Grafting tomato shoots onto tobacco roots in 2003, Rob Baur claimed to be the first to create tomacco, the addictive tomato tobacco hybrid conceived by the Simpsons in 1999. Baur claimed tomacco fruit contained lethal amounts of nicotine, although only leaves were tested.
Demonstrating excellent journalistic thoroughness, news organizations failed to discover that grafted tomacco was first created by Ray Dawson in 1942. Dr. Dawson also measured leaf and fruit nicotine content and published his results in the American Journal of Botany.
Dawson found tomacco leaf nicotine content exceeding typical tobacco leaf content, but tomacco fruit contained insignificant amounts of nicotine. Although Dawson could not confirm the mechanism, we now understand that the failure of tomacco fruit to accumulate nicotine is the result of the location of production and transport of nicotine.
Tobacco roots produce nicotine and transport it to leaves in xylem sap. Once nicotine enters this xylem sap pathway, no cross membrane transport is necessary for transport to leaves. Any graft compatible shoot will accumulate nicotine at the sites of evaporation (leaves) when grafted onto tobacco roots. Likewise, tobacco shoots grafted onto tomato roots, do not contain any nicotine.
Nicotine does not accumulate in tomacco fruit because fruit are not primary sites of evaporation. Without evaporative concentration of xylem sap nicotine, fruit nicotine content in any graft compatible plant would not exceed low xylem sap concentrations. More importantly, water and nutrient accumulation in tomato fruit occurs primarily through phloem. For nicotine to move from xylem sap to phloem sap, nicotine transporters are required that are not present in tomato.
Transgenic tomatoes with leaf nicotine transporters could remobilize leaf nicotine to fruit, but for now addictive tomacco fruit remains a pipe dream.