Chapter 7: Control of Growth and Development

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Have you ever, you know, looked at a house plant sort of reaching for a window, or maybe wondered how a tree just knows it's time to shed its leaves right when autumn arrives?

Plants, they aren't just passive green things a row, they're actually orchestrating this like intricate internal symphony of growth and movement and adaptation.

Today on the Deep Dive, we're going to unlock the hidden language that makes it all happen.

Our guide for this Deep Dive comes from Brian Capon's Botany for Gardeners, third edition.

And look, this isn't just dry academic theory, it's the profound science behind what you actually observe in your garden every day.

It doesn't matter if you're a seasoned farmer or, you know, just looking after a windowsill basil plant, we're moving beyond just what plants do to really understanding how they pull off these remarkable feats right down to their molecular whispers.

Exactly.

And understanding these internal mechanisms, well, it'll give you a much deeper appreciation for the complex, you could almost say, intelligence of plants.

Their incredible resilience, their adaptability, will show you how these scientific principles connect directly to, you know, the things gardeners see every day.

Okay, let's start with those hidden maestros then.

Plant hormones.

For a long time, scientists speculated that plants, well, much like animals, they must produce some special substances to regulate their systems.

What they found, though, was maybe even more fascinating.

Yeah, what's really different about plant hormones compared to animal hormones is where they're made and also their chemical structure.

Think about human hormones, like, say, insulin, right?

Produced in specialized glands.

Right, the pancreas.

Exactly.

Plants don't have glands like that.

Instead, they synthesize their hormones right there in the cells of their general organs, you know, stems, leaves, roots, even flowers.

We've studied five principal ones pretty extensively, but honestly, the plant kingdom is still revealing new chemical messengers.

It's like, imagine plants have their own really sophisticated internal messaging system.

They're sending these chemical signals from one part to another, telling cells precisely when to grow or when to stop, maybe even when to change direction.

Right, and one of the most fundamental things any gardener notices is how plants react to light.

You've definitely seen it.

That striking difference between a robust plant and full sun, short, thick stem leaves close together versus one grown in deep shade or even darkness.

Those shade -grown plants, they often look really stretched out, pale, kind of spindly.

That's called etiolation.

Exactly.

Etiolation.

That's spindly appearance.

That's the plant desperately stretching for any available light.

And the key player here is auxin.

That was actually the very first plant hormone discovered.

Auxin is primarily responsible for phototropism, that classic bending towards a light source we all see.

And the real genius here isn't just that plants bend, it's how they do it.

It's the silent

internal chemical ballet.

So when light hits one side of a stem, auxin actually migrates over to the shaded side.

Oh, interesting.

It moves away from the light.

It moves to the shade, and this buildup causes the cells on that shaded side to elongate, to stretch out faster than the cells on the lit side.

And that literally pushes the stem, making it bend towards the light.

Wow.

So yeah, next time your indoor plant is leaning towards the window, you'll know it's auxin doing its job.

It's incredible how such a simple chemical can orchestrate such a, such a precise visual effect.

Now you mentioned other hormones.

Is there another one involved in this light response?

Yes, absolutely.

Gibberellin.

This hormone, it primarily promotes internode growth.

That's the stretching of the stem sections between the places where leaves attach.

Okay.

The gaps between leaves.

Right.

So in full sun, gibberellin's effect is somewhat held back.

It helps maintain that sturdy sort of squat form.

But in low light, uh -oh, it becomes much more active.

It causes those internodes to stretch dramatically.

Ah, so that pushes the top leaves higher.

Exactly.

To help them reach for sunlight, especially if there's competition from other plants around.

And here's a fascinating little twist on that.

Some tropical rainforest vines, they actually do the opposite.

They bend away from bright light.

Away?

Why?

Towards the dark base of a nearby tree trunk.

This helps them find something to grab onto, some support, so they can climb upwards until they eventually burst out into the canopy above.

That's clever.

Adapting defines the support first.

Okay, so beyond light, plants also navigate their world by responding to Earth's gravity.

That's geotropism, right, or gravitropism.

Both terms are used, yeah.

Geotropism or gravitropism.

It's just remarkable how they manage this without, you know, eyes or any obvious sensory organs we'd recognize.

It really is.

And they have opposing responses.

Most roots are positively geotropic.

Meaning they grow down.

Downwards, exactly.

While stems are negatively geotropic.

They grow up.

Away from gravity.

Right.

And this basic orientation ensures that no matter how a seed lands in the soil upside down, sideways, whatever its root will reliably head down and its shoot will head up.

Imagine how impossible gardening would be if you had to plant every single seed perfectly oriented.

Yeah, that would be a nightmare.

So is oxen involved here too?

Oh yes, oxen plays a crucial role here too.

It's quite clever how it works differently in roots.

So if you lay a stem horizontally, oxen collects on the lower side due to gravity.

And in stems, more oxen means faster cell elongation.

So the lower side grows faster, causing the stem to curve upward.

Makes sense.

Negative geotropism.

But in roots, here's the twist, oxen also collects on the lower side.

But in roots, high concentrations of oxen actually inhibit or slow down cell elongation.

Wait, so high oxen stops growth in roots?

It slows it down significantly.

So the cells on the upper side of the horizontal root, which have less oxen, actually elongate more rapidly.

Ah, pushing the root tip downwards.

Precisely.

Positive geotropism.

It's a really elegant differential response using the same hormone.

That is quite sophisticated for something without, well, without a brain.

And beyond gravity, plants also respond to touch, right?

Degmotropism.

I always think of tendrils on vines.

Exactly.

That's the classic example.

Those slender tendrils coiling around a trellis or another plant for support.

It looks almost like a plant hand reaching out and grasping.

So how did that work?

Well, that coiling happens because the cells on the side of the tendril not touching the support, the outside of the curve, they grow faster than the cells that are making contact.

Again, differential growth.

Is oxen involved?

It's presumed to be involved, yes.

But exactly how that slight pressure triggers the specific hormone movement needed for coiling.

Well, that remains, you know, yet another one of those not quite solved botanical puzzles concerning tropisms.

Fascinating.

Okay.

So those are growth movements towards or away from things.

But plants do other movements too, right?

Like flowers opening and closing.

Or that sensitive plant.

Mimosa.

Yes.

Those are called gnastic movements.

Unlike tropisms, they aren't growth responses oriented by the stimulus direction.

Think of flowers opening in the morning light and closing at night.

Or like you said, the mimosa putica, the sensitive plant.

Yeah, that one's amazing.

Touch it and the lithos just fold up instantly.

Right.

That rapid movement is a seismonastic response triggered by touch or vibration.

It's not growth, but a change in water pressure within specific cells.

It's like the plant is flinching.

Wow.

Okay.

So let's shift gears a bit.

Talk about the cycle of life within plants.

Things like aging,

senescence, and shedding parts like leaves or fruits, which is abscission.

Right.

These aren't just passive decay.

They are highly controlled processes.

It's like plants have an internal clock or maybe an internal program that tells them when it's time to let go or ripen.

So how does a plant orchestrate something as dramatic as say all the leaves falling in autumn or a tomato ripening on the vine?

Hormones again.

Hormones again, precisely.

It's all about a delicate hormonal balance.

You have hormones like oxen, gibberellin, and another one called cytokinin, which actively inhibit aging.

They help maintain cell function, keep things running.

Cytokinin, for example, is really important for promoting cell division, keeping tissues young.

Okay.

So those are the stay young hormones.

Kind of, yeah.

And then on the other side, you have ethylene, which is actually a gas and abscisic acid.

These hormones promote the aging process, senescence, and abscission.

The balance between these two sets, the inhibitors and the promoters, it shifts based on environmental cues like changing temperatures or day length and also internal biochemical signals.

Let's talk about ethylene.

That's the ripening one, right?

Yes.

Ethylene is famous for ripening fruits.

It triggers a whole cascade of biochemical events.

The green chlorophyll breaks down, revealing those vibrant reds, oranges, yellows, protective compounds like tannins decrease, sugars increase.

Making it sweet.

Exactly.

And the structure changes drastically too.

Membranes break down, cell walls soften,

making the fruit appealing for animals to eat and disperse the seeds.

But of course, that same process means that once it's fully ripe, it also deteriorates pretty quickly.

And this is where it gets really useful for you at home.

Understanding ethylene.

Absolutely.

This knowledge is used commercially all the time.

Fruits like bananas are often picked green for shipping, then exposed to ethylene gas just before they hit the stores to ripen them up quickly.

And for your own kitchen, here's a neat trick.

You can speed up the ripening of, say, unripe avocados or tomatoes by putting them in a paper bag with a few pieces of coarsely chopped apple.

An apple.

Why an apple?

Because injured apple tissues, when you chop them, release ethylene gas.

Trapped in the bag, that ethylene speeds up the ripening of the other fruit.

It's the reason why sometimes one ripening apple in a fruit bowl can make everything else ripen faster, too.

Ah.

I knew there was something to that.

Okay, that's a great practical tip.

Yeah.

It's a simple trick, yeah, but rooted in that complex biochemistry.

And the same kind of precise hormonal control applies to leaf -obscission -leaf fall.

Before leaf drops in autumn, a couple things happen.

Chlorophyll breaks down, revealing those autumn colors.

Importantly, cell walls start to weaken in a very specific narrow band of cells right at the base of the leaf stalk, the pedial.

This band is called the obsession zone.

Okay.

Now, during spring and summer, oxygen produced in the leaf itself flows down the pedial and keeps that obsession zone strong and intact.

So oxygen prevents the leaf from falling off.

Basically, yes.

But in autumn, when temperatures drop and days get shorter, oxygen production in the leaf decreases.

At the same time, ethylene production increases in the obsession zone.

This shift, less oxygen, more ethylene stimulates enzymes that break down the cell walls in that zone.

Eventually, the leaf just separates cleanly from the stem.

Wow.

That's incredibly precise.

And this isn't just for deciduous trees.

No, it applies to the time drop of fruits and flowers, too, and even the sequential shedding of older leaves in evergreen plants.

It's a fundamental process.

And again, we've learned to manipulate this.

You mentioned commercial fruit growing.

Right.

Sometimes, growers spray synthetic oxygen on orchards before harvest to prevent premature fruit drop.

This keeps more fruit on the tree until picking time, which obviously increases the yield.

Fascinating how we leverage this internal plant chemistry.

Okay.

Let's connect this biology directly to a very common gardening task,

pruning.

You know, that advice to snip the tips of stems to encourage bushier growth.

That's all about something called apical dominance, isn't it?

Exactly right.

Apical dominance is the term for the physiological process, where the plant's apical bud, the bud right at the very tip of the main stem, actually suppresses the growth of the axillary buds lower down.

Axillary buds being the ones that could become side branches.

Precisely.

Especially the buds higher up the stem, closer to the apical bud.

So when you prune, what are you actually doing hormonally?

You're directly manipulating the plant's internal messaging system.

That apical bud produces oxygen, and it's this oxygen flowing down the stem that exerts the inhibitory effect on those axillary buds.

When you trim off that apical bud, you remove the primary source of that inhibitory oxygen, and that allows the axillary bugs, now freed from suppression, to start developing into side branches.

Buds further down the stem are naturally less affected anyway because the oxygen concentration decreases the further it travels from the tip.

So it's not magic when you pinch a plant to make it fuller.

It's just smart chemistry management.

Pretty much, yeah.

You're just changing the hormonal signals.

Okay.

What about propagating plants?

Making new ones from cuttings?

That relies on adventitious root formation, right?

Roots growing from stems or leaves where they normally wouldn't.

Yes.

And horticulturists know very well that different species have vastly different abilities to form these adventitious roots.

Some plants, you stick a cutting in water or soil, and they naturally produce enough oxygen in the cutting itself to stimulate root development quite easily.

Like willows, maybe.

They root easily.

Willows are a classic example, yes.

But many others, they struggle.

They don't produce enough oxygen on their own, and that's where rooting compounds or rooting hormones come in.

Those powders or gels you dip cuttings into, they're essentially preparations of synthetic oxygen.

Giving the cutting the hormonal boost it needs to make roots.

Exactly.

There's also a technique called layering where you encourage roots to form on a branch while it's still attached to the parent plant, often by wounding the stem slightly and keeping it moist, maybe wrapping it in moss.

Oxen accumulating near the wound plays a key role there, too.

So understanding oxen explains why rooting hormones work and why pinching back plants makes them bushier.

It all connects.

It really does.

You're essentially intervening in the plant's natural hormonal balance to achieve desired gardening outcome.

It's truly remarkable how what started as purely academic curiosity, just searching for these growth regulators, has led to us being able to rewrite nature's script in agriculture and horticulture.

Absolutely.

The applications are widespread.

We now have a whole range of commercially applied synthetic growth regulators.

For example, defoliants.

These are chemicals used to promote leaf abscission.

Cotton growers might use them to make the leaves drop before mechanical harvesting, which makes the process cleaner and easier.

Okay.

Then there are disbutters.

These might be sprayed on certain ornamental trees or shrubs to make the flower buds fall off, preventing the development of unwanted or messy fruits later on.

Right, like on some street trees, maybe?

Could be.

And a really big one in the ornamental plant industry is growth retardants.

These chemicals work by inhibiting the action of gibberellin, the hormone that causes stems to elongate.

Ah, so they keep plants short.

Exactly.

They produce those compact, dwarfed, potted plants like chrysanthemums and poinsettias that are popular.

They look fuller, are easier to ship, and fit better on window sills.

Makes sense commercially.

And of course, we have to mention obicides.

Many weed killers are synthetic growth regulators.

Some, like the famous 2 ,4 -D, are selective.

They mimic auxin, but at concentrations that are toxic primarily to broad plants, leaving grasses relatively unharmed.

Others are non -selective, killing most plants they contact.

It's incredible.

This whole field, built on understanding these tiny chemical messengers, has grown into a massive multi -million -dollar industry.

It really has.

From pure science to major economic impact.

Okay, so we've talked about the hormones and how they work.

But what controls the hormones?

You mentioned environmental cues.

Let's talk about the ultimate conductors of hormonal orchestra.

Environmental timers.

The sophisticated control is exerted by hormones.

They're often regulated by seasonal changes, right?

Especially temperature and day length.

Yes, that's key.

Plants aren't just reacting passively to the present.

They're often anticipating future conditions.

Preparing for what's coming.

Anticipating.

How so?

Well, take temperature.

Many dormant winter buds, particularly in temperate climates, actually require a period of cold temperatures, a chilling requirement, before they can break dormancy and grow in the spring, even if conditions become warm.

So they won't sprout during a warm spell in January.

Exactly.

That chilling period, usually weeks or months below a certain temperature, is thought to stimulate the synthesis of a hormone, quite possibly gibberellin, that's needed for growth to resume later.

This is why certain apple varieties, for example, need maybe 1 ,000 to 1 ,400 hours below about 45 degrees Fahrenheit or 7 Celsius.

If they don't get that chilling, they won't flower or fruit properly.

It limits where they can be grown.

Ah, okay.

So that explains why you can't grow some northern plants in really warm climates.

Precisely.

And related to this is vernalization.

This is a specific process where flowering itself is induced by a cold treatment.

Often the plant, or sometimes even the seed, needs several weeks of cold near freezing temperatures to trigger flowering later on, usually in the spring.

What kind of plants do this?

Classic examples are winter varieties of grains like rye and wheat.

They're planted in autumn, overwinter as small plants, and then bolt and flower in spring.

Also many biennial plants like cabbage, carrots, beets.

They grow vegetatively the first year, need the cold of winter, and then flower in their second year.

And gibberellin is involved here too.

It seems very likely if you take an unvernalized plant one that hasn't had the cold treatment and treat it with gibberellin, often it will bolt and flower.

So the cold probably triggers gibberellin production.

And what about bulbs, like tulips?

Bulbs too.

Many spring flowering bulbs, like tulips and hyacinths, need a significant period of cold while they are dormant to complete flower development inside the bulb.

Tulips might need, say, 13 or 14 weeks at around 50 degrees Fahrenheit, 10 Celsius.

Which is why people in warmer climates often have to pre -chill their tulip bulbs in the refrigerator before planting.

Exactly that.

They're artificially providing the required cold period.

Okay, so temperature is a major timer.

What about the other big one, day length?

Photoperiodism.

Right, photoperiodism.

This is the plant's response to the changing lengths of day and night throughout the year.

It's most famously known for triggering flowering in many species.

And here's that mind -bending fact you hinted at earlier.

Plants are actually measuring the night, not the day.

Isn't that wild?

Yeah.

Despite the term photoperiod and talking about long days or short days, experiments clearly show that it's the length of the uninterrupted dark period that's critical for many plants.

So if you interrupt a long night with just a flash of light?

For a short day plant, that flash can prevent it from flowering because it perceives the night as being broken into two short periods.

For a long day plant needing short nights, that same flash during a long night can actually induce flowering.

Wow.

Okay, so how are plants categorized based on this?

We generally group them into three main categories.

But remember, short and long day are defined relative to each species'

specific critical photoperiod.

It's not one length for all plants.

Right.

It's relative to their own internal threshold.

Exactly.

So short day plants flower only when the day length is shorter than their critical photoperiod, meaning the night is longer than its critical length.

Think poinsettias, chrysanthemums, soybeans.

They typically flower in late summer or fall.

Long day plants flower only when the day length is longer than their critical photoperiod, meaning the night is shorter.

Examples include spinach, dill, lettuce, iris.

They usually flower in spring or early summer.

And then there are day -neutral plants.

These guys flower after they reach a certain stage of maturity or size, pretty much regardless of the day length.

Corn, tomatoes, cucumbers, sunflowers, roses are often day -neutral.

And it's not just about the light trigger, is it?

A plant has to be ready to flower first.

Absolutely.

There's a concept called ripeness to flower.

The plant needs to achieve a certain minimum vegetative size, build up enough food reserves.

It has to be physiologically mature before it can respond to the photoperiodic signal, even if the day length is correct.

A tiny seedling usually won't flower no matter the light conditions.

Makes sense.

Now, if photoperiodism triggers flowering, is there a specific flowering hormone?

The hunt for fluorogen.

This has been a long quest in plant physiology.

Scientists hypothesized a specific flowering hormone, fluorogen, decades ago.

The evidence suggests it exists, for instance.

You can graft a leaf from a plant that has received the correct photoperiod onto one that hasn't, and sometimes the uninduced plant will then flower.

This shows something is moving from the induced leaf.

A signal molecule.

Yes.

But fluorogen has proven incredibly elusive to isolate and identify.

It seems to be effective in extremely low concentrations.

The current thinking is that it might not be a single unique molecule, but perhaps a specific combination or ratio of some of the already known hormones may be interacting with other signaling molecules, like small proteins or RNAs.

It's still an active area of research.

Still some mystery there, but we definitely know how to use photoperiodism

Oh,

absolutely.

Fluoroculturists manipulate photoperiod all the time.

To get chrysanthemums, which are short -day plants, to bloom year -round, they use artificial lighting to extend the day length during natural short days to keep them vegetative, and then use heavy blackout cloths to create artificially long nights when they want them to flower, even in the middle of summer.

It's like creating artificial seasons in the greenhouse.

That's exactly what it is.

It allows for year -round production of plants that would only naturally bloom during specific seasons.

It really highlights how profound photoperiodism is.

It's not just about flowering.

It connects the plant's present condition, the current day length, with predictable future conditions, like the coming winter, or maybe a summer drought.

Precisely.

It ensures that crucial reproductive processes, like flowering and seed development, are initiated early enough so that seeds can mature and disperse before survival becomes difficult or impossible.

It's like plants are not only measuring the passing hours, but they're actually using that information to anticipate and prepare for the future.

Amazing.

So, wrapping this all up,

what does this journey through plant hormones and growth regulation mean for you, the listener?

We've seen how these intricate internal systems work, how seemingly simple things like a plant bending to light or pruning a bush or even putting an apple in a fruit bag are all rooted in these really elegant and complex biological processes.

Yeah, I think it really reinforces the big picture idea.

Plants are incredibly sophisticated organisms.

They're constantly sensing their environment, communicating internally with these hormonal signals, adapting and precisely controlling their growth and development, and understanding these underlying botanical principles.

Well, it doesn't just deepen your scientific knowledge.

It really makes you a more insightful, I think a more effective and a more appreciative gardener.

Absolutely.

We really hope this deep dive into botany for gardeners has given you a whole new appreciation for the hidden life of plants.

And maybe a final thought to leave you with.

Next time you see that house plant bending towards the sun or a tree shedding its leaves in autumn, just take a moment.

Consider that silent, intricate hormonal orchestra working tirelessly just beneath the surface,

constantly conducting and orchestrating every single aspect of that plant's life, always preparing it for whatever comes next.

That's a perfect image, a silent orchestra.

We'll keep exploring the natural world around you, and hopefully you'll see it all with new, more informed eyes now.

Thanks so much for joining us for this deep dive.