Environmental Plant Physiology
Photosynthesis - Aging
krreddy@Ra.MsState.edu
Department of Plant and Soil Sciences
Photosynthesis and Environment
Leaf and Canopy Aging
Goals and Learning Objectives:
• To understand the effects of leaf and canopy aging
on photosynthesis process at leaf and canopy
level.
¾
Photosynthesis and leaf-level aging.
¾
Photosynthesis and canopy-level aging.
Senescence, Aging and Death and Agriculture
¾ Senescence, aging, and death – conceived in the past
as inevitable, negative processes (wear and tear
phenomenon), but now considered as an internal part
of differentiation and development.
¾ Leaf senescence is one of the most conspicuous
processes that has been studied in the context of plant
aging and senescence.
¾ The terminal phase of leaf development cannot be
described simply as a collection of passive and
deteriorative processes during which gradual decline
in vital systems takes place.
Senescence, Aging and Death and Agriculture
¾ Extensive physiological and biochemical studies in the
last few decades on leaf senescence have suggested
that it is highly regulated and active process, which is
characterized by differential and sequential changes in
almost every sub-cellular compartment.
¾ Leaf aging or senescence has implications on
agriculture, affecting crop yield and the shelf life of
leafy vegetables.
Photosynthesis
Photosynthesis -- Aging
Aging
Leaf
Leaf Level
Level
Leaf and canopy development and aging process
Leaf
5-day
12-day
2-day
Canopies
Emerging leaf
Emergence
8-day
Squaring
Flowering
About to abscise
Mature crop
Photosynthesis
Photosynthesis -- Aging
Aging
Leaf
Leaf Level
Level
Deep Inside
Unfolding
leaf
Mature
leaf
Chloroplasts
with limited
thylakoid
development
Chloroplasts
with welldeveloped
thylakoid
Photosynthesis
Photosynthesis -- Aging
Aging
Leaf
Leaf Level
Level
Deep Inside
Chloroplasts contain chlorophyll and
proteins, and these are the centers of
photosynthesis. As leaves grow in size,
these centers of photosynthesizing
chloroplasts develop very well-defined
thylakoids for optimum photosynthesis.
Proteins and pigments that function in the
photochemical events of photosynthesis
are part of the thylakoid.
Photoynthesis and Light Response Curves
50
-2 -1
Photosynthesis, µmol m s
Maximum net photosynthesis
40
LUE
30
20
Light
Carboxylation
Limited
Limited
10
0
LCP where Pn = 0
-10
0
Respiration
500
1000
1500
2000
PPFD, µmol m-2 s-1 Light saturation point
Photosynthesis, µmol CO2 m-2 s-1
Aging - Photosynthetic Light-response Curves
for Cotton Leaves
40
20 days
35
30
27 days
25
20
15
35 days
10
40 days
5
0
0
400
800
1200
1600 2000
PPFD, µmol m-2 s-1
2400
Photosynthesis, µmol CO2 m-2 s-1
Leaf Aging and Photosynthesis
40
30/22°C and 360 µl l-1 CO2
35
30
25
20
15
10
5
0
0
10
20
30
Leaf Age, d
40
50
60
-2
Photosynthesis, µmol CO2 m s
-1
Leaf Aging and Photosynthesis
50
-2
PPFD = 1500 µmol photon m s
-1
40
30
20
10
0
15
20
25
30
35
40
Leaf Age, days
45
50
0.06
0.05
2.0
Light Utilization Efficiency
1.5
0.04
0.03
1.0
0.02
Conductance
0.5
0.01
0.00
15
20
25
30
35
40
Leaf Age, days
45
0.0
50
Conductance, mol H2O m-2 s-1
LUE, µmol CO2 µmol-1 photon
Leaf Aging and Light Utilization Efficiency
and Conductance
Leaf Aging - Photosynthestic Parameters
Expressed as Fraction of Maximum (= 20 d)
Fraction of Maximum
1.5
1.0
Photosynthesis
0.5
0.0
Light Utilization Efficiency
Conductance
15
20
25
30
35
40
Leaf Age, days
45
50
Leaf Aging and Leaf Nitrogen
5
Leaf N, %
4
3
2
1
15
20
25
30
35
40
Leaf Age, days
45
50
Photosynthesis, mg CO2 m-2 s-1
Photosynthesis - Leaf Nitrogen
7
6
5
4
3
2
1
0
1
2
3
Leaf N, %
4
5
Leaf Aging - Leaf Pigments
Chlorophyll and Carotenoids
40
8
35
7
30
6
25
5
20
Carotenoids
4
15
10
15
20
25
30
35
40
Leaf Age, days
45
3
50
Carotenoid, µg cm-2
Chlorophyll, µg cm-2
Chlorophyll
Leaf Aging - Starch and Sugars
2.0
5
Starch
Sugars, %(w/w)
3
1.2
2
Sugars
0.8
1
0.4
0.0
15
0
20
25
30
35
40
Leaf Age, days
45
50
Starch %(w/w)
4
1.6
Leaf Aging and Photosynthesis
¾ As leaves grow in size, leaf net photosynthesis increases rapidly
from leaf unfolding (about zero at leaf unfolding) until the leaf
reaches its potential maximum size. Soon thereafter (from about 20
days from unfolding), leaf net photosynthesis declines linearly.
Light saturation occurs at lower light levels as leaves age.
¾ The onset (from about 20 days) and magnitude (rates) of declines in
leaf conductance and light utilization efficiencies are closely
coupled with leaf maximum photosynthesis rates.
¾ Leaf transpiration rates closely followed photosynthetic rates
throughout the leaf development.
¾ Leaf pigments (chlorophylls and carotenoids) decline from about 35
days from leaf unfolding, at least in cotton.
Leaf Aging and Photosynthesis
¾ The concentration of CO2 inside the leaf (Ci), and the number of
chloroplasts in the mesophyll remain nearly constant during the
aging process, except in the oldest leaves that are about to abscise.
¾Total leaf protein content, RuBP carboxylase and electron transport
activity decline parallel to photosynthesis during the aging process.
¾ Leaf N also declines from about 20 days, but the decline was not as
dramatic as the photosynthetic parameters.
¾ The levels of starch and sugars also decrease during the aging
process suggesting the loss of photosynthetic activity.
¾ Therefore, the physiological deterioration of leaf photosynthetic
activity during the aging process may be more related to declining
Rubisco protein (photosynthetic capacity) and photosynthetic
efficiency (LUE) than to leaf N, leaf pigments and number of
chloroplasts. The leaf N and pigments also play a role at a latter stage.
Leaf Aging and Photosynthesis
Varying light levels during
aging by partial shading the
leaf did not change leaf
photosynthetic
characteristics (LUE and
Pmax.).
Leaf Aging and Photosynthesis
The shape and magnitude of photosynthetic capacity and efficiency
did not change due to leaf location or position when measured at the
same age, at least in cotton.
Photosynthesis, µmol CO2 m-2 s-1
Leaf Aging and Photosynthesis
40
30/22°C and 360 µl l-1 CO2
35
30
25
20
15
10
5
0
0
10
20
30
Leaf Age, d
40
50
60
Leaf Aging and Photosynthesis
¾ In cotton, Pmax is out of sequence
with carbon requirements of bolls:
1. Mainstem leaves reach peak rates
several days before flowering on
the branch and rates are
substantially reduced during bollfilling.
2. The branch leaves subtending
flower/boll reach maximum
photosynthesis (Pmax) about the
time of anthesis.
3. Carbon appears to be translocated
from remote sites to the
developing bolls.
Mainstem leaf
Branch leaf
Flower-bud
Photosynthesis
Photosynthesis -- Aging
Aging
Canopy
Canopy Level
Level
Leaf and canopy development and aging process
Leaf
5-day
12-day
2-day
Canopies
Emerging leaf
Emergence
8-day
Squaring
Flowering
About to abscise
Mature crop
Canopy Photosynthesis, mg CO2 m-2 s-1
Photosynthesis and Light Response Curves
4
Maximum net photosynthesis
3
LUE
2
1
Light
Carboxylation
Limited
Limited
0
Light saturation point
LCP where Pn = 0
-1
0
Respiration
500
1000
PPFD, µmol m-2 s-1
1500
2000
Photosynthesis, mg CO2 m-2 s-1
Canopy Aging - Photosynthesis
7
6
PPFD 1200 µmole m-2 s-1
1995 ambient Temp.
5
4
3
2
1
Flower
Open Boll
360 CO2
0
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Days after Emergence
Photosynthesis - Canopy Growth and Aging
Canopy Light Utilization Efficiency
Canopy Light Utilization Efficiency
(µmol CO2 µmol-1 photons)
0.020
0.018
N - Sufficient
0.016
0.014
0.012
0.010
0.008
0.006
0.004
Flowering
0.002
0.000
20
30
40
50
60
Boll opening
70
80
90
100 110 120 130 140
Days after Emergence
Ptotosynthesis - Canopy Growth and Aging
Canopy Conductance
Canopy Conductance, mm s-1
20
18
N - Sufficient
16
14
12
10
8
6
4
Flowering
Boll opening
2
0
20
30
40
50
60
70
80
90
100 110 120 130 140
Days after Emergence
Photosynthesis
Photosynthesis -- Aging
Aging
Factors
Factors controlling
controlling aging
aging and
and
photosynthetic
photosynthetic process
process
Photosynthesis, mg CO2 m-2 s-1
Canopy Aging - Photosynthesis
Response to Carbon Dioxide
7
6
PPFD 1200 µmole m-2 s-1
1995 ambient Temp.
720 CO2
5
4
3
2
1
Flower
Open Boll
360 CO2
0
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Days after Emergence
Boll-load and Vegetative Growth
Stem Elongation, cm plant-1 d-1
Stem Elongation vs Boll Numbers
5
Linear, y = 3.4 - 0.105 * X, r ² = 0.86
4
3
2
1
0
0
5
10
15
20
Bolls, no. plant-1
25
30
35
Net Photosynthesis, g CO2 m-2 d-1
Canapy Growth - Aging - Daily Net Photosynthesis
Seasonal Trends1995
160
140
1995 Ambient Temp.
120
100
80
60
40
20
0
20 30 40 50 60 70 80 90 100 110 120 130 140 150
Days after Emergence
Canopy Photosynthesis, mg CO2 m-2 s-1
Canopy Growth - Aging - Seasonal Trends
Response to Nitrogen Nutrition
8
7
N-Sufficient
N-deficient treatment imposed
6
5
4
3
2
Flowering
Boll opening
N-Deficient
1
0
20
30
40
50
60
70
80
90 100 110 120 130 140 150
Days after Emergence
Canopy Light Utilization Efficiency
(µmol CO2 µmol-1 photon)
Canopy Aging and Photosynthesis
Canopy Light Utilization Efficiency
0.020
Day of N-deficient treatment
N - Sufficient
0.016
0.012
0.008
0.004
Flowering
Boll opening
N - Deficient
0.000
20 30 40 50 60 70 80 90 100 110 120 130 140
Days after Emergence
Canopy Aging and Canopy Conductance
Response to Nitrogen Nutrition
Canopy Conductance, mm s-1
20
N-deficient treatment imposed
18
N - Sufficient
16
14
12
10
8
6
4
Flowering
Boll opening
2
0
20
N - Deficient
30
40
50
60
70
80
90
100 110 120 130 140
Days after Emergence
Photosynthesis, mg CO2 m-2 s-1
Photosynthesis - Growth - Aging
Response to Water Deficits
9
8
Well-watered
7
6
5
4
3
2
Trt. Start date
Water stressed
1
0
55
Rewatered
60
65
70
75
80
Days after Emergence
85
90
Photosynthesis
Photosynthesis -- Aging
Aging
Canopy
Canopy level
level –– other
other species
species -- soybean
soybean
Canopy Aging and Photosynthesis
Soybean
Photosynthesis
Photosynthesis -- Aging
Aging
Canopy
Canopy level
level –– other
other species
species
Indeterminate
Indeterminate vs.
vs. determinate
determinate plant
plant types
types
Canopy Photosynthesis, mg CO2 m-2 s-1
Canopy Aging - Photosynthesis
Species or Plant type Variation
10
9
Indeterminate Crop: Cotton
Elevated CO2
8
7
6
5
4
3
2
Flowering
Determinate Crop: Sorghum
1
0
0
20
40
60
80
Days after Emergence
100
120
140
Canopy Aging and Photosynthesis
¾ Aging process at the canopy level appears to be a function
of leaf-level processes, but modulated by nutrient
supply/demand which in turn affects growth and
development of various organs including the younger and
more efficient leaves produced at the top of the canopy.
¾ Sustained photosynthetic efficiency and capacity of
canopies require optimum environmental conditions
(water, nutrients, including carbon), to continually initiate
new leaves.
¾ Any management or environmental factor that limits
photosynthesis capacity and efficiency can potentially limit
crop yield.