2022-07-31 by Khushi Goel

[Transcript] IndiaAsksWhy | Why Do Trees Not Stop Growing?

Listen to the full episode, here [00:10] Both- “Hi Everyone”

U- I am Utsuka

J- I am Jigyasa

Both- And you are listening to IndiaAsksWhy.

U- A science podcast supported by IndiaBioScience

J: Where we do the research for you to get smarter.

U: Join us, as India becomes more curious,

Both: one question at a time!

[00:33] J: Last week Jigyasa visited Utsuka’s place. Utsuka took to show Jigyasa her photo album. In one picture, Utsaka and a mango tree in her backyard were roughly the same height. But in another, it had grown way taller. That made them wonder, ‘Why do trees not stop growing?’ And to learn more about trees, they talk to Dr Vinita Gora, a plant scientist from IISER Bhopal about why different trees grow to different heights and ask her about her love for plants.

[01:12] U: Jigyasa, come over here. I really want to show you my photo album. So, this is my picture next to our mango tree when we first moved into our home. And this is a picture next to the same mango tree four years later. Notice any difference in me?

[01:35] J: Yes, yes. I notice. You've grown taller by a foot. But the mango tree? It has grown so much taller! I wonder how trees just keep growing?

[01:53] U: Great question! Before we talk about how trees grow, let's see what we mean when we say "the tree grew". J: Increase in the size of the tree? U: Yes, an irreversible increase in the size is the tree’s growth. And that includes… J: Increase in the height, developing new branches, new leaves, fruits coming up, the widening of the tree trunk... U: And root extending downward and sideways; all this is growth.

[02:25] J: Oh, but I really want to know how the mango tree grew taller than you when you've barely grown a foot in the past few years!
U: Okay. So, you want to talk about the height aspect of growth. The mango tree has a region called the meristematic region at its stem’s tip. [02:44] J: What’s meristem? U: Well, for the mango tree to grow in height, it has to: 1. Make more cells; 2. Turn them into specific kinds of cells; 3. Increase the cell's size. All these three steps happen inside or closer to the meristematic region. J: Okay, let's go one by one. How do they make more cells? U: In trees, or generally in plants, there is a small group of cells called meristematic cells in the meristematic region, whose job is to keep dividing. J: Who "tells" them to keep dividing, Utsuka? [03:24] U: Their DNA, Jigyasa. It says, "Hey, you are a meristematic cell. You need to keep dividing!" Then there are different messengers (aka chemicals) from neighboring cells of meristem which say, "keep making more cells!" Sometimes these "messengers" come from leaves, branches, and even roots! J: Cool! U: The divided cells are not yet specialized. So, the tree's next step is... J: Turn the divided cells into specific kinds of cells! U: Yes, these divided cells -- as they move out of the meristematic region -- become cells of leaves, cells of the stem, and so on. Again, there are specific messengers and genes that "tell" cells what to become. J: The third step is making the cells bigger, you said? U: The third step happens parallel to the second. While cells are becoming specialized, they get bigger by drawing more water and making more cellular components. [04:31] J: Going back to the first step. Do meristematic cells keep making new cells? U: Let's think. What do the dividing cells need? J: Uhm... nutrients? U: Right. Carbohydrates, minerals, water, and so on. And trees don't have access to these nutrient sources all the time. Correct? J: So, if they kept dividing, would they run out of resources to do anything else? [05:00] U: Exactly! So, there are messengers to tell "Uh oh, we are running out of resources. Let's not make more cells for some time. And then there are messengers which would say, "It's winter. There's no point in growing now!" J: Okay. So if growing is so much work, then why should trees keep growing in height? U: Every tree species has a certain tree height as its identity. Mango trees grow in height to get to their resources like sunlight. Trees can’t go to catch sunlight – they are anchored – so, they… J: ...Grow! Hold on. Plants need resources to grow, but they need to grow to get more resources. U: Yes, so they "calculate" when to grow. J: Like? [05:46] U: Trees can "detect" that neighboring trees are shadowing them and tell their meristem, "let's grow taller to catch more sunlight". Recently, plant researchers have found messengers that tell their meristems to stop growing in height when they think, "Ah, we can never bypass this neighbor. Let's not waste precious resources trying to grow any taller". J: Oh, wow! Plants are very clever! But Utsuka, even the tallest of the tallest trees, has to stop growing in height at some stage, right? U: Yes. Water, as we know, is crucial for growth and photosynthesis. As a tree keeps growing taller... J: ...It's harder for them to suck water to the top? U: Exactly! You are super smart, Jigyasa. So, at some stage, all trees stop growing in height. J: If all trees reach the same height? What would happen then? U: I know the perfect person to answer this question! A scientist who studies the story of how plants came to be, Dr. Vinita Gowda - a plant evolutionary biologist!

[06:58] J: And now, it’s time to ask a scientist.

[07:02] J: We are back with Dr Vinita Gowda, a plant scientist from IISER Bhopal. She studies plant evolution, ‘how plants and its pollinators interact’ and so much more than that. Why don't you go ahead, Utsuka and ask your question.

[07:16] U: So, Dr Gowda, our question for you is that, “if the maximum level to which the tree can grow, why is there such a huge diversity in the height of a tree? Why aren’t all of them the same height, basically?”

[07:30] Dr Gowda: The answer to many questions of this is one word, ‘competition’. Because access to the sun is something that is very vital and so is the access to water. That is beyond a certain height it just becomes a cost for the plant to grow taller. As the tree grows taller, the temperature changes at the top, the environment is different, the wind pressures are going to change. And when the wind pressure is high then your branches are gonna break and whenever something breaks then you are losing tissues that you generated so on and so forth. So there will be plants which are herbaceous things.. Most of them don't really like too much sun which are herbs then you have the shrubs which have a little bit of woody tissue and they need a little sun. And even in shrubs there are shade tolerant and shade intolerant and everything in between.

J: So how do these plants decide what height to aim for?

[08:15] Dr Gowda: That information is there in genes. So in some sense this competition for sunlight, competition for space, competition for nutrients means that all organisms cannot be of the same kind. And so competition has resulted in the selection of different traits and one of these traits is tree size or tree girth or tree height, whatever we call it. Basically it is the diversity that is created in nature. If all of them like the same thing then no one would have that resource, there will be a shortage of that particular resource. While if different organisms go for different resources or they modulate that then you can accommodate more people. So I think it is more sort of how the community and how it is structured. Like us, if all of us wanted to be doctors then there would be no patients in some sense. So therefore we have different people. We need artists, scientists, teachers, and social workers. This creates a special place for all of us and therefore it accommodates all of our requirements. So it's just an outcome of this competition and the community and its structures.

[09:11] J: Does that mean that a lot of your work is going in collecting plant samples and studying their DNA to see how their behavior changes with the environment or doesn't change?

[09:21] Dr Gowda: So yeah, my lab is called the ‘Tropical Ecology and Evolution’ lab. All our work is ‘Question-based’. The method is something that we pick as we try to answer the question. So therefore if a question requires me to investigate the DNA or a small fragment of it or the entire genome, we just sort of adjust to the method that is required. So, in one of our recent papers that came out we looked at what drives the diversity in this one specific genus of gingers called Hiricium also known as butterfly gingers or ginger lilies; which is spread in India and goes all the way to southeast asia. To investigate, what drives the diversity in genus we looked at its molecules, which is its DNA, we also looked at the morphology which is the structure.

[10:06] U: So when you were studying this ginger family, did you have to go to the forest to see where and how they are distributed and so on?

[10:13] Dr Gowda: Yeah so you have to go. You can do without going; that is where herbariums come into picture. The reason we went to the forest is because the highest diversity of this ginger is found in the north east of India and we really wanted to see why they are more diverse in the north east of India than the western ghats for instance. Therefore we went to the forest and we collected the plants. So yeah, we take any approach, any instrument that is required that is used by a physicist, chemist, it doesn't matter in my lab. If the question requires that I need to do NMR then we do NMR, if it requires me to do cell culture we do cell culture, if it requires me to look at DNA, we do that if it requires me to do anatomy just disectioning and looking at the structure, we’ll do that. So we are not necessarily restricted by the tools. We are restricted by the question.

[10:52] U: You mentioned that you like to ask very interesting questions about plants, for example, ‘why are these ginger families more diverse in north-east and western ghats?’ So what drives you to ask these questions? Why are you interested in all these?

[11:06] G: I find plants far more fascinating than anything else. I have to say I actually don't relate to animals at all and that is sort of a very harsh statement to say given that I am an animal. I don't relate to my own species in that sense. And I relate to plants partly because I like plants because they represent something very liberal and flexible. They can do anything. For instance, if one of the questions that you already look in my lab is ‘when does a flower decide which sex it wants to be?’ And I like that challenge that anything can happen. And every single time I have said that I know this plant, I know what it's doing. It does something else. So it keeps me sort of excited and I like these kinds of challenges, I guess. I think one thing is that strangely while I do like plants and later more recently I realized that I always like plants. But you know I like how everyone is. There was a time when i thought i wanted to be a chemist, there was a time when i thought i should be a zoologist, a physicist and what not. I have gone through that phase. Interestingly what changed for me, is oftentimes people have stories about how someone motivated them. In my case, like most of the times my decisions were based on the people who were not very encouraging. That it's just what happened that my best teachers in most places were botanists. So once, I sort of realized that I wanted to study plants, I did my bachelors in botany and chemistry and zoology. And then between the two I had really good botany teachers so I decided to do a masters in botany, and all that biotechnology and bioscience that it had. I guess I always revolted against the common thing, and when I was going for masters, biotechnology, biochemistry and microbiology were the in thing and everyone was doing molecular work and I just didn't want to do it. So it was more like a personal revolt that I went to the most basic science and that was botany. And I just said I am gonna show the world that this is really good. So i went to botany and luckily i had some very good teachers there too. And then I kind of realized that I sort of got exposed because of people, my seniors and a few people about this subject called ecology which is usually taught as a side subject or at that time it used to be taught as a secondary subject, a topic within botany or a topic within zoology. And so ecology allowed me to not choose because it could be you know, do plant interactions, plant-animal interactions in the plant biologists but you’ll also be a zoologist. It allowed me this flexibility. And then there was molecular ecology which means that I could also do molecular work. So I think ecology allows me to sort of explore and not necessarily pick a very specific field. So that's when I decided that I wanted to do ecology work and for various reasons I decided to go to the US for my doctoral work. I think it is important when you see something, when you see someone else doing that you should feel a little bit of a tinge of jealousy. You know when I see a very good work that is published, I kind of read it and I go, I wish I had done that and I know where I should be. For me, botany and the kind of science that I do is exactly that. But I think trying is very important. It helps you to focus and decide what is of that interest to you. Because there are so many fields. And therefore you need to weed out the things that do not interest you. Everyone has different interests. And there are jobs where some people don't want to work beyond 9 to 5 in which case you like sitting in a lab and it's 12 o'clock and it's like it's past 5. Then lab work is not your thing. Pick a job that's 9 to 5. So I think my advice would be to see what you don't like and also identify what you like and remember that nothing is easy. If it comes too easy then there is something wrong. So you probably want to know, worry about it and work really hard and be sincere to yourself after all and because science is that and it's our own journey and the questions that we are handling, it's a process and we should just enjoy it.

[14:59] J: That's great, that's a lovely answer.

[15:02] U: So Jigyasa, what did we learn today?

[15:06] J: Trees grow taller because they have a particular region called meristem present at their shoot tip. The DNA in the cells of the meristem decides the height of the trees. The trees can also keep a check on the nutrients and their surrounding trees and communicate to the meristem on whether to grow or not.

[15:24] U: Exactly. Trees are very clever. We also spoke to Dr Gowda and learnt about various techniques that she uses to answer curious questions about plants and trees. She says different tree species have different heights because that diversity is very essential to avoid competition.

[15:49] J: But what we know about trees and plants might change as we get more evidence over time.

[15:56] U: Who knows? One of our listeners might study trees in the future and find out something about them that we never knew before.

[16:10] U: So that’s it for today.

J: If you want to know more about today’s scientist, we have linked his profile in the Show Notes!

U: If you have any questions that you want us to explore, shoot them away to Indiaakswhy@gmail.com

J: If you'd like to directly talk to us directly, then join the fun, join the fun on our Telegram Group. Link in the show notes.

U: For updates on IndiaAsksWhy, follow @IndiaAsksWhy on Twitter, and @India_AsksWhy on Instagram.

J: Shweata N. Hegde and Ruchi Manglunia are the hosts for the podcast.