Original Post 2013-12-31:
A few years ago I was conducting a cold/frost tolerant tomato trial. The primary purpose of the trial didn't have anything to do with cross-pollination or bees. That arose by accident. It seemed like every time I walked past the tomato patch there were a few bumblebees in it. So I started paying more attention. The bumblebees visited many tomato plants, but they were what I have come to call fly-bys. They'd stop at a flower for less than a second, and then be zipping away. When they got to two particular plants, they would stop, and spend up to ten seconds per flower, then fly to the next flower on the plant and spend another ten seconds, etc...
The two cultivars that were highly attractive to the bumblebees were also the two most productive tomatoes in my garden last year. I attribute that partially to the heavy presence of bumblebees. The flowers got pollinated better and thus set heavier loads of fruit that filled out better. One variety started flowering weeks before these plants, but most of the flowers dropped off without forming fruits. I didn't ever see a bumblebee on that plant. I'm wondering if the flowers simply didn't get pollinated?
The two varieties were both Russian varieties 'Jagodka', and 'Nevskiy Red'.
I primarily grow landrace crops. Landraces are genetically diverse crops which have been selected by natural and human selection to thrive for a particular farmer in a particular location. I would like to convert my tomatoes into a promiscuously pollinating landrace population so that I can more easily do mass-selection plant breeding. Because bumblebees were the only pollinator that I noticed that was interested in working the tomato flowers this summer, it seems like I should cater to their needs.
Feedback I received online recommended the following varieties.
Attractive to Bumblebees:- Hillbilly
- Virginia Sweets
- Blondkopfchen
- Tommy Toe
- Stupice
- Croatian Brandywine
Produced lots of pollen:- Indian Stripe
- Danko
- Black Early
- Sungold
- PI 120256 (Not highly attractive to bumblebees in my garden in 2013. Exposed Stigma.)
I decided that I will work on a new project for the next few years: creating a population of tomatoes that is promiscuously cross pollinating. With that goal in mind I started looking at flowers more carefully. I used a 20X magnifying glass to examine the flowers. Yes, I am nerdy enough to carry a magnifying glass with me just about everywhere I go. Here are traits that I noticed that I think might contribute to a successful project.
Simple FlowersThe first thing that was obvious even without magnification is that some tomato flowers have huge numbers of petals. And those petals can be arranged in a manner that prevents bees from getting to the pollen. Here's what I mean by that:
Open Anther ConeWriters generally talk about an 'anther cone' in relation to tomatoes because the anthers are fused together making a cone around the style. Around 10% of the tomato cultivars that I examined had anthers that were not (fully) fused together. They provided a more open architecture to the flower. It seems like it would be easier for foreign pollen to reach the stigma. Here's what that looked like:
Loose Anther ConeThere were differences regarding how tightly the anther cone wrapped around the style. In some cases the diameter of the opening in the anther cone was much wider than the style. In other cases the anther cone was so tight that it looked completely blocked off. It seems like a wide opening would allow more pollen to fall out of the flower to feed bees, and be more likely to be cross pollinated. I didn't document this trait as carefully as I should have. Here's what that looked like:
Extended StigmaThe trait that everyone talks about in relation to tomato flowers and cross pollination. In some cases the stigma was entirely inside the anther cone, in other cases the two were about of equal length, and in some cases the stigma protruded slightly from the anther cone. Cultivars with an exposed stigma were about 15% of the patch. It seems that a more exposed stigma would be more susceptible to cross pollination (If pollinators were attracted to the flower for other reasons). Here's what that looked like:
Free Flowing PollenTwo varieties had pollen that poured out of the flower when vibrated. These were the two that were highly attractive to bumblebees. These represented 5% of the cultivars in the patch. They were my most productive plants. On a different variety that I dissected the pollen looked like jelly. I noticed this free flowing pollen trait in my new potato varieties when I switched from growing sterile plants to growing abundantly fruiting plants. (Bumblebees love my current potatoes.) I'm wondering if a similar trait is at work in tomatoes. Here's what that looked like:
Larger FlowersPerhaps larger petals, bigger flowers, or stronger petioles would make flowers more attractive to pollinators: Especially to some of the larger species of bumblebees.
Self IncompatibilitySome species of wild tomatoes are self-incompatible. Long term, I would like to incorporate that trait into my tomatoes. Tomatillos are an example of a closely related self-incompatible species.
UV MarkersSome flowers may have markings on them that are visible only in the UV range that make them more attractive to pollinators.
When I originally did the evaluation of the tomato flowers, I did not know what to look for, and my vocabulary was a bit loose, so my records are not as easy to understand as I would like. Before I do another evaluation I will create a template, so I can score each of the traits mentioned here. I think that it would be really useful to me to attempt to combine some of these traits into the same plants. An exposed stigma, a loose or open anther cone, and a cloud of pollen in the same flower sounds really exciting. That kind of challenge is what keeps me going as a grower and plant breeder.
I think that I currently have enough germplasm to do this project... More is always better. The two varieties that were highly attractive to bumblebees were planted about 6 feet apart this year. I'm intending to plant large numbers of plants and screen them for any natural hybrids that might show up. One of the varieties that I have grown for years has an exerted stigma. A cross between these could be interesting. I'm thinking that highly attractive to bumblebees or other pollinators (probably implies a loose or open anther cone and free-flowing pollen) and extended stigmas would be a great combination.