Input-Output Analysis of Phloem Partitioning Within Higher Plants
Phloem vasculature within higher plants functions at very high hydrostatic pressure (10 atmospheres). When the pressure is disrupted, there is a surge of flow that almost immediately results in blockage, making experimentation difficult. Therefore, there are few reliable measurements of sap contents and limited understanding of the biophysics of its flow. Consequently we still do not know how partitioning between competing sinks is controlled. In vivo measurement using radioactive tracers is an important tool in the study of phloem function, but has rarely been quantitatively analysed. A detailed time sequence of phloem sap movement through a plant is possible with in vivo measurement of 11C tracer, which is ideal for input-output analysis. Input-output analysis provides a parsimonious description of tracer movement. The only estimates of transport distribution times, pathway leakage, and partitioning between competing sinks that have been reported are based upon input-output analysis of 11C-labelled photosynthate. These quantitative measurements have led to the first mechanistic understanding of phloem partitioning between competing sinks, from which sink priority has been shown to be an emergent property.
KeywordsSeminal Root Phloem Transport Pathway Leakage Label Leaf Tracer Pulse
I am indebted to Drs J.H. Troughton and W.F. Pickard who encouraged me right from the beginning to develop my understanding of input-output analysis, and to Professor P.C. Young for his encouragement and collaboration for over 30 years. Several of his students have had a big input into both data analysis and development of control systems used in the 11CO2 tracer facility. Dr M.R. Thorpe has been a long time collaborator and the sabbatical time of Prof J.F. Farrar spent in our laboratory introduced us to a lot of new plant applications.
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