25. 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.