What determines carbon partitioning between competing sinks?

Carbohydrate sinks have been described by their ability to attract photosynthate, denoted by sink strength, and by their priority rank ordering for supply in the presence of a reduced availability of photosynthate. Sink strength has been defined as the rate of carbohydrate flow into a sink, but this flow rate is also dependent upon supply, other sinks, and resistance to flow of the transport pathway, so it is not a property of the sink alone. It is a property of the entire system. Hence sink strength defined as a flow rate is not a valid descriptor of a sink. However, a simple model of phloem flow based upon Münch's ideas and with saturable unloading has many properties similar to a plant's carbohydrate source-sink relations, including priorities of sinks, and leads to a set of sink descriptors. This model's ability to mimic observed source-sink relations is reviewed here.