For the gardener wishing to keep his own seeds it remains to assess the risks of cross-pollination and natural hybridisation between the different varieties of tomato growing side by side in the same garden.
The stigmata of the tomato become receptive one day before the flower opens out. The pollen begins to spread a little later but still before the flower opens up. The stigmata remain receptive and the pollen continues to spread once the flower is open that is to say for between one day and one week depending upon the conditions.
The level of cross-pollination between tomatoes is dependent upon
a number of parameters.
1. Characteristics peculiar to the variety such as length of style.
The flowers of the tomatoes are fundamentally self-compatible. In the modern varieties the pistil never extends beyond the cone formed by the stamens, which are fused together. The anthers are to be found on the inside surface of the cone and the pollen spreads in the inside. As the flowers are turned down towards the ground the pollen falls on the stigmata, thus self-pollinating.
On the other hand many of the old varieties, as well as the varieties which have inherited the genes of Lycopersicon pimpinellifolium or other wild species of Lycopersicon, have a pistil which emerges above the cone of stamen and which is thus exposed to pollinator insects. These varieties stand a much greater chance of cross-pollinating.
2. Environmental factors affecting the length of the style.
Professor Messiaen, in his excellent work Le Potager Tropical makes the observation that tomatoes grown in the tropics have the tendency to have a longer style. It would seem in fact that the elongation of the style is caused by solar intensity, the length of the days and the ratio of carbon to nitrogen.
3. The presence of pollinator insects.
Most of the time, tomato flowers do not attract pollinator insects, and all the more so if there are alternative sources of pollen more readily available.
In certain areas, however, particularly in the tropics, certain insects are attracted by the flowers that they will then pollinate regularly. An example of these would be the hymenoptera such as Exomalopsis billotti in the West Indies. In North America the solitary bees and bumblebees are the vectors of cross-pollination in particular on the Atlantic seaboard and in California.
4. Distances for isolation.
It is essential to observe the configuration of the flower of each variety and to observe the insect activity in the environment. When the variety has retracted stigmata and when there are no insects to work the flowers, then you do not require any isolation distances.
On the other hand it is perhaps wise to isolate varieties where the stigmata is prominent and all the more so if there are insects present that will work the flowers. The distance recommended in this situation is 30 metres but can be increased up to 200 metres. In this situation you can separate the tomato varieties with plants, which are rich in nectar and pollen. Thus you can, for example, intersperse with squash plants, which greatly attract bees, but bearing in mind that the flowers close up in the afternoon and so you will need to plant another plant as well.
5. Movement of the wind.
The movement of wind around the flowers can have quite an effect on the level of self-fertilisation but apparently very little on the level of cross-pollination.
In areas that are not susceptible to cross pollination, that is to say the major part of the temperate zone, one can expect between 2 and 5% natural hybridisation.
On the other hand in areas that are sensitive, such as the tropics, the percentage of cross fertilisation is much higher. Depending upon the different factors mentioned above the percentage varies from 12% to 47%. Thus, in the tropics, to be on the safe side, it would be wise to leave 500 meters to one kilometer in between two varieties.
Where you want to save the seed of several varieties, the simple solution would seem to be to isolate the varieties in cages formed with mosquito net or fleece.
Each seed is enclosed in a small gelatinous envelope, which contains various chemicals, which keep the seed dormant. Without this protection the seeds would immediately germinate inside the warm wet environment of the tomato. (It is interesting to note just how hot the interior of a ripe juicy fruit can become on a warm summers day).
In the natural environment the ripe fruits will eventually fall to the ground and start to rot. This rotting process will, thus, destroy the protective envelope.
The gardener wishing to keep his own tomato seed must replicate this natural process of fermentation. The method is in fact very simple. The fruits are cut in two and the interior containing the seeds is scooped out into a container.
You can add a little water because this seems to aid the process of fermentation. This mixture is then left for several days until a mouldy layer forms on the top of the container. This fermentation process is in the main triggered by Oospora lactis, which will eliminate bacterial diseases.
The period of fermentation varies according to the ambient temperature. You will need, however, to remain very vigilant in the hot summer days because the fermentation process can take as little as 48 hours.
In this case you risk losing all your seeds if you leave them too long, because once freed of their protective coat they will gleefully start to germinate in what for them is a perfect environment, namely wet and warm!
Once the process of fermentation is complete then you must clean the seeds by placing them in a fine sieve and pass them under running water. The debris and the immature seeds will be washed away leaving only the good seed. You must then place them on a tray, perhaps made with a piece of mosquito net, and then placed in a dry, well ventilated place to complete the drying process.
You are strongly advised to dry them neither on paper, to which they will remain forever stuck, nor in an oven, however low, nor in the sun. The key factor for drying the seeds, any seeds in fact, is not heat but ventilation.
During hot humid weather you really need to use a fan to dry the seed. Also as part of the process you should separate the seeds, which tend to lump several together. The dry seeds should then be stored in glass jars or in paper sachets protected from the damp.
Tomato seeds have a viability of four years but can remain viable for ten or more years.