Class Notes (806,783)
Canada (492,447)
BIOC40H3 (14)
Lecture 3

Plant Physiology - lecture 3.docx

8 Pages
Unlock Document

University of Toronto Scarborough
Biological Sciences
Connie Soros

Plant Physiology – Lecture 3  As we discussed in the last lecture on water and plants, there is an inherent conflict between a plant’s need to take in CO2 for photosynthesis and to conserve water. This is because both CO2 and water must use the same pathway through stomates, with CO2 going in, and water being lost out of the pores  We discussed the special properties of water last lecture, (most of them, dependent on the extensive hydrogen bonding in water molecules)  now we will look at many of the ways that plants have evolved adaptations to move water with in the plant, control water loss from leaves and to replace the water that is lost. WATER IN THE SOIL  Water potential ( ) of soils can be divided into three components 1. osmotic pressure ( ) 2. hydrostatic pressure ( ) 3. gravitational potential ( )  Since solute concentration in soil tends to be very low, the osmotic pressure ) is generally negligible.  For wet soils the hydrostatic pressure ( ) is very close zero, but as soil dries out, the hydrostatic pressure decreases and becomes negative  Remember that water has a high surface tension that tends to decrease air-water interfaces, but because of high adhesive forces in water, it also clings to the surfaces of soil particles. Therefore, as the water content of soil decreases, the water recedes into the interstitial spaces (smallest channels) between soil particles forming air-water surfaces with curvatures that correspond to the balance between the tendency to minimize the surface area of the air-water interface and the attraction of water for soil particles  Water under a curved surface will develop a negative hydrostatic pressure ( ). As soil dries out, water is first removed from the largest spaces between soil particles and subsequently from successively smaller spaces between the soil particles.  As the soil becomes drier, the curvature of the air-water surfaces in pores increases causing the hydrostatic pressure to become increasingly negative (greater tension). The last water potential component is the gravitational potential ( )  Unlike in water, where this parameter is usually negligible gravity plays an important role in soil drainage and gravitational potential is proportional to elevation, higher at higher elevation and lower at lower elevation WATER ABSORPTION BY ROOTS  Effective water absorption by the roots involves intimate contact between the surface of the root and soil  The amount of surface contact between the soil and roots is maximized by root hairs. Root hairs are filamentous outgrowths of root epidermal cells that greatly increase the surface area of the root providing greater capacity for absorption of ions and water from the soil  As mentioned in the first lecture, water enters the roots most readily at the tips (also usually the location of most abundant root hairs), older more mature areas of the root (closer to the base of the plant) have often developed an outer layer of protective tissue called an exodermis (or hypodermis) which contains hydrophobic material (suberin in a casparian strip) in its walls making the these regions of the root less permeable to water  By decreasing the permeability in the older regions of the root, tensions can extend further down into the root system allowing water uptake from the distal (farthest away) regions of the root. PATHWAYS FOR WATER IN THE ROOT  There are three different pathways that water can flow through in the root, the apoplast, the symplast, and the transmembrane pathway 1. Apoplast ­ a continuous system of cell walls, intercellular air spaces of living cells and the lumens (cavities) of non-living cells (eg. Xylem elements and fibers) ­ In the apoplastic pathway of roots, water moves through cell walls and extracellular spaces without crossing any membranes ­ Water does not pass through the living parts of cells. Water movement through the apoplast is obstructed at the endodermis by the casparian strip. The casparian strip is a band within the radial walls of the endodermis that is impregnated with suberin (a wax-like, hydrophobic substance) at this point, water and solutes are forced to pass through the plasma membrane and into the living part of the plant (symplast) ­ Water will pass through the aquaporins (water selective channels of integral proteins within the bilayer of the plasma membrane) ­ Evidence for this is found in studies (eg. Siefritz et al. 2002, Martre et al. 2002) where the genes for aquaporins have been down- regulated, resulting in reduced hydraulic conductivity of the roots and either plants that wilt easily or compensate by producing large root systems ­ Aquaporins can be regulated (ie. Gated) in response to intercellular pH (increased intracellular pH can occur in response to low temperature or anaerobic conditions). An increase cytoplasmic pH alters the conductance of aquaporins in root cells greatly reducing their permeability to water. 2. Symplast ­ Consists of the entire network of cell cytoplasm interconnected by plasmodesmata ­ In this pathway water travels across the root in the living parts of cells via plasmodesmata. 3. Transmembrane pathway ­ Is the route water takes, by entering a cell on one side and exiting on the other side before entering the next cell in the series ­ In this pathway, water crosses the plasma membrane of each cell in its path twice (first entering, then exiting the cell). There may also be transport across the tonoplast (vacuole membrane) GUTTATION  When soil water pressure is high and transpiration rates are low, some plants will develop root pressure and a positive pressure in the xylem (if transpiration is high, water is taken into the leaves and lost to the atmosphere very rapidly so that a positive pressure never develops in the xylem)  When a positive pressure does occur, plants will often produce an exudation of liquid droplets on the edges of their leaves due to root pressure. This phenomenon is called guttation. The positive xylem pressure will cause the release of xylem sap through specialized pores called hydathodes that are associated with vein endings at the leaf margin  Guttation is the dewdrops that you see on tips of grass leaves in the morning. WATER TRANSPORT IN THE XYLEM  Xylem constitutes the longest part of the pathway of water transport, often more than 99.5% of the water transport pathway.  The structure of xylem contributes to the movement of water from the roots to the leaves and negative hydrostatic pressure generated by transpiration in the leaves pulls water through the xylem  Pressure-driven bulk flow is responsible for long distance transport of water in the xylem. The water at the top of a tree develops a large tension (a negative hydrostatic pressure) which pulls water through the xylem. This mechanism is called the cohesion-tension theory of sap ascent because it relies on the cohesive (mutual attraction between water molecules) properties of water in order to sustain large tensions in the xylem water columns  The negative pressure that causes water to move up through the xylem develops at the surface of the cell walls of leaves  The cell walls are composed of hydrophilic cellulose microfibrils and other hydrophilic components, that cause water to adhere (adhesive property) to the wall  As water evaporates from cells within the leaf, the surface of the remaining water is drawn into the small channels (interstices) in the cell wall where it forms curved air-water inte
More Less

Related notes for BIOC40H3

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.