According to lots of “sources” on the internet there’s a Chinese proverb: “the best time to plant a tree was 20 years ago, the second best time is now.” I think “Chinese proverb” just lends the saying the aura of wisdom. It also seems like 19 years ago would be better than today, but that probably wouldn’t make such a good saying.
I did plant two cherry trees in my yard. The elder indigent boarder loves cherries, which, as you may have noticed, cost a lot. He’s well aware that we won’t be getting fruit for a while, but it got him excited about planting trees – that in itself was worth it. He even supervised.
My neighbor already has a few cherries that don’t provide much in the way of sustenance. We’re both hoping that with even more blossoms in the mix we might see some fruit. Sweet cherry trees need cross pollination to set fruit. This is true of a lot of fruit trees, and can cause a little trouble for gardeners with just a few trees.
For the trees it makes perfect sense. Exchange of genetic information allows for new trait combinations. In human type organisms genetic exchange results in children with traits from both parents. In plants it means the same thing: you get a mixture. Sometimes there are even new traits if a mutation occurred in one parent. If this trait is advantageous it will lead to reproductive success. That’s natural selection in a nut shell, so to speak.
Cherries, and other drupes (peaches, plums, pears and almonds) have “perfect” flowers: both male and female naughty bits. That means that a tree makes both pollen and egg cells in every flower. So how come the “self” pollen doesn’t work?
What a great question. I’m glad I asked.
Animals manage to avoid self-fertilization pretty easily. I’ll spare you the details, this is a column about gardening.
Some plants manage to encourage “out crossing” (sex with other plants) using physical differences. For instance one plant might have all male flowers while another has all female. Another option is to release the pollen before the ovules are ready (or the other way around).
Trees in the Prunus genus (the drupes) have gametophytic self-incompatibility (actually pretty common in other genera as well) (Yamane et al J Exp Bot 2003). GSI as it is known relies on a molecular mechanism to ensure that pollen from a tree can’t deliver sperm to its own egg cells.
When a bee lands on a cherry flower and pollen gets deposited on the stigma (the female tissue that receives the pollen) only pollen from another tree will germinate and get to deliver sperm. In plants with this type of incompatibility there is a region of the DNA called the S-locus. As I’m sure you know he DNA is a code, and what it codes for is protein. If the stigma and the pollen have the same protein then the female parent destroys the pollen. If they’re different the pollen grows. Isn’t that cool?
So when you plant cherry trees you have to ensure that there is enough other pollen around that the bees will go from one tree to the next delivering pollen. Every one of those fruits represents a bee carrying a pollen grain from one tree to the next. We sure do owe a lot to those bees.