Fascinating plant hormones

Afew weeks back, in episode three of Beechgrove Garden, I had a wee chat in the glasshouse about seedlings.

More specifically, I talked about why we need to rotate seedlings and why we pinch out certain plants, like sweet peas.

This little segment seems to have touched a nerve with people so I felt compelled to carry on the chat here, and further explore the theme of plant hormones.

Best of all, I’m not having to spend 10 hours in hair and makeup to look human before chatting about it!

Only kidding – I imagine I’d need much longer than that!

The “classic five” hormones every gardener should know about are auxins, cytokinins, gibberellins, ethylene and abscisic acid.

Sounds confusing and scary – but, trust me, they aren’t as intimidating as they sound. Learning about them will change the way you look at plants forever.

It might not be necessary to learn them all off by heart for your wee postage stamp in Buckie, but you could learn something here that changes the way you think – and that’s enough for me.

AUXIN

Auxin was the main hormone I talked about out in the aforementioned segment on Beechgrove Garden.

Phototropism is the phenomenon of plants leaning towards the light. And the reason for phototropism is because of auxin.

Auxin is more prevalent on the dark side of plants. Here it causes faster growth through cell elongation – therefore the dark side of the plant grows faster than the side facing the light.

This means it leans forward. We periodically rotate our seedlings or houseplants in the window to even out the balance and ensure we have straight, upright plants rather than weak, heavily leaning ones.

Auxin is also responsible for apical dominance – or upward growth over outward growth.

The top of a plant stem is called the apical meristem and is full of auxin.

When we pinch out the growing tip, we cut off the supply of auxin. In doing so, we end its dominance, allowing other hormones to come to the fore.

CYTOKININ

Cytokinin comes into play once the overwhelming presence of auxin is reduced and lateral growth – outward growth and branching – occurs.

This is why we pinch out sweet peas once they reach 15cm (six inches) tall.

By pinching out the apical meristem to just above a leaf junction, we cut off the auxin supply – allowing cytokinins to come to the fore and cause the sweet peas to produce lateral branches.

This means we end up with much stronger, bushier plants, which will flower far more.

GIBBERELLIN

Plants have a lot to thank gibberellins for. They appear, on the surface, to have characteristics of all four other plant hormones – a jack of all trades, if you will.

Gibberellins cause cells to elongate in general between stem cells. They play a role in fruit and flower development.

Most notably, they cause seed germination.

ETHYLENE

Bananas are a famous example as a producer of ethylene. We’re advised to pop unripe fruit next to a banana to get it to ripen. This is true.

Bananas will produce ethylene and, interestingly, ethylene begets ethylene – meaning the more there is, the more it causes others to produce.

This is why people also say not to put bananas in the fruit bowl. They cause other fruit to ripen rapidly and it might spoil before you get to use it.

ABSCISIC ACID

This hormone helps plants deal with environmental stressors like drought, heat stress and salinity in soil.

Most famously, it causes seed and bud dormancy.

Germination via gibberellins, growth and elongation via auxin, lateral growth via cytokinins, fruit ripening via ethylene and dormancy via abscisic acid.

Plant hormones play a massive part in all crucial stages within plants, from seed to death.

Take care and happy gardening.

KNOW YOUR STUFF: The main hormone groups in plants can have a big effect in the behaviour of organic matter. Bananas, for example, can spoil other fruit too soon, due to their ethylene.