Journal of Experimental Botany, Vol. 53, No. 376, pp. 1919-1928,
September 1, 2002
© 2002 Oxford University Press
Source–sink partitioning. Do we need Münch?
Received 15 November 2001; Accepted 2 May 2002
INRA, Station Environnement et Grandes Cultures, F-78850 Thiverval-Grignon, France
Abbreviations: Source region parameters have subscript 0, sink 1 and sink 2 parameters have subscripts 1 and 2, respectively. Local concentrations of assimilate: S0; S1; S2. Michaelis constants for sinks 1 and 2: K1; K2; V1; V2. Resistance to concentration-driven flow within the common pathway of a 1-source/2-sink system: R0, and with the separate pathway associated with each sink: R1; R2. Activity fluxes either from source or to sinks: U0; U1; U2.
The simulation of phloem translocation by the Münch theory commonly uses resistances from sources to sinks: the resistances are therefore regarded as important in partitioning. Although resistance is generally a set constant, it is in fact strongly affected by viscosity, and thus the concentration of the transported solute. In this paper, the model of partitioning proposed by Minchin et al. was first corrected for variations in viscosity. The model was further modified, with the source considered as an activity of solute production rather than as a compartment concentration. When so defined, the source cannot differ from the sum of sink activities, largely outdating the source- or sink-limitation concepts. The corrected model confined the effect of resistances on the partitioning to low source activities. In the example of wheat grain filling analysed, such activities would be so low that they would correspond only to pathological conditions. In that case, the use of resistances in modelling is therefore just a mathematical burden, not even easily quantifiable since they are related to anatomical traits that are difficult to access. Leaving out resistances, it becomes easy to calculate the sink activities directly from the source activities, using an intuitive, accessible parametrization. The conditions for such a simplification are discussed.
Key words: Key words: Dry matter distribution, model, sink priority, sink size, sink strength.
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