Blossom Pollination

It's easy to put a tomato plant in some potting medium where it gets ample sun and just water it a couple of times per week. For the most part, you will get a plant that grows and produces leaves. But of course, the whole point of growing a tomato plant is to reap the reward of fruits. And this won't happen if the blossoms don't get pollinated.

First and foremost, you need to have blossoms and they won't appear if the plant is lacking essential nutrients. They will also drop even before blooming if the temperatures are too hot or too cold, or the plant is experiencing other stresses. So once you've got those needs taken care of... what's next?

Pollination!

The tomato is a huge genus, with over 7,500 different varieties! Why is it that we only hear about ~50 varieties at the most when it comes to independent or hobbyist growers?  From what I understand, there are 9 species of tomatoes, only 2 of which are actually edible (so people say 7,500 for the impression it makes, when it's actually not useful).  There is also a considerable gap between commercial and home-gardener cultivars; home cultivars are often bred for flavor to the exclusion of all other qualities, while commercial cultivars are bred for such factors as consistent size and shape, disease and pest resistance, suitability for mechanized picking and shipping, and ability to be picked before fully ripening. And sometimes it's just a matter of availability, where many varieties simply never leave their locale or perhaps have one or more qualities that aren't considered financially attractive enough for wide distribution. The Internet is certainly changing this quite a bit.

Of these many varieties, the heirloom and some hybrid varieties are open pollinators that can self-fertilize. This means that the pollen from a given flower can pollinate the ovary of the same flower. They are not "self pollinating", in that the flower cannot trigger this to happen.  Something externally needs to occur, such as a visit from an insect, rapid vibrations from the winds, or intentional shaking by the grower's hand.

So what exactly are the mechanisms for pollination?  A diagram may be useful here.

Diagram courtesy of BioGraphixMedia

I really like this diagram. Look at how the inside of the tomato echoes the ovary. The ovules become the seeds. This is why "full" pollination is important, as each grain of pollen essentially becomes a seed.  If a flower doesn't get enough pollen, either the fruit produced will be lopsided or the fruit may end up aborted altogether.

Here's another useful diagram with a little more annotation detail:

Diagram courtesy of GeoChemBio

Tomato blossoms go through several stages:
1. First formation -- you see a small growth that looks different from the beginnings of a leaf. It is a branchlet or truss that will produce buds.
2. Bud formation -- buds begin to appear from the truss, which can typically vary from 1 to 6.
3. Enlargement -- the bud continues to grow larger, until at least a half-inch or more.
4. Opening -- the bud opens up, the sepals separating, to partially expose the 5 petals.
5. Widening -- the sepals spread further, allowing the petal star formation to take shape.
6. Ready for fertilization -- the petals are wide open and the anther cone wrapped around the stigma is prominently displayed.
7. Nearing end of fertility -- the petals have opened even wider, looking like the flower is being blasted by air.
8. Fruit or Fall -- if the flower didn't pollinate, it will fall off. Otherwise, it remain and the stem will show signs of thickening. The petals and anther cone withers, while the sepals remain green.
9. Bearing fruit -- the center starts to swell, causing the withered petals push aside or fall off. You will see a small pale green pea, which is the start of a tomato.

Here are some photos taken of blooms from my plants:

Awaiting Fertilization

Petals Starting to Wither -- Pollinated?

Close-up of Anther Cone and Stigma

Aborted Flower

Success! Fertilized Flower Produces Fruit

Petals & Anther Shrivel and Peel Away, Revealing Fruit

I'm not a biologist or expert gardener, but I am fascinated by how nature's biology works especially when I'm actively involved with part of it. The tomato pollination/fertilization cycle is an interesting one. And the more you know about it, the more effective you can be at optimizing the production of fruit!  :-)