GreenLab Course
Production - Expansion
Biomass partitioning
Organ Growth
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Biomass partitioning or allocation is the distribution of growth over the different organs (roots, leaves, stems, fruits, rings).
Biomass partitioning among organ cohorts is not constant and results from competition.
Organs, considered within their specific cohorts, are in competition during their growth.
For a given organ, the ability to accumulate biomass is characterized by a sink, expressed by a value for a given duration, i.e. the organ expansion time.
The set of sink values during organ expansion defines the organ sink strength function, φo.
On a given date t, organ sizes results from biomass accumulation.
Each biomass increase q(t) is defined from the total available biomass Q(t), and the relative ratio of the organ sink φo / Σjφj, with j respectively standing for all the different organ types (all roots, all leaves, etc.).
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q(t) = Q(t) . φo / Σjφj
The value of the sink strength function has to be expressed according to organ expansion, i.e. its age.
Definition of the sink strength function is compatible with definition of the cohort, and can thus be associated with the cohort parameters:
all organs belonging to a given cohort show the same sink value, on any date.
Beta laws are often used to express sink functions, giving a wide range of shapes from two single parameters (see below in the equation sub-section)
Organ weight
We have seen that plant growth is modelled step by step, integrating processes during growth cycles.
Expansion periods are thus expressed as a number of growth cycles.
The sum of all the biomass increments of an organ gives its weight.
The total biomass allocated to a given organ o is thus:
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Qo,p(n) = Σj=n-l;n qo,p(j)
where
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n-l stands for the first appearance cycle of the current organ.
l stands for the expansion duration (in number of cycles) of the current organ.
Organ weight thus results from a dynamic process:
- it results from iterative biomass allocations during all organ expansion periods
- each allocation is defined from the available biomass in the common pool and the organ sink strength
- the organ sink strength varies during organ expansion
Organ weight is thus a convolution, as illustrated in the small example below.
This example shows the growth of the first leaf of a simple plant, over three growth cycles (Images and animation P. de Reffye, CIRAD).
- ( Click on < and > buttons to visualize the biomass allocation steps, on legend to refresh ).