Water Movement through PlantsHORT 301 – Plant Physiology

October 16, 2009Taiz and Zeiger, Chapter 4, Chapter 18 (p. 449-455), Chapter 23 (p. 603-609)

paul.m.hasegawa.1@purdue.edu4.1 Main driving forces for water flow

Soil water holding capacity and availability for uptake by roots

Dependent on soil type and structureGreater surface area/gram – more water holding capacity

Water moves through soil by pressure-driven bulk flowSoil water potential (Ψw): Ψw = solute/osmotic potential (Ψs) + pressure potential (Ψp)

Soil solution Ψs is usually negligibleΨp contributes the most to soil solution Ψw

4.2 Root hairs make intimate contact with soil particles

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Soil hydraulic conductivity during dehydration

Taiz and Zeiger 2006

Water uptake into rootsSecondary root and root hair developmentHydrotropism

4.1 Main driving forces for water flow 4.2 Root hairs make intimate contact with soil particles

Water transport to the xylemApoplastic, and symplastic and transcellular pathwaysAquaporins facilitate symplastic water uptake into roots

4.4 Pathways for water uptake by the root 4.5 Stereo model of the spinach plasma membrane aquaporin (SoPIP2;1)

Water transport through the xylem – root to shootXylem (tracheary) elements – tracheids (angiosperms and gymnosperms) and vessel elements (angiosperms)

4.1 Main driving forces for water flow 4.7 Tracheary elements and their interconnections (Part 1)

4.8 Vessels and tracheids form a series of parallel, interconnected pathways

Xylem element interconnections

Surface tension facilitates water transport from roots to leavesCohesion-tension theory for water movement in the xylemExtensive vascular system in a leaf

4.11 Water pathway through the leaf

Transpiration – water loss from leaf surfaces~95% of plant water loss occurs by evaporation through stomataWater vapor concentration difference along the transpirational pathway drives evaporation

4.11 Water pathway through the leaf

Primary forces that drive water transport:1. Soil - p gradient that drives bulk flow2. Uptake by plant roots - w gradient that facilitates osmosis due mainly to the symplastic s

3. Root to shoot - p gradient resulting from surface tension in the sub-stomatal cavity4. Sub-stomatal cavity to atmosphere – water vapor concentration gradient

4.1 Main driving forces for water flow

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Transpiration occurs primarily through stomata (stoma) About 95% of plant water loss occurs through stomataCuticle and boundary layer are resistances to leaf transpiration

Stomatal complex – pore surrounded by a pair of guard cells that control the aperture size

There are numerous guard cells in a leaf

4.14 Electron micrographs of stomata (Part 2)

Guard cell anatomyTwo guard cells in the epidermis form the aperture of the stomateKidney shaped and dumbbell shaped (most grasses) guard cell pairs

4.14 Electron micrographs of stomata (Part 1)

Guard cell turgor and volume regulate stomatal pore apertureOpening – turgor and increased cell volumeClosing - turgor and volume reduction

4.16 Radial alignment of the cellulose microfibrils in guard and epidermal cells

Raven et al, 2005 Biology of Plants

Light, circadian rhythm, CO2 and drought stress (ABA) regulate stomatal opening/closing