Part 4: Water and assimilate movement
3. Sources and sinks
Each organ in a plant is defined as either as a source or sink.
Source = a plant organ that produces more assimilates than it uses
Sink = a plant organ that uses more assimilates than it produces
An example of a source is a healthy, fully expanded, sunlit leaf.
Examples of sinks are roots, shoot apical meristems (growth tips), new expanding leaves and developing flowers, seeds and fruits.
Some organs can act as a sink at some stages and as sources at others, for example a young developing leaf is a sink as it uses large amount of energy to develop but once it is fully grown and photosynthesising it becomes a source.
The way that the plant shares out the assimilates between the different organs, depending on the sink strength, is called assimilate partitioning.
The method of assimilates moving from sources to sinks is called pressure flow.