What is the greenish-yellow powder you get all over your hands and arms when you pick tomatoes? For the past two summers, this question has been bothering me. But I couldn't find anyone who seemed to know. I asked numerous friends in the biology and horticulture fields, and even several tomato breeders. Most people said "isn't it pollen?" But clearly it isn't coming from tomato flowers — it's all over the plant, on the leaves and stems. One grower called it chlorophyl. Another called it "tomato tar" because it turns black if you don't wash it off quickly enough. 

An interesting characteristic of this substance is how hard it is to get it off your skin. You lather up with soap and water and the suds turn green. Rinse, and lather again, and the suds still turn green. You can wash your hands four or five times and the stuff just keeps coming off. It explains why all your towels and t-shirts get green stains in summer — no matter how many times you wash, there's still some left behind on your skin.

Finally, the mystery of the tomato stuff was revealed. Chris Wien, a horticulture professor at Cornell University, had sent me some information about high tunnel tomato production for an article I was writing for Growing for Market. I emailed to thank him then added, "By the way, do you know the technical term for the greenish yellow powder you get all over your hands when you pick tomatoes?"
Chris emailed right back. "Yes, the green substance is a number of chemicals that are released from hairs situated on the surface of tomato leaves, stems and fruits.  Under a microscope, these look like miniature water towers, and the compounds are inside these glands.  Some of the compounds are called 'acyl sugars'."

Solanum trichome illustration


Finally, I had the right words to Google it with. When I did, I was plunged into the totally unfamiliar world of plant metabolism research. A few hours later, staggering from one barely comprehensible scientific paper to another, I landed on the website of the Solanum Trichome Project, a collaborative genomics project between the University of Arizona, University of Michigan, and Michigan State. That's where I found this beautiful illustration above (by Chris Smith of fivethirtythree studios) of the little hairs that secrete the green stuff that gets all over your skin. And where I learned, in plain English, the meaning and importance of that substance. Here it is, in a nutshell:
The little hairs that cover tomato leaves are technically known as secretory and glandular trichomes (SGTs). About one-third of all the vascular plant species have SGTs. They secrete various secondary metabolites -- that is, substances that aren't used for the growth or reproduction of the plant but have some other function. SGTs contain the essential oils that give herbs their fragrance and flavor. In tomatoes, they produce acyl sugars, terpenoids, and flavonoids.  Acyl sugars are lipids (fats) that are greasy to the touch, insoluble in water and soluble in alcohol. That's why they're so hard to wash off your skin. Terpenoids release the familiar tomato scent when you brush against the plant. Flavonoids are the substances in plants that are getting all the attention for their role in preventing cancer and cardiovascular disease.

These substances are thought to protect plants against environmental assaults including insect attacks, foliar diseases, extreme heat and excessive light. They are of great interest to plant breeders, who hope to use them to develop varieties resistant to late blight, early blight, Septoria leaf spot and other diseases. There is also some research into increasing insect resistance. "Some wild tomato lines from South America have different acyl sugars than the domestic tomatoes, and by crossing them, the breeders can select for compounds that ward off insects," Dr. Wien said.  "Unfortunately, these compounds also give the plants a 'wet dog' smell, so may take some getting used to."

So now I know. The green stuff serves a very good purpose, from the point of view of the tomato plant. And that makes me more kindly disposed to the green stains on my towels and t-shirts in summer.

Lynn Byczynski is the editor and publisher of Growing for Market, a magazine for direct-market farmers.