Water Balance in Plants

Question 1

Plants water %

Answer 1

90-95% water

Question 2

Why do plants need water

Answer 2

• water is necessary component of photosynthesis
• necessary for transport
• provides turgor pressure
• excretion, less so than animals

Question 3

How do terrestrial plants get water?

Answer 3

• absorbed by roots and transported up to rest of plant

- Due to capillary action (adhesion and cohesion and water potential)

Question 4

Capillary action; cohesion

Answer 4

• water molecules stick together

Question 5

Capillary action; adhesion

Answer 5

• water molecules adhere to surfaces like surface of xylem wall

Question 6

Water potential

Answer 6

o Physical property that predicts direction in which water will flow
o Potential refers to water’s potential energy
o Works best when moves from region of high water potential to low water potential
o Matrix potential is the tendency of water to adhere to other materials like soil particles
o Solutes have a negative effect on Ψ by binding water molecules
- Ψ for liquid at sea level 23C is 0
- If solutes are added; - Ψ
o Positive pressure
- Positive pressure has positive effect (ex: expelled from syringe)
- Negative pressure has negative effect (ex: solution withdrawn by syringe)

Question 7

Water flow down water potential gradient

Answer 7

o The more negative the water potential, the more water will flow in by passive diffusion
o Water potential is more negative at top of plant than at bottom

Question 8

Water transport in plants

Answer 8

o Water and minerals are taken up in roots and drawn upwards to leaves because
- Adhesion, cohesion
- Evaporation; negative pressure
o Glucose is made in leaves and stems through photosynthesis and sent down to roots for storage
o Water and minerals pulled up in xylem sap; transported long distances by bulk flow
o Sugars are transported from sites of production (down from leaves and shoots) and storage (up from roots) in phloem sap

Question 9

Xylem vs. Phloem

Answer 9

• Xylem: unidirectional transport, water and minerals, actual tubes are dead cells, no cell walls, thick walls around the xylem are stiffened with lignin
• Phloem: transport in both directions, water and food, living cells with perforated cell walls

Question 10

Controlling water loss

Answer 10

• stomata: pores on the underside of leaves
• Facilitates gas exchange for phorosynthesis (Co2 in, O2 out)
• Main points of water loss: can be opened or closed

Question 11

What if there is no water in soil?

Answer 11

• Water is drawn from cell –> wilting
• Cell regulates by releasing solutes stored in vacuoles to the cytoplasm
• Stomata close –> no photosynthesis; effectively, a form of dormancy

Question 12

Problems with water deficit

Answer 12

• Most sensitive functions are impaired: loss of turgor of cell, lower rate of cell expansion, less cell wall formation and protein synthesis, less chlorophyll synthesis, less photosynthesis, less nitrate reductase (higher waste accumulation)

Question 13

Types of Plants - Mesophytes

Answer 13

• require an average amount of water
• Most plants we see on daily basis
• Water loss from stomata, gained through root
• Deciduous under stress (lose leaves)
• Both perennials and annuals “escape”

Question 14

Types of Plants - Hydrophytes

Answer 14

o Hydrophytes: aquatic, adapted to life in very wet places
- Grow wholly or partially submerged in fresh water
- Little or no cuticle (no need to conserve water)
- Stomata either only on part of leaves not submerged or no stomata
• If no stomata…how do they gas exchange? They get CO2 from dissolved organic compounds
- Underdeveloped transport system
- Aerenchyma (helps them float)
- Heterophylly; presence of different leaf forms on a single plant (surface leaves tend to be round and have stomata on top, submerged leaves then to be streamlined)

Question 15

Types of Plants - Halophytes

Answer 15

o Halophytes: salt-tolerant, potentially lack water
- Less than 1% of flowering plants
- Salt tolerant OR avoidant
- Tolerant species may actually need relatively high salt to grow
- Store water in special tissues (ex: -
- Excrete salt through salt glands
- Draw water into roots by increasing concentration of proline and other amino acids
- Halophytes and the food/climate crisis?
• Food production will have to increase by 50% or more to feed our growing population
• Most agricultural crops require freshwater
• Less than 1% of worlds water is fresh
• May be able to help
- Glasswort, pickleweed can become part of our diet, may help relieve pressure on freshwater
- Seashore mallow; forage, biofuel?
- Cordgrass as forage? Studies show can be used up to 25% of a cow’s diet

Question 16

Mangroves - example of halophyes

Answer 16

• 54 different species, convergent evolution
• Mostly tropical, coastal
• Roots are in ocean floor, stems and roots that emerge from water
• Important as:
- Storm surge protection
- Erosion protection
- Habitat
- Storage of heavy metals
• Impermeable, non-exposed roots
• Water intake through leaves
• Oxygen intake through exposed roots (lenticels)
• Lenticels close when root is underwater
• May sequester salt in roots, stem
• Secrete salt through leaves (salt glands)
• Stores water in leaves roots, stems, cells
• May have hairy leaves to reduce transpiration
• Viviparous
- Embryo grows first to break through seed coat, then out of fruit wall while still attached to parent plant
- Young are sensitive to salt so develop on trees
- How do marine plants Osmoregulate?
• Accumulation of salts in roots
- Lower osmotic pressure draws water in
• Store salts in large vacuoles
• Specialized glands to excrete salt

Question 17

Types of Plants - Xerophytes

Answer 17

• Live where water is scarce
• Hot, dry environments (conserve and store water)
• Drought tolerant
• Occur in habitats with low water availability
  • Deserts or tundras
• Ephemerals after rain (in general xenophytes are ephemerals)
  • Rapid growth, flowering, seed dispersal when it rains and then remain dormant in seed stage until next rain

Question 18

Resurrection plant

Answer 18

• Any plant that can survive severe dehydration
• May revive after months or years
• Ex: rose of Jericho

Question 19

Creosote bush

Answer 19

• Tolerates drought

* Flowers as soon as rainfall occurs, but small waxy leaves to continue photosynthesis

Question 20

Decreasing water loss in desert plants

Answer 20

• Solutions to reduce water loss may reduce co2 uptake and light penetration; could be a problem with many plants, but less so in desert habitats because not so important
• Waxes (cuticle); benefits
- Reduce evaporative loss
- Reduce temp gain
- Reduce UV penetration and predation by infects
*BUT also reduces CO2 uptake and reduce light for photosynthesis
• Stomata in pits (crypts) and hairs
- Reduces water loss through transpiration and increases ability to reabsorb water
• More, smaller stomata (not in cacti)
- Close more quickly, higher concentration of stomata (help takes in extra CO2)

Hair, spines, etc.
• Pubescence
• increase thickness of boundary layer and decrease exchange
• cooling, heating and anti-predator

Question 21

Leaf rolling

Answer 21

• Keeps stomata moist so not too much water is lost and it can be reabsorbed
• May only have stomata on the side which lead rolls

Question 22

Stomata

Answer 22

o pores on underside of leaves
- Facilitate gas exchange for photosynthesis (co2 in and O2 out) 
- Main points of water loss: can be opened or closed 
- Guard cells change shape and cause stomata to be opened or closed 
- Open with
• High light (except CAM plants) 
• Low leaf CO2 
• Hight moisture 
• Favourable temperatures 
• Endogenous rhythms 
- Closes with 
• ABA and abscisic acid 
• Low lead water content 
• Low soil moisture 
• High temperature
• Low relative humidity 
• May undergo partial closure