Plant Water Relations

Anatomy

• Three distinct part of the plant
  • canopy
  • stem
  • roots
• Stomata: Microscopic pores on the surface of the leaves (usually underside) that are the breathing apparatus of the plant. They regulate gas exchange (O2 and CO2) and transpiration.
• Vascular System of plants: It consists of three distinct tissues namely Xylem, Cambium and Phloem.
  1. Xylem (most relevant): moves water up from roots to leaves driven by evaporation pulling water upward (like sucking through a straw). These cells are dead and the flow is only in one direction.
  2. Cambium: ring of dividing cells that produce new Xylem and Phloem as plant grows thicker.
  3. Phloem: moves sugars around (source to sink. Flow can move bidirectionally).

Ecological traits

They are features in a particular species that indicate a function or the performance of that function which can be compared with other species. Example: size of the leaf, Stomata, rooting depth etc.

Water Potential

It is a measure of the potential energy of the water i.e, how “free” or “available” the water is to move.

$$\text{Water potential }(\psi )=\text{Pressure + Osmotic + Gravitational potentials}$$

• Pressure potentials $\to$ It is the existing pressure inside cells.
• $+$ve pressure: pressure is positive, water entering the cells gets pushed out into the rigid walls keeping the plant firm. Otherwise the plant will wilt.
• $-$ve pressure: tension in the xylem. Transpiration causes a suction effect inside the xylem.
• Osmotic potential $\to$ adding solutes in the a water lowers its potential
• Gravitational potential $\to$ potential due to gravity. Taller the plant, higher the potential the plant has to overcome for water transport.

NOTE

Water potential at atm is 0 MPa by convection. So in the plant everything is negative

Flow Velocity

The flow velocities in the xylem can be estimated by the Hagen-Poiseuille equation.

$$v_{f,\mathrm{xylem}}=\underset{K_f}{\underbrace{\frac{r^2}{8\nu L}}}\Delta \psi$$

where,

• $\Delta \psi \to$ Water potential
• $K_f\to$ Hydraulic conductance

NOTE

The Hagen-Poiseuille idealisation assumes smooth cylindrical tubes. It ignores resistances from pits and end walls. However it captures the most essential point that velocity scales with the square of radius.