Lecture 4: Water management, long-distance transport and partitioning

Question 1

Soil water parameters

Answer 1

Saturation: all pores filled with water

Field capacity: Water in larger pores has drained

Permanent wilting point: plants can no longer extract water

Depends on texture and structure

Question 2

Upward movement of water

Answer 2

movement of water+evapotranspiration (xylem): Irrigation

Upward movement of water:
-Xylem
-Transpiration
-water potential

Question 3

How much water is needed?

Answer 3

Precipitation (P)- Evapotranspiration (ET)= 0

When P<ET: Irrigation

Question 4

Measurement of evapotranspiration

Answer 4

Evapotranspiration (mm day-1):
-Reference evapotranspiration ETo
-Crop evapotranspiration under standard conditions (ETc)
-Sink size is the total weight of the sink tissue
-Sink activity is the uptake rate of photosynthates per unit weight of sink tissue.

-Altering either size activity of the sink results in changes in translocation patterns.

Question 5

Partitioning

Answer 5

The different spatial distribution of photosynthates within the plant

Question 6

Partitioning: Synchronicity

Answer 6

-Events in sources and sinks must be synchronised
-Partitioning determines growth patterns

-Balance between shoot growth (photosynthetic productivity) and root growth (water+mineral uptake)

Question 7

Why is it important to study photosynthate partitioning?

Answer 7

-Higher edible yields of crop plants (e.g. grains+fruits)
-(Total yield includes inedible portions)
-Edible vs total biomass ratio: improved yield

Question 8

Photosynthate partitioning

Answer 8

-Increased photosynthesis rate in a source leaf
-Increased translocation from source

Question 9

Where do plants allocate fixed carbon?

Answer 9

1. Synthesis of storage compounds
2. Metabolic utilisation
3. Synthesis of transport compounds

Question 10

Fixed carbon allocation: Synthesis of storage compounds

Answer 10

Starch is synthesised and stored within:
-chloroplasts (temporary storage)
-amyloplasts (long-term storage)

Starch storers: plants that store carbon primarily as starch
-In most species starch is the primary storage form that is mobilised for translocation during the night.

Question 11

Fixed carbon allocation:
Metabolic utilisation

Answer 11

Fixed carbon can be utilised within various compartments of the photosynthesising cell:

ENERGETIC: meet the energy needs of the cell

STRUCTURAL: provides carbon skeletons for the synthesis of other compounds required by the cell

Question 12

Fixed carbon allocation:
Synthesis of transport compounds

Answer 12

-Fixed carbon can be incorporated into transport sugars for export to various sink tissues

-A portion of the transport sugar can also be stored temporarily in the vacuole

Question 13

Crop evapotranspiration (ETC)

Answer 13

Kc = Crop coefficient

ETc= Kc*ETo

Depends on:
-Crop species and variety
-Agricultural methods
-Growth stage

Question 14

Long-distance transport of photosynthates (phloem)

Answer 14

Translocation of photosynthates in the phloem:

-Source: Exporting organs, typically mature leaves that are capable of producing photosynthates in excess of their own needs

Storage organs during the exporting phase: bulbs+tubers

-Sink: All non-photosynthetic organs of the plant that do not produce enough photosynthetic products to support their own growth/storage needs.

Question 15

Assimilation and partitioning of photosynthates:
Photosynthate allocation

Answer 15

The regulation of the diversion of fixed carbon into various metabolic pathways

Question 16

Control points of photosynthate allocation

Answer 16

Starch+sucrose synthesis:
1. Starch synthesis
2. Sucrose synthesis, as well as distribution of sucrose between transport+temporary storage pools

Question 17

Starch+Sucrose synthesis

Answer 17

During the day the rate of starch synthesis in the chloroplast must be coordinated with sucrose synthesis in the cytosol

Question 18

Sucrose vs. starch

Answer 18

There is a limit to the amount of carbon that normally can be diverted from starch synthesis in starch storers

Question 19

Source vs. sink

Answer 19

Translocation to sink tissues depends:
1. On the position of the sink in relation to the source
2. On the vascular connection between source+sink
3. the competition between sinks

(E.g. reproductive tissues (Seeds) might compete with growing vegetative tissues for photosynthates in the translocation)

Question 20

Sink

Answer 20

-Sink strength: the ability of a sink to mobilise photosynthates toward itself; sink strength = sink size * sink activity

Question 21

Case-study: increase yield in wheat

Answer 21

Complex, polygenic genetic trait, with multiple genes contributing to yield potential

It is affected by:
1. the efficiency of sunlight harvest through photosynthesis
2. % sucrose exported from the source leaves for use by non-photosynthetic (carbon sink) organs, including the grain
3. sinks themselves, with unloading+utilisation of sucrose, affecting the partitioning of carbon between different organs

Question 22

The Harvest Index (HI): The proportion of total biomass (B) allocated to the grain

Answer 22

Grain yield (Y) = B x HI
1. HI was improved dramatically during the ‘Green Revolutions’ genes for reduced height

2. Partitioning of a greater proportion of assimilate and other resources to the grain
3. N from fertilisers partitioned to the grain rather than promoting more vegetative growth and making the plants more susceptible to lodging
   -Yields increased while total biomass decreased

Question 23

HI needs and results

Answer 23

The plants need:
-leaves to photosynthesize
-roots to acquire water and minerals
-stalks to support the leaves and heads

Further increases in yield will involve an increase in biomass while HI remans the same, reversing the trend of the last few decades.

-HI cannot be improved much further because it is already lose to 60% in modern elite varieties

Question 24

How can we improve yield?

Answer 24

An increase in carbon fixation through photosynthesis is needed:
-Radiation use efficiency
-Photosynthetic area
-Prolonged photosynthesis (stay-green)
-Resource partitioning.

Question 25

Resource partitioning in wheat

Answer 25

Several sink-oriented strategies:
-Maximising spike fertility
-optimising spike growth to maximise grain number and HI
-Improve lodging resistance
-Improve photosynthate allocation and partitioning