Demai, Chapter Two, Mishnah One
In chapter one, mishnah three, we learned that if one buys certain types of produce from Achziv and north, he does not have to separate tithes, since that region is not liable for tithes from demai. We also are usually not concerned lest the produce was brought there from the land of Israel, in which case it is liable for tithes because it was grown in Israel.
Our mishnah teaches that sometimes tithes must be taken out of certain types of demai produce no matter where they are bought, lest the produce come from the land of Israel.
1) The following things must be tithed as demai in all places: pressed figs, dates, carobs, rice, and cumin.
2) As to rice from outside the land [of Israel], whoever uses it is exempt from tithing it.
Section one: The mishnah refers to products bought from an am haaretz in an area that borders the land of Israel, but was not conquered by those Israelites returning from Babylonia and hence does not officially count as part of the land of Israel. Tithes must nevertheless be taken from these things because it is assumed that they come from the land of Israel, or at least from regions of the land that had been conquered by those returning from Babylonian exile.
Section two: One who buys rice from an am haaretz outside of the land of Israel and uses it there need not separate tithes. However, if he brings it to the land of Israel he must tithe it because it is easily switched with rice that comes from the land of Israel and is obligated to be tithed as demai. We should note that this is Albecks explanation; others explain the mishnah differently.