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Photosynthesis and EnvironmentLeaf and Canopy Aging
Goals and Learning Objectives:
• To understand the effects of leaf and canopy aging on photosynthesis process at leaf and canopy level.
Photosynthesis and leaf-level aging.
Photosynthesis and canopy-level aging.
Senescence, Aging and Death and Agriculture
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.
Leaf senescence is one of the most conspicuous processes that has been studied in the context of plant aging and senescence.
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
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.
Leaf aging or senescence has implications on agriculture, affecting crop yield and the shelf life of leafy vegetables.
Photosynthesis - AgingLeaf Level
Photosynthesis - AgingLeaf Level
Canopies
Emergence
Squaring
Flowering
Mature crop
Emerging leaf2-day 8-day
5-day12-day
About to abscise
Leaf
Leaf and canopy development and aging process
Photosynthesis - AgingLeaf Level
Photosynthesis - AgingLeaf Level
Deep Inside
Unfolding leaf
Mature leaf
Chloroplasts with limited thylakoiddevelopment
Chloroplasts with well-developed thylakoid
Photosynthesis - AgingLeaf Level
Photosynthesis - AgingLeaf 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
PPFD, µmol m-2 s-1
0 500 1000 1500 2000
Pho
tosy
nthe
sis,
µm
ol m
-2 s
-1
-10
0
10
20
30
40
50
Carboxylation
Limited
Light
Limited
LCP where Pn = 0
RespirationLight saturation point
Maximum net photosynthesis
LUE
Aging - Photosynthetic Light-response Curvesfor Cotton Leaves
PPFD, µmol m-2 s-10 400 800 1200 1600 2000 2400P
hoto
synt
hesi
s, µ
mol
CO
2 m-2
s-1
05
10152025303540
40 days
35 days
27 days
20 days
Leaf Aging and Photosynthesis
Leaf Age, d0 10 20 30 40 50 60P
hoto
synt
hesi
s, µ
mol
CO
2 m
-2 s
-1
05
10152025303540
30/22°C and 360 µl l-1 CO2
Leaf Aging and Photosynthesis
Leaf Age, days15 20 25 30 35 40 45 50P
hoto
synt
hesi
s, µ
mol
CO
2 m-2
s-1
0
10
20
30
40
50PPFD = 1500 µmol photon m-2 s-1
Leaf Aging and Light Utilization Efficiency and Conductance
Leaf Age, days15 20 25 30 35 40 45 50
LUE
, µm
ol C
O2 µ
mol
-1 p
hoto
n
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Con
duct
ance
, mol
H2O
m-2
s-1
0.0
0.5
1.0
1.5
2.0
Conductance
Light Utilization Efficiency
Leaf Aging - Photosynthestic ParametersExpressed as Fraction of Maximum (= 20 d)
Leaf Age, days15 20 25 30 35 40 45 50
Frac
tion
of M
axim
um
0.0
0.5
1.0
1.5
Conductance
Light Utilization Efficiency
Photosynthesis
Leaf Aging and Leaf Nitrogen
Leaf Age, days15 20 25 30 35 40 45 50
Leaf
N, %
1
2
3
4
5
Photosynthesis - Leaf Nitrogen
Leaf N, %1 2 3 4 5
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0
1
2
3
4
5
6
7
Leaf Aging - Leaf PigmentsChlorophyll and Carotenoids
Leaf Age, days15 20 25 30 35 40 45 50
Chl
orop
hyll,
µg
cm-2
10
15
20
25
30
35
40
Car
oten
oid,
µg
cm-2
3
4
5
6
7
8
Carotenoids
Chlorophyll
Leaf Aging - Starch and Sugars
Leaf Age, days15 20 25 30 35 40 45 50
Sug
ars,
%(w
/w)
0.0
0.4
0.8
1.2
1.6
2.0
Sta
rch
%(w
/w)
0
1
2
3
4
5
Starch
Sugars
Leaf Aging and Photosynthesis
As leaves grow in size, leaf net photosynthesis increases rapidly from leaf unfolding (about zero at leaf unfolding) until the leafreaches its potential maximum size. Soon thereafter (from about 20days from unfolding), leaf net photosynthesis declines linearly.Light saturation occurs at lower light levels as leaves age.
The onset (from about 20 days) and magnitude (rates) of declines inleaf conductance and light utilization efficiencies are closelycoupled with leaf maximum photosynthesis rates.
Leaf transpiration rates closely followed photosynthetic ratesthroughout the leaf development.
Leaf pigments (chlorophylls and carotenoids) decline from about 35days from leaf unfolding, at least in cotton.
Leaf Aging and PhotosynthesisThe concentration of CO2 inside the leaf (Ci), and the number of chloroplasts in the mesophyll remain nearly constant during theaging process, except in the oldest leaves that are about to abscise.
Total leaf protein content, RuBP carboxylase and electron transportactivity decline parallel to photosynthesis during the aging process.
Leaf N also declines from about 20 days, but the decline was not asdramatic as the photosynthetic parameters.
The levels of starch and sugars also decrease during the agingprocess suggesting the loss of photosynthetic activity.
Therefore, the physiological deterioration of leaf photosyntheticactivity during the aging process may be more related to decliningRubisco protein (photosynthetic capacity) and photosyntheticefficiency (LUE) than to leaf N, leaf pigments and number ofchloroplasts. 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.
Leaf Aging and Photosynthesis
Leaf Age, d0 10 20 30 40 50 60P
hoto
synt
hesi
s, µ
mol
CO
2 m
-2 s
-1
05
10152025303540
30/22°C and 360 µl l-1 CO2
Leaf Aging and Photosynthesis
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 boll-filling.
2. The branch leaves subtending flower/boll reach maximum photosynthesis (Pmax) about the time of anthesis.
3. Carbon appears to be translocatedfrom remote sites to the developing bolls.
Mainstem leaf
Flower-budBranch leaf
Photosynthesis - AgingCanopy Level
Photosynthesis - AgingCanopy Level
Canopies
Emergence
Squaring
Flowering
Mature crop
Emerging leaf2-day 8-day
5-day12-day
About to abscise
Leaf
Leaf and canopy development and aging process
Photosynthesis and Light Response Curves
PPFD, µmol m-2 s-10 500 1000 1500 2000
Can
opy
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
-1
0
1
2
3
4
Carboxylation
Limited
Light
Limited
LCP where Pn = 0
Respiration
Light saturation point
Maximum net photosynthesis
LUE
Canopy Aging - Photosynthesis
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0
1
2
3
4
5
6
7
PPFD 1200 µmole m-2 s-1
360 CO2Flower Open Boll
1995 ambient Temp.
Photosynthesis - Canopy Growth and AgingCanopy Light Utilization Efficiency
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140
Can
opy
Ligh
t Util
izat
ion
Effi
cien
cy(µ
mol
CO
2 µm
ol-1
pho
tons
)
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
N - Sufficient
Flowering Boll opening
Ptotosynthesis - Canopy Growth and AgingCanopy Conductance
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140
Can
opy
Con
duct
ance
, mm
s-1
0
2
4
6
8
10
12
14
16
18
20
N - Sufficient
Flowering Boll opening
Photosynthesis - AgingFactors controlling aging and
photosynthetic process
Photosynthesis - AgingFactors controlling aging and
photosynthetic process
Canopy Aging - PhotosynthesisResponse to Carbon Dioxide
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0
1
2
3
4
5
6
7
PPFD 1200 µmole m-2 s-1
360 CO2Flower Open Boll
1995 ambient Temp.
720 CO2
Boll-load and Vegetative GrowthStem Elongation vs Boll Numbers
Bolls, no. plant-10 5 10 15 20 25 30 35
Ste
m E
long
atio
n, c
m p
lant
-1 d
-1
0
1
2
3
4
5Linear, y = 3.4 - 0.105 * X, r ² = 0.86
Canapy Growth - Aging - Daily Net PhotosynthesisSeasonal Trends1995
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140 150
Net
Pho
tosy
nthe
sis,
g C
O2
m-2
d-1
0
20
40
60
80
100
120
140
1601995 Ambient Temp.
Canopy Growth - Aging - Seasonal TrendsResponse to Nitrogen Nutrition
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140 150
Can
opy
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0
1
2
3
4
5
6
7
8
Flowering Boll opening
N-Sufficient
N-Deficient
N-deficient treatment imposed
Canopy Aging and PhotosynthesisCanopy Light Utilization Efficiency
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140
Can
opy
Ligh
t Util
izat
ion
Effi
cien
cy(µ
mol
CO
2 µm
ol-1
pho
ton)
0.000
0.004
0.008
0.012
0.016
0.020
N - Deficient
N - Sufficient
Flowering Boll opening
Day of N-deficient treatment
Canopy Aging and Canopy ConductanceResponse to Nitrogen Nutrition
Days after Emergence20 30 40 50 60 70 80 90 100 110 120 130 140
Can
opy
Con
duct
ance
, mm
s-1
0
2
4
6
8
10
12
14
16
18
20
N - Deficient
N - Sufficient
Flowering Boll opening
N-deficient treatment imposed
Photosynthesis - Growth - AgingResponse to Water Deficits
Days after Emergence55 60 65 70 75 80 85 90
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0123456789
Well-watered
Water stressedTrt. Start date
Rewatered
Photosynthesis - AgingCanopy level – other species - soybean
Photosynthesis - AgingCanopy level – other species - soybean
Canopy Aging and Photosynthesis
Soybean
Photosynthesis - AgingCanopy level – other species
Indeterminate vs. determinate plant types
Photosynthesis - AgingCanopy level – other species
Indeterminate vs. determinate plant types
Canopy Aging - PhotosynthesisSpecies or Plant type Variation
Days after Emergence0 20 40 60 80 100 120 140
Can
opy
Pho
tosy
nthe
sis,
mg
CO
2 m
-2 s
-1
0
1
2
3
4
5
6
7
8
9
10
Indeterminate Crop: Cotton
Determinate Crop: SorghumFlowering
Elevated CO2
Canopy Aging and Photosynthesis
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.
Sustained photosynthetic efficiency and capacity of canopies require optimum environmental conditions (water, nutrients, including carbon), to continually initiate new leaves.
Any management or environmental factor that limits photosynthesis capacity and efficiency can potentially limit crop yield.
