transport)

9
Water uptake

• Jw Lsr ??w
• Jw - absorption rate, Lsr - conductance in the
  soil-root boundary, ??w - difference in water
  potential between soil and root
• Ma A Lsr ??w
• Ma - amount of water absorbed by the roots, A -
  exchange area of absorption zone
• Availability of soil water is dependent on
  amount of water, amount of solutes, (osmotic
  potential of soil solution), size of capillary
  pores (matric potential of soil).

10
Adaptations of roots for sufficient water uptake

• Hydrotropic growth slower growth on the side
  where is higher soil moisture and more rapid
  growth on the opposite side.
• Root distribution species specific (shallow,
  deep, or multilayer root system)
• Anatomical adaptations endodermis, exodermis,
  etc.
• Physiological adaptations e.g. osmotic
  adjustment
• Soil water potential is mostly less negative than
  root water potential, but under special
  conditions it might be more negative (transport
  of water from plant to soil).
• Plant - lift for water

11
Root morphology and anatomy

• Most rapid absorption is in the apical part of
  the root (5 - 10 cm) where is the highest
  occurrence of root hairs.
• Number of root hairs is huge, their longevity
  short (max. several weeks). They increase
  absorption area considerably.
• Water absorption by older (suberized) root parts
  is much slower. Nevertheless, due to large area
  the amount of water might be about 30 .
• Arbuscular mycorrhiza or ectomycorrhiza
• Great variability in root anatomy

12
Radial water transport in root

• Three pathways
• apoplast (cell walls, intercellular spaces)
• symplast (cell protoplasm connected by
  plasmodesmas)
• across cells
• In water transport (Jr), both the gradient of
  pressure potential (??p) and the gradient of
  osmotic potential (??s) take part, their
  importance is dependent on transport pathway and
  transpiration rate.
• Jr Lr ??w Lr (??p ???s)
• Lr - root conductance, ? - reflection coefficient
  

13
Water transport pathways
14
(No Transcript)
15
Transport pathways

• Apoplastic pathway - under high ??p and high
  transpiration rate, its conductance is high,
  reflection coefficient is low (mass flow of water
  and solutes), it is limited by endodermis and
  exodermis
• Symplastic pathway across cells - ??s is
  important, reflection coefficient is high, its
  conductance is rather low and dependent on
  aquaporins, ABA, Ca2
• Similar transport mechanism radial transport in
  stem, longitudinal transport in root and stem
  tissue with exception of vascular system,
  transport in leaf tissue
• Transport of water is dependent on 1) morphology
  and anatomy, 2) transport mechanism 3)
  water-solute interactions, 4) activity of
  aquaporins

16
Root pressure

• Under high soil moisture and high air humidity
  (low transpiration rate)
• Important for vein filling in spring or after
  embolism
• Gutation, exudation
• Mechanism transport osmotically active compounds
  into xylem creates gradient of osmotic potential
  between xylem and root cells which is driving
  force for water transport into xylem

17
Model of radial transport of water and ions
18
Water uptake by shoot

• The water source rain, dew, fog, air humidity
  near 100
• Under rather low water potential in shoot,
  dependent also on leaf wettability
• For plant water balance is usually more important
  limitation of transpiration than water uptake
• Epiphytic plants (Bromeliaceae, Orchidaceae)