Water Loss and Water Acquisition Flashcards
Where does the majority of water loss occur from?
Stomatal transpiration
Why are primitive plants restricted to wet environments?
Moss gametophytes have no true cuticle; liverworts’ pores tend to remain open, don’t have much control over water loss.
Why does transpiration occur?
Vapor pressure difference between inside of lead and surrounding atmosphere
How do plants control water loss?
Waxy cuticle with pores whose opening can be controlled.
What controls the size of the stomatal aperture?
Guard cells
How does transpiration occur?
Water is lost by evaporation from the substomatal space to the atmosphere via stomata
What determines the rate of transpiration?
Vapor pressure difference
What are radial micelles? What do they do?
Cellulose thickening of guard cell primary wall, restricts expansion of guard cells, allowing them to lengthen but not thicken
What happens when guard cells lengthen?
Stomatal pore opens
Explain the process of stomatal opening using potassium.
- K+ taken up into guard cell vacuoles from surrounding cells
- Water potential lowers, water drawn into guard cells
- Pressure increases inside guard cells, lengthen and separate to produce stomatal aperture
Explain the process of stomatal closing using potassium.
- K+ ions leak out
- Water leaves along potential gradient, guard cells lose turgor
- Guard cells shorten, micelles cause cells to close pore
What factors affect stomatal opening?
Light stimulates opening in many species
Lowering of CO2 due to photosynthesis
Overheating
What factors affecting stomatal closing?
Water loss increases ABA, which promotes K+ leaving guard cells
What is the leaf boundary layer?
Water vapor forms a boundary layer over leaf surface
What is the purpose of the leaf boundary layer?
Reduces vapor pressure difference between sub-stomatal space and surrounding atmosphere, lessening water loss without reducing CO2 uptake
Why does air movement affect transpiration?
Leaf boundary layer is disrupted by wind, even gentle movement
How do pollutants affect transpiration?
SO2 can cause stomata to remain open
Small dust particles can block pores
How does temperature affect transpiration?
Increase in leaf temperature increase vpd between inside and outside
What is the first step of the Calvin cycle?
C3 fixation. C from CO2 added to the 5C RuBP
What does the first step of the Calvin cycle?
2 3C (3-phosphoglycerate) molecules
What is the principle output of the Calvin cycle?
G3P (glyceraldehyde 3 phosphate)
How many G3P molecules are generated and recycled in 3 turns of the C3 cycle?
1 generated, 5 recycled
What is the cost of one C3 glucose?
18 ATP, 12 NADPH
What is assumed about the Calvin cycle cost?
optimal C3 efficiency
What are the problems of RuBisCO?
Slow and poor substrate specificity, leading to photorespiration
When RuBisCO form 3-PGA most efficiently? WHy is this an issue?
Low O2 and high CO2. Our atmosphere has high O2 and low CO2, increases oxygenase activity
How much C is lost in the C3 cycle at 20 degrees Celsius?
Up to 25%
What physiological and morphological approaches have evolved to combat photorespiration costS?
C4, CAM, associated physiology
What are some examples of C4 plants?
Corn, sorghum, sugar cane
What could plants do if they could utilize CO2 more efficiently?
Reduce time or extent of stomatal opening.
Increase water use efficiency
Where is CO2 fixed in the C3 plants?
In the mesophyll chloroplasts
Outline the general idea of C4 plant adaptation.
C4 plants trap CO2 and deliver it to localized RuBisCO
What are benefits of C4 plants?
Increases CO2:O2 ratio around RuBisCO
Reduces photorespiration
Permits more prolonged stomatal closure
Enhanced water use efficiency
Increases rate of photosynthesis
Can fix C at lower concentrations of CO2
Where does C4 fixation occur? Where is this product transferred for C3 fixation?
Mesophyll cells; bundle sheath
Outline C4 fixation
- CO2 enters mesophyll cells and fixed into C4 acid in cytoplasm
- 4 C compund produced
- 4C compound transferred to bundle-sheath chloroplasts where CO2 is released to RuBisCO
- 3C compound is returned to mesophyll cells
What is the enzyme responsible for C4 fixation?
PEPC
How are chloroplasts structured in C4 plants?
Those in mesophyll lack RuBisCO but contains a different carboxylase with higher CO2 affinity
Bundle sheath cells have chloroplasts with no grans and are deficient in PSII (which means low oxygen)
What is the disinctive leaf anatomy of C4 plants?
Kranz anatomy: chloroplast-rich bundle sheath surrounding vascular bundles
What is the drawbacks of C4
More energy intensive: 30+ ATP/glucose (for regenerating PEP, etc.)
What are the trade-offs considered in C4 plants?
Ecological: temperature, water availability
What are examples of C3 plants?
Rice, wheat, barley
How did C4 arise?
Convergent evolution -> 62 independent lineages
Explain CAM.
Import CO2 at night and fix it into malate, malate stored in vacuole and released during the day, yielding CO2 for Calvin cycle
Explain the difference between CAM and C4
C4 separates C3 and C4 in space. CAM separates C3 and C4 in time
What are the costs and benefits of CAM?
Benefit: dramatically reduces water loss (survival and growth in desert conditions)
Cost: energetically expensive (double fixation, regeneration and production of substrates, pumping and accumultating malate against concentration gradient)
What are some examples of CAM plants?
Pineapple, agave, aloe, prickly pear, vanilla
How did CAM emerge?
Convergent evolution
What is dessication tolerance?
Plants dry up but don’t die, allowing them to endure huge reductions in water content. Survive in a dormant desiccated state and recover when sufficient water is available.
How do trichomes help prevent water loss?
Dry up more quickly. White when dry, reflects sunlight and therefore heat.
How does stomatal position reduce water loss?
Adaxial stomata more exposed to sunlight and wind; some plants have high proportion on abaxial (back)
How do stomatal crypts aid in water loss reduction?
More secure boundary layer
How can SA be reduced to limit water loss?
- Buliform cells, first to lose water, pull blades of grass together
- Reduce number of leaves (desert plants, plants shedding leaves during drought conditions)
Give examples of desiccation-tolerant plants.
SHoreline algae, some mosses, ferns, angiosperms
What primitive groups can CAM be found in?
Quillworts, ferns
