Chapter 11. Water and the whole plant

Chapter 11. Water Relations of the Whole Plant

Transpiration

What force drives transpiration?
How do environmental factors (e.g. wind speed) and plant factors (e.g. leaf size) affect the rate of transpiration?
How does water move from the soil to the plant?
How does water ascend from roots to leaves?
Why do plants transpire?

Transpiration = evaporation of water from leaves. T = rate of transpiration (e.g. mL H₂0 / cm2 leaf area / minute).
Review of leaf anatomy

Blade, petiole and veins.
Vascular bundles, mesophyll, epidermis, substomatal space, stomata with guard cells, cuticle (Figure 11.1)

Vapor pressure (symbol e) is the pressure (in Pascals) which would be exerted by an enclosed body of water vapor. Water vapor is mostly determined by the concentration of water vapor molecules and by the temperature.
Fick's Law predicts that water vapor will diffuse from the substomatal cavity (region of high concentration of water vapor molecules) , through the stomata, out to the atmosphere (region of low concentration of water vapor molecules).
The effect of environment and leaf characteristics on transpiration rate.

An analogy from Ohm's Law: Equation 11.3, Figure 11.3.
Atmospheric humidity: affects the numerator of Equation 3.3.
Atmospheric temperature: affects the numerator. High temperature lowers atmospheric RH, increases leaf temperature which increases vapor pressure of substomatal cavity.
Wind: affects denominator by reducing thickness of boundary layer.
Leaf characteristics: small leaves have thinner boundary layers, leaves vary in number of stomata per unit area, stomata may be sunken in pits, protected by hairs, etc. Stomata can open and close. All these affect the denominator.

Transpiration is costly - a good sized tree can transpire ca. 200L/hour.
Transpiration speeds nutrient transport. But uptake by roots is probably the rate-limiting step in plant nutrition.
Transpiration can help cool leaves in hot climates. But convection is as effective. Also leaves with low transpiration rates in hot humid climates grow well.
Transpiration may be the cost plant must bear to photosynthesize as CO₂ is absorbed through stomata.

Plant Anatomy and Water Conduction

The plant vascular tissues are xylem (conducts water and mineral nutrients) and phloem (conducts (photosynthate).
Tracheary elements (lack protoplasts):

Tracheids: Occur in gymnosperms and angiosperms. Single cells, diameters 10-50 microns. Communicate laterally via pit pairs. Secondary walls become lignified, contribute to strength of wood.
Vessel elements: mostly restricted to angiosperms. Wider than tracheids (up to ca. 1000 microns). Organized into vessels. Conduct water more efficiently in accordance with Poiseuille's law (Equation 3.4).

The Ascent of Xylem Water

The problem: how is water transported from roots to leaves, especially in tall trees? A constraint on any hypothetical mechanism is that a water potential difference of 2-3MPA would be required to raise water to the top of the tallest trees.
Some possible mechanisms:

Vacuum pump. If vacuum were generated in canopy, atmospheric pressure would push water up the xylem (like sucking on a straw). Problem: atmospheric pressure (0.1 MPa) can only raise a water column 10.3 m (Figure 11.9).
Root pressure. Acting as osmometers, roots can develop positive pressures, pushing water up xylem. Problem: measured root pressures are in range 0.1-0.5 MPa.
Capillarity. Water will rise in narrow tubes to a height inversely proportional to the radius (Box 11.2). Problem: xylem elements are too wide to support much capillary rise (50 micron diameter tracheid will result in rise of 60 cm).

Transpiration is the driving force. Evaporation from cells walls in substomatal cavity establishes menisci under tension (Figure 11.14). This tension is transmitted to water column right down to the soil, causing it to rise.
Evidence:

Water has sufficient tensile strength to withstand pull (-25 to -30 MPa!).
Tension in xylem can be measured: typically -.5 to -2.5 MPa (sufficient to account for rise).
Water columns in xylem can suffer cavitation and embolisms.

Roots, Soil and Water

Plants and the hydrologic cycle: plants are interposed in the pathway from soil to atmosphere, hence the term soil-plant-continuum.
Soil is complex

Mineral fraction derived from weathering of bedrock or other parent material. Particle size (Table 11.4) affects water retention characteristics through its effect on porosity. Fine, dry soils develop lower matric potentials than coarse, wet soils.
For a particular plant and soil, available water is the amount between field capacity and permanent wilting percentage.

Often comprise >50% of plant biomass. Roots are long and develop tremendous surface areas.
Absorption: mostly through root hairs (see Figure 11.17). Root hairs are mini osmometers.
Transport from root hairs to xylem is apoplastic (mostly) and symplastic.
Water absorption is inhibited by soil anoxia, high CO2 concentrations and (in the lab) respiratory inhibitors.
In saline soils, water uptake can be limited by low solute potential of the soil. Halophytes compensate by reducing solute potential of roots via active transport.