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Interesting question. I do not have any nicotine-specific reference texts in my possession, but a keyword search on [nicotine sources] gives many many hits, mostly in reference to cigarettes and cigarette smoke, but one abstract below provides answers about the foods you mentioned. I was unaware that nicotine content was significant in such common foods as potatoes and tomatoes. Being something of a spicy food fanatic, I was relieved to see that nicotine does not occur in green pepper, at least!

http://www.ncbi.nlm.nih.gov/pubmed/1765327
Dietary nicotine: a source of urinary cotinine.Davis RA, Stiles MF, deBethizy JD, Reynolds JH.
Biochemical/Biobehavioral R&D, R. J. Reynolds Tobacco Company, Winston-Salem, NC 27102.

Foods, principally from plants in the family Solanaceae, and a number of teas were examined for the presence of nicotine. Dietary nicotine would give rise to cotinine in urine and compromise estimates of exposure to tobacco smoke that depend on urinary cotinine. All foods were homogenized, extracted and analysed for nicotine and cotinine by gas chromatography with nitrogen-sensitive detection (GC) and/or GC/MS (mass spectrometry). Weak acid and aqueous extracts of the teas were analysed in a similar manner. Nicotine was not detected (less than 1 ng/ml of extract) in egg plant or green pepper. The average values for nicotine in tomato and potato were 7.3 ng/g wet weight and 15 ng/g wet weight, respectively. Black teas, including regular and decaffeinated brands, had nicotine contents ranging from non-detectable to greater than 100 ng/g wet weight. Instant teas yielded the highest nicotine contents observed (up to 285 ng/g wet weight). The possible sources of nicotine in these foods are discussed. A range of potential values for urinary cotinine concentrations (0.6 to 6.2 ng/ml) was calculated based upon estimated average and maximal consumptions of these foods and beverages. Because of the potential for exposure to nicotine by way of these routes, the use of urinary cotinine as a biomarker of exposure to environmental tobacco smoke may be compromised.

http://www.abc.net.au/quantum/poison/nicotine/about.htm
Most tobacco today comes from the plant Nicotiana tabacum, but there are 66 other species of plants that contain nicotine. 19 of these are native to Australia. In fact Australian Aboriginees may have been the first people to use tobacco. We know they made use of at least 5 species. The leaves were powdered, often mixed with ash and chewed. The nicotine acted both as a stimulant and an assuager of hunger so it was carried on long journeys through the desert.

http://www.quitsmoking.com/kopykit/reports/nicotineaddiction.htm
The average cigarette generally contains about 8.4 milligrams of nicotine and 15 milligrams of tar. Tobacco smoke also contains as many as 4,000 other naturally occurring gases, particles and compounds, including carbon monoxide, carbon dioxide, hydrogen cyanide, traces of arsenic and carcinogens.

Hi, sorry to take so long getting back to you! I was out of town without computer access most of this week. Anyway, as to your question, 1,000,000,000 (or 10⁹) ng = 1 g, or, 1 ng = 10^�? g. So, 8.4 mg would be

8.4 mg X 1 g X 1,000,000,000 ng = 8,400,000 ng

Thus, a cigarette contains a very large amount of nicotine. A single cigarette contains about one gram of dry tobacco. The nicotine scale for the food samples would probably be better if they were in ng of nicotine per gram of dry food substance in order to make a better comparison. So what is needed is the percent water in these foods. According to http://www.h2ouniversity.org/html/3-5_facts_life.html, tomatoes are about 90% water and potatoes are 80% water.

Since one gram of a "wet" tomato is 90 percent water and 10% other substances, the amount of nicotine in a dried tomato is as follows:

7.3 ng X 1 g (wet) = 73 ng / g (dry)

1 g (wet) 0.10 g (dry)

This compares to 8,400,000 ng / g in dry cigarette tobacco. For potatoes, the amount of nicotine is similarly calculated to be 15 ng / 0.20 g (dry) = 75 ng / g (dry).

According to http://www.ag.ohio-state.edu/~news/story.php?id=2407, eggplant is 92% water. And according to http://www.foodfit.com/healthy/archive/healthyNutriSmarts_waterveggies.asp, green peppers are 93% water. Come to think of it, I guess I didn't need the pepper info, but there are other percentages given there also.

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