9.1: Transport in the xylem of plants

Transpiration The loss of water vapour from leaves and other aerial parts of the plant. 90% of water absorbed by the roots is lost by transpiration.

Structure of the leaf Note: Waxy cuticle Stoma Guard cells Spongy mesophyll

Stomata and guard cells 1.Why can stoma only be closed temporarily? 2.What causes the stoma to open and close? 3.Why do they change shape? What substances are involved?

Structure of stem & root

Potometers Complete the virtual transpiration lab.

The Xylem The structure of the xylem leads to very efficient water transport within the stem, maintaining structure even at very low pressures. Features include: -Long continuous tube -Thickened walls -Walls embedded with lignin (strong polymer) -Non-living (flow of water is passive) -Cohesive properties of water molecules and adhesion between the water molecules and the walls of the xylem allow water to be transported efficiently.

Water loss When water is evaporated from the leaf, adhesion causes water to be drawn through the cell wall from the nearest available source (xylem) to replace the lost water. This evens applies at low pressure, the force of adhesion between the cell wall and water is strong enough to suck water out of the xylem. Consequently a pulling force is created by the water in the xylem – Leaf, stem, roots. This is called transpiration-pull and is strong enough to pull water up the stem against the pull of gravity. Transpiration is a passive process, what factors affect the rate of transpiration?

Active transport Roots absorb water by osmosis Mineral concentrations are usually higher in root tissue compared to the soil. Therefore, protein pumps in the plasma membrane of root cells active pumps ions into the plant. There are separate pumps for each mineral type. Some plants, e.g. orchids, require minerals which bind to soil particles so they have formed a relationship with a fungus. The fungus grows around the root hairs of the plant, absorbing minerals bound to soil particles (phosphate), in return, the plant supplies sugars and nutrients to the fungus.

Water conservation Adaptations for water conservation include: 1.By being ephemeral – very short life cycle to be completed when water is plentiful. 2.By being perennial – store water in specialized leaves/stems/roots, consequently these plants live for a long time. 3.Marram grass (Ammonphila arenaria) has a rolled leaf (see slide 12), Stomata are in pits, hairs on the inside of the roll. All of these adaptations are an attempt to reduce water loss by evaporation. 4.Xerophytes are adapted to very dry environments. E.g. cacti. -Very small leaves (spines) -Pleats to allow stem to expand/contract -Waxy cuticle covering stem -Few stomata -Stoma open at night

Finally…. Halophytes survive in very salty (saline) soils. Adaptations include: 1.small, scaly leaves/spines 2.Leaves are shed when water is scarce 3.Water storage in specialized cells in leaf 4.Thick cuticle – layered epidermis 5.Sunken stomata 6.Long roots 7.Able to remove excess salt with specialized cells.