Into the rabbit hole

I love the fact that trees don’t read books and that the laws of nature don’t always seem to apply to nature. I love this, and to serve as a constant reminder I have my very own illiterate disobedient tree in my backyard. It is a cutting from an old apple tree. The parent tree is so old that nobody is quite sure what it is – it comes from a time when fruit trees had great names; Peasgood-Nonsuch and Broxwood Foxwhelp. Several years ago, we took five cuttings from the parent tree and I have one of these bound and lashed to my side fence; espalier – oh the indignity.

What makes my tree the ultimate non-conformer is that it is completely out of sync with the parent plant and its four sisters – it has become the black sheep of the genetically identical family. It retains its leaves for months longer than the rest of them – it has just gone past the shortest day, and it still has leaves on. Positioning and situational variance could account for some of that, but the other sisters and the mother plant all shed their leaves and break their buds as if they were still one – which in a way they are, even though they grow in different places miles apart.

Clearly something bad must have happened to my apple tree, some trauma, some life changing event; maybe my tree had gotten in with the wrong crowd? Maybe my tree had been switched at grafting…

After an entire minute of consideration, maybe two but I doubt three (it is just an apple tree after all), I decided that my genetically identical family was not actually genetically identical, and the answer was the root-stock. We took cuttings from the parent tree and had them grafted onto root-stock. Obviously, there was something about the root-stock used on my tree that was different from all the rest. Problem solved. Almost.

I’m not sure when or even why I started thinking about this again, but I did and the more I did, the less logical my solution seemed; what have roots got to do with leaf senescence? How can a root-stock, which has been ‘blinded’ [as had all of its buds cut out], which holds virtually no photosynthetic tissue and by default mainly exists under the ground, influence when the leaves fall off the tree? Problem not solved.

The answer turns out to be hormones and in particular abscisic acid and cytokinin. These particular hormones (and we are talking hormones in the widest sense) just happen to be produced in the roots.

We used to think abscisic acid (ABA) controlled abscission – after all that is how it got its name. But the current school of thought is that ABA has more to do with environmental protection and dormancy than the shedding of parts (abscission). Cytokinin controls a bunch of things including senescence (biological ageing). Cytokinin moves from roots to leaves, and if it is produced in high enough qualities can contribute to the delay of senescence. When the translocation of ABA from roots to shoots is blocked or slowed, the onset of leaf senescence can be retarded. Put these together and we get a tree that retains its leaves.

Positioning and situational/environmental variance influence the production of ABA – cytokinin stops the leaves from growing old – the longer the tree maintains leaves the longer those leaves can produce energy. Problem solved.

But, now I have a new problem. Why? Why does my plant retain its leaves? Is it under so much environmental pressure that it is intentionally retaining photosynthetic tissue in a desperate attempt to survive, or is it seated in some kind of environmental wonderland where it has no need to shed photosynthetic tissue because it can grow for as long as it wants to?

Why does the answer to every question produce more questions? Where will this rabbit hole go, and do I want to go there?