When 5, which is 6 becomes 8

I have a wisteria that grows and grows, but where it goes nobody knows. I train it and shape it to follow a plan but it still it grows to follow its nose… and I don’t know why.

As a self-respecting arborist, I dislike vines – with the exception of grapevines and hops, but I like them for their fruit more so that their vegetative state.

That said, wisteria only just makes it onto the arborist vine hate list because it doesn’t have tendrils, hooks, aerial rootlets or adhesive pads. It climbs by wrapping its branches around whatever support it can find. New growth kind of wanders off into space and flails about searching for something to hold onto (a bit like someone that has enjoyed the fruit of grapes and hops a little too much…). As the new growth extends, it makes sweeping, rotating movements called circumnutation until it finds something to hold onto. Once a bit of new shoot comes into contact with something solid, contact-sensitive hormones are triggered in the outer layer of tissue (epidermal cells), growth extends on the opposite side of the shoot causing the shoot to wrap itself around the thing of support; this is called thigmotropic movement. The shoot then lignifies and stiffens to prevent unwinding and the growing tip continues to wander about some more. Apart from the ancient Greek, the process of climbing is pretty cool.

The process of beginning the climb is also very cool. When on the ground, a vine will grow away from the light – it will grow into the darkness to find something to grow up. Once it has gained purchase, it will start to grow towards the light. The process of growth towards and away from light is called phototropism, and is controlled by hormones (auxins). Positive and negative phototropism, the same process but with different response mechanisms causing growth in a different direction – very efficient and again very cool.

But don’t for a moment think that I’m beginning to like wisteria – respect maybe, but not like. My wisteria has issues, my wisteria won’t grow backwards; ‘I train it and shape it to follow a plan but it still it grows to follow its nose’. There seems to be something in it, that prevents it from growing back towards its source.

Picture this; my wisteria grows up a wall to a given height, and then is attached to wires running left and right – kind of like a capital ‘T’. It happily grows left and right away from the trunk (its source) and has done so for several years. In places it has become thin, so I’ve selected branches and turned then back so they grow towards the trunk. But the new growth from those branches sprouts and then grows away from the trunk… I cut it and bend it and hurt it so, I force it there but it does not care – it just grows to follow its nose… and I don’t know why.

I don’t think it is mocking me with geotropism (growth in response to gravity) – and yes, I am sure that it’s mocking me; vines do that! In botany, we normally associate geotropism with downward growth in roots – but with an arboricultural bent (quite literally) response growth on one side of a trunk or on branches is also managed in part by geotropism. Starch-filled amyloplastic organelles called statoliths respond to repeated movement (shaken gravity – if you will). Amyloplasts in the cells stimulate growth to firm up the area – to make the movement stop. But I don’t think geotropism or phototropism is the reason why my wisteria cannot grow backwards.

I pondered chemotropism (response to particular substances) for a while. To be honest, I didn’t actually know that chemotropism was a thing. I was aware that roots respond either positively or negatively to the presence of certain chemicals and/or electrical charges in the soil. I was also aware that pollination in part is controlled by chemical acceptance or rejection (i.e. the stigma accepting the correct pollen grain and allowing it to germinate) but I didn’t know there was a tropism name for it. But I don’t think chemotropism is at play – I think it’s something else.

Back in the day there were five classes of plant hormones the controlled growth, development and responses to the environment. Then there became a sixth class; Brassinosteroids (BRs). Not happy with six, a seventh class (Jasmonates or JAs) and then an eighth class of plant hormones has been identified; Strigolactones.

The answer to my question seems to lie in the works of stri-go-lactones. At this stage, my brain started to hurt. I had only just discovered that chemotropism was a thing, then I discover that 5 plant hormones were really 6 but are actually 8. Strigolactones seem to be responsible for stimulating branching (among other things) and may explain why my wisteria will not grow backwards. It is a hormone thing, and that’s good enough for me.

I have a wisteria that grows and grows, and where it goes strigolactones know…

• Written for the ARB Magazine (UK)