A fruit is a sink because it produces very little of all the sugars it requires and depends on a source for all the inorganic nutrients it contains.

Vegetables, e.g., spinach leaves are a different matter. Only young leaves are sinks, when they import sugars as well as nutrients. Soon afterwards, however, they become sources of sugars. For all the inorganic nutrients they require, they remain a sink.

Thank you sir

See this link:

http://eprints.icrisat.ac.in/12165/1/RP-04818.pdf

Thank you very much sir

Sink is very important to reflect proper activities of sources available and it provides needful energy for developing plant parts or organs.

It is a result of the climate of growing, selecting varieties, use of fertilizer types ....