GreenLab Course
Development
Modelling development and viability in rhythmic growth
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In the case of rhythmic growth, axis development and viability have to be considered as a dual scale process:
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at whole axis level, built from serial growth units, and
at growth unit level, built from serial phytomers.
Rhythmic Axis Development
The distribution of the number of phytomers in a growth unit always follows a unimodal or a bimodal curve shape.
The unimodal case (full pre-formation or full neo-formation)
The unimodal case is related to a single functioning process, either pre-formed or neo-formed.
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The difference lies in the fact that, in the pre-formed case,
the phytomer expands building the full growth unit, while in the neo-formed case,
phytomers appear one after the other until a stop related to a season or to meristem
death or flowering (sympodial growth).
These unimodal shaped distributions can be fitted to binomial laws.
Growth unit construction is seen as a Bernoulli process with a probability b of each phytomer appearing, applied to a given number of growth cycles N.
The bimodal case (pre-formation followed by neo-formation)
The bimodal case is related to a mixed pre-formed / neo-formed growth unit building process.
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A first section of the growth unit is built from preformed phytomers, followed then by neo-formed phytomers.
This situation is common in many temperate species (poplar tree, wild cherry tree, etc.) but also in tropical species (e.g. cacao tree).
For a given physiological age, such bimodal distribution shapes can thus be fitted with two binomial laws and the probability pn of having neo-formation.
pn can be simply assessed by a ratio standing for the proportion of neo-formation occurrences.
However, the neo-formed section may show (-or not) significant dispersion for the number of cycles.
This neo-formed section can thus be adjusted to a binomial or to a negative binomial law.
It is interesting to note that, on a single individual, the neo-formed part decreases with the physiological age. The neoformed part can be systematically seen on the trunk, more or less established on the secondary axis, and not expressed at all on short axes.
On some species, binomial Laws parameters are rather stable according to the physiological age, and the various distributions can be fitted by the variation in the proportion of neo-formation.