1. Water Movement through Plants HORT 301 – Plant Physiology October 16, 2009 Taiz and Zeiger, Chapter 4, Chapter 18 (p. 449-455), Chapter 23 (p. 603-609) paul.m.hasegawa.1@purdue.edu

2. Soil water holding capacity and availability for uptake by roots Dependent on soil type and structure Greater surface area/gram – more water holding capacity

3. Water moves through soil by pressure-driven bulk flow Soil water potential (Ψ w ): Ψ w = solute/osmotic potential (Ψ s ) + pressure potential (Ψ p ) Soil solution Ψ s is usually negligible Ψ p contributes the most to soil solution Ψ w B A

4. Soil hydraulic conductivity during dehydration Taiz and Zeiger 2006

5. Water uptake into roots Secondary root and root hair development Hydrotropism

6. Water transport to the xylem Apoplastic, and symplastic and transcellular pathways Aquaporins facilitate symplastic water uptake into roots

7. Water transport through the xylem – root to shoot Xylem (tracheary) elements – tracheids (angiosperms and gymnosperms) and vessel elements (angiosperms)

8. Xylem element interconnections

9. Surface tension facilitates water transport from roots to leaves Cohesion-tension theory for water movement in the xylem Extensive vascular system in a leaf

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

11. Primary forces that drive water transport: 1. Soil -  p gradient that drives bulk flow 2. 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 cavity 4. Sub-stomatal cavity to atmosphere – water vapor concentration gradient 1 2 3 4

12. Transpiration occurs primarily through stomata (stoma) About 95% of plant water loss occurs through stomata Cuticle and boundary layer are resistances to leaf transpiration Stomatal complex – pore surrounded by a pair of guard cells that control the aperture size

13. There are numerous guard cells in a leaf

14. Guard cell anatomy Two guard cells in the epidermis form the aperture of the stomate Kidney shaped and dumbbell shaped (most grasses) guard cell pairs

15. Guard cell turgor and volume regulate stomatal pore aperture Opening – turgor and increased cell volume Closing - turgor and volume reduction

16. Raven et al, 2005 Biology of Plants

17. Light, circadian rhythm, CO 2 and drought stress (ABA) regulate stomatal opening/closing