Water Balance of Plants - 3

Question 1

Where does water leave in plants and how does water reenter the plant

Answer 1

water leaves through the leaves which allows water to enter from the roots and soil and move up the xylem conduits through bulk flow

Question 2

Water content and rate of movement in soils depend on what two factors and why

Answer 2

soil type and structure
because they influence pressure gradient and hydraulic conductivity

Question 3

water absorption increases due to increases surface area of what

Answer 3

root hairs

Question 4

what defines soil texture

Answer 4

the diameter
ex. sand is 1mm or larger
clay is 2um or less

Question 5

What is the difference between clay and sand soil

Answer 5

clay has greater surface area because of smaller particles and therefore have smaller channels in between them and cause water to be retained longer

Question 6

Explain the difference in pressure potential for wet and dry soils

Answer 6

pressure potential is higher in dry soils because they are fighting harder to get water from plant because they have less of it

Question 7

what causes gravitational potential to increase in soil

Answer 7

increase in elevation

Question 8

How does water move through soil

Answer 8

by bulk flow, from regions of higher soil water content to lower

Question 9

rate of water flow in soils depending on what two factors

Answer 9

1. size of the pressure gradient
2. hydraulic conductivity of the soil = measure of ease which water movies through (which once again varies with water content and soil type)

Question 10

does clay or sand have higher hydraulic conductivity

Answer 10

sand because easier because bigger particles so less turns in the pathway

Question 11

Define root hairs

Answer 11

filamentous extensions of root epidermal cells that greatly increase SA of root and soil water contact

Question 12

Explain the variability of permeability of root hairs

Answer 12

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)

Question 13

What pathways does water move through in roots

Answer 13

apoplast, symplast, and transmembrane pathways

Question 14

Define apoplast pathway

Answer 14

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

Question 15

Define casparian strip

Answer 15

obstructs apoplast pathway and its function is to insulate water from exiting the xylem

Question 16

symplast pathway

Answer 16

water moves through network of cell cytoplasm that is interconnected by plasmodesmatea (gates) that is a facilitated movement

Question 17

transmembrane pathway

Answer 17

water moves through cell membrane of each cell, twice, and need solute potential. facilitated by aquaporins and requires energy from respiration

Question 18

Root pressure is most likely to occur when soil water potentials are ____ and transpiration rates are ____

Answer 18

high
low

Question 19

Define guttation

Answer 19

occurs when plants develop root pressure frequently and produce liquid droplets on the edges of leaves.
this pressure can reduce effects of cavitation

Question 20

what consists of the longest part of the pathway of water transport in plants

Answer 20

xylem

Question 21

define tracheids

Answer 21

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

Question 22

Define vessel elements

Answer 22

shorter segments than tracheids that allows water to move through pits and perforation plates

Question 23

What are the two types of xylem conduits

Answer 23

1. tracheids
2. vessel elements

Question 24

What allows water to move throughout the plant starting from the soil

Answer 24

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

Question 25

What are the two mechanisms for water transport through plants and what are their impacts

Answer 25

1. cohesion-tension theory (most important)
2. root pressure (minor role)

Question 26

Define cohesion-tension theory

Answer 26

water is mostly attracted to each other and therefore water is pulled up through the xylem by negative pressure (tension)

Question 27

Explain root pressures role in water transport throughout the plant

Answer 27

water is pushed through the xylem by positive pressure and only occurs under certain conditions (lots of water and low transpiration)

Question 28

What are the 3 steps to cohesion-tension theory

Answer 28

1. solar radiation input to change phase of water into vapour
2. vapour diffuses out of leaf down a vapour concentration gradient
3. tension is created in the cell walls where evaporation initially occurs and tension pulls water up through the plant

Question 29

Explain the evaporation of water from the xylem

Answer 29

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

Question 30

Define cavitation

Answer 30

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)

Question 31

Define vulnerability curve

Answer 31

quantify species susceptibility of cavitation and impact (hydraulic conductivity vs. imposed level of xylem tension)
as tension increases, conductivity decreases

Question 32

how do vulnerability curves change in environments of moist or dry habitats

Answer 32

decrease in xylem conductivity occurs at lower tensions in moist habitats than dry

Question 33

What are the three ways in which plants minimize consequences of cavitation

Answer 33

1. finite length of vessels and tracheids
2. root pressure causes pressure to shrink gas bubbles
3. secondary growth causes development of new xylem every year

Question 34

waxy cuticle

Answer 34

covers leaf surface and is effective barrier against water loss

Question 35

where are stomata present

Answer 35

present on upper and lower surfaces, mostly lower surfaces

Question 36

what causes hydraulic resistance to vary in leaves

Answer 36

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)

Question 37

What is the largest factor of transpiration

Answer 37

temperature

Question 38

What two things does transpiration depend on

Answer 38

1. difference in water vapour concentration between leaf air spaces and external air (to create the gradient)
2. diffusional resistance of the pathway (which includes stomatal resistance and boundary layer resistance)

Question 39

Define boundary resistance layer

Answer 39

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

Question 40

What determines the boundary layer resistance

Answer 40

wind speed and leaf size
when wind is still, boundary layer is think, and stomatal apertures have little affect on transpiration

Question 41

what anatomical and morphological aspects of leaves can affect the thickness of the boundary layer (3)

Answer 41

1. hairs on surface of leave = microscopic windbreak
2. sunken stomata are sheltered from the wind
3. size and shape of leaves and orientation influence the way the wind crosses the surface of the leaf

Question 42

what is stomatal resistance governed by

Answer 42

stomata open or closed
when open, provide a low resistance pathway for diffusion of gases across the epidermis

Question 43

guard cells

Answer 43

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

Question 44

define cellulose microfibrils

Answer 44

play an essential role in opening and closing the stomatal pores and are fanned out radially from the pore.

Question 45

What is the process of guard cells opening

Answer 45

ion uptake which decreases osmotic potential and water moves into the guard cell. as water enters, turgor pressure increases and the stomata opens

Question 46

what are the first two dominant stimulis of stomatal opening

Answer 46

temperature then light

Question 47

when water is abundant, what is the functional solution to limit water loss while CO2 is the temporal regulation of stomatal apertures

Answer 47

stomata open during the day and closed or minimized at night

Question 48

_____ plays a partial role in light-dependent stomatal opening
provided by what evidence

Answer 48

photosynthesis
DCMU is a photosynthetic electron transport inhibitor and when DCMU is present, it also partially inhibits stomatal opening

Question 49

define transpiration ratio

Answer 49

the ratio of water loss to photosynthetic carbon gain
therefore measures the effectiveness of plants in moderating water loss and allowing CO2 uptake

Question 50

What are the 3 factors that results in the large ratio of H20 efflux to smaller CO2 influx

Answer 50

1. concentration gradient that drives water is 50x larger
2. CO2 is larger and diffuse through air slower than water vapor
3. CO2 must cross the plasma membrane, cytoplasm, and chloroplast envelope = higher resistance