Water uptake, water transport and transpiration

1. Water uptake, water transport and transpiration

2. Things to know from today’s lecture How water molecules show cohesion and adhesion and why this is important in water transport The pathways of water movement in a plant What water potential is, how to measure it, how it varies in a plant during a day and what effect this variation may have Basic aspects of leaf energy budget How stomatal opening is controlled Trichomes

3. Hydrogen Bonds and Cohesion Water molecules have weak negative charges at the oxygen atom and positive charges at the hydrogen atoms. H H O + H O H H H O O H H H O H Positive and negative regions are attracted. The force of attraction, dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four other water molecules – the force of Cohesion. The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many hydrogen bonds form, the resulting Cohesion is sufficiently strong as to be quite stable. Adhesion is the tendency of molecules of different kinds to stick together – by a similar process. Water sticks to cellulose molecules in the walls of the xylem, counteracting the force of gravity. http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html

4. How water moves through the plant At 22oC (72F) and 50% Relative Humidity yair = 100 MPa negative Typically yleaf=-1 to - 4MPa Water potentials of connected tissues defines rate of water flows through a plant. ysoil= 0.01 to - 0.1 MPa Water potential indicates how strongly water is held in a substance. It is measured by the amount of energy required to force water out. Water potential , referred to as y (psi), is measured in megapascals, Mpa, (SI, SystÈme Internationale) units. For pure water at standard temperature and pressure (STP) y = 0 Mpa.

5. Upper epidermis Photosynthetic cells (mesophylll) Stoma leaf vein lower epidermis xylem phloem Water Uptake in Growth Regions Growing cells also remove small amounts of water from xylem vascular cylinder hair cell soil particle Water molecule cortex endodermis 1 Driving Force is Evaporation 2 Cohesion in Xylem Fig. 30.9, p. 523 3 Water Uptake from Soil by Roots

6. Measuring water potential The pressure bomb! Compressed air

7. Field measurements of  Forest laboratory in south west Scotland Measurement every hour for 7 days

8. Diurnal pattern of shoot water potential Midnight Midday 500 400 Transpiration 300 Mg/sec/tree 200 100 Shoot water 0  potential -1 MPa -2 30 Jul 31 Jul 1 Aug 2 Aug 3 Aug 4 Aug 5 Aug 6 Aug During daylight water loss from foliage exceeds water gain from soil so shoot water potential decreases.On sunny days  reaches –2 Mpa

9. Cessation of physiological processes: Cell growth and wall synthesis are very sensitive and may stop at -0.5 MPa Photosynthesis, respiration and sugar accumulation are less sensitive. They may be affected between -1 and -2 MPa

10. The energy budget of foliage Only 1-3% of radiation is used in photosynthesis Some radiation is reflected and some energy is re-radiated Radiation input In addition to radiation input leaf temperature can also be affected by wind speed and humidity because these conditions affect rate of cooling If Tleaf > Tair then the leafwarms the air Evaporative cooling depends upon latent heat of evaporation

11. 3.0 Wind speed influences transpiration 2.5 2.0 1.5 Transpiration flux, g H2O/cm2 leaf surface/second X10-7 1.0 0.5 Stomatal aperture, m The boundary layeraround a leaf extends out from the leaf surface. In it air movement is less than in the surrounding air. It is thick in still air, and constitutes a major resistance to thefluxof H2O from the leaf. A slight increase in wind speed will reduce the boundary layer, and increase transpiration. Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly  http://forest.wisc.edu/forestry415/lecture6/windspd.htm

12. Review of osmosis Diffusion of water across a selectively permeable membrane from a hypotonic to a hypertonic solution Hyper - above Hypo - below Water crosses the membrane until the solute concentrations are equal on both sides

13. Control of stomatal opening and closing Guard cells actively take up K causing water to enter by osmosis. The guard cell’s walls are unevenly thickened causing the cells to bow as they becomes turgid

14. Trichomes increase boundary layer resistance Peltate trichomes Trichome: hairlike projection from a plant epidermal cell. Trichomes do have other functions Coleus Foxglove Olive Curatella americana

15. Laboratory measurement of transpiration A laboratory potometer 1. Fill the potometer by submerging it – make sure there are no air bubbles in the system. 2. Recut the branch stem under water and, keeping the cut end and the potometer under water, put the cut end into the plastic tubing.

16. Components of experiments There is usually a THEORY behind each experiment A statement predicting alternative responses: “If thisis done that will happen otherwise it will not.” 1. Hypothesis 2. Treatment A specific, designed, manipulation sufficiently accurate to detect response to the treatment 3. Measurement The same measurement is made but the treatment is not applied. This provides the essential contrast. 4. Control Enables the degree of response to be defined and helps to protect against obtaining results by chance 5. Replication Required to establish the degree of certainty that can be attributed to a result, e.g., repetition with the same and different species 6. Repeated An experiment has: And must be