Water Balance of Plants - 3 Flashcards
Where does water leave in plants and how does water reenter the plant
water leaves through the leaves which allows water to enter from the roots and soil and move up the xylem conduits through bulk flow
Water content and rate of movement in soils depend on what two factors and why
soil type and structure
because they influence pressure gradient and hydraulic conductivity
water absorption increases due to increases surface area of what
root hairs
what defines soil texture
the diameter
ex. sand is 1mm or larger
clay is 2um or less
What is the difference between clay and sand soil
clay has greater surface area because of smaller particles and therefore have smaller channels in between them and cause water to be retained longer
Explain the difference in pressure potential for wet and dry soils
pressure potential is higher in dry soils because they are fighting harder to get water from plant because they have less of it
what causes gravitational potential to increase in soil
increase in elevation
How does water move through soil
by bulk flow, from regions of higher soil water content to lower
rate of water flow in soils depending on what two factors
- size of the pressure gradient
- hydraulic conductivity of the soil = measure of ease which water movies through (which once again varies with water content and soil type)
does clay or sand have higher hydraulic conductivity
sand because easier because bigger particles so less turns in the pathway
Define root hairs
filamentous extensions of root epidermal cells that greatly increase SA of root and soil water contact
Explain the variability of permeability of root hairs
the older/mature roots are less permeable because they develop a modified layer of cells with hydrophobic material to enable water uptake (dont lose water in all parts of roots)
What pathways does water move through in roots
apoplast, symplast, and transmembrane pathways
Define apoplast pathway
water moves through cell walls and intercellular airspaces of non-living cells. this pathway is obstructed by the casparian strip which forces the water to move into symplast pathway in order to enter xylem
Define casparian strip
obstructs apoplast pathway and its function is to insulate water from exiting the xylem
symplast pathway
water moves through network of cell cytoplasm that is interconnected by plasmodesmatea (gates) that is a facilitated movement
transmembrane pathway
water moves through cell membrane of each cell, twice, and need solute potential. facilitated by aquaporins and requires energy from respiration
Root pressure is most likely to occur when soil water potentials are ____ and transpiration rates are ____
high
low
Define guttation
occurs when plants develop root pressure frequently and produce liquid droplets on the edges of leaves.
this pressure can reduce effects of cavitation
what consists of the longest part of the pathway of water transport in plants
xylem
define tracheids
elongated spindle shaped cells that overlap vertical to one another and water flows through pits. pits of tracheids will line up and form pit pairs that dont include secondary cell wall to provide lower resistance
Define vessel elements
shorter segments than tracheids that allows water to move through pits and perforation plates
What are the two types of xylem conduits
- tracheids
- vessel elements
What allows water to move throughout the plant starting from the soil
prsesure-driven bulk flow moves water throughout the xylem and the decrease of water potential at evaporation sites in leaves allows for the ascent of water from the soil throughout the plant
What are the two mechanisms for water transport through plants and what are their impacts
- cohesion-tension theory (most important)
- root pressure (minor role)
Define cohesion-tension theory
water is mostly attracted to each other and therefore water is pulled up through the xylem by negative pressure (tension)
Explain root pressures role in water transport throughout the plant
water is pushed through the xylem by positive pressure and only occurs under certain conditions (lots of water and low transpiration)
What are the 3 steps to cohesion-tension theory
- solar radiation input to change phase of water into vapour
- vapour diffuses out of leaf down a vapour concentration gradient
- tension is created in the cell walls where evaporation initially occurs and tension pulls water up through the plant
Explain the evaporation of water from the xylem
water is pulled from xylem into the mesophyll (cells in the cell wall) which causes a water potential decrease and xylem tensions pull up water from the soil which requires a continuous liquid-filled pathway
Define cavitation
the filling of tracheid or xylem vessels with air causing the formation and expansion of gas bubbles due to tension in the surrounding liquid. cavitation results in embolisms (gas filled voids)
Define vulnerability curve
quantify species susceptibility of cavitation and impact (hydraulic conductivity vs. imposed level of xylem tension)
as tension increases, conductivity decreases
how do vulnerability curves change in environments of moist or dry habitats
decrease in xylem conductivity occurs at lower tensions in moist habitats than dry
What are the three ways in which plants minimize consequences of cavitation
- finite length of vessels and tracheids
- root pressure causes pressure to shrink gas bubbles
- secondary growth causes development of new xylem every year
waxy cuticle
covers leaf surface and is effective barrier against water loss
where are stomata present
present on upper and lower surfaces, mostly lower surfaces
what causes hydraulic resistance to vary in leaves
varies in response to growth conditions and exposure to low leaf water potentials (ex. shade) (ex. if have higher leaf vein architecture means increase resistance)
What is the largest factor of transpiration
temperature
What two things does transpiration depend on
- difference in water vapour concentration between leaf air spaces and external air (to create the gradient)
- diffusional resistance of the pathway (which includes stomatal resistance and boundary layer resistance)
Define boundary resistance layer
the resistance due to the layer of undisturbed air next to the leaf surface, which water vapour must diffuse through to reach the air of the atmosphere
What determines the boundary layer resistance
wind speed and leaf size
when wind is still, boundary layer is think, and stomatal apertures have little affect on transpiration
what anatomical and morphological aspects of leaves can affect the thickness of the boundary layer (3)
- hairs on surface of leave = microscopic windbreak
- sunken stomata are sheltered from the wind
- size and shape of leaves and orientation influence the way the wind crosses the surface of the leaf
what is stomatal resistance governed by
stomata open or closed
when open, provide a low resistance pathway for diffusion of gases across the epidermis
guard cells
control stomatal opening
two surround the stomatal pores
function as hydraulic values that are effected by light intensity and quality, temperature, leaf water status, and intracellular CO2 concentrations
define cellulose microfibrils
play an essential role in opening and closing the stomatal pores and are fanned out radially from the pore.
What is the process of guard cells opening
ion uptake which decreases osmotic potential and water moves into the guard cell. as water enters, turgor pressure increases and the stomata opens
what are the first two dominant stimulis of stomatal opening
temperature then light
when water is abundant, what is the functional solution to limit water loss while CO2 is the temporal regulation of stomatal apertures
stomata open during the day and closed or minimized at night
_____ plays a partial role in light-dependent stomatal opening
provided by what evidence
photosynthesis
DCMU is a photosynthetic electron transport inhibitor and when DCMU is present, it also partially inhibits stomatal opening
define transpiration ratio
the ratio of water loss to photosynthetic carbon gain
therefore measures the effectiveness of plants in moderating water loss and allowing CO2 uptake
What are the 3 factors that results in the large ratio of H20 efflux to smaller CO2 influx
- concentration gradient that drives water is 50x larger
- CO2 is larger and diffuse through air slower than water vapor
- CO2 must cross the plasma membrane, cytoplasm, and chloroplast envelope = higher resistance