Lecture 14: Plants in arid environments

Question 1

Desert examples:

Answer 1

• Mohave
• Arizona
• Sahara
• Atacama

Question 2

Water supply to deserts varies…

Answer 2

• varies over time
• – seasonal (reg rainfall at particular time of year)
• – occasional
• varies with topography
• varies in distribution in the soil profile (rainfall accumulates in riverbed or depressions before draining several metres to stable water table)

Question 3

more water needed at ____ temperatures because of increased ______

Answer 3

higher

evapotranspiration

Question 4

water potential =

Answer 4

• the work required to raise bound water to the level of pure water
• pure water = 0WP
• wp in soils and plants is therefore negative

Question 5

plant water potential =

Answer 5

• osmotic potential
• turgor pressure in plant cell
• matric potential
• – osmotic dominates in plant cells

Question 6

soil water potential =

Answer 6

• osmotic potential
• gravitational potential
• matric potential
• • matric potential most important in soils

Question 7

dry air water potential =

Answer 7

approx -100 MPa

Question 8

water potential gradient drawing water from soil to plant to air

Answer 8

soil around -0.6, as you move through plant centre to tips wp decreases (-0.9 to -4 in day) this to air is mass difference (dry air -100MPa)

Question 9

plant adaptions to low water supply

Answer 9

• avoidance

- – short term acclimation to water deficit –> using stomatal control of water loss

Question 10

leaves are warmer than the air temperature, heating of leaves drives

Answer 10

transpiration by increase wp gradient

Question 11

___ Are key in controlling water loss

Answer 11

stomata

Question 12

gradual drop of water potential of leaves, roots and soil

Answer 12

wiggly graph.

• as soil grad drops across seven days
• in day leaf and root drops and comes back up
• leaf lower than root
• at night all three come back together
• as soil wp decreases, leaf must get lower in the day to draw water up

Question 13

limit water loss from leaves: (i.e. in desert plants)

Answer 13

• cuticle helps keep water
• hide stomata,
• • i.e in a crypt w hairs or just sunken
• reduce photosynthetic rate as a cost

Question 14

Desert life and CAM metabolism

Answer 14

• prevent water loss
• photosynthesise in day, carbon exchange at night (take in CO2 at night)
• cell becomes more acidic at night, less acidic in day
• shut stomata in the day

Question 15

both C3 and CAM plant example

Answer 15

• facultative CAM plant
• Mesembryanthemum crystallinum
• they start as C3 plant, max photosynthesise,
• when dry switches to CAM, prevent water loss but continues to grow

Question 16

desert plant: morphological adaptions

Answer 16

• store water

- change root morphology

Question 17

desert plant: two types of adaptions

Answer 17

• physiological
• • CAM
• • stomata sunken/crypt
• morphological
• • store water
• -change root morphology

Question 18

water storers have ___ roots

Answer 18

shallow

Question 19

water storage = & example

Answer 19

• stem water storage
• example Barrel cactus
• • no leaves
• • high SA:V
• • ribbing allows expansion and contraction & increases SA
• • waxy cuticle
• • CAM metabolism
• • large water buffers heat exchange

Question 20

barrel cacti spines =

Answer 20

• modified leaves
• protects from herbivores
• shade surface

Question 21

water storage in leaves examples and why is it good

Answer 21

agave and aloe

– increases window in which u can photosynthesise

Question 22

Nobel 1977: Barrel cactus

Answer 22

• investigated barrel cactus
• plant reacts to unpredictable rainfall
• roots near surface, captures rain, photosynthesises for as long as it then can
• during drying = restricts time of day it photosynthesises (stomata are open)
• low temp optimum for photosynthesis as photosynthesises at night when desert is cold

Question 23

adaptive roots

Answer 23

• Search deep: spindling deep root (tap root), when finds water it proliferates
• Search widely: spread out root system
• search widely and deeply: BOTH