Plants are dynamic! Many physiological processes occur in each part of the plant Materials are transported through specialized conducting systems Energy is harnessed from the sun through photosynthesis to forms sugars and other storage products Stored foods are broken down to yield chemical energy

1. Tibebu Belete Chane
Plant Physiology
How Plants Respond to Environmental Stimuli)

2. Plant movements
(How Plants Respond to Environmental Stimuli)
❖Higher plants, being fixed to soil,
cannot move from place to place.
However….. within the plant body various
mechanisms are in constant motion.
❖ Certain parts of the plant body (in response
to external stimuli) show physical
displacement called movement

3. Types plant movements
1. Tropic Movements
➢Tropisms - plant growth toward or away
from a stimulus such as light or gravity.
2. Nastic Movements - response to
environmental stimuli that are
independent of the direction of the
stimulus. Pre-determined response
➢ Nastic movement
➢ Tropic Movement
➢ Turgor movement

4. ➢When the upper side of an organ
grows faster than the lower side,
the movement is called epinasty.
(e.g., downward curling of leaf,
opening of sepals of flower.
➢When the lower side grows more
rapidly than the upper side, it is
called hyponasty. (e.g. upward
curling of leaf blade)
Nastic Movement (cont…)

5. 3. Turgor movements
❖When more water is present in the cell
it is fully expanded and becomes rigid
or hard - turgidity.
❖When less water is present inside the
cell, it is not fully expanded and
remains soft - flaccid.
➢For example, the leaves bend in hot
summer due to excessive
transpiration on account of the loss of
turgidity of cells of the leaf.

6. Plants move in response to:
* light
* touch
* chemicals
* temperature
* gravity
* H2O

7. Factors that cause movement are called stimulus.
The place that receives the stimulus on the plant
is called the perception site.
The plasma membrane is necessary for the
message to be able to pass from one place to
another so that the plant can respond properly.

8. Tropic responses
Directional movements by growth in
response to a directional stimulus

9. .
There are many types of tropisms :
❖ Phototropism
❖ Geotropism
❖Thigmotropism
❖ Hydrotropism
❖ Chemotropism
❖ Thermotropism
TROPISM
Plant growth in response to a stimulus
(“tropo” – ”turn”)
Can be positive or
negative

10. Phototropism
(“Photo” – light)

11. Location of light “sensing” in shoot tip
This experiment
was conducted by
Charles Darwin in
1880 Aluminum cap
No bending
Only shoot tip exposed

12. Location of light “sensing” in shoot tip
• What Darwin stated:
– “we must therefore conclude that some influence is transmitted from
the tip to the more basal regions of the shoot thereby regulating
growth and inducing curvature”
• “influence” was the first reference to “hormonal regulation”
in plants
• “influence” = IAA

13. Growth movement

14. Phototropism
❖Phototropic responses involve bending of
growing stems toward light sources.
➢Individual leaves may also display
phototrophic responses.
• auxin most likely involved
Auxin, produced in the
apical meristem, moves
downward by active
transport into the zone
of elongation and
generates growth by
stimulating elongation
In a stem, the shaded side contains
more auxin and grows longer,
which causes the stem to grow
towards the light. The plant
does NOT bend towards the light. It
grows because the auxin causes the
cells to elongate on the shaded side,
so this side grows more. This
unequal growth of the two sides,
results in the growth of the stem
towards the light.

16. EARLY EXPERIMENTS ON PHOTROPISM SHOWED
THAT A STIMULUS (LIGHT) RELEASED CHEMICALS
THAT INFLUENCED GROWTH
Results on growth of coleoptiles of canary grass and
oats suggested that the reception of light in the tip of
the shoot stimulated a bending toward light source.

17. IMPORTANCE OF
PHOTOTROPISM
Enables leaves to be in the best position
possible to receive adequate light for
photosynthesis
http://www.darienps.org/teachers/otterspoor/botany/tropism
s/Gravitropismwlight.jpg

18. GEOTROPISM
Geotropism/ Gravitropism is the growth
of a plant in response to gravity.
Positive Geotropism
It is the growth of a plant towards the center of
the earth-down with gravity.
Ex. roots growing down
Negative Geotropism
It is the growth of a plant away from the
center of the earth-opposite from the pull of
gravity
Ex. stems grow up
Positive Geotropism
Negative Geotropism
(“Geo” – Earth)

19. Gravitropism
❖present at germination
✓auxins play primary role
❖Increased auxin concentration on the lower side in stems
causes those cells to grow more than cells on the upper
side.
➢stem bends up against the force of gravity
✓negative gravitropism
❖Upper side of roots oriented horizontally grow more
rapidly than the lower side
➢roots ultimately grow downward
✓positive gravitropism

20. Gravitropism = Geotropism
Both auxin and gibberellins are
involved
If a stem is horizontal, auxin
produced at the apical meristem
moves down the stem and
concentrates on its lower side.
Growth of the lower side is greater
than that of the upper side, and the
stem bends upwards

21. Auxins and gravitropism
➢ Auxins in root tips have the
opposite effect to those in shoot
tips – they stop elongation of
cells.
➢ Encourages positive geotropism
➢ Helps roots to anchor the plant
in place

22. IMPORTANCE OF GEOTROPISM
❖ Pulls roots down to anchor a plant
❖ Roots can get needed water and minerals if they
stay in the soil

23. http://www.youtube.com/watch?v=iFCdAgeMGOA&featu
re=related
CORN TROPISM

25. Plants Respond to Touch
❖Thigmotropism (“Thigmo” – “touch”)
❖ is directional growth response to touch/contact with an
object.
• tendrils
• Vines growing on a wall or fence

26. Thigmotropism

27. SEISMONASTY - a nastic response resulting
from contact or mechanical shaking
Mimosa pudica L. (sensitive plant)

28. Turgor Movements
❖Turgor movements are reversible changes in the position of
plant parts due to a change in internal water pressure and
are often, but not always, started by contact with objects
outside of the plant.
❖The movement of Mimosa pudica (Touch-me-not) leaves
when touched is an example of turgor movements. Another
example of this is the reaction seen in the Venus fly trap
when an insect lands inside its leaves.
http://www.youtube.com/watch?v=PRo4rg07_gg&feature=fvwp&NR=1
http://www.youtube.com/watch?v=o3hk9Sz--h4

31. Hydrotropism (hydro-water)
1. A plant’s growth and response to water.
2. A plant’s roots will grow in the direction
of water concentration.
3. Positive = Growth toward water source
Negative = Growth away from water
source

32. THERMOTROPISM
to bend toward or away from heat
(“Therm” – “heat”)
Positive Thermotropism
toward heat
Negative Thermotropism
Away from heat

33. Importance of Thermotropism
In hotter weather helps prevent water loss
through stomata

34. Chemotropism
movement caused by chemical stimuli
(“Chemo” – chemical)
Positive Chemotropism
toward high nutrient soil (healthy soil)
Negative Chemotropism
Away from low nutrient soil (unhealthy soil)
Helps control and regulate growth and development
of plant

35. Nyctinasty
❖sleep movements
❖prayer plant - lower leaves
during the day and raises
leaves at night
▪ shamrock (Oxalis)
▪ legumes
Credit:(http://employees.csbsju.edu/ssaup
e/biol327/Lab/movie/movies.htm)
❖Movement of higher plants in
response to the onset of
darkness, or a plant
"sleeping

36. Photoperiodism
➢ Photoperiodism is the response of
plants to changes in the
photoperiod, or the relative length
of daylight and night.
➢ To respond to changes in the
photoperiod, plants maintain a
circadian rhythm, a clock that
measures the length of daylight
and night.
Many flowering plants initiate flowering
in response to changes in photoperiod.
They may be long-day plants, short-day
plants, or day-neutral plants. Poinsettias
for example are short-day plants.

37. PLANT STRESS PHYSIOLOGY

38. STRESS PHYSIOLOGY
❖Stress physiology is an important
branch of environmental physiology
concerned with plant adaptiveness to
the environmental extremes.
❖In a plant environment, there are
several stressful conditions, such as
deficiency of water, unfavorable
temperature, salinity etc.

39. Stress and strain terminology
❖Stress: An external factor acting adversely on an organism
❖Strain: Any physical or chemical change produced by a stress
❖Elastic strain: A reversible physical or chemical change produced by a
stress
❖Plastic strain: An irreversible physical or chemical change produced by
a stress
❖Stressor/Stress factor: Any factor that causes injury or stress stimulus
❖Stress response: Stress stimulus with ensuing state of adaptation

40. ❖ Eustress: It is an activating, stimulating stress that increase the
physiological activity of a plant and thus a positive element for plant
development.
❖Distress: It is a severe and a real stress that causes damage and thus has a
negative effect on the plant and its development
❖ Zero stress: The stress that is just insufficient to produce a plastic strain
❖Stress resistance: Ability of the plant to survive under adverse
environmental condition is termed as stress resistance (adaptation,
avoidance and tolerance)
❖Elastic resistance: Ability of the plant to prevent reversible or elastic strain
(physical or chemical change) when exposed to a specific stress

41. ❖Adaptation refers to heritable modifications in structure or
function that increase the fitness of the organism in the stressful
environment. It is also called protection. e.g. CAM plants to
desert
❖Acclimation refers to non-heritable physiological modifications
that occur over the life of an individual. These modifications are
induced by gradual exposure to the stress. The process of
acclimation is known as hardening.

42. Human stress: strong emphasis on
psychological / emotional
processes with consequences for
physical well being
What is stress?

43. What is stress?
➢Stress is a significant
deviation from the
conditions optimal for life
and obtaining changes and
responses at all functional
levels of the organism.
One indicator of heat stress when corn leaves roll
in the early morning in areas where there is
adequate soil moisture, at southeast Nebraska
field. (Photo by Gary Lesoing)
➢Stress in plants can be
defined as any external factor
that negatively influences
plant growth, productivity,
reproductive capacity, or
survival

44. Plant stress is not psychological!
➢but what exactly is plant stress?
Drought
Storm
damage
Acid rain
Insect damage

45. Why are we interested in plant stress?

46. Plant stress
❖“A stressful
environment is any
environment that is less
than optimal for plant
growth”
❖Problems:
❖Does an “optimal”
environment exist?
❖If so, are plants always
stressed?

47. Non-optimal conditions versus plant stress
❖There is always a limiting factor. Every change of an
environmental factor influences plant growth and
development. Is this stress?
Frederick Frost Blackman FRS (1866 –
1947)- the law of limiting factors

48. ➢Stresses; external conditions that adversely affect growth,
development or productivity
➢Stress is a disadvantageous influence exerted on a plant by
external abiotic or biotic factor(s), such as infection, or heat,
water and anoxia
➢Is stress the “external condition” or the “plant response”?
Plant stress?

49. Stress and “external conditions”?
❖Conditions that comprise a stress for one plant
species are not necessarily a stress for another
species.
❑ Is a temperature of 5-10°C a stress?
Chilling stress maize No chilling stress peas
❖ Consequently, stress conditions are very important in terms
of biodiversity and plant distribution.

50. ❖Abiotic stress is a physical (e.g.,
light, temperature) or chemical
insult that the environment may
impose on a plant
❖“Abiotic stress is the primary
cause of crop loss worldwide,
reducing average yields for most
major crops by more than
50%...” (Wang et al., 2003)
❖Biotic stress is a biological
insult, (e.g., insects, disease) to
which a plant may be exposed
during its lifetime.
Type of plant stress

51. Sources of Plant Stress (Stressor)
❖Radiation (low and high)
❖Temperature (low and high)
❖Wind
❖Water logging
❖Salt Stress
❖Moisture stress/Drought
❖Minerals
❖Human being
❖Pathogen, insect and weed

52. Natural stress factors
❖High irradiance (photoinhibition,
photooxidation)
❖Heat (increased temperature)
❖Low temperature (chilling)
❖Sudden and late frost
❖Water shortage (desiccation problems)
❖Natural mineral deficiency (e.g.
nitrogen shortage)
❖Long rainy periods
❖Insects
❖Viral, fungal, and bacterial pathogens

53. Anthropogenic stress factors
➢air pollutants (e.g., SO2, NO, NO2, NOx)
➢ozone (O3) and photochemical smog
➢formation of highly reactive oxygen species (1O2,
radicals O2* and OH* , H2O2)
➢photooxidants (e.g. peroxyacyl nitrates) • acid rain, acid
fog, acid morning dew
➢acid pH of soil and water
➢mineral deficiency of the soil, often induced by acid rain

54. ➢oversupply of nitrogen (dry and wet NO-
3 deposits)
➢heavy metal load (lead, cadmium, etc.)
➢overproduction of NH+
4 in breeding stations (uncoupling of
electron transport)
➢increased UV radiation (UV-B and UV-A)
➢increased CO2, global climate change
Anthropogenic stress factors

55. Consequences of stress/ stressor
Symptoms of
water logging
❑Wilting
❑Reorientation of
growth processes
❑ Inhibition of nutrient
ions transport by
roots
❑Inhibition of shoot
growth
❑Hyper-trophic growth

56. Effect of Drought on
Plants
❑Functioning of stomata
❑Carbohydrate metabolism
in green leaves
❑Photosynthetic activity
❑Osmotic pressure
❑Permeability
❑Biochemical effects
Consequences of stress/
stressor

57. Effects of Saline soil
❑Osmotic Effects
❑Specific Ion Effects
❑Effect on Phytohormone Levels
❑Effect on Transpiration
❑Effect on Photosynthesis
❑Effect on Biomass Production
❑Effect on Cell Division &
Expansion
Consequences of stress/ stressor

58. Response of plants to stress
❖Stress response describes the response of the cell, plant or
ecosystem, following exposure to a stressor
❖Stress tolerance- an extension of the range of conditions under
which an organism can operate, i.e. prevention of distress
❖Plants have evolved different strategies
➢Stressor escape
➢Stressor avoidance
➢Stressor resistance
➢True stressor tolerance

59. Stressor escapers
❖Stress escape via opportunistic growth and
reproduction cycles
Flowering desert in California

60. ❖ Early flowering (left)
and late flowering
(right) sorghum
cultivars under late-
season drought stress.
The late cultivar will
not flower at all due
to stress
Stressor escapers

61. Stressor escapers
❖Escape stress-exposure via opportunistic
growth and reproduction cycles

62. True- stressor tolerance
❖A relatively rare phenomenon whereby plants
are able to survive exposure of physiological
systems to a stress.

63. Craterostigma plantagineum
dehydration and rehydration

64. Greenhouse phenotyping experiments for cowpea drought tolerance:
drought stress was applied for (A) 1, (B) 8, (C) 15, and (D) 22 d.

65. True- stressor tolerance
❖Vascular plants have developed extensive
morphological (roots, cuticle, reduced leaf
area) and physiological systems to prevent
cellular dehydration stress

66. Stressor avoidance
❑Stress avoidance; tolerance
based on mechanisms that
prevent exposure of the cell
content to stressors
❑Species have evolved
mechanisms that can hasten,
retard, or decrease the action of a
stressor on cellular systems
Ferocactus-Mexico

67. What happens when a plant is
exposed for the first time to a
stressor?

69. Stress tolerance
❑Stress acclimation is a non-heritable/
physiological / biochemical response
leading to increased tolerance.
❑Stress adaptation is an evolutionary/
genetic process leading to increased
tolerance

70. The end!