Plant Growth Regulators: What are Plant Hormones [Horticulture 101 Series]

so far this semester we've learned a lot of different ways to help plants grow we know that the duration of light or the quality of light uh the media that we use uh the nutrients that we add can all affect how a plant is able to grow and develop uh plant growth Regulators sometimes called pgrs are just one more way that we as horticulturalists can use our understanding of plant anatomy and physiology to help plants develop in ways that we want um unlike nutrients that are coming from the outside plant hormones phyto hormones or pgrs

plant growth Regulators um can be made by plants themselves in different situations and in different parts of the plant for example oxen is always present in the very tip the growing merism of plants which is why we can remove it by pinching back plants we also have synthetic pgrs um we have things that we can spray on Plants um that mimic different plant growth Regulators or these hormones that plants create um as well as mask them so that they aren't um able to do uh or send the signal to do uh what plants um you

know what that hormone controls so these pgrs U we can use to start growth stop growth modify the bushiness of a plant or the lateral growth of a plant we can break dormy or make seeds more D dorment um there's five known plant hormones that are typical there there's more than that that are known but these are the five that they're typically used in the Horticultural industry I will say that um there are some that are used a lot more than others for example rooting hormone uh contains plant growth hormones and that's something that we

typically do use for um uh cuting whereas we might use cyto Canin only in tissue culture uh and and micropropagation uh ethylene gas is typically used post Harvest so uh it has to do with the ripening and maturity of fruit uh whereas we might see obic acid used primarily in seed um seed production and Seed treatments um so these hormones while they're naturally present in plants at different times um they are manipulated in our industry in different ways oxin which are contained in the tip of the plant are associated with stem elongation this is what

tells a plant um hey you need to grow as tall as you can as fast as you can so that you can crowd out the other plants so that you can get enough light that's actually kind of a anthropomorphized view of it I guess so plants aren't necessarily competing consciously with those plants around them but if you're the tallest plant chances are you'll get the most Sun so oxin control what we call apical dominance that apical meisten that tip of the the growing CH here uh to grow upwards it's produced in the tips of the

plant and it can U migrate it can go from cell to cell to cell um when we pinch back the top of that we get rid of that uh need for a plant to grow uh as tall as we can and instead and other hormones cyto canines that are are produced in the roots tell the plant to uh to grow bushy to grow lateral branches instead of um the apical one another cool thing that um oxen is responsible for is phototropism in our experiments with photosynthesis we saw how plants will bend towards the light this

happens because as that oxin is sent cell to cell to cell it's telling each of those cells uh to elongate remember to grow up um but if one side gets more of that than the other then they're not going to grow at the same rate so if this side grows faster than this side right it's going to end up curving we see this because um that that oxen is actually uh photophobic meaning it moves away from the light technically and so there's more oxin on the side that's shaded and therefore the Shady Side grows longer

and the side with oxin um without oxin um or with less oxin doesn't grow as long so we see that bending this was actually an interesting experiment that Darwin did so if you pinch off the top of your plant you should see its response to light delayed an interesting experiment that um you won't have to do today but it's a fun lab for the future uh another thing that oxen's responsible for is the plants ability to sense gravity look at this colius here see how it's been laid down on its side now even if I

put um now light um has an impact but plants also are able to sense gravity uh so that they know to grow up and so again that oxen moves to the lower side and that creates uh that creates that curvature that we see in gravitropism so oxin is a pretty busy um busy little hormone there here's some Nanos and tobacco and again we're seeing that the oxin are telling this plant to grow super tall and if we were to cut that down right here we would be removing that oxin at least temporarily until that new

maristone can develop and start secreting that oxin and in the meantime we'd begin to see that auxiliary growth and that's why that pinching back that removal of the terminal bud is removing the oxin and in doing so it creates um bushier rather than taller plants oxin is also very important for root development and um so in a lot of our cuttings that we do um we use a synthetic form of oxin in order to dip our cuting into to create and initiate root um there's actually natural forms of oxen um for example in willow trees

in the genus uh salic uh they produce a lot of oxin naturally and so if you you uh needed a cheap and organic rooting hormone you can take little um pieces of stem from um willow trees or shrubs and cut them up and soak them in water uh for a few hours um and that's good for you know a fair amount of cuting as long as you use it right away so oxin a very very important um pgr uh gibberellin uh gibberellin help stimulate flowering um and there's synthetic forms of gibberellin uh that we can

kind of brush on to plants to help increase flowering now we can do this through light as we learned light duration long day and short day plants can help initiate flowering we can also um help encourage flowering uh because of temperature differentials in the greenhouse um we don't go into that in that class but daytime and nighttime temperatures and how you manipulate those uh can also encourage flowering but you can also um kind of paint on some synthetic generate gibberellin to initiate flowering um gibberellin can help with initiation of mination and they also similar to

oxin cause elongation but specifically gibberellins occur because of elongation in between the nodes of a stem if you remember our stem Anatomy the inter node is kind of the place on a stem um um that's in between where little buds or branches or leaves might form um this is produced throughout the plant whereas oxin is produced in the tip take a look at these two plants so jerelin cause internodes to stretch and this comes into play a lot if you've ever left like a a tarp on the grass for too long or a blanket and

you notice that when you pull back that blanket or tarp the grass is um it's yellow right it stops producing chlorophyll um but it also it also is really long and stretchy uh much longer than the grass around it um so with highlight intensity we don't see that kind of stretching whereas in low light um the gibberellin kick in to help stretch the interner nodes if you can think about it as a survival mechanism the gibber Els are saying hey I don't have any light right now I can't grow and I'm going to die without

it so uh rather than putting my energy into creating chlorophyll um which breaks down actually really quickly over time plants are continually making new chlorophyll I'm going to put my energy into this gibberellin which is going to cause my intern nodes to stretch so that I can get around this thing that's blocking me um like the tarp for example in order to capture the light um in the in the greenhouse right we can have plants that are too close together so they're not getting enough light they're shading each other and this results in um weaker

plants that break when we ship them and so um we have a product on the market it's called B9 and it's a growth regulator that causes the gibberellin um to stop it inhibits gibberelic um acid uh the gibberellin from being produced and in doing so instead of having nice long tall plants we actually have more compact growth and if you think about the appearance of a plant um you're not really buying it for the stems you're looking at that foliage or the flow and so uh we use this sometimes in Greenhouse production um poas are

a good example where instead of having Long Tall growth we've got nice compact bushy growth cyto enzy mentioned um often work very much um against oxin whereas oxin are produced in the tips uh the cyto canines are often produced in the roots and their job is to push out lateral growth um and also the formation of new cells and cell cell differentiation so cyto Canin we typically see in the use of tissue culture production um this is often added uh additionally and it can come naturally uh for example cyto Canin occur naturally in things like

coconut water uh and things like that or synthetically in a powder that we mix into this medium to this augur uh in order to to grow and rep replicate cells um it can cause um cell division uh and it can also uh help cells decide what kind of cells they're going to be and help them differentiate um so cyto kinin we um we don't see as much outside of micropropagation but we are starting to see it I'm seeing it in facial creams and wrinkle creams and things like that um because this plant hormone is associated

with h with cell growth um so again just kind of comparing cyto Canin to oxin if you remove the oxin from the top the cyto canines kick in to promote lateral growth um whereas the oxin typically inhibit inhibit the cyto kindes instead causing apical dominance or for that plant to go straight up ethylene gas okay so ethylene gas is the reason why you've heard that expression One Bad Apple can spoil the whole bunch and that's because uh ethylene gas is produced in ripening fruit and what it does is it encourages ripening of others or sence

aging maturity uh and so if you've ever had like um green tomatoes and we get those all the time in Colorado it's October the frost is coming I still have plenty of green tomatoes I can take my green tomato and because this is a gas right it's not like a chemical um that's that's passed through osmosis from cell to cell it's actually a gas that kind of surrounds uh the parts of the plant which is like the stems and ripening fruits and dying leaves of a plant um it can be released and captured so I

take a brown paper bag I put in my green tomatoes and I throw in a piece of ripe fruit whether that's another tomato or an apple or something um this is actually used commercially um so what this does is basically is it ages or matures a plant it thickens the stem stems it can break down chlorophyll it can um weaken cell membranes it can soften cell walls it can cause flowers um to more quickly um develop their fruits and that fruit to ripen more quickly now that's great if you want to speed up production um

but syence particularly like in an annual um kills plants and so it's great not to have like dead and dying plant material inside your Greenhouse with others not to mention pest problems um but you can see a practical application actually here's another experiment just if um these two plants here you see that quick ripens but there is an upside to etheling gas in fact if you've ever eaten a banana um if you've ever eaten a banana uh chances are it's been ripened using ethylene gas this is a naturally occurring odorless flavorless um very safe gas

but um bananas bananas are like the leading fruit in the US it's crazy they've only been in the US about 100 years um great book about U the banana I'll put on on our reading list but um bananas if you ship them ripe are going to be bruised in nasty and so they're usually picked green and shipped um unripened uh when they get to a kind of a distribution center for groceries um they're they're put into kind of a sealed container where they uh introduce ethylene gas and that actually helps them ripen up quickly for

market so they don't have to keep them in storage um the last one we'll talk about is abscissic acid uh and this is the plant stress hormone and this is something that often kicks in to let a plant know that hey um conditions are not favorable for example uh an adaptation for really dry EnV enironment might be to um have a plant's stomata remain closed during the hottest parts of the day and so obic acid would be the chemical hormone that's like hey it's pretty hot here you're going to lose water through transpiration if you

open up your stomata even to get the CO2 to undergo photosynthesis so let's keep them Clos why don't we uh it also in uh keeps seeds dormant um until uh conditions are favorable for germination such as temperature and moisture um so again with uh growth Regulators plants produce these but we also have um uh applications for them in our greenhouse and so we've got um synthetic ones for helping rooting uh here's a couple of different um industry brand names uh and we've also uh used them for growth retardant for example with jellins to keep plants

compact or to like jib a chamelia when you need something to bloom you can apply it um for the most part the um the use of these can be somewhat expensive so we don't typically see them outside of applications like rooting hormone um uh unless you're in a fairly large commercial um uh environment here's some uh things to keep in mind when growing them and um safety as well um some of the synthetic uh pgrs are are not terribly um good for you so make sure that you're following safety applications should you choose to use

them