GreenLab Course
Production-Expansion
Biomass partitioning
Organs
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Sinks and Sources
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Organs, considered within their specific cohorts, are in competition during their growth.
For a given organ, the ability to accumulate biomass is characterized by its sink strength.
We can consider that all organs show this capability. They are sinks.
Assumption.
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Organs are sinks during their expansion. The sink strength function, characterizing organ biomass
demand during organ development (a given number of growth cycles) is usually not constant,
but becomes null when expansion is finished.
It means that there is no organ maintenance "cost" in this model.
Conversely, some organs are considered as having the ability to produce biomass
They are sources. They show this capability as soon they appear, and until they reach their functionning time. The capability of the source is characterized by its source strength.
Assumption.
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In our model presentation (and implementation), we consider that only leaves show the ability
to be sources.
The fonctioning time is assumed to be greater than the expansion time.
In theory, each sink organ could also be a source and the model formalism could easily be extended to allow it.
Beta laws are often used to express sink and source strength functions, giving a wide range of shapes from two single parameters (see further in equation sub-section)
Typical organs in the GreenLab model
The list of organs is not exhaustive.
While leaves are the only source organ, usual sink organs are internodes, leaves, female and male fruits, rings, roots.
However, their respective number of cohorts, numbers within cohort and sink strength may be significantly different from each other.
Roots.
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In this model, the root system defines a single unique organ.
Its sink strength is defined by a single constant ratio (typically 15 to 20%) of plant demand.
Despite this crude approach, the model show satisfactory results on agricultural plants such as beetroot.
Female and male fruits.
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On some crop plants, it may be convenient to distinguish between male and female sexual organs.
Their phenology can be quite different as well as their relative ability accumulate biomass.
Thus, their relative sink strength may be significantly different, in terms of both strength and dynamics.
For instance on maize, tassels and cobs should be considered as different organs.
Internodes
- Internodes are sink organs and not considered as being potential sources.
The biomass allocated to internodes defines in fact the pith volume (and its dimensions).
Internode organs do not include wood rings, resulting from another process.
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Secondary growth, and especially ring width establishment, may be a critical biomass consumer, and must thus
be considered as a sink organ.
Computation of the biomass allocated to wood rings is rather complex (see elements below)
Other specific organs.
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As an example, on cotton, the leaf organ must be split into two organs, the blade (sink and source)
and the petiole (sink only). Such should be the case for all plants whose petioles are significant biomass
consumers and show a different sink strength function compared to the blade.