Water Relations Steady- State
“Tho’ vegetables have not an engine, which, by its alternate dilatations and contractions, does in animals forcibly drive the blood through the arteries and veins; yet has nature wonderfully contrived other means, most powerfully, to raise and keep in motion the sap. I shall begin with an experiment upon roots, which nature has providently taken care to cover with a fine thick strainer; that nothing shall be admitted into them, but what can readily be carried off by perspiration, vegetables having no other provision for discharging their recrement.” Stephan Hales, Vegetable Staticks, 1727
Plant Growth: Getting to the Root of the Problem Root: Function: Absorb water Absorb nutrients Filter out the unwanted Anchorage Storage Communicate with shoot Structure: Surface Area Endodermis Cortex Growing root tips
Zone of Maturation Continued Root growth is critical to root function.
Cortex Epidermis Stele Root Cross Section
cortex endodermis pericycle xylem phloem Endodermis is location of filter
Growth Longest roots recorded are for mesquite, Prosopis glandulosa (80’)
Primary roots of an adult rye plant Secale cereale were measured and found to be 380 miles in length.
Cohesion-Tension TheoryMechanism of water movement in xylem is driven by changes in from soil through plant to air Note that even at near 100% RH, air still more negative than leaf Thus: water flows from leaf to air However, even at air RH 100%, the slightest air movement across the leaf lowers air to less than in leaf so water flows from leaf to air
During all this pulling, hydrogen bonds hold water molecules together in columns inside xylem tubes = cohesion The very negative of the air tugs on the water column, causing the H2O molecules to move up through the plant. (Water molecules, not Disney symbols) Air Rhizoshere (rootzone)
Cohesion/tension explains how water can travel upwards against gravity in a plant. Transpiration at leaves Water molecules pulled up stem to replace molecules lost to air Tension on water in xylem Water pulled into roots
Water into the Root Roots have evolved to increase water absorption area by formation of root hairs. New root hairs have to be constantly produced to have water uptake. Damaged or diseased roots do not produce root hairs, severely limiting their ability to take up water.
Disease and Water Movement Many fungal or bacterial pathogens cause diseases with a characteristic symptom of wilt. The wilting comes because the pathogen enters the vascular tissue and as it grows, it clogs the water-conducting vessels. Cutting a stem and seeing discolored vascular tissue is a good “clue” that helps diagnose disease. In herbaceous stems a vertical cut is made just under the epidermis of the stem. If there is an infection, you can see a “streaking” in the vascular tissue.
Cavitation or Embolism Air bubble (vapor lock) in the xylem, break in the water chain NOT GOOD - stops water flow through that column in its tracks and often forever Practical application: Cut flowers often can’t take up water because of cavitation at cut ends of xylem - leads to the idea of cutting stems underwater.
Water Loss from the Leaf • Stomates- pores in the leaves, primary way plants regulate transpiration (water loss)
Stomatal Control • 3. Plant Water Status • sensed by the roots • when soil dries and soil approaches root , roots cannot take up water to meet plant demands, plants begin to loose water faster than it is taken up • in response to water loss, roots then synthesize ABA • ABA signals stomata to close to decrease water loss • water status is the overriding environmental factor that controls stomatal opening/closing
Plant Adaptations to Save Water • 1. Sunken Stomates • Area of higher RH develops • in the “pit” which reduces the • VPD between leaf and air. • 2. Stomates on underside of leaves • The upper side of leaves are exposed to light which warms the leaf and increases VPD causing more water evaporating if stomates are on the upper surface.
Plant Adaptations to Save Water • 3. Hairy leaves • hairs serve as a wind break to maintain an undisturbed layer of air around the leaf (boundary layer) • reduces VPD at leaf surface • 4. Osmotic adjustment • plant will automatically add solutes to cells which causes to drop which draws water into the cell.
Plant Adaptations to Save Water • 5. CAM Metabolism (succulents and some orchids) • stomates closed during day, open at night • at night CO2 enters the leaves • CO2 then converted and stored as an acid • during day, CO2 released and used in photosynthesis
Plant Adaptations to Save Water • 6. C4 Metabolism (warm-season grasses such as corn, turfgrasses) • CO2 converted to acid • acid ‘shuttled’ to special cells for photosynthesis • CO2 released for photosynthesis • location of special cells reduces photorespiration which ‘wastes’ CO2 in non-C4 plants