The soil-plant-atmosphere continuum (SPAC) is the pathway for water moving from soil through plants to the atmosphere. Continuum in the description highlights the continuous nature of water connection through the pathway. The low water potential of the atmosphere, and relatively higher (i.e. less negative) water potential inside leaves, leads to a diffusion gradient across the stomatal pores of leaves, drawing water out of the leaves as vapour.

1. Plant water Continuum
Presentation by:
Haider Ali Malik
BBOF16M008
Department of Botany
University of Sargodha

2. Soil-plant-atmosphere continuum (SPAC)
 The soil-plant-atmosphere continuum (SPAC) is the pathway for
water moving from soil through plants to the atmosphere.
 Continuum in the description highlights the continuous nature of
water connection through the pathway.
 The low water potential of the atmosphere, and relatively higher (i.e.
less negative) water potential inside leaves, leads to a diffusion
gradient across the stomatal pores of leaves, drawing water out of
the leaves as vapour.

3.  As water vapour transpires out of the leaf, further water molecules
evaporate off the surface of mesophyll cells to replace the lost molecules
since water in the air inside leaves is maintained at saturation vapour
pressure.
 Water lost at the surface of cells is replaced by water from the xylem,
which due to the cohesion-tension properties of water in the xylem of
plants pulls additional water molecules through the xylem from the roots
toward the leaf.

4. Fundamental Principles
• Two general principles are indispensable for studying the SPAC:
(1) the conservation principles that take the form of mass and energy ‘budgets,’ and
(2) the transport principles that relate the flow of some quantity to the difference or
of other quantities that influence or ‘force’ the flow and describe the ‘state’ of the
exchange process.
• The principles of conservation of mass and energy are the backbone of
studies of the SPAC. Because mass and energy can take many forms as they
throughout the SPAC, and even interact with each other, budgets are constructed
to quantify the important stores and flows of important life-enabling constituents
such as water, carbon, or energy.
• A budget is simply the application of the conservation principles (mass or energy)
to a specific system that must be carefully defined.

5. Role of Soil in Continuum
 Plant growth depends on the use of two important natural
resources, soil and water.
 Soil provides the mechanical and nutrient support necessary for
plant growth and Water is the major input for the growth and
development of all types of plants.
 Soil provides food and fertilizers to the crops where as water
mobilize the organism of plant growth and helps in introducing
food and fertilizers to crops.
 Soil provides the room for water to be used by plants through the
roots present in the same medium.

6. Water is the Link for Soil, Plant, and
Atmosphere Continuum
Water and plants
 Water availability limits the productivity in many ecosystems
 Functions within a plant
 Most of plant fresh weight comes from water (up to 90%)
 Provide structure and support
 Source of oxygen release form photosynthesis
 Medium for transporting nutrients, metabolites, and plant hormones
 Lost by transpiration through stomata
 Inevitable consequence of photosynthesis

7. Water and plants
 Water availability limits the productivity in
many ecosystems
 Functions within a plant
 Most of plant fresh weight comes from
water (up to 90%)
 Provide structure and support
 Source of oxygen release form
photosynthesis
Medium for transporting nutrients,
metabolites, and plant hormones
 Lost by transpiration through stomata
 Inevitable consequence of photosynthesis

8. Leaf gas-exchange

9. Stomata
Opening and closing are dynamically regulated

10. Ascent of Sap: How does water move to the tree top?
Koch et al. (2004) Nature 428: 851
• By suction
• Water in the xylem is under tension
• Water evaporating from the leaves (transpiration) creates
this tension (i.e., suction)
• Cohesion among water molecules provides a continuous
water column
• Dixon and Joly (1894)
• Cohesion-Tension theory

11. Soil-Plant-
Atmosphere
Continuum
(SPAC)

12. Conductive Vessel Element in Mountain Mahogany Wood (SEM x750).
This image is copyright Dennis Kunkel at www.DennisKunkel.com

13. Water movement in the soil-plant-atmosphere continuum
Water moves from higher to
lower water potential, so
Yatmos < Yleaf < Ystem < Yroot < Ysoil

14. Soil water potential

15. Soil water adheres to soil particles of different sizes and kinds.
This adhesion represents a “tension”, or YP < 0.
In most soil solutions, solutes are dilute so YS ≈ 0.
Exceptions: saline soils, salt marshes.
YW = YS + YP + Yg
YS ≈ 0
YP < 0
Yg≈ 0
So, for most soils
YW = YP
Fig. 4.2

16. Soils differ in characteristic particle size.

17. The more contact a volume of water has with the soil surface, the greater
the tension with which it is held.

18. Water is held more tightly in small crevices.
YP = -2T/r
Where r = radius (m) of curvature of meniscus, and
T = the surface tension of water,7.28 x 10-8 MPa m
r1
r2

19. YP = -2T/r
1. As soils dry, water is held in small pore
spaces (r decreases) so soil water
potential decreases
2. Soils with smaller characteristic particle
size (e.g. clay vs. sand) tend to have lower
water potential.
3. More difficult for plants to extract
water from clay than sand

20. YP = -2T/r
Example: calculate YP for r = 1 x 10-6 m and 1 x 10-7 m.
About -0.15MPa for 1µm, and -1.5 MPa for 0.1 µm.

21. Getting water from the soil into plant
Yroot < Ysoil

22. How water reaches from roots to leaves?
Pathway for water movement from roots
to leaves.
Fig. 4.3
 Water can travel from
the soil to the root
xylem by two distinct
pathways
o symplastic pathways
o apoplastic pathways

23.  The less-suberized
growing tips of roots have
higher water uptake rates
than older portions of the
root.

24.  Water flows from
roots to leaves via the
xylem
 Xylem is a network of
specialized cells
called tracheary
elements.
 Gymnosperms have
tracheids.
 Angiosperms have
vessel elements &
sometimes tracheids.

26. Xylem cavitation
Embolisms that stop water transport can
form in tracheary elements when xylem
pressure is sufficiently negative to pull in air
through a pit.

27. May 17, 2003 North of San Francisco Peaks

28. September 20, 2003 North of San Francisco Peaks
PJ Woodland Juniper Woodland

29. The xylem network is extremely intricate in leaves.
Fig. 4.8

31. The wet walls of leaf cells are the
sites of evaporation.
Where does water evaporate inside leaves?

32. As for soils, a more
negative YP develops as
leaf cell walls
dehydrate and water is
held in smaller pore
spaces.
YP = -2T/r

33. The most widely accepted model of water transport
through the xylem is the “cohesion-tension model”.
1. A negative pressure or tension is generated in
leaf cell walls by evaporation (transpiration).
2. The cohesive property of water means this tension
is transmitted to water in adjacent xylem and throughout
the plant to the roots and soil.

34. Water potential and water flow in plants in a
nutshell
Water moves from one part of the plant to another
down a water potential gradient. Different components
of ψ are important at different stages.
 Soil to roots: matric potential.
 Roots to stems: pressure potential.
 Stems to cells: osmotic potential.
 Cells to stomata: vapor potential.

35. Precipitation (Yg)
Infiltration
Overland flow
(erosion, nutrients)
Percolation
Throughfall)

36. Evaporation from
leaf surfaces
Evaporation from
soil
Transpirational water loss
to the atmosphere
Evapotranspiration
Water transport
through the plant
Water uptake by
plant roots

37. Surface soil
water Yt = - 0.8 MPa
Surface roots
Yt = - 1.1 MPa
Leaves
Yt = - 1.5 MPa
Atmosphere
Yt = - 30 MPa
Water in the soil-plant-atmosphere continuum J = L (Dyt/l)
Water moves along a
gradient of decreasing
water potential
What happens
at night?
Stomata close –
which term does
this affect?
How would water
potential gradient
respond?