Graft hybrid attempt

Graft hybrids are a topic of much controversy, has been studied only briefly, and at this point in time the concept is very poorly understood.

For years it was believed that DNA did not cross the graft union further than a few adjoining cells, then it became clear that metabolites and some DNA could cross the graft union but there was controversy over how far this would travel or if any changes could be inherited by seeds.  Then it started to look as though DNA transfer can be heritable and passed on through seeds to some extent.

While there is still a lot of controversy over this topic, it is now pretty clear that DNA can cross the graft union under some situations and may cause heritable changes to be incorporated into seeds.

Graft hybridisation is not the same as cross pollinating where half of the DNA from each parent is passed on.  We are not sure how much DNA from each parent is passed on, and it appears in many cases that 0% is passed on.  At the moment there are more questions than there are answers.  I find it fascinating.

Cross pollinating strawberry and raspberry to create a hybrid has a low rate of success and is rather time consuming.  I can see why so few people have ever attempted this.  Around two years ago I made a large number of attempts, planted large numbers of seed, and only have a very small number of fruiting intergeneric hybrid plants to show for my efforts.

Given that heritable changes to DNA can sometimes be incorporated through grafting, it got me thinking.  Perhaps it is possible to use graft hybridisation to cross strawberry and raspberry plants?  Perhaps I graft the two, allow the scion to flower/fruit, collect the seeds, if I plant the seeds they may display some form of genetic cross over between the two plants.

To attempt this, the first question is one of what part to graft.  It may be possible to graft the growing tip of a raspberry onto strawberry roots, I am self-taught at grafting and that sounds difficult.  I worry that I am not up to this kind of grafting and would likely just kill both plants.  So this may not make crossing the two any faster or easier than simply cross pollinating a lot of flowers.

Or, would it be best to graft strawberry scion onto a raspberry plant?  Some strawberry plants can go from seed to fruit in 5 months or less, saving and germinating strawberry seeds is simple.  I guess it is possible to excise the strawberry’s growing bud and graft that to a raspberry somehow, again this sounds like a difficult graft for someone with few grafting skills, so I would probably end up killing both plants.

Assuming that it is even possible to graft a rasberry and a strawberry, this graft may or may not take all that easily as there may be issues with mild incompatibility.  I am self-taught, and don’t have a laboratory, so will be working in the field where insects and birds and children frequently visit and the weather can be downright hostile at times.  If I have any hope of succeeding I need to work with something that is simple to graft to increase my chances of success.

Strawberry runners grafted onto a raspberry cane

I decided to try and graft strawberry runners onto a raspberry cane.  Runners give me a large area to work with so if I mess up I can cut off that part and try again, I can do this several times if needed, until I make a neat cut on just the right angle.  You can't really see in the photos, but some runners are long and others are short, this was partly to see if that made a difference and partly because I had to cut some runners shorter as I made the cut wrong a few times.  I did this with a few plants but only took pictures of the first one.

With runners the young plantlet is still developing, if grafted onto a more mature raspberry cane it will be a kind of ‘mentor graft’ which is said to facilitate DNA exchange.  Given the length of a raspberry cane I can graft large numbers of runners and increase the chances of one working. I ended up grafting more than is in the photographs, about half a dozen runners per raspberry cane.  I wanted to do more but ran out of time and effort.

I grafted several strawberry runners onto a few raspberry plants.  I am unsure if ploidy makes any difference to success rates of grafting two different genus so I used both diploid plants (2n = 2x = 14).  The runners were very thin and difficult to use, but it was all I had available at the time.

To the best of my knowledge, no one has ever attempted anything like this using strawberry and raspberry.  There have been a few attempts using various Solanaceae species that have confirmed genetic material can cross the graft union and these changes can be heritable in the seeds, and at least one intriguing graft hybrid experiment using plums, but nothing with Rubus and Fragaria.

My plan was to graft strawberry runners onto raspberry canes, then when the strawberry plant was large enough I would allow it to root in soil.  Potentially some raspberry genes may have crossed into that strawberry and may be heritable.  I would then allow that strawberry (which had previously been grafted onto a raspberry) to flower and set fruit, I would collect and plant seed, and then see if there were any noticeable difference in those seedlings.

I assumed that if this worked there would be a low rate of genetic cross over, so I was planning on growing out a lot of seedlings.

I ran out of time on grafting day and kept some of the runners not grafted but sitting in a glass of water.  They all died after a day, so gave me hope that my poor technique grafting may have been successful.  All of my grafts looked bad for a day or two, then they perked up.  This was a very encouraging sign.

After 8 days there was a preventable mishap.  One hot day most of my strawberry raspberry grafts died.  The plants were in a plastic bag to increase humidity, but it also made a little solar oven which cooked them.  They looked fine in the morning, and were crispy and dead in the evening.  I think it was due to the heat rather than incompatibility issues as the raspberry hosts dropped a lot of their leaves that day.

After that incident I had one single grafted runner that was still alive and growing well.  It was small but growing fast, so I assumed the graft had taken.  This runner had a small plant that then sent out a further runner, so I was confident this had worked.  This grafted runner remained alive for 28 days after the initial heatwave and was developing well, I put a pot of soil under it so it could send down roots, then another heatwave hit and it died before it had a chance to root.

My little strawberry runner survived for 36 days after I had grafted it and had a nice looking little plant ready to set down roots.  This was extremely frustrating.  The rest of summer was record breakingly hot so I didn't try again.  I even lost a lot of my raspberry plants that were growing in the garden.

I hope to give this another try in spring if I have time.  The fact that the last runner lived so long was encouraging, but at this point I don't know if this could work, let alone if my grafting skills are good enough to succeed.  Unfortunately either no one has tried this before, or they haven't bothered making their results accessible, so I will have to do it and learn for myself. 

If I give this another try I plan to write a blog post of the results.  Even if I am unsuccessful it is a good result and worth learning from.  I wish someone else had tried this properly and either failed or succeeded, that way I wouldn't have to start from scratch.  Let's be honest, if this is not going to work then I am just wasting time.