plant 5+6

Question 1

Control of Flowering

Answer 1

• Photoperiodic control of flowering
• Phytochrome – a key photoreceptor
• Circadian rhythms and the biological clock
• Senescence

Question 2

When do apical meristems become floral meristems?
Signal causing switching depends on plant:

Answer 2

• Developmental age.
• Temperature (Vernalization).
• Day/night length (photoperiod).
• Combination – inductive day length may hasten flowering but not be essential (e.g. Arabidopsis).

Question 3

Types of response to photoperiod

Answer 3

1. LD plants, e.g. henbane, spinach, clover: flowering triggered in spring or early summer.
2. SD plants, e.g. Maryland Mammoth, poinsettia, chrysanthemum: flowering triggered in late summer or autumn. 3. Many plants are day-neutral, (e.g. maize, tomatoes): age and temperature interact to regulate flowering.

Question 4

Where is the stimulus perceived?

Answer 4

• Shifting a leaf to short days promotes flowering.
• Shifting the meristem to short days has no effect.
• Detached leaves can be induced to flower and can ‘pass on’ the flowering signal when grafted.
• Diffusible signal: “florigen” (Chailakhyan, 1936).
• Florigen recently identified in rice and Arabidopsis as protein synthesised in phloem companion cells and translocated through phloem. See Pennisi E (2007) Science 316:350-351. Probably regulates transcription.

Question 5

How is the stimulus perceived?

Answer 5

Red light (660 nm) most effective for interrupting dark period to inhibit flowering and is reversed by far-red light (720 nm).
Daylight contains more red- than far-red.
Photoreceptor for red light in plants is phytochrome.
Standard criterion for phytochrome effect: far-red light reverses effect of red light.

Question 6

Is it the length of the day or night that determines flowering?

Answer 6

• Flowering is induced by Florigen, a small protein produced in companion cells in the leaves.
• Most plants require induction by a specific photoperiod for flowering.
• Light is perceived by the phytochrome.
• Pfr phytochrome is produced from Pr phytochrome under red light (day)
• Pfr phytochrome inhibits flowering in short day plants and induces flowering in long day plants.
• Plants gauge the length of the night NOT the length of the day.

Question 7

Circadian rhythm

Answer 7

Clock enables organisms to anticipate changes in environment – adaptive advantage.
Examples of rhythmic phenomena:
* Leaf movements.
* Stomatal opening.
* Stem growth.
* Membrane potential.
* Transcription (includes some genes involved in photosynthesis and in flowering 36% of gene in Arabidopsis are under circadian control).

Question 8

Three main criteria define clock-controlled rhythms:

Answer 8

1. Rhythm persists in absence of external cues.
2. Rhythm can be reset by external signals (e.g. light).
3. No lasting effect of temperature on timing of rhythm.

Question 9

Senescence and programmed cell death – final stages of development

Answer 9

Some cells, organs and tissues undergo programmed cell death during normal development (brown areas).
Highly coordinated dismantling of cell components (proteins, lipids, DNA, pigments).
N, P, C, minerals are redistributed for new growth or storage

Question 10

What controls senescence?

Answer 10

During senescence, some metabolic pathways are turned off and new pathways (mainly catabolic) are activated. Senescence can be pre-programmed or triggered by environmental signals, e.g. nutrient stress, pollution, UVB, pathogens.
Senescence involves plant hormones, especially ethylene.

Question 11

SAGs: senescence associated genes; ~50 identified.

Answer 11

1. Genes that regulate initiation of senescence or rate of progress, e.g. ‘stay green mutants’ show delayed senescence – higher crop yield.
2. Genes encoding enzymes of catabolism, e.g. proteases

Question 12

Plant responses to environmental stress:

Answer 12

The response can be:
* Stress-type specific or general
* Localized or systemic
Different types of responses:
* Short-term responses, happen within minutes.
* Mid-term responses, happen within hours .
* Long-term responses, happen within days or even weeks.
The different environmental stresses act independently
The environmental signals are integrated by the plant cell to generate an integrated plant response.

Question 13

Two types of responses:

Answer 13

Resistance: the plant changes its physiology and adapts its metabolism to alleviate stress effects.
Avoidance: the plant perceives the stress signal but ignores it and accepts the cost in terms of growth.
Plants gauge stress level and activate the appropriate response.
There are critical levels for each stress.

Question 14

Three types of plants:

Answer 14

Stress-sensitive plants, in this case the plant might respond to the stress but the response might not be enough or inadequate.
Stress-resistant plants, in this case the plant deploys adequate responses to mitigate the stress effects.
Stress-tolerant plants, in this case the plant is constitutively prepared for the stress effects.

Question 15

Water stress

Answer 15

Water stress = stress caused by critical levels of water that trigger adaptive responses and cause reduction in growth.
Water stress as consequence of water deficit

Question 16

Effects of water-deficit:
Water deficit causes severe structural and metabolic disturbances:

Answer 16

• Osmotic imbalances in the cell. Water loss results in increased solute concentration, which affects enzyme activities.
• Cellular membranes become porous to solutes upon rehydration.
• Reduced photosynthesis caused by limited CO2 diffusion into the leaf because of stomatal closure and inhibited photosynthetic machinery.
• Changed root and shoot growth as consequence of limited photosynthesis.
• Accelerated “ageing”, plants develop rapidly towards flowering, seed formation and death. They usually produce less seeds or smaller seeds.

Question 17

Crassulacean acid metabolism

Answer 17

Stomata open mainly during the night for reduced water loss during the day. CO2 is taken up during the night, stored as organic acids and metabolised during the day behind closed stomata.

Question 18

Morphological and anatomical responses to water stress

Answer 18

• Rapid stomatal closure to limit evapotranspiration.
• Lower stomatal density on the new leaves.
• Leaf expansion is reduced to limit the evapotranspirational area.
• Leaves produce more wax on their surface to have higher insulation to water vapour.
• Plants show enhanced root extension into deeper moist soil.
• Some plants become succulent.
• Old leaves are rapidly lost.

Question 19

Adaptations to drought
Reduced metabolism:

Answer 19

Welwechia mirabilis grows in the Namib desert. This plant reduces its metabolism to the minimum and as soon as it receives water the leaves become green and start growing.

Question 20

Adaptations to drought
Evasion:

Answer 20

Some plant species from dry areas evade drought by having a very short life cycle, they germinate, grow, flower and set seeds before the end of the wet season.

Question 21

Adaptations to drought
Opportune leaf production:

Answer 21

Xerophytes are particularly resistant to drought, they produce leaves only when there is sufficient water for photosynthesis

Question 22

Adaptations to drought
Extended Roots:

Answer 22

Some plants from dry areas develop very long roots that go deep in the ground (-50m)
These adaptations are usually accompanied by high costs in terms of reduction in growth and productivity.

Question 23

Metabolic adaptations:

Answer 23

• Reduced metabolism
• Production of alcohol fermenting enzymes to produce ATP from sugar degradation.

Question 24

Morphological adaptations:

Answer 24

Pneumatophores = root extensions in the air used to take up O2.
Aerenchyma = large parenchymatic cells around the vascular system used to transport O2 from the shoots to the roots.

Question 25

salt stress

Answer 25

Salt stress is the consequence of high non metabolized ions in soil or in water. The most common salt stress is caused by Na+ and by Ca2+. Also, high concentrations of Cl- , SO4 2- and carbonates can cause significant stress.

Question 26

High salt is found mainly in:

Answer 26

• Coastal salinity marshes (0.5 M Na+ , solute potential of – 2.7MPa).
• Lakes where evaporation exceeds precipitation
• Excessively irrigated soils.
• Ground water under reduced rainfall

Question 27

salt acts at 4 levels

Answer 27

1. It changes soil properties by reducing soil porosity reducing its aeration and hydraulic conductance.
2. It generates low water potential causing indirect water stress reducing water and nutrient uptake.
3. It induces cellular toxicity of non metabolized ions like Na+ and Clresulting in enzyme inhibition
4. It causes ionic imbalances between cellular compartments. Salt stress affects severely photosynthesis and plant growth. In contrast to water deficit roots stop growing rapidly under salt stress in most plants.

Question 28

Glycophytes

Answer 28

plants sensitive to salt stress, usually die at salt concentrations lower than 100mM NaCl. E.g. bean, rice, soybean, maize.

Question 29

Salt-tolerant non halophytes:

Answer 29

plants that tolerate relatively high salt concentrations higher than 200mM NaCl. E.g. tomato, citrus, wheat.

Question 30

Halophytes

Answer 30

plants adapted to salinity and that can resist salt concentrations higher than 500mM NaCl, e.g. ice plant and salt cress.

Question 31

Mechanisms to escape/alleviate salt stress effects:

Answer 31

• Production of compatible solutes to increase the osmotic force driving water into the cell.
• High ion selectivity, toxic ions are not taken up by the roots.
• These roots have high capacities of ion extrusion that allow the removal of any toxic ions taken up
• The cell have ion transporters that allow the sequestration of ions in the vacuole.

Question 32

Adaptations to salinity:

Answer 32

• Osmoregulation
  Thellungiella halophila a close relative of Arabidopsis grows on 0.5M NaCl, produces high levels of proline.
• Bladder cells
  Mesembryanthemum crystallinum has specialised epidermal cells with massive vacuoles called bladder cells. These cells sequestrate large amounts of salt (1M Na+ )
• Glands
  Are used to excrete salt to the surface of the leaf, e.g mangroves.
  Substantial amount of crystallised salt appearing on the surface of a mangrove leaf.

Question 33

effect of high temp

Answer 33

• Increased evapotranspiration and induced wilting of the plant.
• Increased respiration.
• Increased photorespiration
• Reduced photosynthesis.
• Enzyme inhibition.
• Excessive membrane fluidity

Question 34

effect of low temp

Answer 34

• Reduced metabolism because of reduced enzyme activities.
• Reduced membrane fluidity and even crystallization and destabilisation of membranes.
• Reduced water availability in the free form in the cell.
• Reduced photosynthesis and growth.

Question 35

stomatal physiology

Answer 35

Anatomy of stoma = stomate (plural = stomata)
Stomata are composed by two guard cells, surrounded by subsidiary cells
Guard cells and subsidiary cells (specialized epidermal cells) = the stomatal complex or stomatal apparatus
The number of stomata per leaf area (stomatal density) varies between species. Stomatal index = number of stomata in relation to total number of epidermal cells.

Question 36

mode of function of stomata

Answer 36

Guard cells function as hydraulic valves that open and close stomata.
The increase in volume of guard cells as a consequence of osmotic water uptake causes them to stretch opening the stomatal pore.
Stomatal closure is controlled by water loss by guard cells and their relaxation as a consequence of decreased hydrostatic Pressure

Question 37

Stomatal opening: expansion of guard cells
During stomatal opening:

Answer 37

K + enters guard cells driven by the electrochemical gradient caused by the H+ ATPase activity.
Cl- ion also enters the cells. Both ions are transported into the vacuole.
Malate participates in creating the osmotic force leading to water movement into the cell and its expansion and acts as Counter-ion to K+
There is about 5 folds more K+ in guard cells in the open state compared to the closed state. K+ is transported from the surrounding subsidiary cells

Question 38

Sucrose and malate:

Answer 38

Sucrose is transported from mesophyll cells to guard cells causing stomata to open. It acts as osmoticum and energy provider.
Malate is produced from starch breakdown and is used as counter-ion of K+ during stomatal opening

Question 39

Stomatal closure:relaxation of guard cells
During stomatal closure:

Answer 39

Movement of Ca2+ into the guard Cells as well as the change in cytosolic pH leads to K + efflux and the reduction of osmotic pressure and water movement out of the guard cell.

Question 40

Regulation of stomata
Internal CO2 concentration

Answer 40

High internal CO2 concentrations in the substomatal chamber, lead to stomatal closure even under high light.
The guard cells gauge the [CO2 ] int and respond to it rather than the concentration of CO2 in the atmosphere.
[CO2 ] int is controlled by the amount of CO2 that enters through stomata and the assimilation rate.
Stomatal closure as a consequence of increased [CO2 ] in substomatal cavity due to increased respiration; this also results in increased leaf temperature.
The mechanism by which guard cells sense [CO2 ] is still unknown.

Question 41

Regulation of stomata
Light = Stomatal opening

Answer 41

• Direct effect
• Indirect effect through photosynthesis
  Blue light induces stomatal opening. the signal is perceived by xanthophylls.
  Darkness = stomatal closure
  Light effects are mediated by the phytochrome or the cryptochrome.
  Stomata from CAM plants respond differently to blue light.

Question 42

Regulation of stomata
Water vapour

Answer 42

Water loss elicits stomatal closure through two mechanisms:
* Hydropassive closure: as a result of direct water loss by evapotranspiration by guard cells
* Hydroactive closure: as a result of induced Ca2+ ion influx into guard cells

Question 43

Regulation of stomata
Abscisic acid (ABA)

Answer 43

is considered to be the mediator of the regulation of stomata by water stress. ABA is either produced in the guard cells or is transported from the surrounding cells or from the roots

Question 44

Regulation of stomata
Circadian control

Answer 44

Stomata from plants having C3 or C4 photosynthesis open during the day and close during the night. Stomata from crassulacean acid metabolism plants open mainly during the night.

Question 45

Stomatal regulation and water loss
Boundary layer resistance
Boundary layer thickness determined by:

Answer 45

– Wind speed.
– Leaf morphology, e.g. hairs, sunken stomata
Stomatal aperture has relatively little effect when air is still.

Question 46

Transpiration ratio – water loss and carbon gain

Answer 46

• Concentration gradient driving water loss is ~ 50 times that of CO2 influx: low [CO2 ] in air and high [H2O] in leaf. * CO2 diffuses more slowly than H2O in air (heavier molecule).
• CO2 has to cross more membranes - resistance of CO2 influx.