Physiology of Plant Production
under Controlled environment
Physiology of Plant Production
under Controlled Environment
(PLS 475A/575A Session 14)
Chieri Kubota
Dept. of Plant Sciences
Efficiency of crop production
Harvest Index = (Dry mass of harvestable organs)/(Total dry
mass of the plant)
„
Transplants, leafy vegetables
„ Translocation
of photosynthates:
Source/sink relationship
Handout: Ho, L.C. 1988.
Metabolism and
compartmentation of
imported sugars in sink
organs in relation to sink
strength. Annual Review
of Plant Physiology and
Plant Molecular Biology
39:355-378.
Photosynthesis
„
Biochemical processes to produce
carbohydrate from water and carbon
dioxide using light energy.
„
Simple reaction equation:
Net
photosynthetic rate ≈ whole plant growth
rate ≈ yield
„
Fruiting vegetable production, root crops
Net
photosynthetic rate ≈ whole plant growth
rate ≠ yield
Understanding mechanisms of translocation of
photosynthates (carbohydrate) is critical.
Photosynthesis occurs in
chloroplasts in plant cells.
Thylakoids
chloroplast
H2O + CO2 + Light energy → (CH2O) + O2
Reactions in photosynthesis
Photochemical reaction (Step 1)
„ O2 evolution by oxidation of water and
production of NADPH (Step 2)
„ ATP synthesis (Step 3)
„ Fixation of CO2 and production of
carbohydrate (Step 4)
„
Steps 1 to 3：Thylakoid reaction (light
reaction)
Step 4: Calvin cycle (dark reaction)
1
Thylakoid reaction
Calvin cycle
ADP+Pi
Cytochrome
complex
1C
CO2 fixation
6 CO2
ATP
(ribulose-5-biphosphate)
6ADP
Cytb/f
Ｑ
P700*
2e-
P680*
PQ
PC
5C
fd
2e-
2e-
P680
H 2O
P700
Thylakoid
membrane
3C
6 Ru5P
(ribulose-5-phosphate)
5C
H+
Carbohydrate
C6H12O6
synthesis
Phosphooxydation
and reduction by
ATP and NADPH
12 GAP
(phosphoglyceraldehyde)
Sink-source
relationship to
understand
mechanism of
translocation of
carbohydrate.
Fruits and flowers (reproductive organs), meristem,
or other storage organs (roots, stems, tubers): Sink
Evidences of Sink-Source
interaction
Control
80
12NADP+
+ 6H2O
starch
Translocation
100
12ADP
12NADPH
3C
CO2
sucrose
12ATP
Mature leaves (vegetative organs): Source
Feedback inhibition of photosynthesis
Net photosynthetic rate
(% of control)
(3-phosphoglycerate)
synthate
complex
2e-
Accumulation of sucrose
in leaves inhibits
photosynthesis. This is
known as feedback
inhibition of
photosynthesis.
12 PGA
6ATP
ATP
Photosystem I
½ O2
+H+
„
RuBPCase
(rubisco)
6 RuBP
NADP NADPH
Photosystem II
Partial or complete removal of sink organ
(i.e. fruits) reduced leaf photosynthesis.
„ Partial removal of source organ (leaves)
while remaining fruits load increased the
remaining leaves photosynthesis.
„
60
Plants with sink removal
40
20
0
0
24
48
72
96
Time after removal of sink organs (h)
Decrease in photosynthesis by removing sink organs (ears) of wheat (King et
al., 1967)
2
Sink-source relationship
Source is a net exporter or producer of
photoassimilate; it exports more assimilate
than it requires for its own.
„ Sink is a net importer or consumer of
photoassimilate.
„ Photoassimilates are translocated from a
source to a sink.
„
Mechanism of translocation in the
phloem
„
„
Pressure-flow hypothesis: Sugars are
translocated in the phloem by mass transfer
along a hydrostatic (turgor) pressure gradient
between the source and sink.
Phloem loading and unloading play a major
role in regulating both translocation rate and
partitioning of assimilates between competing
sinks.
Chemical composition of phloem exudate
and xylem sap from stems of tobacco
(Nicotiana glauca) (Hocking, 1980)
Loading
Amino acid
Unloading
A diagram of pressure flow
(a driving force of
translocation of
photoassimilates
Phloem sap flow facts
Speed = 350 – 700 mm h-1
„ Flow rate = 4 – 13 g h-1 cm-2 (cross
sectional area)
„ pH = 7.8 – 8.4
„ Unloading and loading rates are the
limiting factors of translocation.
Phloem
mg/L
10808.0
Xylem
mg/L
283.0
Nitrate
N/D
N/A
Ammonium
45.3
9.7
Potassium
3673.0
204.3
Phosphorus
434.6
68.1
Other ions
956.6
378.7
Sucrose
155000-168000
N/D
Hexose
N/D
N/A
Total dry matter
170000-196000
1100-1200
pH
7.8-8.0
5.6-5.9
Loading of sugars at the source.
„
Plasmodesmata
(symplast)
Sucrose
Sucrose
Phloem
sieve elements
Sucrose
Cell wall space
(apoplast)
Sucrose
Sucrose
Source cell
3
Unloading of sugars at the sink.
Assimilate distribution (allocation
and partitioning)
Regulation of assimilate distribution affects
productivity and yield.
„ Plant microenvironments should be
controlled so as to enhance more
assimilate distributed to harvestable
organs.
„
vacuole
Plasmodesmata
(symplast)
Sucrose
Sucrose
Sucrose
glucose
fructose
Sucrose
Phloem
sieve elements
Sucrose
Cell wall space
(apoplast)
Sink cell
Allocation of assimilate in leaves
Leaf metabolism and biomass
„ Storage
„ Export from the leaf
„
Partitioning of assimilate among
sinks
Sink strength
„ Distance between sink and source
„
NOTE: If the number of sinks is reduced, a
correspondingly higher proportion of the
photoassimilates is directed to each of the remaining
sinks. Idea of fruit pruning is based on this.
Sink strength
„
Idea to express capacity of a sink to
accumulate metabolites. It is often given
as the product of sink size and sink activity,
as follows:
Sink Strength = Sink Size X Sink Activity
„
However, it is a conceptual value and not
actually well quantified.
Sink competition
„
Flowering young tomato plants
roots>young leaves>in-florescence
„
Fruiting tomato plants
fruits>young leaves>flowers>roots
(Ho,1988)
4
Distribution of photoassimilates in plants
(Marshall and Sager, 1976)
Tomato cultural practices
„
Leaf pruning
„
Fruit pruning
„
Toping (termination of apical buds)
>> Removal of _______ to increase _______.
>> Removal of _______ to increase _______.
>> Removal of ________ to increase _______ and to
terminate both vegetative and reproductive growth.
„
Removing suckers
>> Removal of ______ to increase ________.
Soy beans
Potato
Tomato
Wheat
5