For a a couple of years we have been baffled by flats of tomato seedlings that contain plants that have the traditional divided leaves and also plants with the more unusual potato leaves. The leaf shape should be consistent to the variety, so we chalked it up to planter error. After all we usually have 5 or more volunteers planting about 7000 tomatoes. However, it keeps happening, and in purchased seed. I was confused, as I had always been told tomatoes don’t out-cross. So I did a little bit of web-work to try to find out what is going on.
First we need to understand a little about pollination in tomatoes. Heirloom tomatoes are open-pollinated, which means they are pollinated naturally, either by insects or wind. Hybrids are produced through selectively controlled hand-pollination. Tomato flowers are considered perfect in botanical terminology, which means every flower has both male and female parts. All tomatoes are self-fertile, so each plant can accept the pollen that it has produced, but they can also out-cross or accept pollen produced by other tomato plants.
In order to preserve the distinctive characteristics in a variety, the pollen fertilizing the tomato flowers needs to come from the same variety. So Brandywine tomatoes should only be fertilized with pollen from other Brandywines to ensure that saved seeds are Brandywine. But if every tomato can accept any tomato pollen, how do we ensure that our Brandywines don’t cross with the Mortgage-lifter variety planted in the next row?
In general tomato growers do not worry too much about this issue because of two idiosyncracies in tomato biology. First, the anthers that produce the pollen are wrapped around and, in most varieties, extend a little above the the female parts of the flower, so that each flower’s own pollen falls directly onto the stigma (female flower structure that accepts the pollen). Second, the stigma becomes receptive and the pollen ripens before the flower actually opens. This means that in most cases all of the fertilization happens before the flower ever opens and can accept pollen from other plants. Tomatoes are mostly functionally self-fertile and there is little opportunity for out-crossing.
Of course, there are always exceptions. Some of the older and long-season varieties tend to have longer styles (structure that holds the stigma) that extend above the pollen-producing anthers, so they are more likely to out-cross. This is especially true in varieties that contain genes from tropical vareties, wild-type, or currant-type varieties.
Out-crossing also seems to be more dependent on pollinator visits than wind, so the more pollinating insects you have the more transfer of pollen is likely to occur. Flower size is important too. Several people in various tomato forums reported that they saw more out-crossing in large flowered and double-flowered varieties. This makes sense, since pollinators probably prefer to visit larger flowers. A study on cherry tomatoes, showed larger flowers tend to attract more pollinators and also show more out-crossing. According to Jeff McCormick, at Southern Exposure Seed Exchange, in most of the US we can expect about 2-5% of our tomatoes to outcross with other varieties unless we control pollination.
So how does this relate back to my potato leaved seedlings? In many cases these are older varieties, so I suspect that these types are more susceptible to out-crossing in the field. The growers didn’t take adequate precautions to limit out-crossing.
There are several suggestions for limiting out-crossing. One is to put distance between your varieties. McCormick suggests about 10 feet between varieties as adequate for most home gardeners. Another option is to “bag” individual flowers or plants so that they are not visited by pollinators. Another, and I think the most practical, is to give your tomatoes a good shake, each day so that they are more likely to drop their own pollen onto their own flowers.
- Guide to tomato flower structure: http://www.kdcomm.net/~tomato/Tomato/xingtom.html
- McCormick, Jeff. Isolation distances for tomatoes: http://www.southernexposure.com/isolation-distance-tomatoes.p.html
- Kennell, Holly (1995) The Gardener. This report is available online through the Washington State University Extension Service, Gardening in Western Washington–Library–Tomato Pollination