Water uptake water transport and transpiration Things to

Water uptake, water transport and transpiration

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

Hydrogen Bonds and Cohesion Water molecules have weak negative charges at the oxygen atom and positive charges at the hydrogen atoms. H O + H H Positive and negative regions are attracted. O The force of attraction, dotted line, is called H H H O O a hydrogen bond. Each water molecule is H hydrogen bonded to four other water H H O H 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/Biology. Pages/H/Hydrogen. Bonds. html

How water moves through the plant 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. At 22 o. C (72 F) and 50% Relative Humidity yair = 100 MPa negative Typically yleaf = -1 to - 4 MPa ysoil = 0. 01 to - 0. 1 MPa Water potentials of connected tissues defines rate of water flows through a plant.

Upper epidermis Photosynthetic cells (mesophylll) Stoma leaf vein lower epidermis 1 Driving Force is Evaporation xylem phloem Growing cells also remove small amounts of water from xylem Water Uptake in Growth Regions 2 Cohesion in Xylem vascular cylinder hair cell soil particle Water molecule cortex endodermis 3 Water Uptake from Soil by Roots Fig. 30. 9, p. 523

Measuring water potential The pressure bomb! Compressed air

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

Diurnal pattern of shoot water potential Midnight Midday 500 Transpiration Mg/sec/tree 400 300 200 100 Shoot water potential MPa 0 -1 -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

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

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

Transpiration flux, g H 2 O/cm 2 leaf surface/second X 10 -7 3. 0 Wind speed influences transpiration 2. 5 The boundary layer around a 2. 0 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 1. 5 major resistance to the flux of H 2 O from the leaf. 1. 0 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 0. 5 Stomatal aperture, m http: //forest. wisc. edu/forestry 415/lecture 6/windspd. htm

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

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

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

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.

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