Sinfonian's Square Foot Garden had an article today that talked about tomato flowers and I started to comment, but it just got too long, so I'm putting it here for everyone. As many of you know I'm doing a little tomato experiment growing F2 seed that I saved, so I've been doing a little research in tomato breeding and seed saving and have come across a lot of info.
There are two kinds of tomato flowers. Most modern tomatoes have the first kind. These have short styles. The anthers (the male part of the flower that produces pollen) form a cone around the style and the stigma (the female part that accepts the pollen at the end of the style) is totally enclosed. Bees can't get to the stigma to pollinate it, so they are totally self pollinating. Bees actually do help with pollination here. Studies have shown that the frequency of the bees buzzing is perfect to loosen the pollen and make it fall on the stigma. Wind also helps and shaking the flowers can help too. If you grow in a greenhouse, shaking the plants is mandatory for fruit set.
The second kind of tomato flower has long styles. The stigma sticks out past the cone of the anthers. These don't always self pollinate. They can be pollinated by bees in the usual way. They can cross with other tomatoes. Sometimes they self pollinate like the other tomatoes (and they don't need bees for this, if the flower is pointing down the pollen can still fall on the stigma). However the seed collected from these tomatoes may not be true to type.
So how can you tell if your tomato has long or short styles? Most modern varieties have short styles, but as I found out earlier you can't be sure. My Sungolds from which I saved F2 seed might have crossed with my others since they have long styles. The wild tomatoes all have long styles. According to Suzanne Ashworth's book, Seed to Seed, all potato leaved tomatoes have long styles as do beefsteaks when they have double blossoms. If you want to save seed from your tomatoes it is always a good idea just to check the flowers first. Just look at the cone formed by the anthers. Does the style stick out past it or not? If it does, you have to isolate your plant if you want to save seed. I'll probably do this by making little remay bags for the flowers, since mine will all be planted in one 20'x20' garden. Oh and when you check those styles, make sure it is from newly opened blossoms. Once they are pollinated, the anther tube will start to open and you will see the style then.
Though tomatoes pollinate themselves just fine, that doesn't mean all your tomatoes will automatically be pollinated and if they aren't pollinated no tomato will be formed unless it is parthenocarpic (if you want to read about that go to Sinfonian's SFG and click the link to the article he references). What causes a tomato not to pollinate? Temperature is the major culprit. Low temperatures can cause the flowers to be malformed (55°F 13°C) and pollen is usually not viable under 50°F 10°C. I don't see high temperature problems here in the Boston area, but the same thing happens when the temperature goes over 90°F 32°C.
Please note that those temperatures are an estimate. There is a lot of genetics at play about what temperatures the flowers are viable. Some have been bred for temperature extremes and can produce where others can't. Strange things can happen to the plant at temperature extremes too. If your tomatoes don't self pollinate at high temperatures, the flowers can sometimes elongate their styles in an attempt to try to cross with viable pollen. This is a very cool survival mechanism since the resulting seed will always have some genes for high temperature survival. Though if you want true to type seed, don't save seed when temperatures get that high.