Thursday, August 21, 2008

Grafting Round 1: The epiphytic tomato

Who knew tomatoes could be epiphytic ( epiphyte: a plant that derives its moisture and nutrients from the air and rain and grows usually on another plant)? Only under overzealously-controlled, extremely unnatural conditions, i.e., my inadvertent tomato sauna/terrarium. This was my first attempt at grafting, and I learned a lot during and since. I was possibly marginally close to success. Here's the story:

I'll try to explain the techniques and reasons for grafting in another post. Briefly, grafting is growing the top of one plant on the roots of another. The plant used for its vigorous and disease-resistant root system is called the rootstock, and the plant you want to actually grow - in the case of tomatoes, the variety you want to harvest and eat - is called the scion. A successful graft means that the rootstock stem fuses with the scion stem, xylem and phloem channels connect and water/nutrients pass up/down, and the plants become one. The most important thing is for the two stems to be the same width, so the xylem and phloem line up; they are only around the outer part of a stem and need to have contact in order to heal together.

While there's several techniques for joining scion to rootstock, for my initial trial, I was entranced by the idea of growing two varieties of tomatoes off of one plant. That meant inserting the main stem of the scion into the rootstock main leader (but not cutting off the rootstock leader), with the goal of the scion eventually becoming another leader. It's a more complicated graft for a beginner to do successfully than something like tube grafting (I think), so of course I pick this as my first attempt in which I have one try to get it right. However, the technique, called "approach grafting", has some built-in insurance: when slicing the two stems together, all roots are left intact until the graft "takes" and new growth forms. Here's a diagram (from ASGAP). In order to keep 2 leaders - one rootstock, one scion - I would eliminate "Cut 1".

I planted two varieties close to each other in the same pot: Stupice, a nice paste heirloom, and better boy, a hybrid. The plan was to insert the main stem of the Stupice into the main stem of the Better Boy, since Better Boy has the VFN resistance.

Here's where it got fun:
1) While I was away at a conference, the Stupice significantly outgrew the Better Boy. The plants were already planted next to each other, but the stem diameters no longer matched. Brilliant.

Plan change:
refer to the figure to the right (from Britannica Online, cleft grafting): can't do approach graft (method 2, although I didn't know about method 1 at the time); I have lop off the Better Boy stem, slice it into a wedge shape, and stick it into the Stupice stem (method 5). I didn't know it at the time, but this is what Britannica Online calls "side cleft" graft and what a TAMU workshop calls "side" graft.

2) Everything I read emphasized the absolute need for same stem diameter. Not only does this affect the height at which you insert the scion, but quite a large slice into the rootstock stem is required to accommodate the scion stem if they are the same diameter. So I ended up slicing through most of the rootstock stem. Actually, I had to cut a wedge out of the rootstock with the same angles as the scion wedge. I found out it's really hard to control a razor blade in soft plant tissue. I cut really deep.
If only I knew: according to TAMU, side grafts are recommended when the stems aren't the same diameter. Makes sense now.... if only the TAMU site showed up in regular google search results, not just image results...

But, I managed to get good contact between scion and rootstock. I secured the graft with a clothespin - it was the right size and tension (not too tight) and, most importantly, I had it on hand - and used kabob sticks as support stakes taped around the plant pot (you can see them in the last couple of pictures) to keep any cover off the plant itself. The next step was to let the plant heal by providing an atmosphere where it has to do as little work (transpiration) as possible: somewhere dark, warm, and humid.

Thus, my first healing chamber: the plant on a flooded seed tray with a black plastic garbage bag around it and a seedling heat mat under it. I put the plant pot inside another pot so it sat above the standing water in the seed tray. The support stakes kept the garbage bag away the plant, at least mostly. The heat mat heated the chamber enough to create a very humid, warm atmosphere. I opened the bag to vent the chamber a couple of times a day. It was closed pretty tightly by cinching the bag and clipping it with a binder clip.

Upon flashlight examination over the subsequent week, the graft never wilted too much and was looking good. However, the clothespin covered the graft union so I could never see if it remained in good contact,etc. The chamber remained at a steady 80F and very wet - it was pretty much my very own bedroom rainforest. I really didn't know how long to keep the plant in the chamber, so after 7 days - the time recommended for smaller grafts in online guides - I began leaving the bag slightly open for an hour or so at a time. But each time, the scion wilted a bit, so I thought maybe the graft wasn't ready for the real world yet. I started to notice some edema (swelling of cells) on the rootstock leaves after about a week, which was probably a sign of too high humidity and temperature (but which the online guides said would be temporary). Since the scion was still wilting when I reduced the temperature and humidity, I felt I couldn't move the plant out of the healing chamber quite yet. Maybe the molding wooden support stakes should have been another hint I created a little tropical rainforest for my tomato, and what I saw next was bound to happen:

Adventitious roots! No! Here, you can see that the scion put out its own roots (right above the clothespin). And why not? There's almost 100% relative humidity, quite confusing. (A few times, the garbage bag would get so weighed down with clinging moisture inside that the support stakes would poke through, and the heavy bag would be resting on the plant. Also bad.)

So after 14 days, I decided some tough love was in order; the plant would die if I didn't give it some light. Here's my 2nd phase healing chamber:

This is in a relatively low-light window. I poked a few small holes in the ziploc bag, and spritzed the inside with a spray bottle of water twice a day. As long as it stayed wet, the scion looked perky!

Finally a week later - it had now been 3 weeks - I loosened the ziploc and set it in a window with more light, and the scion promptly wilted past the point of no return. Yep, after all that - done in a day or two. But, at least I could now remove the clothespin and look at the graft union. What I found was that there was no union. The scion just had adventitious roots, and the rootstock cut had callused over so it could never heal back together. I think the first healing chamber was a little too hot, causing enough sap production in the wounds to force apart the scion from the rootstock at the graft union. The Stupice was still alive, barely (what a trooper), so I called this experiment over and made it a stem splint with part of a straw.

The Stupice is hanging on, but with the vigor of a guy long overdue for a triple bypass. No surprise since I cut almost all the way through the stem - it's running on probably 1/4 of it's xylem/phloem channels. It's pretty pathetic. It keeps growing straight up, with a few little leaves, but has a fruit forming now! The mature tomato will totally break the stem, but I'm going to let it go because hey, who am I to alter nature ;)

Lessons Learned:

1)if trying an approach graft again, plant the two together in the same pot at grafting time, not before, to assure same stem diameter.

2) chill out on the healing chamber conditions. Less than 80F, definitely less than 100% relative humidity. Wondering if maybe it doesn't have to be completely dark; not every guide I've read now recommends total darkness (incl. TAMU, which recommends something extremely similar to my 2nd phase chamber).

3)I need a sturdier structure for the chamber, so it won't touch the plant. While I think it was the sap production that pulled the graft apart, any plant movement might disturb the graft union; an evaporation-laden plastic bag wants to fall on the plant no matter how much I don't want it to.

4)I should try a simpler grafting technique (tube grafting?):
a) so I can claim a success
b) the clothespin was heavy (did it tug on the graft?) and blocked view of the graft union. Tube grafting uses a clear tube which holds the stems together firmly.

Round 2 recap coming soon...

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