Lecture 4: Water management, long-distance transport and partitioning Flashcards
Soil water parameters
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
Upward movement of water
movement of water+evapotranspiration (xylem): Irrigation
Upward movement of water:
-Xylem
-Transpiration
-water potential
How much water is needed?
Precipitation (P)- Evapotranspiration (ET)= 0
When P<ET: Irrigation
Measurement of evapotranspiration
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.
Partitioning
The different spatial distribution of photosynthates within the plant
Partitioning: Synchronicity
-Events in sources and sinks must be synchronised
-Partitioning determines growth patterns
-Balance between shoot growth (photosynthetic productivity) and root growth (water+mineral uptake)
Why is it important to study photosynthate partitioning?
-Higher edible yields of crop plants (e.g. grains+fruits)
-(Total yield includes inedible portions)
-Edible vs total biomass ratio: improved yield
Photosynthate partitioning
-Increased photosynthesis rate in a source leaf
-Increased translocation from source
Where do plants allocate fixed carbon?
- Synthesis of storage compounds
- Metabolic utilisation
- Synthesis of transport compounds
Fixed carbon allocation: Synthesis of storage compounds
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.
Fixed carbon allocation:
Metabolic utilisation
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
Fixed carbon allocation:
Synthesis of transport compounds
-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
Crop evapotranspiration (ETC)
Kc = Crop coefficient
ETc= Kc*ETo
Depends on:
-Crop species and variety
-Agricultural methods
-Growth stage
Long-distance transport of photosynthates (phloem)
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.
Assimilation and partitioning of photosynthates:
Photosynthate allocation
The regulation of the diversion of fixed carbon into various metabolic pathways
Control points of photosynthate allocation
Starch+sucrose synthesis:
1. Starch synthesis
2. Sucrose synthesis, as well as distribution of sucrose between transport+temporary storage pools
Starch+Sucrose synthesis
During the day the rate of starch synthesis in the chloroplast must be coordinated with sucrose synthesis in the cytosol
Sucrose vs. starch
There is a limit to the amount of carbon that normally can be diverted from starch synthesis in starch storers
Source vs. sink
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)
Sink
-Sink strength: the ability of a sink to mobilise photosynthates toward itself; sink strength = sink size * sink activity
Case-study: increase yield in wheat
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
The Harvest Index (HI): The proportion of total biomass (B) allocated to the grain
Grain yield (Y) = B x HI
1. HI was improved dramatically during the ‘Green Revolutions’ genes for reduced height
- Partitioning of a greater proportion of assimilate and other resources to the grain
- 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
HI needs and results
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
How can we improve yield?
An increase in carbon fixation through photosynthesis is needed:
-Radiation use efficiency
-Photosynthetic area
-Prolonged photosynthesis (stay-green)
-Resource partitioning.
Resource partitioning in wheat
Several sink-oriented strategies:
-Maximising spike fertility
-optimising spike growth to maximise grain number and HI
-Improve lodging resistance
-Improve photosynthate allocation and partitioning
