Lecture #8 Soil-Water-Plant Flashcards
Sandy soils:
Clay Soils:
Humus:
- Large soil particles and pores
- Fine soil particles and pores
- binds soil particles into large aggregates, gives structure
Gravitational Water
all water that can freely drain under gravity in the soil
Capillary Water
Water held by capillary forces withing soil pores against gravity, beneficially stored within rootzone for plant use
clay soil 40% retention
sand soil <15%
Hygrosopic
water bound to soil particles by adhesion, not ready for plant use, clay soils have more
Positive pressure potential
when watertable is above location the water is under positive pressure
Negative potential/ tension/ capillary pressure
watertable below location
How is capillary pressure caused
by presence of menisci ie. air-water interface
surface tension of water works to minimize sureface area of interface
How does capillary pressure affect plants
water released to roots and remaining water recess into smaller pores to reduce surface area
as radius of curvature decreases, capillary pressure increases (becomes more negative)
the smaller the pores, more negative capillary gets and greater effort by plant roots to get water
How does water move through soils
moves as “bulk flow”
moves from high water potential to low water potential
governed by darcys law
capillary barrier
when soil becomes more dry, the K (permeability) drops rapidly until reaching capillary barrier
it is a layer of dry soil that does not permit water to enter under unsaturated conditions
where does water mostly enter roots
near the root tip
What are root hairs
hairs on root that help increase contact area
Suberization
when mature parts of plants become impervious to water
helps draw water from new roots further from base of plant
3 pathways of water flow in roots
apoplast
symplast
transmembrane
Apoplast
non plastic, include cell walls and intercellular material
water does not cross any plasma membranes and it is mainly an exo-cellular pathway (goes around cells)
Symplast
continuous network of interconnected plasmodesmata (membrane lined channels connecting adjacent cells)
transport via cell to cell pathway
Transmembrane
occuring across a cell membrane, may cross vacuolar membrane (Tonoplast)
Which is preferred pathway of water in roots?
the path of least resistance
directed by:
water potential gradient
resistance to water flow
What happens at endodermis
Casparian strip (hydrophobic band of suberin) obstructs apoplast pathway
endodermis suberized in mature parts of plant
blocking flow forces water/solutes to pass thru plasma membrane
down regulating the expression of aquaporin genes can lead to…
decrease hydraulic conductivity of roots, causing plants to wilt easily and develop extensive root system
how is control of permeability by aquaporins caused?
forcing water to move symplastically
factors controlling water uptake
low temp
anaerobic conditions
respiratory inhibitors
how is “gate” closed?
decreased respiration causing increase in cytoplasmic PH
The “gate” function controlled by…
intercellular pH and Ca++ levels
of stem is cut will sap still come out?
yes because there will be positive pressure coming out (0.05-0.2 mpa) cause by water drawn into xylem leading to positive hydrostatic pressure
if plant cut mid day then…
transpiration is high so water is drawn out and less pressure is built up
hydathodes
pores where positive xylem pressure causes sap to ooze out from
least resistant pathway for water transport through plants?
xylem
treachery elements in xylem?
tracheids: in angiosperm and gymnosperms
vessel elements: in angiosperms some gymnosperms and some ferns
tracheid and vessel elements functions
elongated spindle shapes cells
pits overlap
pit pairs provide low resistance pathways for water movement between tracheids through pit membrane
vessels
shorter/wider than tracheids
connected at ends by perforation plates
located at extreme ends connected by pits (on the lateral walls)
Mechanism of transport through xylem
primarily pressure-driven bulk flow
independent of solute concentration gradient
bulk flow governed by Poiseuille’s law
