Plant Water Relation
Easy Overview
Plants don't drink water — they pull it. From the ground, up through their stems, to the highest leaves. How? Without a heart or muscles? This chapter is about the physics wizardry plants use to move water around. It's basically plumbing, but way more impressive.
Water potential — the driving force
Water moves from areas of high water potential to low water potential. Think of water potential as enthusiasm for moving. Pure water has maximum enthusiasm. Add solutes (salt, sugar) and enthusiasm drops. So water flows from pure (high potential) to salty (low potential). Plants use this gradient to pull water in.
Absorption of water — roots do the work
Root hairs are the VIP entrance for water. They have a huge surface area and are in direct contact with soil particles. Water enters via osmosis — the root cells have more solutes than the soil, so water rushes in. From there, it travels through the cortex via apoplast (through cell walls) or symplast (through cell interiors).
Ascent of sap — pulling water upward
Water travels up the xylem vessels like a straw. But there's no pump at the bottom. The pull comes from the top — transpiration. As water evaporates from leaves, it creates a negative pressure that yanks water up from the roots. This is the transpiration pull-cohesion-tension theory. Water molecules stick together (cohesion), so when one gets pulled, the whole column moves.
Transpiration — the necessary evil
Leaves lose water through stomata as water vapor. This is transpiration. It's 'necessary' because it pulls water up and cools the plant. It's 'evil' because the plant loses precious water. Plants balance this by opening stomata during the day (when they need CO₂ for photosynthesis) and closing them at night or during drought.
Stomatal mechanism — the gatekeepers
Stomata are pores made of two guard cells. When guard cells take up water and become turgid, the stoma opens. When they lose water, it closes. Potassium ions flow in and out to control this. It's like a bouncer at a club — K⁺ ions let water in, the guard cells swell, the door opens.
Key Points
- •Water potential (ψ) = solute potential + pressure potential; water moves from high ψ to low ψ
- •Root hairs absorb water by osmosis; high solute concentration in root cells drives entry
- •Apoplast (cell walls) and symplast (cytoplasm) are two water transport pathways in roots
- •Transpiration pull + cohesion of water molecules + adhesion to xylem walls = water rises
- •Cohesion-tension theory explains ascent of sap in xylem
- •Transpiration cools plants and enables mineral transport but causes water loss
- •Stomata open when guard cells are turgid (K⁺ influx), close when flaccid (K⁺ efflux)
- •Guttation: water loss in liquid form through hydathodes (occurs at night/high humidity)
Practice Questions
- Explain the cohesion-tension theory of water transport in plants.
- What is water potential? How does it determine the direction of water movement?
- Describe the structure of stomata and explain how they open and close.
- Differentiate between apoplast and symplast pathways of water transport.
- Why is transpiration called a necessary evil? Explain with reasons.